Maple syrup urine disease

Maple syrup urine disease (MSUD), also known as branched-chain ketoaciduria or branched-chain alpha-keto acid dehydrogenase deficiency, is an inherited metabolic disorder caused by deficiency of the branched-chain alpha-keto acid dehydrogenase (BCKD) enzyme complex. This enzyme complex is essential for the breakdown of three branched-chain amino acids (BCAAs): leucine, isoleucine, and valine. When this enzyme is deficient, these amino acids and their toxic byproducts accumulate in the blood, urine, and tissues, giving the urine a characteristic sweet odor resembling maple syrup. The disease primarily affects the nervous system, as elevated leucine levels are particularly neurotoxic and can cause severe brain damage. Several clinical forms exist. The classic (severe) form presents in the neonatal period, typically within the first few days of life, with poor feeding, lethargy, vomiting, and a distinctive sweet-smelling urine. Without prompt treatment, affected infants rapidly develop encephalopathy with seizures, opisthotonus, respiratory failure, coma, and potentially death. Intermediate, intermittent, and thiamine-responsive forms also exist, which present later in life and may have milder symptoms but can still cause metabolic crises during periods of physiological stress such as illness, surgery, or fasting. MSUD is caused by pathogenic variants in the BCKDHA, BCKDHB, or DBT genes, which encode the E1α, E1β, and E2 subunits of the BCKD complex, respectively. Rare cases involve mutations in the DLD gene encoding the E3 subunit. Treatment involves lifelong dietary restriction of branched-chain amino acids, with careful monitoring of plasma amino acid levels. During acute metabolic crises, emergency management includes intravenous glucose, insulin, and sometimes dialysis to rapidly lower leucine levels. Some patients with the thiamine-responsive form may benefit from thiamine supplementation. Liver transplantation has emerged as a definitive treatment option that can restore sufficient enzyme activity to allow an unrestricted diet and prevent further metabolic crises, though it does not reverse existing neurological damage. Newborn screening programs using tandem mass spectrometry have significantly improved early detection and outcomes.

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