Some of the earliest described cases of this disorder had a multiple carboxylase deficiency, not isolated deficiency of 3-methylcrotonyl-CoA carboxylase. In patients with the isolated deficiency the major abnormal urinary metabolites are 3-hydroxy-isovaleric acid and 3-methylcrotonylglycine. There are three clinical presentations. In the neonatal or early-infantile form there is hypotonia, seizures, feeding difficulties and vomiting. Prognosis is poor. In the late-infantile form there is a Reye-like illness with seizures, hyperammonaemia and hypoglycaemia, and in the juvenile form there is vomiting and dehydration often following increased protein intake. The prognosis is better in these later onset forms.

Enzyme Test: 3-methylcrotonyl-CoA carboxylase is assayed to diagnose this disease.

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