Maple syrup urine disease (MSUD)

Alternative names Branched-chain ketoacid decarboxylase (BCKD) deficiency
About the condition Maple syrup urine disease is an amino acid disorder. It is a hereditary disease that is caused by a missing enzyme needed to metabolise the branched-chain amino acids (leucine, isoleucine, and valine) found in protein. These amino acids accumulate in the blood and are toxic to the brain.
Inheritance Autosomal recessive
Year screening started in WA 2005
Incidence in Australia Approximately 1 in 100,000 births
Enzyme defect Deficient activity of the enzyme complex involved in the oxidative decarboxylation of the alpha-keto acid derivatives of leucine, isoleucine, and valine.
Symptoms if untreated MSUD usually causes problems very early in the newborn period, such as poor feeding, vomiting, lethargy, hypoglycaemia, seizures, opisthotonic posturing, and coma. If not treated, the disease soon leads to death. The infant's urine has a sweet maple syrup smell, which gives the condition its name. There are some less severe forms of the condition, the mildest of which may go undetected for months or years. These may be associated with intellectual disability.
Metabolites on bloodspot screening
  • Increased leucine and isoleucine
  • Increased alloisoleucine (second-tier test)
Diagnostic tests
  • Plasma amino acids
  • Urine organic acids
  • BCKDHA, BCKDHB, and DBT gene analysis
Treatment
  • Early assessment and treatment are important and infants may need intensive care initially
  • Leucine, isoleucine, and valine restricted diet
  • Essential amino acid supplementation
  • Evaluate for possible thiamine responsiveness
Screening issues Mild forms of MSUD may be missed as the branched-chain amino acids may not be increased in the first weeks of life.