Primary cerebellar agenesis presenting as isolated cognitive impairment.

Primary cerebellar agenesis is a rare entity. To the best of our knowledge, eleven living cases have been reported till date. Most of these were associated with some degree of motor impairment. We present a case of cerebellar agenesis in a child who presented with cognitive abnormalities leading to poor performance at school. No motor impairment was seen. Among the eleven cases reported earlier, only one case showed lack of motor impairment.

Introduction

Primary cerebellar agenesis is rare as only eleven living cases have been reported till date to the best of our knowledge. Our case is unique in that the child presented with isolated cognitive impairment with no motor impairment.[123] The clinical presentation is highly variable. Mild to profound motor impairment, mental and behavioral abnormalities have been described in these patients.[2] We report a case of primary cerebellar agenesis presenting with isolated cognitive impairment. This case is among the few cases which shows that it is possible to have normal motor function even in the absence of the cerebellum and that the cerebellum has an important role to play in nonmotor functions.

Case Report

A 5-year-old male child was brought to the pediatric outpatient department on account of parental concern for impaired academic performance at school. A detailed history was elicited, and physical examination was carried out. The child was born of a full-term normal vaginal delivery. There was no history of consanguinity. The perinatal period was uneventful. There was no history of developmental delay. The child only had problems with learning leading to poor performance at school. A detailed neurological examination was carried out. Power was Grade IV in all limbs with normal deep tendon reflexes. Cranial nerves were normal. There was no dysarthria, ataxia, nystagmus, or intention tremor. The child had problems with remembering names and identifying colors and objects. He was referred for magnetic resonance imaging (MRI) study of the brain in view of learning disability and high degree of parental anxiety. A routine MRI brain was carried out. T1-weighted images in the axial and sagittal plane, T2-weighted images in the axial and coronal plane, and axial fluid-attenuated inversion recovery images were obtained. Images revealed a normal-sized posterior fossa which was empty and showed cerebrospinal fluid (CSF) signal intensity on all sequences [Figures 1 and 2]. The tentorial attachment was normal and not superiorly placed. The straight sinus and sinus confluence were normally located. The brainstem (pons and medulla) was hypoplastic as seen on sagittal T1-weighted images [Figure 3]. Small remnants of the cerebellum were seen anteriorly adjacent to the pons and likely represented residua of the anterior quadrangular lobes [Figures 1 and 2]. The supratentorial brain was normal. The cortical gyri and corpus callosum were normal. There was no hydrocephalus. The above imaging findings were consistent with a diagnosis of primary cerebellar agenesis. The child is on follow-up with the pediatric department.

Figure 1

Axial T1-weighted magnetic resonance image shows an empty, normal sized posterior fossa (large white arrow) filled with hypointense cerebrospinal fluid. There is no mass effect. Remnants of the quadrangular lobes are seen anteriorly (small white arrows)

Figure 2

Axial T2-weighted magnetic resonance image shows an empty, normal sized posterior fossa (large white arrow) filled with hyperintense cerebrospinal fluid. Remnants of the quadrangular lobes are seen anteriorly (small white arrows)

Figure 3

Sagittal T1-weighted magnetic resonance image shows hypoplastic brainstem (white arrow) with empty posterior fossa (black arrow)

Discussion

Primary cerebellar agenesis is an extremely rare entity. Sometimes, small remnants of the cerebellum may be present on account of which the term subtotal cerebellar agenesis has also been used in the literature. Earlier on, this condition had only been reported in autopsy cases. The first case of primary cerebellar agenesis was described by Combettes in 1831.[14] To the best of our knowledge, eleven living cases of cerebellar agenesis have been reported till date.[123] The clinical presentation is highly variable. Cerebellar agenesis has been reported to be associated with profound motor impairment. However, normal or near normal motor function has also been reported.[1] Our patient also did not have any significant motor impairment. This may point toward neuronal plasticity which allows the supratentorial brain to compensate for cerebellar function. A case of cerebellar agenesis has been reported to show improvement in motor function over the years,[2] which lends support to the proposed theory that the remaining brain may compensate for cerebellar function over a period of time. Moreover, small remnants of the palaeocerebellum may be present, which could compensate for the motor function. A review of imaging in a case of cerebellar agenesis reported earlier in a man who died in 1951 without any motor impairment revealed a small residual cerebellum.[5] Remnants of cerebellar tissue were also seen in a case of cerebellar agenesis in a 22-year-old male by Velioglu et al.[6] Our patient had remnants of the anterior quadrangular lobes which may account for the lack of motor manifestations.

Recent studies have revealed the role of cerebellum in important nonmotor functions including attention, language, working memory, and learning, which explains cognitive and behavioral abnormalities in cerebellar agenesis.[78]

Diagnosis of cerebellar agenesis in living cases is based on MRI of the brain. Conventional MRI reveals a normal-sized posterior fossa with CSF signal intensity. Either no cerebellar tissue or small remnants of the quadrangular lobes may be seen. There is no mass effect. The tentorial attachment and straight sinus are normally located with no superior displacement. The pons and medulla are hypoplastic due to the lack of afferent input from the cerebellum. Diffusion tensor imaging reveals the lack of efferent and afferent cerebellar fibers. Magnetic resonance angiography reveals bilateral absence of the cerebellar vessels.[1]

Primary cerebellar agenesis needs to be differentiated from secondary degeneration of the cerebellum (vanishing cerebellum) in Chiari II malformation. Chiari II is characterized by a small posterior fossa unlike the normal sized fossa in primary cerebellar agenesis.[1] In addition, there are multiple scattered remnants of the cerebellum and hydrocephalus. Meningomyelocele is seen in more than 90% of patients. It may also be associated with partial or complete callosal agenesis and other anomalies. The other differential diagnosis for primary cerebellar agenesis is Dandy Walker syndrome and its variants. In that case, the posterior fossa is enlarged with superior displacement of the tentorium, straight sinus, and torcular herophili. Variable degrees of cerebellar vermian hypoplasia are seen. In primary cerebellar agenesis, the posterior fossa is normal in size with no displacement of surrounding structures.[9]

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