Dizygotic twins with 3-hydroxy-3-methylglutaric aciduria; unusual presentation, family studies and dietary management.

A 4-month-old infant with hypotonia and macrocephaly was diagnosed as having 3-hydroxy-3-methylglutaric aciduria, using gas chromatography and mass spectrometry and confirmatory enzyme studies. The same diagnosis was made on his asymptomatic non-identical twin. Examination of the pedigree is consistent with an autosomal recessive mode of inheritance. Dietary treatment improved the symptoms of the propositus, but did not prevent episodes similar to Reye's syndrome in both twins. One such episode closely followed immunisation and our experience suggests that children with this disorder should be observed carefully following immunisation. These episodes were accompanied by an overflow of a wide range of abnormal metabolites. Examination of the urine for organic acids should be considered in infants with unexplained hypotonia and macrocephaly, especially if accompanied by abnormal biochemical indices.