Feline hyperammonemia associated with functional cobalamin deficiency: A case report

Ammonia is a major neurotoxic substance associated with the complex pathogenesis of hepatic encephalopathy. Although several primary and secondary conditions have been reported to cause hyperammonemia, in veterinary medicine this condition is considered primarily associated with hepatic disease or portosystemic shunting. Only a few cases of inherited urea cycle enzyme deficiency and organic acid metabolic disorders have been reported in cats with hyperammonemia. To the best of our knowledge, this is the first report of hyperammonemia in a cat caused by accumulation of methylmalonic acid (MMA) secondary to functional cobalamin deficiency. A 2-year-old spayed female Turkish Angora cat exhibited postprandial depression with a 3-month history of hyperammonemia. Serum protein C and bile acid concentrations were normal. Plasma amino acid analysis revealed a deficiency of urea cycle amino acids. Although the serum cobalamin concentration was markedly high, there was no evidence of inflammatory, hepatic, or renal disease or neoplasia on blood, ultrasonographic, and computed tomographic examination. Gas chromatography-mass spectrometry revealed a high MMA concentration in the urine. Based on the results, functional cobalamin deficiency was diagnosed. Following oral amino acid supplementation and initiation of a low-protein diet, the serum ammonia level returned to normal and the postprandial depression improved. Urea cycle amino acid deficiency secondary to functional cobalamin deficiency presumably caused hyperammonemia due to MMA accumulation in this case.