Classic maple syrup urine disease

Classic maple syrup urine disease (MSUD), also known as branched-chain ketoaciduria or branched-chain alpha-keto acid dehydrogenase deficiency, is a severe inherited metabolic disorder caused by deficiency of the branched-chain alpha-ketoacid dehydrogenase (BCKD) enzyme complex. This enzyme is essential for breaking down the branched-chain amino acids (BCAAs) leucine, isoleucine, and valine. When the enzyme is deficient, these amino acids and their toxic metabolites accumulate in the blood, urine, and tissues, causing serious damage primarily to the central nervous system. The disease is named for the characteristic sweet, maple syrup-like odor of the urine, cerumen, and sweat of affected individuals. Classic MSUD is the most severe form and presents in the neonatal period, typically within the first few days of life. Affected newborns appear normal at birth but rapidly develop poor feeding, lethargy, irritability, and a distinctive sweet odor. Without prompt treatment, the condition progresses to seizures, respiratory failure, cerebral edema, coma, and death. Elevated leucine levels are particularly neurotoxic and can cause irreversible brain damage. The disease 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. Treatment of classic MSUD requires lifelong dietary restriction of branched-chain amino acids, with careful monitoring of plasma BCAA levels. During metabolic crises—often triggered by illness, fasting, or physiological stress—emergency management includes intravenous glucose, insulin therapy, and sometimes dialysis to rapidly lower leucine levels. Liver transplantation has emerged as a definitive treatment option that can restore sufficient BCKD enzyme activity to allow an unrestricted diet and prevent further metabolic crises, though it does not reverse pre-existing neurological damage. Newborn screening programs using tandem mass spectrometry have significantly improved early detection and outcomes. With early diagnosis and meticulous metabolic management, individuals with classic MSUD can achieve improved neurodevelopmental outcomes, though they remain at risk for metabolic decompensation throughout life.

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