A unique case of derangement of vitamin B12 metabolism.

The case is described of a child, age 6 1/2 years, with retarded mental development, mild neurological signs and abnormal metabolism of sulphur-containing amino acids and methylmalonate, due to an inborn error in the formation of vitamin B12 coenzymes. The patient was treated for almost three years with hydroxycobalamin, folic acid, pyridoxine and choline. Though physical growth was normal, she continued to demonstrate a moderate degree of mental retardation. A brother of the patient died at the age of 5 years, probably of a similar, but undiagnosed, disorder. As far as we are aware there are only four other reported cases similar to the case described here. Two of these patients died and in other other two the defect was so mild that no treatment was necessary and who, in fact, showed appreciable improvement during the follow-up period, which to date amounts to 3 years and 3 months. For reasons detailed in the discussion, it is suggested that the diagnosis of homocystinuria is not complete until studies of folate and vitamin B12 metabolism are undertaken at the same time, so as to identify the metabolic defect(s) responsible for the condition.