Maple syrup urine disease type 1B is an inherited metabolic disorder characterized by the lack of an enzyme needed to break down certain building blocks of proteins.
Without the enzyme to break down these amino acids, they accumulate and cause damage to the body. Symptoms can range from mild to severe, with the classic type being the most severe type.
Individuals with the other types often present with milder symptoms but can also go through periods of exhibiting more severe symptoms. Symptoms include urine that smells like maple syrup, poor feeding, vomiting, lack of energy, abnormal movements, and delayed development. Maple syrup urine disease typically presents in infants shortly after birth but can also present later in infancy or in early childhood.
This condition is caused by pathogenic (disease-causing) variants in the BCKDHB 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, individuals of Ashkenazi Jewish descent have a 1 in 97 chance to be a carrier. Carriers of maple syrup urine disease typically do not experience any symptoms.
Other names for this condition include CKD deficiency, branched-chain alpha-keto acid dehydrogenase deficiency, branched-chain ketoaciduria, and ketoacidemia.
Resources:
Maple Syrup Urine Disease Family Support Group
National Organization for Rare Disorders (NORD)
Revised July 2022
