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Abstract
THE contribution of ammonia to the pathogenesis of hepatic encephalopathy has been widely investigated.1, 2 Hyperammonemia and cerebral dysfunction in the absence of liver disease and portal hypertension have been reported in patients with acquired3 or congenital4, 5 deficiency of hepatic enzymes of the Krebs urea cycle and in patients who have had ureterosigmoidostomy.6 7 8 9 However, in the latter group coexistent liver disease was present and could have contributed to encephalopathy.6 7 8 9 We report the case of a patient who had recurrent hyperammonemic encephalopathy despite adequate urea-cycle enzymes and normal liver structure. Laboratory studies showed a venous-blood ammonia level of 86 μmol per liter and normal serum aspartate aminotransferase, alkaline phosphatase, total serum bilirubin, prothrombin time, and glucose. The elevated pH of the urine relative to the pH of venous blood in the bladder favored nonionic diffusion of ammonia directly into the inferior vena cava via internal iliac veins.12 In essence, this diffusion created a spontaneous shunt allowing ammonia to bypass the liver and reach the brain in sufficient concentration to produce encephalopathy. The occurrence of hyperammonemic encephalopathy in our patient is analogous to that reportedly produced through experimental obstipation with a rectal balloon in patients with ureterosigmoidostomy.6 7 8 9 However, in those reports, manifestations of hepatic disease -- jaundice, cirrhosis, or abnormal liver-function tests -- were noted, and subtle forms of congenital hyperammonemia syndromes had not been entirely excluded as possible factors.6 7 8 9 In conclusion, hyperammonemia should be sought in patients with unexplained encephalopathy not complicated by clinical jaundice; if found, a possible source of abnormal entry of ammonia directly into the systemic circulation should be considered in addition to the possibilities of severe liver disease and congenital or acquired enzyme deficiencies.