Maple Syrup Urine Disease (MSUD) is a genetic disorder where the body can’t break down certain amino acids, leading to a distinctive smell in urine. Without a specific enzyme, these amino acids build up and harm the body.
There are four types of MSUD: classic (most severe), intermediate, intermittent, and thiamine-responsive.
Classic MSUD appears in the first week of life. Babies with this type can’t digest proteins properly and experience symptoms like vomiting, seizures, and brain swelling. If untreated, it can be fatal. Intermediate MSUD is less severe than classic but still requires strict diet management. Intermittent MSUD is rare and symptoms are occasional, often triggered by illnesses or high-protein foods. Thiamine-responsive MSUD might improve with vitamin B1 supplements, but it’s not a complete solution.
MSUD is managed with a low-protein diet and sometimes special liquid formulas. Regular monitoring by doctors is vital to adjust the diet. Any signs of brain swelling need immediate medical attention. During illnesses, patients might need special diets to avoid hospitalization. Thiamine-responsive MSUD patients may get thiamine supplements, although the full impact is still uncertain. Treatment and diet management are lifelong.
MSUD is caused by pathogenic (disease-causing) variants in the BCKDHA (type 1a), BCKDHB (type 1b), or DBT (type II) gene and exhibits autosomal recessive inheritance. This means that both parents must be carriers to have a 25% chance to have a child with the condition. The risk of being a carrier is based on a person’s ancestry or ethnic background. For example, 1 in 97 individuals of Ashkenazi Jewish descent are carriers for MSUD type 1b.
Other names for this condition are branched-chain alpha-keto acid dehydrogenase deficiency, branched-chain ketoaciduria, and ketoacidemia.
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Revised October 2023
