Maple syrup urine disease

Alternative names 

Maple syrup urine disease is an inherited disease of amino acid metabolism that causes acidosis, central nervous system symptoms, and urine that may smell sweet like maple syrup.

Causes, incidence, and risk factors

Maple syrup urine disease (MSUD) is caused by the inability to metabolize the branched-chain amino acids leucine, isoleucine, and valine. The disease is called MSUD because urine from affected people smells like maple syrup.

In the most severe form, MSUD causes severe acidosis during the first week of life. This is characterized by progressively poorer feeding, Vomiting, seizures, Lethargy, and finally coma.

Untreated infants may die in the first few weeks of life in severe forms of the disease. MSUD also occurs in an intermittent form and a mild form. Even in the mildest form, infections can cause mental retardation and bouts of acidosis.


  • Family history of MSUD or unexplained infant death  
  • Urine which smells like maple syrup  
  • Feeding difficulties  
  • Lethargy  
  • Vomiting  
  • Seizures  
  • Coma  
  • Avoiding food

Signs and tests

  • Urine amino acids (elevated levels of the amino acids leucine, isoleucine, and valine)  
  • Plasma amino acids (elevated levels of leucine, isoleucine, and valine)  
  • Ketosis (elevated levels of ketone bodies in urine and plasma)  
  • Acidosis (excess acid in blood)

Treatment of the acute episode:

  • Acute acidosis is treated to restore normal pH.  
  • Because this is a protein intolerance disease, protein is cut from the diet .  
  • High doses of intravenous fluid, sugar and fat are given to prevent dehydration and provide energy to stimulate protein synthesis, which lowers the levels of the amino acids which cannot be broken down.  
  • Peritoneal dialysis or Hemodialysis are used to remove the high levels of amino acids.  
  • A special diet free of branched-chain amino acids is started immediately.

Long range treatment requires a special diet. Strict compliance is necessary to prevent neurological damage. This requires close supervision by a registered dietitian and a physician, and parental cooperation. The diet includes a synthetic infant formula with low levels of the amino acids leucine, isoleucine, and valine.

Frequent blood testing for amino acid levels allows doctors and dieticians to adjust the balance of these branched-chain amino acids so that they are neither deficient nor in excess. Affected people must remain on this diet permanently.

Expectations (prognosis)
If left untreated, life-threatening neurological damage may result. Even with dietary treatment, stressful situations and illness can still cause bouts of acidosis. Death may occur during these episodes. With strict dietary treatment, children have grown into healthy adulthood.


  • Neurological damage such as low IQ, if poorly treated  
  • Possibly fatal acidosis episodes

Calling your health care provider
Call your health care provider if you have a family history of MSUD and are planning to start a family. Also call immediately if you have a newborn who exhibits symptoms suggestive of maple syrup urine disease.

Genetic counseling is suggested for prospective parents with a family history of maple syrup urine disease. Some states, such as Pennsylvania, screen all newborns for signs of MSUD. You may receive a call from your pediatrician stating that your baby has an abnormal blood test result for MSUD. A follow-up blood test for amino acid levels should be done right away to find out if your baby does have the disease.

Johns Hopkins patient information

Last revised: December 8, 2012
by Brenda A. Kuper, M.D.

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