Metabolic etiologies in West syndrome.

West syndrome (WS) is an early life epileptic encephalopathy associated with infantile spasms, interictal electroencephalography (EEG) abnormalities including high amplitude, disorganized background with multifocal epileptic spikes (hypsarrhythmia), and often neurodevelopmental impairments. Approximately 64% of the patients have structural, metabolic, genetic, or infectious etiologies and, in the rest, the etiology is unknown. Here we review the contribution of etiologies due to various metabolic disorders in the pathology of WS. These may include metabolic errors in organic molecules involved in amino acid and glucose metabolism, fatty acid oxidation, metal metabolism, pyridoxine deficiency or dependency, or acidurias in organelles such as mitochondria and lysosomes. We discuss the biochemical, clinical, and EEG features of these disorders as well as the evidence of how they may be implicated in the pathogenesis and treatment of WS. The early recognition of these etiologies in some cases may permit early interventions that may improve the course of the disease.

Disorders with inborn errors of metabolism have been found in approximately 3%‐22% of infants with infantile spasms

Infantile spasms may be diagnosed in up to 12% of pediatric patients with phenylketonuria

Currently, in many cases, diagnosis of infantile spasms may precede the diagnosis of the metabolic disorders

Infantile spasms have been associated with numerous metabolic disorders, although the association with many of them can be sporadic

Early diagnosis of metabolic disorders may lead to early initiation of appropriate treatment

Epileptic encephalopathies are a group of disorders for which epilepsy or epileptic activity plays the main role in disease phenotypes such as severe cognitive and behavioral impairments.1, 2, 3 As an early life encephalopathy with an average age of onset of 6 months, West syndrome (WS) is characterized by infantile spasms (IS), hypsarrhythmia, an interictal electroencephalography (EEG) pattern with irregular, high‐amplitude slow waves on a chaotic epileptic background, and neurodevelopmental decline; the presence of 2 of these symptoms confirms the diagnosis.4, 5, 6 IS is often used to describe the syndrome but can also be used in the literature to describe the characteristic flexion or extension spasms that manifest in this syndrome. More recently, and to also address the patients who have late‐onset spasms or continue to have spasms beyond the infantile age, it has been proposed that the term “epileptic spasms” be used when referring to these seizures. In patients with WS, IS disappear over time but are often replaced by various types of seizures. WS may also develop into Lennox‐Gastaut syndrome. Many of the infants with IS develop signs of autism spectrum disorders. Indeed, most patients with the disease show poor outcome, with neurodevelopmental or neuropsychiatric symptoms.3, 5, 6, 7

The incidence of IS is 2 to 3.5 infants per 10,000 live births, and the majority of affected infants (~64%), have structural, metabolic, infectious, or genetic etiologies, whereas in the remainder, no etiologies can be identified.5, 8, 9 In a cohort of 251 infants with IS,9 the breakdown of etiologies, according to the general classification scheme proposed by the International League Against Epilepsy (ILAE),2, 10 included metabolic (4.8%), genetic (14.4%), genetic‐structural (10%), structural‐congenital (10.8%), structural‐acquired (22.4%), and infectious (2%). Genetic examination of 356 trios consisting of parents and probands with IS and Lennox‐Gastaut syndrome revealed that 12% of the probands carried de novo genetic alterations.11 However, prognosis is better in infants with IS due to unknown etiology following early treatment compared to those with structural and metabolic etiologies.5, 8, 12

In this review, we discuss the contribution of metabolic etiologies in the pathology of WS in light of the recent literature. In addition, we describe in vitro and in vivo models of the metabolic disorders included in this review. We have included only articles in English language that were retrieved from PubMed using the search keywords “Infantile Spasms,” “West syndrome,” or “epileptic spasms” to identify relevance to IS. We used keywords “Inborn Errors of Metabolism” or “Metabolic Etiologies” or specific names for each syndrome or gene to retrieve articles on metabolic etiologies of spasms. In our review, we included all etiologies that manifested with IS, recognizing that, for rare diseases, it may be difficult to establish definite causative associations, since these are often based on case reports. Therefore, the level of evidence and strength of association of the various disorders with IS is not the same across these disorders. Articles describing work on both in vivo and in vitro models were accepted for review. Furthermore, separate searches for animal models (“rat” or “mouse” or “mice” or “animal model”) of specific metabolic syndromes were retrieved for review to determine if they produced a phenotype consistent with epilepsy or IS.

Inborn errors of metabolism may include metabolic errors in organic molecules involved in amino acid and glucose metabolism, fatty acid oxidation, metal metabolism, pyridoxine deficiency or dependency, or acidurias in organelles like mitochondria and lysosomes, as well as other rare diseases such as leukodystrophies13, 14, 15, 16, 17, 18, 19 (Table 1). In a Chinese cohort of 60 patients with IS, metabolic disorders were found in 47%, among whom 22% had inborn errors of metabolism.19 In a retrospective study from Saudi Arabia consisting of 80 patients with IS, hereditary neurometabolic disorders were diagnosed in 12.5%, although the high rate of consanguinity (75% among IS patients with metabolic disorders) may have contributed to the high incidence in this population.20 In a larger U.S. cohort (251 infants with IS), metabolic etiologies were identified in 4.8%.9 In the United Kingdom Infantile Spasms Study (UKISS) study, metabolic etiologies were reported in 3.1% of IS patients: 2 newborns with hypoglycemia and 2 infants with pyridoxine dependency or mitochondrial disorder (n = 127 responders).21 The variability in the frequency of such metabolic etiologies may be due partially to the ethnic origin or to the different diagnostic batteries used for detection of such abnormalities. Early diagnosis of certain metabolic etiologies may permit the early initiation of appropriate treatments of WS, which, in certain cases, may ameliorate symptoms or may even lead to the resolution of the disease, such as in phenylketonuria (PKU)22 or cobalamin (vitamin B12) deficiency.23 Such cases emphasize the importance of genetic or metabolic screening of newborns when they are suspected to have metabolic diseases.17, 18, 19, 24, 25

Table 1

Metabolic etiologies in IS

Metabolic errors in organic molecules
Amino acid metabolism
Phenylketonuria	Nonketotic hyperglycinemia
Glucose metabolism & transport
DEND	Persistent symptomatic hypoglycemia (specific syndromes)	GLUT1 deficiency
Fatty acid oxidation
SCAD deficiency
Metal metabolism
Menkes disease
Vitamin B6‐, Pyridoxine dependency, & pyridoxal 5′P deficiency
Acidurias
d‐Glyceric Aciduria	Methylmalonic acid & cobalamin deficiencies
Homocysteinemia	Propionic acidemia
Metabolic errors in organelles
Mitochondrial disorders
PDHC deficiencies	Leigh syndrome	Alpers‐Huttenlocher disease
Lysosomal storage diseases
Hurler syndrome	Niemann‐Pick disease
Other diseases
Leukodystrophies	Biotinidase deficiency	DBP deficiency
Biotinidase deficiency	Williams‐Beuren Syndrome	CDG
Molybdenum cofactor deficiency	Primary carnitine deficiency	Isovaleric acidemia

CDG, Congenital disorders of glycosylation; (DBP) D‐bifunctional protein; DEND, Developmental Delay, Epilepsy, and Neonatal Diabetes; GLUT1, Glucose Transporter 1; PDHC, Pyruvate dehydrogenase complex; SCAD, Short‐chain acyl‐coenzyme A dehydrogenase enzyme deficiency.

Metabolic Errors in Organic Molecules

Deficiencies in amino acid metabolism

Phenylketonuria and related disorders

Phenylketonuria (or PKU) is the most prevalent inborn error of metabolism, which is caused by autosomal recessive mutations in the phenylalanine hydroxylase (PAH) gene leading to mental retardation, seizures, and motor deficits if left untreated.26, 27, 28 PAH mutations result in defect(s) in oxidation of phenylalanine (Phe) to tyrosine, a precursor of the catecholamines dopamine, norepinephrine, and epinephrine. Excess Phe metabolites inhibit dihydroxyphenylalanine (DOPA) decarboxylase, 5‐hydroxytryptophan decarboxylase, and glutamic acid decarboxylase (GAD), and interfere with myelin formation.29, 30 Increased Phe levels can be toxic to the brain, and untreated individuals develop irreversible mental dysfunction. A simple blood test can determine Phe levels and therefore PKU diagnosis.31 Although dietary restriction of Phe is the most common treatment for PKU and should be started immediately after its diagnosis, tetrahydrobiopterin (BH4) in mild cases and large neutral amino acids mainly for adult PKU cases are alternative therapies, although they are not as potent as dietary restriction; for review.27, 28, 32

In an adult population of 3,714 individuals with PKU, the prevalence of epilepsy was 5.2%.33 However, in a comprehensive retrospective study from China evaluating 503 PKU patients that included pediatric patients, seizures were found in 107 PKU patients (21.3%). 62 of them (12.3%) had WS with typical (76%) or modified (24%) hypsarrhythmic pattern recognized on EEG.22 Of interest, 82% of those patients manifested WS before PKU diagnosis could have been established. A positive correlation was present between the age of dietary restriction initiation and incidence of WS in patients, indicating that early diagnosis and treatment could prevent the evolution of WS. Supporting this, 5 patients receiving combined treatment showed amelioration in myelination on their follow‐up magnetic resonance imaging (MRI).22 Other studies also reported PKU patients presenting with hypsarrhythmic EEG and/or IS34, 35, 36, 37 (Table 2).

Table 2

Disorders of inborn errors in metabolism: clinicopathological findings and association with WS

Metabolic errors in West syndrome
Metabolic disorder	Mode of inheritance—metabolic or gene defect	Main clinical and laboratory abnormalities	Brain pathology	Association with IS	Treatment effect on IS
Metabolic errors in organic molecules
Amino acid metabolism
Phenylketonuria (PKU)	AR – PAH26, 27, 28 AR ‐ DHPR Deficiency39, 40	↑ Phe22, 34, 35, 37 Mental retardation22, 34, 35, 36 *↑ Biopterin, ↓ DHPR activity41 Hypotonia36, 37, 41 Developmental delay37, 41	Cerebral atrophy41 Delayed myelination22, 41 Lesions in white matter, basal ganglia, cerebellum, brain stem37	IS with typical or modified hypsarrhythmia has been reported in up to 12% of PKU patients when pediatric population is included.22, 35, 36, 41 In a study, 82% of patients with PKU were diagnosed with IS prior to the diagnosis of PKU.22 30.4% (7/23) of patients with PKU had hypsarrhythmia and 5 of them reported IS in.35	Protein restriction: 8/62 cases: trigger IS22 17/62 cases: effective (with 77.8% relapse)22 Protein restriction + VPA and/or NTZ: 11/62: effective (18.2% relapse)22 Protein restriction effective in 2 cases, reduction in spasms in 1 case, not effective in 1 case35 (1) Corticotropin, (2) corticotropin + protein restriction (1) only reduction in spasms, (2) effective in 1 case36 Protein restriction, VGB, VPA, Benzodiazepines and prednisolone: no effect in 1 case37 (1) Protein restriction, L‐DOPA, 5‐HT, (2) Protein restriction, CZP (3) Protein restriction, hydrocortisone and neurotransmitter support (1) No effect, (2) Partial effect on myoclonus (3) effective in 1 case41
Nonketotic hyperglycinemia (NKH)	AR ‐ GLDC, AMT or GCSH52, 53, 54, 67	↑ Glycine67, 68 Mental Retardation68 Hypotonia67, 68 Abnormal BAEP66 Abnormal visual responses66	Brain atrophy67 Bilateral subcortical heterotopia67 Choroid plexus cysts68	Case reports with IS, hypsarrhythmia54, 65, 66, 67, 68, 343 IS/hypsarrhythmia are considered common among neonatal NKH survivors beyond neonatal age.17, 54, 64 0.7% (1/134) of IS patients with metabolic screening in9 had NKH.	NTZ & PHT No effect in 2 cases65 AEDs (unmentioned) No effect in 3 cases67 Sodium benzoate, ketamine, dextromethorphan, VGB, PHB, PHT, CZP, diazepam, ACTH Only reduction in seizures, developmental delay persists in 1 case68 Case reports of vigabatrin‐induced acute encephalopathy reported in two infants with NKH343
Glucose metabolism and transport
Developmental delay, epilepsy and neonatal diabetes (DEND)	AD ‐ KCNJ1177, 80, 81, 82	Neonatal Diabetes77, 80, 81, 82 Developmental delay77, 80, 81, 82 Hypotonia77, 80, 81 Ptosis77, 80 Dysmorphic features77, 80	Normal81	Case reports of IS & hypsarrhythmia77, 80, 81, 82	(1) Insulin, (2) PHB, VGB, PHT, ACTH (3) ACTH and glucocorticosteroids + Sulfonylurea (1) effective against DM, (2) not effective, (3) effective in 1 case but the patient died after initial amelioration80 (1) Insulin, (2) Pyridoxine, PHB, VGB, TPR, LEV, clobazam, VPA, (3) Sulfonylurea (1) effective against DM, (2) only transient and partial reduction of the seizures, (3) Effective in 1 case81 Sulfonylurea effective in 1 case82
Persistent symptomatic hypoglycemia probably associated with several underlying etiologies that may have contributed to the outcome	N/A	↓Glucose20, 87, 88, 344 Developmental delay20, 87 Microcephaly20 Hypotonia20	Normal20, 87, 88 Uni‐ or bilateral parietooccipital cyst87, 88 Abnormal occipital/parietal signal20, 88	IS & Typical/modified hypsarrhythmia20, 87, 88, 344 IS87 IS & bilateral epileptic activity87	(1) Glucose infusion and prednisolone, (2) +diazoxide and nifedipine, (3) +ACTH, (4) no effect ACTH, positive effect with PHB 1: No effect, 2: Effect on hypoglycemia, (3) Effect on spasms, (4) continued therapy in 1 case344 (1) Diazoxide, chlorothiazide, (2) +VPA, ACTH, prednisolone (3) VGB, Lamotrigine (1) Effect on hypoglycemia, (2) No effect on spasms, (3) controlled seizures in 1 case87 (1) Diazoxide, (2) VGB, prednisolone (3) VPA, TPR (4) LEV (1) Effect on hypoglycemia, (2) no effect on spasms (3) reduction in spasms, (4) reduction in seizures in 1 case87 (1) Diazoxide, chlorothiazide, (2) prednisolone, currently CBZ (1) Effect on hypoglycemia, (2) reduction in spasms in 1 case87 (1) Pancreatectomy, (2) ACTH (1) DM development (2) IS stopped, developmental delay persists in 1 case87 (1) Pancreatectomy, (2) VGB (1) DM development, (2) N/A87 VGB Spasms resolved but focal seizures continued in 1 case, not effective in 1 case20
GLUT1 deficiency	Deficiency in glucose transporter104, 106 SLC2A1 mutation112	↓Glucose104, 111 ↓Lactate104 ↓ CSF/blood glucose104, 111, 112 Developmental delay111 Microcephaly111	Normal111, 112	Convulsions, IS111 IS‐flexion & hypsarrhythmia112 Rare association with IS	(1) ACTH, (2) Pyridoxine, biotin folinic acid, (3) Ketogenic diet 1&2: no effect, 3: effect on IS and seizures in 1 case111 Ketogenic diet Effect on seizures, developmental delay persists with slight improvement in 1 case112
Fatty acid oxidation
SCAD deficiency	AR‐ACADS116	↑ Ethylmalonic acid117 Developmental delay117	Hypoplastic corpus callosum without splenium and rostrum117 Abnormal cortical gyri117	One case report with generalized tonic‐clonic seizures, IS & hypsarrhythmia117	(1) ACTH, (2) NTZ, TPR, gamma globulin, ZNS (1) partial effect, (2) no effect in 1 case117
Metal metabolism
Menkes disease	XR ‐ATP7A125, 127, 128, 130, 131, 132, 133	↓ Cu & ↓ Ceruloplasmin123, 125, 128, 130 Developmental delay123, 130 Mental retardation123, 130 Hypotonia123, 125, 128 Hypopigmentation in skin123, 130 Fragile hair125	Ventricular dilatation123 Abnormal cerebral vessels125, 128, 129 Cortical atrophy125,128,129 Delayed myelination129	IS are common in Menkes disease; all infants with Menkes disease had IS in126 IS, multifocal clonic seizures & Hypsarrhythmia123 IS with/out slow and asymmetric background activity with multifocal spikes and slow wave discharges125 IS & (modified) hypsarrhythmia127, 129, 130 IS & diffuse irregular slow waves and spike waves126 IS, partial seizures & hypsarrhythmia128 IS, focal seizures & hypsarrhythmia129	(1) Cu‐His therapy (2) PHB, primidone, PHT, CZP, VPA (1) Partial effect only initially, (2) Not effective in 1 case123 Cu‐His, VGB/NTZ Effect on Cu level but only partial effect on IS in 2 cases125
Vitamin B6, pyridoxine dependency and pyridoxal 5′P deficiency
Vitamin B6, Pyridoxine dependency and Pyridoxal 5′P deficiency	AR – PNPO164, 165 AR ‐ ALDH7A1166, 167, 168, 169, 170	Developmental delay165, 169 Mental retardation168 Tonic‐clonic convulsions, myoclonic jerks following pyridoxine withdrawal test168 ↑ Pipecolic acid169	Atrophy165 Increased subarachnoidal space165	Myoclonic, multifocal seizures, IS‐flexion & suppression‐burst activity165 Generalized tonic clonic seizures & hypsarrhythmia in 3 cases168 IS, hypsarrhythmia169 13% (7/53) of patients with pyridoxine‐dependent and/or responsive seizures had IS.345 64% of late‐onset patients with pyridoxine‐responsive and/or dependent seizures had IS.345 0.8% (1/127) of patients with IS had pyridoxine dependency.192	13% (7/53) of patients with pyridoxine‐dependent and/or responsive seizures had IS.345 Pyridoxine, benzodiazepine, VGB, TPR, ACTH, PHB, CZP, VPA, clobazam, pyridoxine, ketogenic diet No effect in 1 case (PLP‐never tried)165 (1) VGB, (2) ACTH, (3) Pyridoxine (1 & 2) Not effective, (3) effective but autism spectrum disorder persists in 1 case169 Pyridoxine Effective in 3 cases169
Acidurias
D‐Glyceric aciduria (D‐GA)	AR – GLYCTK176	↑ Glyceric Acid176, 177, 178, 179 Hypotonia179 Autistic behavior179 Motor and mental retardation179	Delayed myelination179 Cerebral atrophy179 Increased T2 signal from mesencephalon, pontine tegmentum, bilateral dentate nuclei, globus pallidus and thalami179	One case report with IS.179 Epileptic fits & multifocal generalized activity with hypsarrhythmia179	(1) VGB, (2) ACTH, (3) fructose‐restricted diet + VGB + diazepam (1) Not effective, (2 & 3) partially effective, dietary restriction resolve MRI abnormalities in mesencephalon, thalami and globus pallidus, in 1 case179
Methylmalonic Aciduria & B12 Deficiency	AR – MUT183, 186, 187, 188, 189	↑ MMA19, 189, 190 ↓ B1223, 190, 191 Developmental delay19, 23, 189 Macrocytic anemia190 Pernicious anemia191 Hypotonia23, 189, 190 Abnormal BAEP19	Abnormal signal from 2 globi pallidi189 Cerebral atrophy191 Delayed myelination191	IS & hypsarrhythmia19, 23, 189, 190 10% of IS patients in19 had methylmalonic aciduria 0.7% (1/134) of IS patients with metabolic screening in9 had methylmalonic aciduria	Biotin, L‐carnitine, BH4, Vitamin B12, B1 and B2 Not effective in 1 case, partially effective on seizures in 2 cases, effective in 3 cases19 Low protein diet, L‐carnitine, hydroxycobalamin and VGB, lamotrigine, hydrocortisone First 3 treatments are effective on MMA and the rest effective on IS, hypsarrhythmia resolves by time, developmental delay persists in 1 case189 Cyanocobalamin, PHB, ACTH Effective in 2 cases23 ACTH and Vitamin B12 Effective in 1 case190 B12 Triggered IS that is responsive to anti‐seizure treatment191
Homocysteinemias	AR –MTHFR200, 201, 202, 204	↑Homocysteine (maybe also ↓ B12)200, 201, 202, 204 ↓ CS activity200, 201, 202 Developmental delay204	Demyelination204	Case report with IS & hypsarrhythmia and drug resistant epilepsy with episodes of status epilepticus.204	Betaine, methionine, folic acid, corpus callosotomy, vagal nerve stimulation Not effective in 1 case204
Propionic Acidemia	AR – PCC215, 216, 217, 218	Glycinuria222 Propionic academia222 Deficient propionyl CoA activity222	N/A	Case report with myoclonic seizures & hypsarrhythmia222	ACTH and later protein restriction Seizures stopped but developmental delay persisted in 1 case222
Metabolic errors in organelles
Mitochondrial disorders
Pyruvate dehydrogenase complex (PDHC) deficiencies	PDHC deficiency X‐linked‐PDHC E1α1, 229, 230, 231, 232, 233, 234, 235, 236	Lactic acidosis233, 234, 235, 236 Developmental delay231, 233, 235, 236 Hydrocephalus231 Hypotonia231, 233, 235, 236 Abnormal facies231, 235, 236 Microcephaly231, 233, 235, 236	Cortical atrophy231, 233	Seizures & hypsarrhythmia231 IS & hypsarrhythmia233, 234, 235 IS‐flexion & multiple spike and slow wave discharges236 30% (9/30) of female patients with PDHC had WS.234 3% (1/30) of male patients with PDHC had WS.234	High fat diet, biotin, lipoic acid, thiamine Not effective in 1 case231 Thiamine, bicarbonate, biotin, folic acid Not effective in 1 case231 Thiamine + ketogenic diet and Thiamine + DCA Mildly effective in 1 case and effective in the same case, respectively, but developmental delay persists233 DCA + Vitamin B1 Effective in 3 cases, not effective in 3 cases234 DCA + Vitamin B1 + Vitamin b6 + ZNS Effective in 1 case234 Multiple therapies [Vitamin B6, VPA + CZP, Thiamine, DCA] B6‐not effective, the rest‐mild effect in 1 case235
Leigh and Leigh‐like syndrome	Nuclear (including PDHC) or mitochondrial variants of genes involved in cellular energy production20, 245, 246, 247, 248, 249, 250, 251	Developmental delay20, 246, 248, 249, 250, 251 ↑ Lactate or pyruvate20, 246, 250, 251 Hypotonia20, 246, 251 Spasticity246 Optic atrophy/retinopathy246, 251 Cardiac problems246 Cytochrome c oxidase deficiency250	Basal ganglia lesions [80%]246, 251 Thalamus lesions [40%]246, 250 SN lesions [40%]246, 249 Putamen and brain stem lesions248, 249, 250 Cerebral white matter myelination abnormalities249, 250	IS‐flexion or extensor & hypsarrhythmia20, 246, 248, 249, 250, 251 Clonic convulsions, myoclonic seizures, apneic seizures, hemiconvulsions, opsoclonus‐myoclonus246, 251 Tonic seizures249 Tonic‐clonic convulsions250 0.7% (1/141) of IS patients with genetic testing had Leigh disease in9 2/80 (2.5%) of patients with IS had clinical Leigh‐like syndrome in20	VGB Not effective in 1 case20 ACTH Effective in 2 cases, not effective in 1 case246 CZP Effective in 2 cases,250 Effective only in myoclonus in 1 case251 Vitamin B6 Mildly effective in 1 case249 CBZ and ACTH Effective in seizures in 1 case, developmental delay persists250 VGB, +Hydrocortisone or +CZP Effective on IS in 3 cases, developmental delay persisted in all251
Alpers‐Huttenlocher disease	Respiratory chain complex 1 deficiency260 FARS2262, 263 POLG mutations259, 260, 261	Liver dysfunction259, 260, 261 Mental retardation260 Spasticity260	Generalized, symmetric cortical atrophy260	IS, hypsarrhythmia, myoclonic jerks (complex I deficiency)260 27% (3/11) reported patients with FARS2 mutations have IS and hypsarrhythmia, one also had focal seizures262, 263 No known cases of POLG mutations and IS	N/A
Lysosomal storage diseases
Hurler Syndrome Niemann Pick Disease	Low a‐L‐iduronidase (IDUA) activity)269 AR – SMPD1270	↓α‐L‐iduronidase activity (Hurler syndrome)269 ↓ SMPD enzyme (NPD)270 Psychomotor retardation269 Opaque cornea269 Diminished bone density269	Dilatation of ventricles269 Hydrocephalus269	Case reports of IS & hypsarrhythmia269, 270	VGB, TPR, Corticotrophin, VPA, surgical intervention No effect in 1 case269
Other diseases
Leukodystrophies Alexander disease Krabbe Disease	AD ‐ GFAP282, 283 AR ‐ GALC284, 285	Developmental delay282, 283	Demyelination282 White matter degeneration277, 278, 279, 280 Spongy vacuolation277, 278, 279, 280	Case reports IS IS‐flexion & hysparrhythmia282 IS & convulsions283	VGB Effective on IS but developmental delay and other seizures persist in 1 case282 ACTH Effective only on IS in 1 case but developmental delay persists283
Biotinidase deficiency	AR ‐ BTD291, 294 ↓ BTD activity289, 290, 291, 292, 293, 294	Red skin rashes290, 292 Lactic aciduria290, 292 ↑ alanine293 Hypotonia293, 294 Alopecia293, 294 Optic atrophy293 BAEP abnormalities293, 294 Developmental delay293, 294	Normal293 Atrophy294	Case reports IS IS & burst suppression pattern293 Only IS and convulsions293 IS‐flexion spasms &hypsarrhythmia294	Corticotropin & PHB Partially effective on IS in 1st case293 Biotin supplement Effective on IS but persisting developmental delay in 1st and 2nd case293 Corticotropin Partial effect on IS in 2nd case293 PHB +VPA No effect on convulsions in 2nd case293 (1) PHB +VPA, (2) Phenytoin, (3) Biotin+VGB, (4) Biotin + VGB + TPR (1) No effect on IS, (2) effective on IS, (3) Partial effect in 1 case (4) Effective in 1 case, developmental delay persists294
D‐Bifunctional protein (DBP) deficiency	AR ‐ HSD17B4298, 299, 300 ↓ DBP activity298, 299, 300, 301, 302	Hypotonia302	Abnormal periventricular white matter302 Septum pellucidum cyst302 Hypoplastic corpus callosum302	Case report of IS & modified hypsarrhythmia302	• (1) Pyridoxine, (2) Clordemetildiazepam, (3) VPA, Clobazam, lamotrigine (4) VGB (1) no effect, (2) no effect, (3) no effect, (4) only reduction in spasms302
Williams Beuren syndrome (WBS)	AD, Deletion at WBS area of 7q307, 308, 309, 310	Facial elfin appearance310, 311 Cardiac anomalies310, 311 Developmental delay310, 311	Normal310, 311	Case reports of IS & hypsarrhythmia310, 311	ACTH, CLZ, VGB, VPA Over 50% effect on IS in first case310 ACTH, VPA Effect on IS in second case310 (1) Pyridoxine, VPA, (2) ACTH (3) ZNS, NTZ, PB (4) TRH (5) PHB, ZNS (1) No effect (2) effective for IS, but progressive ventricular hypertrophy, (3 & 4) partially effective, effect only on IS in 1 case311
Congenital Disorders of Glycosylation (CDG), Molybdenum Cofactor Deficiency (MCD), Primary Carnitine Deficiency (PCD)
Congenital Disorders of Glycosylation (CDG)	Deficient glycosylation319, 320	Abnormal vision319 Developmental and motor delay319	↓ white matter319 abnormal myelination319	Case report of IS & hypsarrhythmia319	N/A
Molybdenum cofactor deficiency (MCD)	AR ‐ MOCS 1 and 2324 AR‐GPHN324 Toxic sulfite accumulation324	Developmental delay20 Hypotonia20 Microcephaly20 ↑sulfocysteine20 ↓uric acid20	Delayed Myelination20	Case report of IS‐flexion spasms & hypsarrhythmia20	VGB, LEV, VPA Not effective in 1 case20
Primary carnitine deficiency (PCD)	Defective fatty acid oxidation323	Developmental delay20 Hypotonia20 ↓carnitine20 ↑ammonia20	Atrophy20	Case report of IS‐extension spasms & asymmetric hypsarrhythmia, focal seizures20	VGB, CLZ, LEV Not effective in 1 case20
Isovaleric acidemia	IVD enzyme deficiency335, 336	↑IVG335, 336 Lactic acidosis335 Hyperammonemia336	Normal335	Case report of IS‐flexion spasms & hypsarrhythmia335	(1) VGB (2) VGB + ACTH (for IS), Protein restricted diet + L‐carnitine + hydroxycobalamin supplement (for isovaleric acidemia) (1) Not effective, (2) effective in 1 case335

ACADS, Acyl‐CoA dehydrogenase C‐2 to C‐3 short chain; ACTH, Adrenocorticotropic hormone; AD, Autosomal dominant; AED, Antiepileptic drug; ALDH7A1, Alpha‐aminoadipic semialdehyde dehydrogenase; AR, Autosomal recessive; ATP7A, Adenosine triphosphatase; BAEP, Brainstem auditory evoked potentials; BH4, tetrahydrobiopterin; BTD, Biotinidase; CBZ, Carbamazepine; CDG, Congenital disorders of glycosylation; CS, Cystathionine synthase; CSF, Cerebrospinal fluid; CZP, Clonazepam; DCA, Dichloroacetate; DEND, Developmental Delay, Epilepsy, and Neonatal Diabetes; ?GA, ?‐Glyceric Aciduria; DM, Diabetes mellitus; FARS2, Phenylalanine‐tRNA synthetase; GALC, Galactosylceramidase; GFAP, Glial fibrillary acidic protein; GLUT1, Glucose Transporter 1; GLYCTK, D‐Glycerate Kinase; 5‐HT, 5‐Hydroxytryptamine; IDUA, Alpha‐L‐iduronidase; IS, Infantile spasms; KCNJ11, Potassium voltage‐gated channel subfamily J, member 11; L‐DOPA, Levodopa; LEV, Levetiracetam; MCD, Molybdenum cofactor deficiency; MRI, Magnetic resonance imaging; MTHFR, Methylene tetrahydrofolate reductase; MUT, Methylmalonyl mutase; NPD, Niemann‐Pick disease; NKH, Nonketotic hyperglycinemia; NTZ, Nitrazepam; PAH, Phenylalanine hydroxylase; PCC, Propionyl‐coenzyme A carboxylase; PCD, primary carnitine deficiency; PDHC, Pyruvate dehydrogenase complex; PHB, Phenobarbital; Phe, Phenylalanine; PHT, Phenytoin; PKU, Phenylketonuria; PNPO, Pyridoxine‐5'‐phosphate oxidase; POLG, catalytic subunit of mitochondrial DNA polymerase gamma; SCAD, Short‐chain acyl‐coenzyme A dehydrogenase enzyme deficiency; SLC2A1, Solute carrier family 2 member 1; SMPD1, Sphingomyelin phosphodiesterase 1; SN, Substantia nigra; TPR, Topiramate; TRH, Thyrotropin releasing hormone; VGB, Vigabatrin; VPA, Valproic Acid; WBS, Williams‐Beuren syndrome; WS, West Syndrome; ZNS, Zonisamide.

Another but rare cause of PKU is dihydropteridine reductase (DHPR) deficiency, where decreased or no enzyme activity leads to abnormalities in BH4 generation. DHPR deficiency has been associated with autosomal recessive guanosine triphosphate cyclohydrolase I (GCHI), DHPR, or 6‐pyruvoyl tetrahydropterin synthase (PTPS) gene deficits.38 BH4 is a cofactor of PAH and therefore deficiency of BH4 may lead to elevated Phe, despite normal PAH levels. In such cases, dietary Phe restriction may not suffice to correct the symptoms, and BH4 supplementation is required. BH4 is also involved in the tyrosine and tryptophan hydroxylation, and therefore BH4 deficiency results in reduced norepinephrine, dopamine, and serotonin synthesis, which may further contribute to the neurologic symptoms even if Phe levels are corrected.38, 39, 40 DHPR deficiency may further reduce the activity of folate.38 Therefore, the presence of normal PAH levels in patients with elevated Phe or the persistence or progression of neurologic symptoms despite the correction of Phe levels may suggest the need for diagnostic tests for DHPR deficiency. Diagnostic tests may then include DHPR activity levels; analysis of pterins in urine, blood, or cerebrospinal fluid (CSF); and CSF levels of metabolites of monoamines (homovallinic acid [HVA], 5‐hydroxyindoleacetic acid [HIAA]).38 If DHPR deficiency is documented, consideration of additional specific or supplemental treatments for this disease including BH4, folinic acid, or drugs that supplement the monoaminergic deficits should be considered, in addition to Phe dietary restriction.38

DHPR deficiency may also lead to IS and hypsarrhythmia, even though this is one of the rare etiologies41 (Table 2). Phe restriction and neurotransmitter supplement improved a patient's outcome42; however, in a patient with IS, an additional therapy with steroids was needed to control spasms and hypsarrhythmia.41

Early neurological sequelae (ie, delayed myelination) may be overlooked depending on the onset of Phe accumulation37 since myelination is a continuous process starting from the last days of gestation and reaching through adulthood (for review43). In addition, the results of the PKU newborn screenings need to be interpreted cautiously, considering that false‐positive or, rarely, false‐negative results may also be seen.37, 44, 45

In vitro and in vivo experimental PKU models have provided significant insights (Table 3).46, 47, 48, 49, 50, 51 l‐Phe competes both for the glutamate binding region of the alpha‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazolepropionic acid (AMPA) and kainate receptors, and the glycine‐binding site of the N‐methyl‐d‐aspartate (NMDA) receptors in rat neuronal hippocampal cultures, depressing glutamatergic transmission in the presence of very high Phe levels.47 This effect may precipitate seizures, in certain situations, when Phe levels are normalized after its dietary restriction reversing the depressed glutamatergic transmission.47, 49 In vivo models induced by increasing the Phe levels and/or the use of Phe hydroxylase inhibitors have been used but their phenotype resembles more to BH4 deficiency. Therefore, models such as Pah enu2 mice, which carry chemically induced mutations in the PAH gene, through the administration of ethylnitrosourea (enu), have provided an alternative option to model PKU of genetic etiology.46, 50, 51 The adult Pah enu2 model mimics the clinical pathology as well as presents with a decreased threshold to audiogenic seizures.46, 49, 50 Expression studies from these mice also show NMDA receptor subtype–specific changes in glutamate receptors, with increases in NMDA receptor subtype 2A (NR2A) and decreases in NR2B, as well as increased expression of AMPA receptor subunits glutamate receptor 1 (GluR1) and GluR2/3.48

Table 3

In vivo and in vitro models of metabolic diseases associated with WS

Metabolic disease	Disease model and main features	Spontaneous seizures or spasms
Phenylketonuria (PKU)	Rat hippocampal neuronal cultures exposed to high Phe depression of glutamatergic transmission47 Pahenu mice ↓ threshold to audiogenic seizures46, 49, 50 ↑expression of NR2A&B, GluR1&2/348	NR NR
Nonketotic hyperglycinemia (NKH)	Glycine on cultured hippocampal rat slices ↑ excitability & neurotoxicity55 Glycine on auditory brain stem rat slices ↑ transmitter release56 95% ↓ Gcs mice & Amt deletion in mice Neural tube defects69, 70 29% ↓ GCS mice Longer electroshock induced tonic–clonic seizures71 High locomotor activity and anxiety behavior71 Severe neuronal injury after focal cerebral ischemia72 Glyt1 KO mice abnormal respiration reversed by strychnine73	NR NR NR NR NR
DEND	Expression of KCNJ11 mutations in Xenopus oocytes Interfere with KATP sensitivity or sulfonylurea efficacy78 Kir6.2 KO mice Myoclonic jerks, tonic convulsions, & death in response to hypoxia84 SNr firing rates increase during hypoxia in vitro84 Overexpression of SUR1 in mice Protective against kainic acid–induced seizures85	NR NR NR
Symptomatic hypoglycemia	Insulin‐induced hypoglycemia in rats Neuronal damage in the hippocampus, caudate nucleus, spinal cord, and cerebellar Purkinje cells96 In immature rats: neuronal damage in cortex, DG, thalamus, and hypothalamus97 In PN25 rats ketogenic diet protects against neuronal damage if started at PN2199 In fasting rats: decreased latency to seizures103 Insulin & SEZ‐induced hypoglycemia in juvenile rats Seizures100 Hypoglycemia in hippocampal slices from juvenile mice Seizures responsive to NMDA & non‐NMDA antagonists & midazolam, decreased synaptic transmission, spreading depolarizations101 Slices from developing rats exposed to repetitive hypoglycemia decreased LTP102	NR seizures seizures
GLUT1 deficiency	Heterozygous Glut1 mice Generalized/focal onset electrographic seizures, rhythmic or polyspikes, SWDs, no behavioral change113 Behavioral arrests during fasting113 Expression of SLC2A1 variants in Xenopus oocytes Mild to dramatic glucose transport changes115	Seizures NR
SCAD deficiency	BALB/cByJ mice impaired thermoregulation in cold121 Slices from BALB/cByJ mice Cerebral edema, astrocytic swelling, neuronal mitochondrial injury120 Strial slices from ethylmalonic acid–administered rats Deficient redox homeostasis & ↓ Na+/K+ATPase activity122	NR NR NR
Menkes disease	Mobr mice impaired axonal development & synaptogenesis in the cerebral cortex and the hippocampus, ↓ peptidylglycine alpha‐amidating monooxygenase activity in slices137 Phenotype partially reversed by human Menkes transgene139 Inhibition of atp7a expression in Drosophila cell–specific amidation impairments141	NR NR
Vitamin B6‐, Pyridoxine dependency & pyridoxal‐5′‐phosphate deficiency	TNAP KO mice seizures, ↑ PLP, ↓ GABA that are reversible by PLP but not pyridoxine depending on the mice background172 TNAP+/+ and heterozygous mice Pyridoxine‐deficient diet‐induced seizures173	Seizures Seizures
ᴅ‐Glyceric aciduria (DGA)	Rats with high fructose & fat diet type II diabetes180 Glyctk KO mice	NR NR
Methylmalonic aciduria & B12 deficiency	Intrastriatal MMA injection in rats Clonic & tonic–clonic seizures, ↓duration of seizures with preadministration of MK‐801, alpha‐tocopherol or ascorbic acid193, 194 ICV MMA in rats clonic convulsions that can be reduced by pyridoxine, baclofen, and muscimol; ↓GAD activity that can be prevented by pyridoxine & MK‐801196 CD320/TCbIR KO mice cognitive deficits & anxiety198 smaller pyramidal cells, impaired LTP in CA1, ↓ GluR1198 Membrane preparations from MMA‐treated rat cortex/brain homogenates preincubated with MMA ↓Na+/K+ATPase activity, prevented by glutathione195	Seizures Seizures NR No
Homocysteinemias	Heterozygous Mthfr KO mice Impaired short‐term memory in WT animals born to this phenotype & apoptosis in the hippocampus213, 214 Systemic administration homocyteine in rats & mice convulsive seizures205, 206 Intraperitoneal homocysteine injection status epilepticus & flexion seizures in immature rats210 Spikes & spike waves without behavioral correlation in adults rats210 ICV application of homocysteic acid in PN12 rats Generalized tonic–clonic seizures, prevented by pre‐NBQX & AP7 application211 Subcutaneous chronic homocysteine injection in rats ↓ Na+/K+ATPase activity in hippocampal synaptic membrane preparations212 Homocysteine application on rat organotypic cortical & hippocampal slice cultures Excitotoxicity207	NR Seizures Seizures Seizures NR NR
Propionic acidemia	ICV propionic acid injection in adult rats Dystonia, hyperactivity, caudate spiking, ↑ severity of kindling‐induced seizures223 ↑ oxidative stress, ↓glutathione & glutathione peroxidase activity & neuroinflammation ex vivo223 Oral propionic acid application in PN 21 rats ↓ glutathione & glutathione peroxidase activity, ↑ IL6, TNFα, HSP70 & caspase 3, DNA fragmentation, ↓GABA, serotonin & dopamine226	NR NR
PDHC deficiency	PDHC E1 α null mutation in mice Low PDC activity, reduced litter size238 All males die & 50% females survive after only brain‐targeted mutation, females show ↓neuronal density & neuropil fibers, abnormal neuronal localization in the gray matter, ↓lipid synthesis & irregular myelination, adult females have neurological deficits239 PDHC E1 α knockdown in striatum & SN in rats abnormal amphetamine‐induced rotation240 Systemic deletion of exon 8 from PDHC E1 α ↓ brain weight, de novo lipid synthesis, ↓ proliferation, differentiation and migration of newly generated neuronal precursor cells in cerebellum, impaired dendritic development of Purkinje cells ex vivo Impaired acoustic startle reflex242 E2 impairment in zebrafish Ketogenic diet reversed abnormal vision, lactic acidosis & lethargy243	NR NR NR NR
Leigh syndrome	Ndufs4 KO mice Delayed growth, progressive lethargy, ataxia, blindness, progressive loss of startle response & death at week 7 due to systemic KO, hypoxia reversed neurodegenerative changes & ↑ life span, rapamycin also extended life span, prevented brain injury, ↑ GABA, dopamine and free fatty acid levels, ↓glycolytic intermediates254, 256, 257 Similar outcome as in the systemic knockout, in addition, gliosis & microglial activation due to KO only in glial cells and neurons255 ND2 deletion in Drosophila Rapamycin increases the life span but leads to fat storage impairments258	NR NR
Alpers‐Huttenlocher disease	D257A knockin mice (PolyAmut/mut) Early aging, ↓ body size & weight, death264, 265 Early lethality & mtDNA depletion if KO at preimplantation stage267 Polg−/− zebrafish larvae survive till juvenile age266	NR NR
Lysosomal storage diseases	Idua KO mice High glycosaminoglycan in urine, flat face, abnormal lysosomal storage in glial cells, vacuolation in Purkinje cells, neurons in the cortex, no behavioral alterations272 Human IDUA injection into putamen reverses brain pathology273 ICV human IDUA injection prevents brain pathology & spatial learning deficits274 ASM KO mice High sphingomyelin, no ASM activity, ↓body weight & death272 Mutations expressed in this background show high level of expression in brain & higher residual ASM activity than ASM KOs272	NR NR
Leukodystrophies	Mice carrying WT+1 mutated hGFAP No Rosenthal fibers286 Mice carrying WT + multiple mutated hGFAP Rosenthal fibers in adult age, severe convulsions & high mortality rate when challenged with kainate286 Mice with hGFAP Hypertrophic astrocytes, Rosenthal fiber‐like inclusions, upregulation of small HSPs287	NR NR NR
BTD deficiency	Rats fed with biotin‐deficient diet Alopecia of lower back, delayed growth, longer BAEP latency295 BTD KO mice fed with biotin‐deficient diet Hypotonia, demyelination, axonal degeneration, impaired motor neuron function, lethargy, limping, ventriculomegaly, slow growth, weight loss reversed with biotin supplement296, 297	NR NR
DBP deficiency	MFP2 KO mice Failure to thrive, astrogliosis & microglial activation in gray matter, death300, 303 Nestin‐Mfp2−/− mice mild neuroinflammation & axonal impairments in Purkinje cells, motor deficits305 dbp knockdown in zebrafish Rescue by murine Dbp306	NR NR NR
Williams‐Beuren syndrome (WBS)	Fzd9 deletions in mice both heterozygous & homozygous deletions lead to ↓ threshold of pentylenetetrazole‐induced seizures & abnormalities in hippocampal structure, only homozygous deletions lead to spatial learning impairments315 WBS distal deletions in mice cognitive impairment316 WBS distal & proximal deletions in mice defective motor skills316 Heterozygous WBS complete deletions in mice ↓ brain weight, dendritic length & spine density in the hippocampus, GFAP+ cells in amygdala, ↑ immature cells in DG, craniofacial and cardiac abnormalities, spatial learning deficits317 Slices from these mice show unstable LTP in CA1, ↓ BDNF in CA1‐CA3318	NR NR NR NR
Congenital disorder of glycosylation (CDG)	Mgat2‐null mice 20% of the animals show transient paralysis & tremors resembling epileptic seizures322	Seizure mimics
Molybdenum cofactor deficiency (MCD)	Mocs1 KO mice survival 1 to 11 days, human MOCS1 & cPMP injections ↑life span325, 326 Mocs2 KO mice Survival for 11 days, apoptosis in the hippocampus327 Cortex & brainstem, inactivation of all molybdenum cofactor‐dependent enzymes, accumulation of hypoxanthine, xanthine & S‐sulfocysteine ex vivo327	NR NR
Primary carnitine deficiency (PCD)	Dietary restriction of carnitine in rats 50% reduction of carnitine, no other changes328 Dietary restriction of carnitine with butyrobetaine hydroxylase inhibitor in rats Liver anomalies329	NR NR
Isovaleric acidemia	Isovaleric acid application onto rat cortical synaptic membrane preparations ↓ Na+/K+ ATPase activity337 Isovaleric acid application onto rat brain homogenates & mitochondrial preparations ↑ protein oxidation338 IVG application onto rat brain homogenates & mitochondrial preparations Lipid peroxidation & ↓GSH levels338	NR NR NR

AMT, Aminomethyltransferase; AP7, 2‐amino‐7 phosphonoheptanoic acid; ASM, acid sphingomyelinase; ATP7A, Adenosine triphosphatase; BAEP, Brainstem auditory evoked potentials; BNDF, Brain‐derived neurotrophic factor; BTD, Biotinidase; DG, Dentate gyrus; Fzd9, Frizzled 9; GABA, Gamma‐aminobutryric acid; GAD, Glutamic acid decarboxylase; GFAP, Glial fibrillary acidic protein; GLYT1, Glycine transporter 1; GCS, Glycine cleavage system; GluR1,2,3, AMPA receptor subunits glutamate receptor 1,2,3; GLUT1, Glucose Transporter 1; GLYCTK, D‐Glycerate Kinase; GCSH, Glycine cleavage system H protein; HSP, Heat shock protein; ICV, Intracerebroventricular; IDUA, Alpha‐L‐iduronidase; IL6, Interleukin 6; IVG, Isovalerylglycine; KATP, Adenosine triphosphate (ATP)‐sensitive inward‐rectifier potassium channels; KCNJ11, Potassium voltage‐gated channel subfamily J, member 11; Kir, Inward rectifier potassium channel; KO, Knockout; MFP‐2, multifunctional protein‐2; Mgat2, Mannosyl (Alpha‐1,6)‐Glycoprotein Beta‐1,2‐N‐Acetylglucosaminyltransferase; MMA, Methylmalonic acid; Moblo , Mobr and Modp, Murine mottled blotchy, brindled and dappled; MOCS1 and MOCS2, Molybdenum cofactor synthesis protein 1 and 2; mtDNA, Mitochondrial DNA; MTHFR, Methylene tetrahydrofolate reductase; NBQX, 2,3‐dihydroxy‐6‐nitro‐7‐sulfamoyl‐benzoquinoxaline‐2,3‐dione; ND2, NADH dehydrogenease 2; NDUFS4, NADH : ubiquinone oxidoreductase subunit S4; NMDA, N‐methyl D‐aspartate; NR2A and NR2B, NMDA receptor subtype 2A and 2B; LTP, Long‐term potentiation; PAH, Phenylalanine hydroxylase; PDHC, Pyruvate dehydrogenase complex; Phe, Phenylalanine; PLP, Pyridoxal phosphate; POLG, catalytic subunit of mitochondrial DNA polymerase gamma; PN, Postnatal day; SEZ, Streptozotocin; SLC2A1, Solute carrier family 2 member 1; SLC6A9, Solute carrier family 6 member 9; SN, Substantia nigra; SNr, Substantia nigra pars reticulata; SUR1, Sulfonylurea 1; SWDs, Slow wave discharges; CD320/TCblR, Transcobalamin II receptor; TNAP, Tissue non‐specific alkaline phosphatase; TNF? ,Tumor Necrosis Factor ?; WBS, Williams‐Beuren syndrome; WT, Wild type.

Glycine Encephalopathy or Nonketotic Hyperglycinemia

Glycine encephalopathy or nonketotic hyperglycinemia (NKH) is caused by autosomal recessive mutations in mostly P‐protein (a pyridoxal phosphate‐dependent glycine decarboxylase, GLDC) and/or T protein (a tetrahydrofolate‐requiring enzyme, aminomethyltransferase, AMT) and very rarely H‐protein (hydrogen carrier protein, GCSH), which are components of the mitochondrial glycine cleavage system (GCS).52, 53, 54 As a result, glycine levels increase. Glycine is an inhibitory neurotransmitter, especially in the spinal cord, but also acts as a positive regulator of NMDA receptors in central neurons, and is shown to increase excitability and lead to neurotoxicity in cultured hippocampal rat slices,55 as well as to enhance transmitter release in rat auditory brainstem slices.56 Mutational screening of NKH‐related genes in 69 families with NKH patients identified mutations in GLDC and AMT genes in 75% to 83% individuals with neonatal or infantile onset NKH but in none of those with late‐onset NKH.54

NKH symptoms may start to appear in neonatal, infantile, or a later period, with a better prognosis in the latter.53, 54 Transient NKH was also reported with a good prognosis.57, 58, 59 The affected individuals show elevated plasma and CSF glycine levels, and present with hypotonia, seizures, and developmental delay.60, 61, 62 In a retrospective study reporting on the natural history of 65 patients with NKH, seizures were reported in ~90% of patients, most (68%) of whom had neonatal‐onset seizures.63 Among the patients with NKH who manifested seizures, 29% had burst suppression on EEG and 5% had hypsarrhythmia.63 Most of the literature on IS in NKH is in the format of case reports, with very few reports on larger cohorts of patients with NKH. In general, expert reviews of the clinical course of neonatal NKH and a cohort of 56 families with neonatal NKH described the emergence of IS and/or hypsarrhythmia as a common feature of NKH among infants who survive beyond the neonatal period.17, 54, 64

NKH patients manifesting with WS have been reported54, 65, 66, 67, 68 (Table 2). Because the prognosis is very poor in neonatal‐onset NKH, Korman et al.68 treated, immediately after birth, a prenatally diagnosed male newborn carrier of a GLDC mutation with sodium benzoate and ketamine (NMDA receptor antagonist; Table 2). Although the treatment prevented the neonatal hypotonia and apnea it did not ameliorate the long‐term epilepsy and neurocognitive outcomes.

To create a model for NKH, Pai et al.69 used mice with 95% decreased Gldc expression with dramatically reduced GCS activity that led to neural tube defects in homozygote mice (Table 3). Similar defects are also reported from use of mice lacking GCS activity through deletions in the Amt gene.70 However, neither report mentions any seizure phenotype. On the other hand, in a model of mild‐type glycine encephalopathy, mice with 29% reduced GCS activity (low GCS) show a longer duration of tonic or clonic seizures induced by electroshock, and higher locomotor activity and anxiety‐related behavior, which can be partially reversed by NMDA antagonists specific to the glycine‐binding site of the receptor.71 Likewise, after focal cerebral ischemia, mice with low GCS activity demonstrate greater neuronal injury compared to wild‐type (WT) animals.72

The glycine transporter 1 (Glyt1) knockout (KO) mice have been used to model features of glycine encephalopathy.73, 74 Homozygous mutations in solute carrier family 6 member 9 (Slc6A9) encoding Glyt1 are lethal within the first postnatal day (PN), mostly due to respiratory deficits, but strychnine, a glycine receptor antagonist, reverses the abnormal respiratory activity in brainstem slices from these mice.73 An association between NKH and GLYT1 was suspected in a patient with no GCS abnormality but absence of glycine transport system on autopsy.75 A homozygous SLC6A9 mutation has recently been identified in an atypical NKH patient, a 15‐month‐old girl presenting with normal EEG and most of the disease symptoms except encephalopathy.76

Deficiencies in glucose metabolism and transport

Developmental delay, epilepsy, and neonatal diabetes

Permanent neonatal diabetes mellitus has been associated with autosomal‐dominant mutations in the potassium voltage‐gated channel subfamily J, member 11 (KCNJ11) gene encoding the pore‐forming subunit (Kir6.2) of the adenosine triphosphate (ATP)–sensitive inward‐rectifier potassium channels (KATP). In the presence of ATP, KATP channels close and trigger insulin secretion from pancreatic beta cells.77 Closure of these channels leads to membrane depolarization and opening of l‐type voltage‐dependent calcium (Ca2+) channels that subsequently increase intracellular Ca2+ and therefore insulin exocytosis. Some of the diabetic neonates present also with developmental delay and epilepsy,77 a syndrome that is known as DEND (developmental delay, epilepsy, neonatal diabetes).78, 79, 80, 81, 82 Hypsarrhythmia has been reported in a total of 4 DEND patients77, 80, 81, 82 (Table 2), 3 of whom presented with IS and one with tonic–clonic seizures.80, 81, 82 Sulfonylurea therapy (acting by binding to the sulfonylurea receptor part of the KATP and closing them—leading to insulin secretion) mitigated neurological symptoms and psychomotor development in these patients.80, 81, 82 However, one patient died following initial amelioration.80

KATP channels are expressed not only in pancreas but also in skeletal muscle, cardiac muscle, and brain tissue (for review: ref 83). Therefore, epilepsy, including IS when present, was suggested to be linked to the genetic defect in DEND syndrome, rather than the outcome of neonatal diabetes.77, 79, 80 The rarity of reported cases of IS in DEND and the lack of cohorts exploring the incidence of IS in patients with DEND do not allow for strong arguments that there is a specific association of this syndrome with IS rather than epilepsy in general. However, not all patients carrying mutations have neurologic features.79 Despite the fact that the location of K 6.2 mutations has a strong influence on the severity of symptoms,78 DEND is a rare disease and more information is needed to confirm these findings.

In vitro functional analyses of different mutations leading to neonatal diabetes and DEND using Xenopus oocytes indicate that although a mutation (R201C) in KCNJ11 directly acts on ATP sensitivity of KATP, 2 other mutations linked with DEND (Q52R and V59G) lead to conformational change that keeps channels in an open state and therefore interferes with the sulfonylurea efficacy (Table 3).78 In vivo EEG and electromyography (EMG) recordings from K 6.2 KO mice show myoclonic jerks, severe tonic convulsions, and death when challenged with hypoxia, whereas WT animals recover normally.84 In vitro recordings from SNr neurons from the same model displayed no difference in firing rates of the neurons compared to the neurons from WT animals; however, the firing rates increased during hypoxia in the former, whereas they decreased in slices from WT animals, indicating the inhibitory effect of these channels in SNr.84 Likewise, overexpression of the other subunit of KATP channels, sulfonylurea 1 (Sur1), in mice protects against kainic acid–induced seizures and the neuronal damage.85 Although the protective role of these channels in different parts of the brain during hypoxia is known,86 there are no known reports of spontaneous seizures in these models.

Persistent symptomatic hypoglycemia

Patients with persistent neonatal hypoglycemia and specific syndromic etiology such as those that may require pancreatectomy for persistent hyperinsulinemia may develop IS following variable latent time.87, 88 In earlier studies, neonatal hypoglycemia (low blood glucose levels, <2.6 mmol/L) was reported in some of the IS patients in 2 long‐term population studies in Finland.89, 90 However, at the time these studies were carried out, there were limited diagnostic tools to determine the reason behind the persistent hypoglycemia while limited treatment options existed, such as since diazoxide administration and pancreatectomy were developed later. In almost all cases in which IS occurred, there were structural brain abnormalities, indicating that the abnormal MRI finding was the entry criterion in the available retrospective study91 (Table 2).

Increased awareness of the significance of identifying the causes of hypoglycemia and the institution of targeted treatments helped to decrease the incidence of persistent symptomatic hypoglycemia as an etiologic factor in IS.89, 92

Hypoglycemia may have profound effects on the brain where it is used as a primary energy source, even though the exact mechanism leading to IS has yet to be discovered. The effects may include but are not limited to mitochondrial energy deficiencies and increased susceptibility to hypoxic‐ischemic brain injuries,88, 93 and may interfere with any of the proposed mechanisms of IS/WS development.7 The effects of hypoglycemia may be age specific. Experimental data from immature rats and dogs point out that immature animals are more resistant to induced hypoglycemia if there are no other underlying abnormalities, owing to their decreased energy dependence, higher endogenous carbohydrate storage and use of alternative energy sources, as well as compensatory mechanisms such as increased cerebral blood flow and glucose transport into the brain. However, hypoglycemia in the presence of hypoxia‐ischemia may be detrimental (for review94).

A number of models, both in vitro and in vivo, that mimic the hypoglycemic condition are studied in different animals (Table 3).95 Results of one of the studies that evaluated insulin‐induced hypoglycemia in rats indicate that neuronal damage is triggered when EEG becomes isoelectric EEG, regardless of the glucose level.96 The damage is correlated with the duration of the isoelectric time, in that the longer the time spent in the isoelectric state, the greater the brain injury.96 In an attempt to establish an animal model for brain injury caused by neonatal hypoglycemia, Zhou et al.97 later showed that immature rats exposed to prolonged insulin‐induced hypoglycemia had neuronal death and that the damage was seen mainly in cortex, dentate gyrus (DG), thalamus, and hypothalamus but not in the CA1 and CA3 regions of the hippocampus. In contrast, a previous study that used mature animals showed damage in the hippocampus, caudate nucleus, spinal cord, and cerebellar Purkinje cells.96 These findings underscore the impact of age when animals are exposed to hypoglycemia on both neuronal injury and its location.98 In addition, the ketogenic diet provides the brain with an alternative energy source in the absence of glucose, and decreases the neuronal injury when started at the weaning age (PN21) in rats exposed to insulin‐induced hypoglycemia at PN25.99 In juvenile rats with insulin‐ and streptozotocin‐ (SEZ, an antibiotic that is toxic to the pancreatic beta cells) induced hypoglycemia, seizures and neuronal damage were found to be similar to that in nondiabetic rats treated with only insulin, indicating the relation of seizures to decreased levels of glucose.100 Slices from juvenile mice exposed to hypoglycemia showed seizures and subsequent decline in synaptic transmission that were accompanied by spreading depolarizations in the hippocampus, and only seizures could be abolished by NMDA and non‐NMDA‐receptor antagonists and midazolam.101 Likewise, following repetitive hypoglycemia, slices from developing rats showed decreased long‐term potentiation (LTP) in the hippocampus, suggesting a cognitive impairment, even though the basal synaptic transmission was intact in these animals.102 On the other hand, adult rats that had free or limited access to food 14–16 h before the insulin‐induced hypoglycemia, had decreased latency to seizure development in the fasting group, although this effect was related to the reduced expression of KATP channels in SNr due to fasting.103

Glucose transporter 1 deficiency

Deficiency of glucose transporter 1 GLUT1, the primary transporter in the blood–brain barrier allowing glucose entrance to the brain, was first reported by De Vivo et al.104 in 2 infants having drug‐resistant epilepsy, developmental delay, hypoglycorrhachia (a very low CSF glucose level), and low lactate level. No genetic analysis is performed but western blot analysis of erythrocytic glucose transporters (EGTs) showed decreased interaction with cytochalasin B, a mycotoxin that binds to GLUTs and inhibits glucose transport. EGTs show close homology to brain vascular endothelial cell GLUTs and are therefore suggested to represent the activity of brain GLUTs.105 Mutations in GLUT1 (SLC2A1, solute carrier family 2 member 1) underlie the pathology of the GLUT1 deficiency syndrome, which comprises infantile‐onset epilepsy, developmental delay, movement disorders, and acquired microcephaly.106, 107, 108 Various seizure types may occur, including generalized tonic or clonic, atypical absences, atonic, myoclonic, myoclonic astatic, or focal. 108, 109, 110 GLUT1 deficiency syndrome has rarely been reported in IS patients who responded to the ketogenic diet fully or partially,110, 111, 112 including a patient with a missense mutation in SLC2A1 112 (Table 2).

Heterozygous haploinsufficiency of the Glut1 gene in mice (heterozygous Glut1 mice) mimics most of the clinical phenotype of GLUT1 deficiency without the histopathological changes, whereas homozygous deletion of the gene is lethal during embryonic development (Table 3).113 In heterozygous Glut1 mice, generalized or focal‐onset electrographic seizures with rhythmic spikes or polyspikes and slow wave discharges or bursts of 2‐3 Hz spike and slow‐wave discharges (SWDs) were noted with no apparent disruption of the animal's behavior. During fasting, however, 1‐4 seconds of 6 Hz SWDs associated with behavioral arrests were also seen.113 The authors conclude that lactate deprivation is the main reason for pathology in the presence of GLUT1 deficiency113 since glucose is used mainly by astrocytes to maintain the glutamate‐glycine cycle and is converted into lactate, which is in turn used by the neurons as a main energy source.114 In Xenopus oocytes, functional testing of 7 variants in the SLC2A1 gene, which is also associated with 1% of the genetic generalized epilepsies, shows mild to dramatic glucose transport changes that are not always correlated with the phenotypes seen in the patients, indicating that in some cases, modifier genes and environmental factors also play a role in GLUT1 deficiency.115 These models have already shown a predisposition to spontaneous seizures, even though no IS have been reported, and the possible presence of disease modifiers.

Deficiencies in fatty acid oxidation

Short‐chain acyl‐coenzyme A dehydrogenase enzyme deficiency

Short‐chain acyl‐coenzyme A (CoA) dehydrogenase enzyme deficiency (SCAD) is an autosomal recessive disorder with mutations in the acyl‐CoA dehydrogenase C‐2 to C‐3 short chain (ACADS) gene wherein infants' increased levels of butyrylcarnitine and urinary ethylmalonic acid are accompanied by heterogeneous clinical symptoms such as hypotonia, hypoglycemia, and lethargy, and later in life by developmental delay and seizures.116 To our knowledge, there is one WS case with SCAD resistant to treatment in the literature117 (Table 2).

The BALB/cByJ mouse is described as a model for human SCAD during routine screening of mutated mice for metabolic disorders (Table 3).118, 119 Histopathologically, cerebral edema, astrocytic swelling, and neuronal mitochondrial injury120 and clinically, impaired thermoregulation in cold conditions are reported121; however, no seizure phenotype is mentioned. Because it was known that the ethylmalonic acid accumulates in SCAD deficiency, a study investigated the effects of its direct administration into striatum in young rats and found that striatal slices from these rats have deficient redox homeostasis and decreased sodium potassium ATPase (Na+/K+ATPase) activity, both of which may lead to neuronal impairment in neurotransmission.122

Deficiencies in metal metabolism

Menkes disease

IS have been commonly reported in patients presenting with Menkes disease123, 124, 125, 126, 127, 128, 129, 130 (Table 2). Bahi‐Buisson et al.126 reported that IS manifested during the intermediate stage of Menkes disease in all their patients. Menkes disease is an X‐linked recessive disease with low serum copper (Cu) levels due to mutations in an adenosine triphosphatase (ATP7A) gene encoding P‐type ATPase, which transports nutritional Cu into organelles.131, 132, 133 In the absence of Cu, dysfunctional Cu‐dependent enzymes involved in blood clotting, superoxide removal, myelin, and dopamine/melanin and norepinephrine synthesis (ie, blood clotting factors V and VII, superoxide dismutase and ceramide galactosyl transferase, lysyl oxidase, and dopamine beta hydroxylase) lead to variable outcomes such as anemia, vessel abnormalities, muscle weakness, and neurologic effects.127, 134 Nevertheless, Cu supplementation is inefficient as a therapy in Menkes disease.125, 129 Even when the disease was first described in 1962, severe neurological symptoms and seizures were observed in 5 related patients.131 However, the main mechanism leading to IS is still unknown (for review135).

Variations in the murine mottled (blotchy and dappled, Mo blo and Mo dp) locus are suggested to be the homolog of the human Menkes disease–causing ATP7A gene.136, 137 Neurological aspects of Menkes disease, including seizures, are reflected at most by the brindled mottled mouse (Mobr), although no spasms were reported (Table 3).137 In these mice, axonal development and synaptogenesis are impaired in the cerebral cortex and the hippocampus, and in addition, peptidylglycine alpha–amidating monooxygenase activity is reduced, indicating an impaired amidation that is important for functioning of neuropeptides as glycine, cholecystokinin, and neuropeptide Y.137, 138 Some of these changes in the brindled Mobr can be rescued by the human Menkes transgene, although the Cu concentrations are not fully reversed to normal.139 In addition, recently, non‐mammalian models of Menkes disease have also been established. The normal brain expression of atp7a in a zebrafish model is very low,140 whereas amidation impairments in a Drosophila model are similar to the murine model.141 These findings from animal models already indicate the global alterations that lead to the Menkes disease phenotype and pathology similarities in different organisms.

Vitamin B6‐, pyridoxine dependency, and pyridoxal‐5′‐phosphate deficiency

Reports of nervous system symptoms, epileptiform activities, or seizures induced by pyridoxine (vitamin B6) deficiency and of the seizure‐suppressing effect of pyridoxine have been described for rats and humans.142, 143, 144, 145, 146 A Japanese study testing pyridoxal phosphate (PLP, active form of pyridoxine) as a treatment in WS showed efficacy in 12.7% of patients including the ones with no underlying metabolic defect, prompting a treatment trial with high‐dose PLP in all WS infants.147 Similarly, Ohtahara et al.148 and Pietz et al.149 report successful use of PLP in 11.8% of WS patients and of high‐dose pyridoxine in 30% of IS patients, respectively, regardless of the prenatal etiology (ie, tuberous sclerosis complex [TSC], chromosome abnormalities, or brain malformations; symptomatic or idiopathic). Pyridoxine is still one of the first treatment choices in WS in Japan due to its safety and because of the side effects of other treatments.150, 151, 152 However, new reports implement the use of first‐line treatments (eg, adrenocorticotropic hormone [ACTH] or vigabatrin [VGB]), as there is no significant difference in efficacy of pyridoxine treatment compared to other medications.153

The exact mechanisms of pyridoxine or PLP deficiency and vitamin B6 dependency, as well as therapeutic action of these compounds, are still unknown in seizure generation considering that even in certain patients with unknown etiology epilepsy,146 pyridoxine or PLP treatment can be effective. Vitamin B6 is converted into pyridoxine and its active coenzyme form, PLP, which is a cofactor of a number of enzymes including GAD catalyzing the conversion of glutamate to γ‐aminobutyric acid (GABA).154 Low GABA levels are seen in some patients with neonatal seizures that were responsive to vitamin B6 and pyridoxine.155, 156 GABA is the main inhibitory neurotransmitter in the central nervous system (CNS), but also plays a role in oxidative energy metabolism.157 Due to this relation, seizures are attributed to functional deficiencies in GAD1 and GAD2 genes and GABA synthesis158, 159, 160 but no significant linkage has been found in affected families.161, 162, 163

Autosomal recessive mutations in the pyridoxine‐5′‐phosphate oxidase (PNPO) gene that expresses the enzyme that converts pyridoxine and pyridoxamine to active PLP underlie PLP‐responsive (but pyridoxine‐unresponsive) neonatal epileptic encephalopathy in 5 patients.164 A patient with IS of unknown etiology who had normal CSF pyridoxal‐5′‐phosphate, carried a PNPO mutation that resided in a conserved sequence with damaging effects, as determined by 2 functional online analysis programs, polymorphism phenotyping v2 (PolyPhen) and sorting tolerant from intolerant (SIFT). 165 DNA sequencing from 13 individuals between 17 months and 15 years of age who had pyridoxine‐dependent seizures revealed mutations in the α‐aminoadipic semialdehyde dehydrogenase (ALDH7A1) gene that encodes antiquitin which is involved in lysine catabolism.166 The location of the mutations in the ALDH7A1 gene contributes to the phenotype of the patients.167 In addition, unlike patients carrying PNPO mutations or having PNPO deficiency, in patients with ALDH deficiency, no increases in threonine or glycine in serum or CSF are found.

There are patients with ALDH7A1 mutations and IS although there is a patient with antiquitin dysfunction but without a mutation suggesting an unknown mechanism in this deficiency168, 169 (Table 2). Of interest, patients with folinic acid deficiency, also a known but rare cause of seizures, may carry ALDH7A1 mutations.170 Although this condition is seen mostly with neonatal seizures, urine antiquitin measurement is advised when no underlying cause is present for IS.171

KO mice for tissue‐nonspecific alkaline phosphatase (TNAP) show seizures, elevated PLP and reduced levels of GABA all of which can be reversed by the application of PLP but not pyridoxine (Table 3).172 Interestingly, pyridoxal treatment is influenced by the genetic background of the mice, being effective fully only in C57BL/6 mice, not effective in inbred 129/Sv and partially effective in hybrid B6129.172 Litters born to both TNAP+/+ mice and heterozygous mice who are fed by pyridoxine‐deficient diet also show epileptic seizures.173 PLP and primidone suppress zebrafish seizures induced by Ginkgo biloba, leading to PLP and GABA reductions despite the fact that this model is not a specific model for PLP deficiency.174

Acidurias

d‐Glyceric aciduria

ᴅ‐Glyceric Aciduria (ᴅGA) was first described in a 2‐year old boy having symptoms similar to NKH such as high urinary glycine, myoclonic jerks, and motor and developmental retardation, but also with hyperglyceric acidemia and hyperglycericaciduria.175 Autosomal recessive mutations in the d‐glycerate kinase (GLYCTK) gene were identified in 3 patients including the ᴅGA patient first described.176 Regardless of the high urinary ᴅ‐glyceric acid content, individuals that are fully healthy or present with only mildly affected fructose metabolism have been reported.176, 177, 178

A 6‐month‐old boy with healthy sisters from first‐degree, consanguineous parents was diagnosed with WS and was followed until 3 years of age179 (Table 2). A fructose‐free diet provided partial amelioration, with decreases in autistic behavior and resolution of abnormal signal intensity in mesencephalon, thalami, and globus pallidum on MRI; however, it is not clear how defects in fructose metabolism led to neuronal symptoms, and particularly WS.

No seizures are reported in rats that are exposed to a high‐fructose and high‐fat diet (Table 3).180 Glyctk KOs that are created by the clustered regularly interspaced short palindromic repeats (CRISPR) and Glyctk small interfering RNAs (siRNAs) to silence the gene are available; however, no model with a seizure phenotype has been identified to date.

Methylmalonic aciduria and cobalamin (B12) deficiencies

Deficient methylmalonyl CoA mutase (MUT) that converts propionyl‐CoA from amino and fatty acids into a Krebs cycle component, succinyl‐CoA, leads to accumulation of methylmalonic acid (MMA) and its derivatives. These derivatives are toxic to kidneys and brain where they affect particularly the basal ganglia.181, 182, 183 5′‐Deoxyadenosylcobalamin is a cofactor of MUT; therefore its absence or deficient synthesis may also result in methylmalonic aciduria.183, 184, 185 The disease progression, and treatment options and their success may differ depending on the location of autosomal recessive mutations affecting either the synthesis pathway of the apoenzyme or the cofactor.183, 186, 187, 188 IS due to MMA accumulation or B12 deficiency is reported in several patients with various responses to vitamin supplements19, 23, 189, 190, 191 (Table 2). In bigger cohorts, the frequency of methylmalonic aciduria among the reported underlying etiologies in infants with IS varies between 0 and10%.9, 19, 192

Rats show clonic and tonic–clonic seizures after intrastriatal MMA injection, and the duration of these seizures are reduced by preadministration of MK‐801, alpha‐tocopherol, or ascorbic acid, indicating the possible role of NMDA receptors and free radicals in the MMA‐induced phenotype (Table 3).193, 194 Membrane preparations from both cerebral cortex of the subcutaneously MMA‐treated rats and brain homogenates of untreated rats that are preincubated with MMA indicate decreased Na+/K+ATPase activity, which is prevented by ex vivo simultaneous glutathione application.195 On the other hand, intracerebroventricular (ICV) injection of MMA leads to clonic convulsions and lower GAD activity in rats, and although the duration of convulsions can be reduced by pyridoxine, baclofen, and muscimol, decrease in GAD activity can be prevented by pyridoxine and MK‐801.196

Of the few mice models that mimic cobalamin deficiency, transcobalamin receptor (CD320/TCbIR) KOs present with neurological impairments that are similar to the human cobalamin deficiency.197, 198, 199 These mice show cognitive deficits and anxiety behaviorally, whereas in vitro examination indicates smaller pyramidal cells and impaired in the CA1 region of the hippocampus together with lower GluR1 expression, indicating an AMPA receptor–mediated LTP deficit.198 However, seizures have not been observed.

Homocysteinemias

Increased levels of homocysteine in blood and urine in patients presenting with neurological symptoms were reported to be due to cystathionine synthase (CS) deficiency or decreased activity of methylene tetrahydrofolate reductase (MTHFR) enzyme due to autosomal‐recessive MTHFR mutations.200, 201, 202 Insufficiency of cobalamin (or vitamin B12), a cofactor of MTHFR, may also result in homocysteinemia. When homocysteine is not converted to cystathione via CS or not reduced to methionine via MTHFR, it accumulates in many tissues and interacts with a number of molecules including cytochrome C, hemoglobin, and immune mediators. Therefore, homocysteine affects a range of functions including energy metabolism, oxidative reactions, and immunity (for review see203).204 Although the direct link between homocysteinemia and seizures or spasms is not clear, it is known that systemic administration of homocysteine leads to convulsive seizures in adult rats and adult mice and excitotoxicity in organotypic cortical and hippocampal slice cultures from rat brain.205, 206, 207 In addition, homocysteine accumulation is associated with vascular and neurodegenerative diseases, and also with epilepsy (for review208). Folic acid and cobalamin supplements are known to decrease blood homocysteine levels.209

A 14‐month‐old girl from a nonconsanguineous parents presenting with IS, hypsarrhythmia, and developmental delay was shown to have MTHFR deficiency. None of the interventions including betaine, methionine, vitamin supplements, various antiseizure drug combinations, corpus callosotomy, and vagus nerve stimulator implantation blocked her seizures204 (Table 2).

Intraperitoneal homocysteine injection of rats of different age groups ranging between 7 and 90 days show age‐specific seizures (Table 3).210 In immature rats, status epilepticus and flexion seizures are apparent, whereas in adult rats, spikes and spike waves without a behavioral correlation are observed. Intracerebroventricular administration of homocysteic acid in 12‐day‐old rats leads to generalized tonic–clonic seizures that are completely prevented when NMDA nonselective and selective 2,3‐dihydroxy‐6‐nitro‐7‐sulfamoyl‐benzoquinoxaline‐2,3‐dione (NBQX) and 2‐amino‐7‐phosphonoheptanoic acid (AP7), respectively, are administered before homocysteic acid.211 Likewise, subcutaneous chronic injection of homocysteine in rats from PN6 to PN28 decreases Na+/K+ATPase activity in the synaptic membrane preparations from the hippocampus ex vivo.212 Few animal models present with homocysteinemia but the Mthfr KO mouse is the closest phenotype to the one seen in humans.197 Heterozygote mice show a moderate increase in homocysteine, whereas homozygotes have a 10‐fold increase and they have developmental retardation.213 Of interest, WT animals from heterozygote hyperhomocysteinemic mothers show impaired short‐term memory assessed by a behavioral test, and slices from these mice indicate apoptosis in the hippocampus.214

Propionic acidemia

Autosomal recessively inherited mutations in propionyl‐coenzyme A carboxylase (PCC) genes resulted in PCC deficiencies, leading to deficient catabolism of various amino and fatty acids and cholesterol.215, 216, 217, 218 Due to the resulting accumulation of propionyl‐CoA, secondary hypoglycemia, hyperammonemia, and hyperglycinemia, it is considered as a form of ketotic hyperglycinemia216 but can also manifest without metabolic symptoms.219 Patients are presenting with failure to thrive, vomiting, ketoacidosis, lethargy, and neurological symptoms ranging from developmental delay to seizures and stroke‐like episodes. Treatment involves protein‐restricted diet and carnitine load to help propionylcarnitine excretion, although interventions may not prevent or reverse all the neurological symptoms.220, 221

In a 4‐month‐old girl with hypsarrhythmia and propionic acidemia associated with deficient PCC activity in her leukocytes and cultured skin fibroblasts, ACTH treatment stopped her seizures and normalized her EEG. Dietary protein restriction also was reported as beneficial, but she developed ketoacidosis triggered by poor feeding at 11 months of age and developmental delay222 (Table 2).

ICV administration of propionic acid in adult rats led to dystonia, hyperactivity, spiking in the caudate, and worsening of kindling‐induced seizures; in vitro analysis showed increased oxidative stress and reduced glutathione and glutathione peroxidase activity along with neuroinflammation (Table 3).223 Of interest, the authors suggest that this model may serve as a model for autism spectrum disorder (ASD), as it shows impairments of social behavior, social cognition, and sensorimotor ability.224, 225 In addition, in vitro evaluation of the brains from 21‐day‐old rats that received oral propionic acid show decreased glutathione and glutathione peroxidase activity, elevated levels of neuroinflammatory markers, interleukin 6 (IL‐6), tumor necrosis factor alpha (TNF‐α), interferon gamma (IFN‐γ), and heat shock protein 70 (HSP17), and proapoptotic marker caspase 3, along with DNA fragmentation as well as decreased levels of GABA, serotonin, and dopamine.226 Despite the fact that IS, especially due to symptomatic origin, is also a known risk factor for ASD,5, 227 no seizures are reported in this model.

Metabolic Errors in Organelles

Mitochondrial disorders

Pyruvate dehydrogenase complex deficiency

Pyruvate dehydrogenase complex (PDHC) comprises multiple enzymes that catalyze oxidative decarboxylation of pyruvate to acetyl‐CoA, an irreversible and rate‐limiting reaction in the aerobic glucose metabolism in mitochondria.228 PDHC enzymes are expressed in many cell types, and deficiency of the enzymes leads to lactic acidemia as well as neurological complications, since the brain is a highly energy dependent organ.228, 229 Lactic acidosis was found to be due to the mutations in the pyrophosphate‐dependent pyruvate decarboxylase (E1) α subunit of the PDHC,230 and these mutations were shown to be the most prominent ones in patients with mild to severe neurological symptoms.229, 231 Depending on the genomic localization of the mutations on the X chromosome and also the random X inactivation in different tissues, PDHC E1 α subunit deficiency may have variable effects on tissues with sexually dimorphic features.229, 231, 232

There are several reports of patients presenting both PDHC deficiency and WS with variable responses to treatment231, 233, 234, 235, 236 (Table 2). A preference for female patients with PDHC to develop WS has been reported.234 Genotype‐phenotype variability has been observed in patients with PDHC deficiency receiving similar treatments, which could be due to different genetic substrates or variable developmental effects of PDHC deficiency and its treatments.237

Mice with a null mutation of PDHC E1 α subunit, which removed exon 8 (Pdha1 (Deltaex8)) from the gene globally, have low PDC activity and reduced litter size, whereas developmental delay was reported in the mice that had the Pdha1 (Deltaex8) allele predominantly (Table 3).238 When the same mutation was targeted only to the developing nervous system, using a nestin‐cre promoter, male mice died prenatally and only 50% of female mice survived.239 These female mice had decreased neuronal density and neuropil fibers, abnormal localization of the neurons in the gray matter, reduced lipid synthesis, and irregular myelination. Of interest, the affected female mice survived into adulthood without discernible neurological deficits. Knockdown of the PDHC E1 α subunit via the delivery of siRNAs into striatum and substantia nigra of rats led to abnormal amphetamine‐induced rotation240 indicating degenerative changes in the nigrostriatal system.241 A recent investigation of a systemic deletion of exon 8 from PDHC E1 α subunit in mice showed general decrease in brain weight, de novo lipid synthesis, reduced proliferation, differentiation, and migration of newly generated neuronal precursor cells in prenatal and postnatal periods in cerebellum and impaired dendritic development of Purkinje cells.242 The locomotor activity was normal in these mice; however, they had impaired acoustic startle reflex indicating mild motor abnormalities. In a zebrafish model of PDH deficiency via the impairment of the E2 subunit, ketogenic diet reversed the abnormal vision, lactic acidosis, and lethargic swimming behavior.243

Leigh syndrome (subacute necrotizing encephalomyopathy)

Leigh syndrome was first described in 1951 based on the autopsy findings of focal, bilateral subacute necrotic white matter lesions in a boy presenting with somnolence, deafness, blindness, and spastic limbs.244 The lesions are now known to include basal ganglia, thalamus, cerebellum, and brainstem; the symptoms and signs include hypotonia, feeding problems, and psychomotor delay.245 Mutations leading to this phenotype can be autosomal or X‐linked, as in the case of mitochondrial PDHC or respiratory chain mutations, although patients without these mutations were also reported.245, 246, 247

Leigh and Leigh‐like syndromes presenting with IS/WS were described in several patients due to PDHC mutations20, 246, 248, 249, 250, 251 (Table 2). Of interest, Tsuji et al.246 reported 2 sisters with Leigh syndrome, where only one developed WS and in another family both brothers with Leigh syndrome had WS later, adding further to the complexity of pathology in Leigh syndrome with WS patients.

Because mutations in the NADH:ubiquinone oxidoreductase subunit S4 (NDUFS4), encoding a mitochondrial complex I protein, are also related to Leigh and Leigh‐like syndromes,252, 253 Ndufs4 KO mice were generated (Table 3).254 These mice were behaviorally normal during the first 4 postnatal weeks but their growth was delayed; they progressively became lethargic, ataxic, and blind; lost their startle response; and died by week 7.254 Selective KO of Ndufs4 in neurons and glia present with similar phenotype as complete KO; however, slices from these mice also showed gliosis and microglial activation in brainstem and cerebellum.255 Of interest, hypoxia (11% O2) reversed neurodegenerative changes and increased the life span of Ndufs4 KO mice.256 In addition, rapamycin (an inhibitor of mechanistic target of rapamycin [mTOR]) extended the life span, prevented brain injury, increased the low levels of GABA, dopamine, and free fatty acids, and decreased the glycolytic intermediates without having an effect on oxidative stress markers in Ndfus4 KO mice.257 Likewise, rapamycin also increased the life span of animals but led to fat storage impairments in a Drosophila model of Leigh syndrome via the complex I subunit NADH dehydrogenase 2 (ND2) deletion.258 Alterations in glycolytic intermediates and fat storage in these models suggest that rapamycin exerts its effects via the changing metabolism, a hypothesis that is yet to be proven.257, 258

Alpers‐Huttenlocher disease

Alpers‐Huttenlocher disease is a rare disorder characterized by microcephaly, refractory seizures, and liver dysfunction that may result from deficiencies in the PDH enzyme in citric acid cycle or respiratory chain or mutations in mitochondrial DNA polymerase gamma (POLG) of FARS2 gene (Phe‐tRNA synthetase 2).259, 260, 261 Three patients with Alpers‐Huttenlocher disease and WS were identified in a study examining the etiological factors in 20 patients with mitochondrial dysfunction confirmed by biochemical and morphologic examinations260 (Table 2). Despite all 3 Alpers‐Huttenlocher disease patients having mitochondrial complex I deficiency, it was not inherited in any of them. Among 11 reported cases with FARS2 mutations, 3 (27%) had WS and hypsarrhythmia.262, 263 No reports of patients with POLG mutations and IS are known to the authors.

The mitochondrial mutator mice, carrying a D257A knockin mutation at the catalytic subunit A of the Polg gene that removes the proofreading ability of the enzyme, show instability of the mitochondrial DNA (mtDNA) with increased mutation rate and deletions in the mitochondrial DNA (mtDNA), leading to early aging (Table 3).264, 265 Homozygous PolgA mice manifest reduced body size and weight around the 20th week, prematurely age at around 48 weeks, and die by 61 weeks of life.264, 265 Heterozygous PolgA mice do not show abnormalities in aging.264 A zebrafish model was developed where Polg−/− larvae survived until juvenile age (4 weeks after fertilization).266 A different study suggested that early KO of PolgA at the preimplantation stage results in early lethality by embryonic day 11 and depletion of mtDNA in mice.267

Lysosomal storage diseases

Lysosomal storage diseases comprise a group of disorders including neuronal ceroid lipofuscinosis (NCL), Gaucher disease, Tay‐Sachs disease, Hurler syndrome, Niemann‐Pick disease, and a few others resulting from deficiencies in lysosomal metabolic enzyme(s) (for review16, 268). These diseases may feature neuromuscular, respiratory, endocrine, cardiovascular, and morphological symptoms, although most patients may appear normal neonatally because the excess, harmful metabolites that are produced by the dysfunctional enzymes can be removed via the placenta during gestation.268 Mutations in the lysosomal enzymes are mostly autosomal recessive, or in few exceptions such as Fabry disease, Hunter syndrome and Danon disease, are X‐linked.268

There is a report of a 5‐month‐old boy with IS and a low α‐l‐iduronidase (IDUA) activity confirmed to have a severe type of mucopolysaccharidosis I, a lysosomal storage disease called Hurler syndrome269 (Table 2). Another recent study reported a 5‐month‐old patient with IS having a form of Niemann‐Pick disease (NPD) due to sphingomyelin phosphodiesterase enzyme deficiency caused by compound heterozygous mutations in sphingomyelin phosphodiesterase 1 (SMPD1), also known as the acid sphingomyelinase (ASM) gene270 (Table 2).

There are different experimental models for various types of lysosomal storage diseases (for review271) (Table 3). For instance, Idua KO mice generated by targeted disruption of exon 5 of the gene in mice embryonic stem cells that are injected later into blastocysts have deficient IDUA activity.272 These mice show some phenotypic features such as flat face and increase in glycosaminoglycan in urine, which are similar to those seen in human patients with Hurler syndrome. Abnormal lysosomal storage is detected only in glial cells at 4 weeks, but as the KO mice age, vacuolation in Purkinje cells and also neurons in the cortex are prominent without behavioral alterations.272 When 6 to 7 week old Idua KO mice are injected with viruses containing human IDUA only into their putamen, the brain pathology is reversed.273 Furthermore, ICV injection of a similar viral construct into 4‐ to 6‐day‐old mice brains also results in widespread expression that prevents the brain pathology and also spatial learning deficits in Idua KO mice.274 Asm KO mice, on the other hand, show high levels of sphingomyelin accumulation and no detectable ASM activity, and die after 6 to 8 months with half of the body weight of controls and heterozygotes at the same age.275 Also described is a transgenic model for NPD in mice with mutations in complete ASM KO background that may allow evaluation of the effects of different mutations.276

Other Diseases

Leukodystrophies

Leukodystrophies present with white matter degeneration, spongy vacuolation, loss of myelin, and resulting motor dysfunction in the early stages of the diseases. Some of these diseases namely Alexander disease, Krabbe disease, X‐linked adrenoleukodystrophy, hereditary diffuse leukoencephalopathy with spheroids, and metachromatic leukodystrophy also feature seizures and epilepsy.16

Autopsy investigations of brain specimens from patients with IS and cognitive dysfunction show spongy dystrophies or leukodystrophies.20, 277, 278, 279, 280 Some patients with Alexander and Krabbe disease were reported to have IS/WS281, 282 (Table 2). Autosomal dominant mutations in glial fibrillary acidic protein (GFAP) resulting in build‐up of Rosenthal fibers in astrocytes may be associated with Alexander disease,283 whereas autosomal recessive mutations in galactosylceramidase gene (GALC) leading to lysosomal glycosphingolipid accumulation was shown to lead to Krabbe disease.284, 285

Mice expressing WT and only one copy of the human GFAP (hGFAP) mutation (R239H) causing Alexander disease showed no Rosenthal fiber formation (Table 3).286 However, mice expressing WT and multiple copies of the transgene do show Rosenthal fibers only in adult age, have severe convulsions and a high mortality rate when challenged with kainate but show no spontaneous seizures and leukodystrophy.286 Hypertrophic astrocytes, Rosenthal fiber‐like inclusions, and upregulation of small HSPs are seen in mice carrying the hGFAP transgene without mutations but overexpressed GFAP.287 In a similar model, microarray analysis show increased immune and stress response, neuronal death, and dysfunction.288

Biotinidase deficiency

As an inherited disorder in an autosomal recessive manner, biotinidase (BTD) enzyme deficiency leads to abnormalities in the recycling of the vitamin biotin, which is an important cofactor of carboxylase enzymes that help in the catabolism of complex organic acids, fatty acid synthesis, and gluconeogenesis.289, 290, 291 The resulting metabolic dysfunction clinically presents with red skin rash, lactic aciduria, hearing loss, hypotonia, seizures, and developmental delay, and biotin supplements can be used to treat the symptoms.290, 292

Three patients with full or partial BTD deficiency and IS partially responsive to biotin supplement have been reported; however, mild to moderate developmental delay persisted in all patients293, 294 (Table 2). Rats fed with a biotin‐deficient diet have alopecia of the lower back, delayed growth, and longer latencies in brainstem auditory evoked potentials (BAEPs), indicating an alteration in the auditory system (Table 3).295 BTD KO mice, fed with a biotin‐deficient diet show hypotonia, demyelination, axonal degeneration, impaired motor neuron function, lethargy, limping, and ventriculomegaly, in addition to slower growth and weight loss, and these symptoms are reversed with biotin supplement.296, 297

D‐bifunctional protein deficiency

D‐bifunctional protein (DBP) deficiency is one of the peroxisomal disorders with poor prognosis, leading to defective oxidation of fatty acids and synthesis of bile salts due to mutations in the DBP gene also known as paroxysmal multifunctional enzyme type 2 (HSD17B4) or multifunctional protein‐2 (MFP‐2).298, 299, 300 High levels of very long chain fatty acids in the plasma and cultured fibroblasts, hypotonia, facial dysmorphism, neuronal migration defects, demyelination, developmental delay, and seizures may be present (for review301). A patient with drug‐resistant WS having DBP deficiency has been reported302 (Table 2).

Multifunctional protein (MFP2) KO mice mimic the metabolic disturbances at birth leading to failure to thrive and death, but they are not hypotonic and do not show any neuronal migration defects (Table 3).300, 303 The surviving MFP2 KO mice, however, have astrogliosis and microglial activation in the gray matter, which is not seen in humans.304 In mice, the selective MFP2 deletion either from all neuronal cells (Nestin‐Mfp2−/−) or only from oligodendrocytes (Cnp‐Mfp2−/−) lead to mild neuroinflammation and axonal impairments in Purkinje cells correlated with motor deficits only in Nestin‐Mfp2−/− mice, indicating no oligodendrocytic involvement in this brain pathology.305 Complete MFP2 KO mice also show severe proneuroinflammatory activation. More recently, morphological impairments, delayed growth, and abnormal neuronal development in the dbp knockdown zebrafish model is rescued by murine Dbp, indicating the benefit of modeling the disease in different organisms.306

Williams‐Beuren syndrome

Williams‐Beuren syndrome (WBS) also known as Williams syndrome is due to variable microdeletions within the WBS area on the long arm of chromosome 7 containing 26 to 28 genes such as, Elastin (ELN) and Syntaxin 1A (STX1A), and may present with various features as hypercalcemia, hypothyroidism, impaired glucose tolerance, growth abnormalities, characteristic facial appearance, mental retardation, and cardiac anomalies.307, 308, 309 The exact contribution of the involved deleted genes to the WBS phenotype is yet to be elucidated. Treatment is only symptomatic, directed toward the specific clinical presentation of the disease.307

In WBS, due to deletions in the WBS area of the 7th chromosome, WS has been reported in 3 children whose spasms were fully or partially treated with ACTH and anti‐seizure drugs, although the developmental impairment is progressive310, 311 (Table 2). Of interest, the deletions in the membrane‐associated guanylate kinase inverted‐2 (MAGI2) gene in the very same region of chromosome 7 were previously shown to be associated with IS.312 The scaffolding protein encoded by MAGI2 is also associated with Stargazin protein. MAGI2 mutations are the cause of epilepsy in the stargazer epilepsy model, which presents with generalized cortical spike and wave discharges accompanied by behavioral arrest and complex bilateral neocortical discharges.312, 313 A recent study evaluating the candidate genes residing on chromosome 7 in a zebrafish knockdown model reported haploinsufficiency of ywag gene, residing in the telomeric region of the chromosome and encoding 14‐3‐3 protein gamma, and related this gene with IS and cardiomegaly seen in WBS patients.314 Because the WBS area encompasses a number of genes, the first experiments focused on both hetero‐ and homozygous single gene deletions in this region to elucidate the contribution of each gene to the phenotype (for review192). Of these modeled genes, both heterozygous and homozygous deletions of one of the Wnt receptor–coding frizzled 9 (Fzd9) gene, involving in the neurotrophic processes, led to decreased pentylenetetrazole‐induced seizure threshold and abnormalities in hippocampal structure, whereas the homozygous deletions also result in spatial learning impairments in mice (Table 2).315 The large‐scale deletions in proximal and/or distal regions in the mouse WBS area present with cognitive impairments in distal deletions and defective motor skills in double, heterozygotes (proximal and distal deletions), showing a closer relation to the human phenotype.192, 316 Heterozygous, almost‐complete deletions of the WBS region in mice reduce brain weight and dendritic length and spine density in the hippocampus and GFAP‐positive cells in the amygdala; increase the number of immature cells in the DG; and lead to craniofacial and cardiac abnormalities.317 In addition, complete deletion mice show spatial learning deficits, and slices from these mice indicate unstable LTP with no changes in presynaptic function or AMPA and NMDA receptor activity in CA1 but low brain‐derived neurotrophic factor (BNDF) levels in CA1‐CA3 regions.318

Congenital disorders of glycosylation, molybdenum cofactor deficiency, and primary carnitine deficiency

Several disorders such as congenital disorders of glycosylation (CDG), molybdenum cofactor deficiency (MCD), primary carnitine deficiency (PCD), and isovaleric acidemia (IVA) were also reported in patients with WS.

CDG is a group of disorders with deficient glycosylation of proteins and lipids, leading to their abnormal folding, transport, stability, and activity.319, 320 A patient with abnormal vision, IS, hypsarrhythmia, abnormal myelination, reduced white matter, and developmental and motor delay was shown to have a deficient glycosylation, classified as CDG‐li, as investigated by serum transferrin isoelectric focusing319 (Table 2). This defect impairs the transfer of mannosyl residues at the cytosolic side of endoplasmic reticulum, leading to deficient oligosaccharide biosynthesis. Mouse models for 2 different groups of CDGs (CDGI and CDG II) are currently available (for review321). However, transient paralysis and tremors that resemble the epileptic seizures are reported only from 20% of mannosyl (alpha‐1,6)‐glycoprotein beta‐1,2‐N‐acetylglucosaminyltransferase (Mgat2)–null mice modeling the CDG type IIa (Table 3).322 A further evaluation is needed to understand the pathology in this model. MCD and PCD were reported in 2 different WS patients in a study that investigated the metabolic etiologies in 80 children.20 Both patients were nonresponsive to antiseizure drugs and showed severe developmental delay (Table 2).

Molybdenum is a cofactor of xanthine dehydrogenase, sulfite oxidase, and aldehyde oxidase and in the absence of it, all these enzymes are affected leading mainly to toxic accumulation of sulfite.17 Carnitine is involved in the transport of long‐chain fatty acids and peroxisomal oxidation products, such as acetyl‐CoA, into mitochondria for oxidation. Deficiency in carnitine transport leads to PCD with defective fatty acid oxidation and fatty acid accumulation. 323

Because few mutations in molybdenum cofactor synthesis protein 1 and 2 (MOCS1 and MOCS2) genes and one mutation in gephyrin (GPHN) gene leading to MCD are reported, several animal KO models are described (for review324). Mocs1 KO mice survive 1 to 11 days with no convulsions, ataxia, or structural changes in the brain (Table 3).325 Systemic injection of adenoviruses carrying human MOCS1 cDNA increases the life span of the Mocs1 KO mice that are treated via hepatic injections of cyclic pyranopterin monophosphatase (cPMP), a precursor in the molybdenum cofactor synthesis, until they are old enough to get a systemic injection.326 Mocs2 KO mice also survive for 11 days on average; pathological and biochemical studies show apoptosis in the hippocampus, cortex, and brainstem; inactivation of all molybdenum cofactor‐dependent enzymes; and accumulation of hypoxanthine, xanthine, and S‐sulfocysteine.327

Dietary restriction of carnitine in rats leads to 50% of reduction in its physiological levels, indicating an ongoing endogenous synthesis but induces no major anatomical or behavioral changes (Table 3).328 Dietary restriction, together with supplement of butyrobetaine hydroxylase inhibitor that blocks the last step of the carnitine synthesis results in carnitine deficiency within 3 weeks in rats; however, only liver anomalies are reported.329 Of interest, when combined with acyl‐CoA, acylcarnitines, including acetyl‐l‐carnitine, have neuroprotective effects in various disorders such as330 Alzheimer's disease331 and ischemia‐hypoxia–induced brain injury332 (for review333). Carnitine deficiency may result in loss of these protective effects. In addition, the link between PCD and autism is investigated.334

Isovaleric acidemia due to deficient isovaleryl‐CoA dehydrogenase (or IVD) enzyme was confirmed in cultured fibroblasts from a boy in presenting with vomiting, lethargy, and WS335 (Table 2). IVD is involved in the catabolism of leucine, and in IVD deficiency, this amino acid cannot be degraded, leading to excess excretion of isovalerylglycine (IVG).336 To our knowledge, currently there is no model mimicking isovaleric acidemia; however, application of isovaleric acid or IVG onto cortical homogenates of 30‐day‐old rats indicates no changes in citric acid cycle or creatine kinase activity, whereas synaptic membrane preparations from these homogenates show that IA decreases Na+/K+ATPase activity via peroxide radicals (Table 3).337 In a similar in vitro experiment using brain homogenates and mitochondrial preparations, IVG but not IA leads to lipid peroxidation and reduced GSH levels in a mitochondria‐independent way. On the other hand, IA triggers protein oxidation.338 Despite the fact that no other phenotype is defined in these experiments, in vitro alterations are suggestive of neurodegenerative transformations.

Diagnosis of Metabolic Etiologies

As discussed earlier, the frequency of metabolic disorders in the diagnostic evaluation of infants with IS varies among studies, ranging between 3% and 47%, depending on the cohort, the type, and extent of diagnostic investigation for such causes, and the criteria used in each study to distinguish “metabolic disorders” from “genetic etiologies” or “inborn errors of metabolism.”9, 19, 20 In infants with IS, metabolic disorders for which we have more evidence for a higher association with IS include PKU, NKH, Menkes disease, pyridoxine responsive or dependent seizures, methylmalonic aciduria, and mitochondrial disorders (Table 2). In general, such diagnostic tests may include routine clinical chemistry (eg, electrolytes, glucose, ammonia, lactate, liver function, or creatine kinase), amino acids, monoamines, or organic acids in blood, urine, or cerebrospinal fluid, specific assays for the activity of certain enzymes, genetic testing for nuclear or mitochondrial DNA defects, or biopsy. Especially newborn screenings from a basic biochemical test to rapid tandem mass spectrometry or genetic tests if indicated may lead to early detection of these abnormalities even before the appearance of the symptoms.339, 340 Early identification may also assist in the fast clearance of toxic metabolites (ie, hyperammonemia) and could improve the course of the disease, as in PKU,22 cobalamin deficiency,20, 23, 339, 340, 341 or pyridoxine‐responsive seizures. It is beyond the scope of this review to detail the diagnostic workup indicated for each condition or provide guidelines for the physician. The clinical suspicion for an underlying metabolic or genetic etiology plays a significant role in initiating the specific workup, particularly in regions where cost and availability may be a prohibiting factor, which could also affect the reported incidence of such etiologies. Clinicians are therefore guided by other features, symptoms, or signs of these disorders, including neuroimaging findings or expected rate of specific metabolic disorders in the relevant population to direct the diagnostic workup by assessing the cost‐benefit relationship of each test before ordering extensive diagnostic metabolic workups. We refer the readers to textbooks or specific reviews that address the diagnostic features and findings of such disorders.16, 17 MRI or routine metabolic studies, including glucose and lactate/pyruvate, or organic and amino acids analysis, may help guide further diagnostic decisions. Genetic testing with specific epilepsy panels or whole‐exome sequencing, which may also identify defects in certain neurometabolic genes, are increasingly available. More specific diagnostic tests for metabolic disorders may be requested based on clinical suspicion and available diagnostic workup.

Associated features that may help the diagnostic evaluation include the neurological exam (eg, hypotonia or hyperreflexia) or presence of movement disorders (Table 2)17, 20, 339 Clinical examination may give hints such as dysmorphic features and neonatal diabetes (eg, DEND),77, 80 skin hypopigmentation (eg, Menkes disease),123, 130 facial elfin appearance (eg, Williams‐Beuren syndrome),310, 311 abnormal facies (eg, PDHC deficiencies),231, 235, 236 alopecia (eg, biotinidase deficiency),293, 294 abnormal visual responses (eg, NKH)66 or alterations in auditory‐evoked potentials (eg, NKH, methylmalonic aciduria, and biotinidase deficiency),19, 66, 293, 294 optic atrophy (eg, Leigh syndrome and biotinidase deficiency).246, 251, 293

Biochemical screening is typically the first approach to diagnose the metabolic abnormalities. In blood—increased Phe (PKU), glycine (NKH, propionic acidemia), homocysteine (homocysteinemia), propionate (propionic acidemia), lactate (Leigh syndrome, biotinidase deficiency, isovaleric acidemia), ammonia (PCD, isovaleric acidemia) or lactate; or decreased glucose (GLUT1 deficiency), Cu and ceruloplasmin (Menkes disease), B12 (B12 deficiency), carnitine (PCD); in urine—increased ethylmalonic acid (SCAD deficiency), glyceric acid (D‐GA), methylmalonic acid (methylmalonic aciduria), propionate (slightly propionic acidemia), sulfocysteine or decreased uric acid (MCD), isovalerylglycine (isovaleric acidemia); and in CSF—increased pipecolic acid (pyridoxine dependency) give hints about respective diseases. Decreased DHPR (DHPR deficiency), cystathionine synthase (homocysteinemia), alpha‐l‐iduronidase (Hurler syndrome), SMPD (Niemann‐Pick disease) or abnormal propionyl CoA (propionic acidemia), cytochrome c oxidase (Leigh syndrome), and enzymatic activities can also be used as confirmatory analyses. Anemia (B12 deficiency) and neonatal diabetes (DEND), cardiac problems (Leigh syndrome, Williams‐Beuren disease), and liver dysfunction (Alpers‐Huttenlocher disease) are also some symptoms of the respective diseases (Table 2).20, 339, 340, 341

MRI findings may be normal in some cases as DEND, GLUT1 deficiency, or isovaleric acidemia, or provide either nonspecific features (atrophy, delayed myelination, white matter abnormalities) or specific alterations such as in acidurias (abnormal signals from globus pallidi) or Leigh syndrome (basal ganglia and SN lesions) (Table 2).20, 339, 340, 341 Dysgenesis of corpus callosum and gyral abnormalities may also be present in certain patients with NKH.342

Conclusions

Metabolic etiologies are one of the major contributors to the evolution of WS pathology. Because WS presents in early ages when most of the developmental changes are occurring, where both temporal and spatial coherence are critical, especially for the brain maturation, the presentation and progression of the symptoms and responses to the treatment are variable in patients having inborn errors of metabolism together with WS. The IS/WS phenotype has not been reported yet in several of the existing experimental models for metabolic diseases; nevertheless, these models provide invaluable information about putative pathogenesis and prevention as well as development and fine tuning of newer and better models. Despite the lack of knowledge in the exact pathology of many metabolic errors leading to IS/WS, diagnosis of the metabolic deficiency may decelerate the progression of WS, ameliorate patients' symptoms, and in some cases even treat WS patients. Therefore, screening for disorders of inborn metabolic pathways including physical and neurological examination, biochemical and/or genetic investigations, and MRI (where possible), are helpful in diagnosis. The development of animal models is critical not only for diagnosis but also for improving our knowledge about the pathways involved in WS generation.

Conflict of Interest

S.S. has no conflicts of interest. S.L.M. has no conflicts of interest with regard to this review. He is serving as Associate Editor of Neurobiology of Disease and is on the editorial boards of Brain and Development, Pediatric Neurology, and Physiological Research. He receives from Elsevier an annual compensation for his work as Associate Editor on Neurobiology of Disease and royalties from 2 books that he coedited. He received a consultant fee from UCB for participation in a Data Safety Monitoring Board. He has also received honorarium for participation in an advisory board meeting of Mallinckrodt, but there is no conflict of interest with regard to the contents of this review. A.S.G. has no conflicts of interest with regard to this article. She is co‐Editor in Chief of Epilepsia Open and has received royalties for publications from Elsevier. She has also received honorarium for participation in an advisory board meeting of Mallinckrodt, but there is no conflict of interest with regard to the contents of this review. We confirm that we have read the Journal's position on issues involved in ethical publication and affirm that this report is consistent with those guidelines.