Maple syrup urine disease (MSUD) is a metabolism disorder passed down through families in which the body cannot break down certain parts of proteins. Urine in persons with this condition can smell like maple syrup.
Maple syrup urine disease (MSUD) is caused by a gene defect. Persons with this condition cannot break down the amino acids leucine, isoleucine, and valine. This leads to a buildup of these chemicals in the blood.
In the most severe form, MSUD can damage the brain during times of physical stress (such as infection, fever, or not eating for a long time).
Some types of MSUD are mild or come and go. Even in the mildest form, repeated periods of physical stress can cause mental retardation and high levels of leucine.
Wendel U, Ogier de Baulny H. Branched-chain organic acidurias/acidemias. In: Fernandes J, Saudubray J-m, van den Berghe G, Walter JH, eds. Inborn Metabolic Diseases: Diagnosis and Treatment. 4th ed. New York, NY: Springer; 2006:chap 19.
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