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. Laborat ory tests showed hyperammonaemia (arterial level 186 mu mol/l) and slightly elevated prothrombin time. The patient was treated with sodium benzoate, l actulose and a protein-restricted diet. This resulted in an insufficient de crease in blood ammonia levels. Metabolic investigations were unrevealing a part from a slightly elevated urinary glutamine concentration. Liver tissue showed steatosis and mildly decreased activity of N-acetylglutamate syntha se suggesting partial deficiency. Treatment with N-carbamyl glutamate did n ot affect serum ammonia levels. Colour Doppler sonography and MR angiograph y demonstrated a patent ductus venosus. After surgical ligation of the duct us venosus, serum ammonia levels returned to normal and mental and motor pe rformance improved markedly. Conclusion In late onset hyperammonaemia, partial N-acetylglutamate synthas e deficiency and portocaval shunt should be ruled out.