Acute hyperammonaemic encephalopathy.

A 58-year-old man with chronic hepatitis B and alcoholic liver cirrhosis was admitted following massive haematemesis with a resultant haemorrhagic shock from bleeding oesophageal varices. Despite emergency variceal ligation with haemodynamic stabilization, the patient remained stuporous and developed two generalized tonic–clonic seizures. Neurological examination revealed intact brainstem reflexes, extensor response of the left upper limb to pain stimuli and absent plantar responses.

The differential diagnoses at that point in time included hypoxic ischaemic encephalopathy, non-convulsive status epilepticus, Wernicke encephalopathy and hyperammonaemic encephalopathy. His electroencephalogram showed background slowing with no epileptiform activity; urea and electrolyte measurements, glucose and thyroid function test were unremarkable.

MRI brain revealed a strikingly bilaterally symmetrical pattern of cortical hyperintensities on T2 and diffusion-weighted images (DWI) (Fig. 1A and B) involving the frontal regions, especially the cingulate gyri (arrow) and insula cortices (arrowhead). See Figure 2 for corresponding apparent diffusion coefficient (ADC). These MRI findings are typical of the regional lesion distribution in acute hyperammonaemic encephalopathy. The diagnosis of hyperammonaemic encephalopathy, most likely precipitated by variceal bleeding complicating pre-existing liver disease, was confirmed by the elevated serum ammonia level of 90 µmol/L (9–35 µmol/L).

Figure 1:

(A) MRI brain T2-weighted sequence revealed T2 hyperintensity in the bilateral cingulate gyri (white arrow) and insula cortices (arrowhead). (B) DWI sequence showed hyperintensity in the same distribution as the T2 hyperintensitiy in (A).

Figure 2:

ADC image showed corresponding ADC reduction in regions with DWI hyperintensity as previously seen in Fig. 1B.

Although DWI is most commonly used to detect acute cerebral infarction, bilateral abnormal cortical lesions may also be seen in many acute non-stroke conditions such as encephalitis, seizures, hypoxic ischaemic encephalopathy, hypoglycaemia and hyperammonaemia [1]. The symmetrical vulnerability of the insula and cingulate gyri suggests a systemic, metabolic disease, which was helpful in suggesting hyperammonaemic brain damage rather than other possible diagnoses or ischaemic infarction [2]. The corresponding ADC can be reduced acutely with subsequent normalization [3].

Hyperammonaemic encephalopathy may result from chronic liver disease, and also from uncommon inborn errors of metabolism such as citrullinaemia [4] and ornithine transcarbamylase deficiency [5], and drugs such as valproic acid [6]. Recognizing the distinctive findings of acute hyperammonaemia on DWI can be very helpful for early diagnosis and treatment to prevent potentially irreversible neurological damage and death [6].