Neurodevelopmental outcome of long‐term therapy of urea cycle disorders in Japan

In Japan, urea cycle disorders (UCDs) are one of the most frequent inborn errors of metabolism, estimated to have a prevalence of 1 per 50 000 live births. In an attempt to develop more effective treatment and enhance the quality of life, we investigated the clinical manifestations and prognosis of 216 patients with UCDs diagnosed and treated between 1978 and 1995. These included 92 cases of neonatal‐onset UCD and 116 of late‐onset UCD. Two cases of ornithine transcarbamylase (OTC) deficiency in males and 2 cases of argininosuccinase (AL) deficiency were diagnosed prospectively. By far the most common disorder was OTC deficiency, accounting for 2/3 of all cases. At the end of 1995, the 5‐year survival rate was 22% for the neonatal‐onset type and 41% for the late‐onset type. Among the 20 long‐term survivors with neonatal‐onset UCD, 18 (90%) had moderate to severe neurodevelopmental deficits; this contrasts with 13 of 47 (28%) survivors with the late‐onset type. In analysing 108 UCD cases, peak blood ammonia level during the first hyperammonaemic attack was correlated with neurodevelopmental outcome. When the concentration of blood ammonia was less than 180 µmol/L (5 times normal), there was no severe neurological damage. When the concentration of blood ammonia exceeded 350 µmol/L (10 times normal) at the first hyperammonaemic attack, the patients died or had severe neurological deficits. Our data point to the importance of early diagnosis and aggressive treatment.