Disorders of ornithine metabolism

Hyperornithinaemia due to ornithine aminotransferase (OAT) deficiency results in gyrate atrophy of the choroid and retina (GA). Although the progression of the retinal degeneration is highly variable, most GA patients lose all functional vision in middle age (45-65 years). Treatment includes an argininerestricted diet and a trial of pharmacological doses (250-500 mg/day) of pyridoxine (vitamin B). Long-term compliance with an arginine-restricted diet, especially when started at a young age, can reduce ornithine accumulation and slow visual loss and chorioretinal degeneration. Creatine supplementation may be indicated to replenish tissue levels, but this question has not yet been adequately addressed. Rarely, OAT-deficient neonates present with hyperammonaemic encephalopathy due to impaired urea cycle function caused by substrate limitation, with associated hypoargininaemia and hypoornithinaemia.