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The disorder is very rare with less than 20 cases reported in the literature.

Clinical manifestations usually become apparent in the neonatal period or during infancy but the diagnosis may not be made until childhood. Some of the reported patients also displayed hypoglycaemia, spastic quadriparesis, microcephaly, progressive neurological deficit, seizures, vomiting, atrophy of the basal ganglia, severe hypotonia and hepatomegly.

The syndrome is caused by mutations in the AUH gene (chromosome 9) encoding 3-methylglutaconyl-CoA hydratase, an enzyme involved in leucine degradation.

As the clinical picture is variable and nonspecific, diagnosis can be made by assay of 3-methylglutaconyl-CoA hydratase activity in fibroblasts or leukocytes, quantitative analysis of urinary organic acid excretion or, more recently, analysis of bodily fluids by NMR spectroscopy.

Patients with 3-MGA type I can be distinguished from those with other forms of 3-MGA (types II, III and IV; see these terms) by the distinctive pattern of metabolite excretion: 3-methylglutaconic acid levels are highly elevated (higher than those detected in other forms of 3-MGA) whereas methylglutaric acid levels are usually only slightly elevated, and there is a high level of 3-hydroxyisovaleric acid excretion (not present in other forms of 3-MGA).

Prenatal diagnosis should be possible through detection of high levels of 3-hydroxyisovaleric acid in the amniotic fluid or through enzyme analysis of cultured amniocytes.

The syndrome is inherited as an autosomal recessive trait.

Treatment is largely symptomatic but dietary management with a modest leucine restriction and supplementation with L-carnitine may be beneficial in some cases.