Practical management of combined methylmalonicaciduria and homocystinuria

Combined methylmalonicaciduria and homocystinuria is a disorder of intracellular cobalamin metabolism that remains a challenge to the physician unfamiliar with the diagnosis. We have followed six patients with combined methylmalonicaciduria and homocystinuria (four males, two females, age 4.2-24 years) for a median of 6.5 years (range 4-9 years). Age at diagnosis was between 18 days and 14 months in early-onset cases (n = 4) and 15 and 19 years in late-onset cases. Predominant clinical features include microcephaly, hydrocephalus, seizures, and white-matter changes on magnetic resonance imaging in early-onset cases. The white-matter changes may be secondary to impaired methylation owing to a lack of readily available methyl groups. Spastic quadriparesis and diplegia are long-term sequelae in late-onset cases. Management consists of hydroxycobalamin intramuscular injections, oral folate, betaine, and carnitine supplementation. Dietary protein restriction may be necessary when metabolic control remains difficult. The implementation of an emergency regimen should alleviate episodes of metabolic decompensation and reduce the rate of hospital admissions.