Acute hyperammonemic encephalopathy

• Occurs due to increased production of ammonia/ reduced elimination of ammonia.

• Pathogenesis : In hyperammonemic state, ammonia crosses blood brain barrier induces excess glutamine, an osmolyte, causing water influx leading to cerebral edema. Ammonia also interferes with oxidative metabolism resulting in excessive lactate levels.

• Seen in the background of cirrhosis or non-cirrhotic conditions like genetic disorders/ drugs/ infections. In our case patient had high plasma concentration of valproic acid and drug induced hyperammonemia.

• Typical MRI findings – Bilateral symmetrical cortical signal abnormalities on FLAIR and DWI/ADC, predominantly in insula and cingulate gyrus with sparing of occipital and perirolandic cortices. Basal ganglia, thalami and brain stem are additional sites of involvement. It is also emphasized that signal changes distribution is independent of origin of hyperammonemia.

• T1 hyperintense bilateral globus pallidi may be a clue to the underlying hepatic disease.

• The increased intracellular osmolality caused by hyperammonemia results in a reduction of choline and myoinositol peaks and a rise in the glutamine and glutamate peaks on proton MR spectroscopy.

• Differential diagnoses: Hypoxic ischemic encephalopathy, hypoglycemic encephalopathy, viral encephalitis, autoimmune encephalitis, and Creutzfeldt-Jacob encephalopathy.

• Cortical changes are potentially reversible on early treatment.

• May result in atrophy of insula and cingular gyrus.

References:

1.Reis E, Coolen T, Lolli V. MRI Findings in Acute Hyperammonemic Encephalopathy: Three Cases of Different Etiologies. Journal of the Belgian Society of Radiology. 2020;104(1):9.

2.U-King-Im JM, Yu E, Bartlett E, Soobrah R, Kucharczyk W.  Acute hyperammonemic encephalopathy in adults: imaging findings. AJNR Am J Neuroradiol. 2011;32(2):413-418.

3.Rosario M, McMahon K, Finelli PF. Diffusion-weighted imaging inacute hyperammonemic encephalopathy. Neurohospitalist. 2013;3(3):125-130.