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Maple syrup urine disease
Reviewed by Medicinsk Expert•Last updated: 2026-10-22•Condition

Maple syrup urine disease

Maple syrup urine disease is an inherited disorder where the body cannot properly process certain amino acids, leading to a distinctive sweet odor in infants' urine and severe neurological complications if untreated.

Overview

Maple syrup urine disease is an inherited disorder in which the body is unable to process certain protein building blocks (amino acids) properly. The condition gets its name from the distinctive sweet odor of affected infants' urine. It is also characterized by poor feeding, vomiting, lack of energy (lethargy), abnormal movements, and delayed development. If untreated, maple syrup urine disease can lead to seizures, coma, and death.

Maple syrup urine disease is often classified by its pattern of signs and symptoms. The most common and severe form of the disease is the classic type, which becomes apparent soon after birth. Variant forms of the disorder become apparent later in infancy or childhood and are typically milder, but they still lead to delayed development and other health problems if not treated.

Frequency

Maple syrup urine disease affects an estimated 1 in 185,000 infants worldwide. The disorder occurs much more frequently in the Old Order Mennonite population, with an estimated incidence of about 1 in 380 newborns.

Causes

Mutations in the BCKDHA, BCKDHB, and DBT genes can cause maple syrup urine disease. These three genes provide instructions for making proteins that work together as part of a complex. The protein complex is essential for breaking down the amino acidsleucine, isoleucine, and valine, which are present in many kinds of food, particularly protein-rich foods such as milk, meat, and eggs.

Mutations in any of these three genes reduce or eliminate the function of the protein complex, preventing the normal breakdown of leucine, isoleucine, and valine. As a result, these amino acids and their byproducts build up in the body. Because high levels of these substances are toxic to the brain and other organs, their accumulation leads to the serious health problems associated with maple syrup urine disease.

Researchers are studying other genes related to the same protein complex that may also be associated with maple syrup urine disease.

Associated Genes

  • BCKDHA
  • BCKDHB
  • DBT
  • PPM1K

Inheritance

This condition is inherited in an autosomal recessive pattern, which means both copies of the gene in each cell have mutations. The parents of an individual with an autosomal recessive condition each carry one copy of the mutated gene, but they typically do not show signs and symptoms of the condition.

Other Names for This Condition

  • BCKD deficiency
  • Branched-chain alpha-keto acid dehydrogenase deficiency
  • Branched-chain ketoaciduria
  • Ketoacidemia
  • MSUD

Frequently Asked Questions

Dietary management is the cornerstone of MSUD treatment. It involves a strict, lifelong diet that severely restricts the intake of the branched-chain amino acids (BCAAs): leucine, isoleucine, and valine. This means avoiding or carefully limiting protein-rich foods like meat, eggs, dairy products, nuts, and legumes. Special BCAA-free medical formulas and foods are used to provide essential nutrients while controlling BCAA levels. Regular blood tests are necessary to monitor amino acid levels and adjust the diet as needed.
Even with strict dietary management, individuals with maple syrup urine disease require lifelong monitoring and management. Potential long-term complications can include learning difficulties, developmental delays, and neurological issues, especially if BCAA levels are not consistently controlled. Acute metabolic crises, often triggered by illness or stress, can still occur and require immediate medical intervention to prevent brain damage.
In newborns, classic maple syrup urine disease typically appears within the first few days of life. Key early signs include poor feeding, frequent vomiting, increased sleepiness or lack of energy (lethargy), and a distinctive sweet, maple syrup-like odor in their urine or earwax. If you notice these symptoms, especially the sweet odor, it's crucial to seek immediate medical attention as untreated MSUD can rapidly lead to seizures, coma, and be life-threatening.
Maple syrup urine disease is an autosomal recessive inherited disorder. This means that a child must inherit two copies of a mutated gene (one from each parent) to develop the condition. If you already have one child with MSUD, both parents are carriers. For each subsequent pregnancy, there is a 25% (1 in 4) chance that the child will inherit two mutated genes and have MSUD, a 50% chance they will be a carrier like the parents, and a 25% chance they will not have MSUD and not be a carrier. Genetic counseling is highly recommended for families to understand these risks.
Source: MedlinePlus, National Library of Medicine

MedlinePlus brings together authoritative health information from the National Library of Medicine (NLM), the National Institutes of Health (NIH), and other government agencies and health-related organizations.

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