A 6-year-old boy with hyperammonaemia: Partial N-acetylglutamate synthase deficiency or portosystemic encephalopathy?

We describe a 6-year-old boy admitted with lethargy and somnolence. Laboratory tests showed hyperammonaemia (arterial level 186 μmol/1) and slightly elevated prothrombin time. The patient was treated with sodium benzoate, lactulose and a protein-restricted diet. This resulted in an insufficient decrease in blood ammonia levels. Metabolic investigations were unrevealing apart from a slightly elevated urinary glutamine concentration. Liver tissue showed steatosis and mildly decreased activity of N-acetylglutamate synthase suggesting partial deficiency. Treatment with N-carbamyl glutamate did not affect serum ammonia levels. Colour Doppler sonography and MR angiography demonstrated a patent ductus venosus. After surgical ligation of the ductus venosus, serum ammonia levels returned to normal and mental and motor performance improved markedly. Conclusion: In late onset hyperammonaemia, partial N-acetylglutamate synthase deficiency and portocaval shunt should be ruled out.