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Definition

Maple syrup urine disease (MSUD) is a disorder in which the body cannot break down certain parts of proteins. The urine of people with this condition can smell like maple syrup.

Causes

Maple syrup urine disease (MSUD) is inherited, which means it is passed down through families. It is caused by a defect in 1 of 3 genes. People with this condition cannot break down the amino acids leucine, isoleucine, and valine. This leads to a buildup of these chemicals in the blood.

In the most severe form, MSUD can damage the brain during times of physical stress (such as infection, fever, or not eating for a long time).

Some types of MSUD are mild or come and go. Even in the mildest form, repeated periods of physical stress can cause intellectual disability and high levels of leucine to build up.

Symptoms

Symptoms of this disorder include:

• Coma
• Feeding difficulties
• Lethargy
• Seizures
• Urine that smells like maple syrup
• Vomiting

Exams and Tests

These tests may be done to check for this disorder:

• Plasma amino acid test
• Urine organic acid test
• Genetic testing

There will be signs of ketosis (buildup of ketones, a by-product of burning fat for energy) and excess acid in the blood (acidosis).

Treatment

When the condition is diagnosed, and during episodes, treatment involves eating a protein-free diet. Fluids, sugars, and sometimes fats are given through a vein (IV). Dialysis through your belly or a vein can be done to reduce the level of abnormal substances in your blood.

Long-term treatment requires a special diet. For infants, the diet includes a formula with low levels of the amino acids leucine, isoleucine, and valine. People with this condition must remain on a diet low in these amino acids for life.

It is very important to always follow this diet to prevent nervous system (neurological) damage. This requires frequent blood tests and close supervision by a registered dietitian and physician, as well as cooperation by parents of children with the condition.

Outlook (Prognosis)

This disease can be life threatening if untreated.

Even with dietary treatment, stressful situations and illness can still cause high levels of certain amino acids. Death may occur during these episodes. With strict dietary treatment, children have grown into adulthood and can remain healthy.

Possible Complications

These complications can occur:

• Neurological damage
• Coma
• Death
• Intellectual disability

When to Contact a Medical Professional

Contact your health care provider if you have a family history of MSUD and are planning to start a family. Also call your provider right away if you have a newborn who has symptoms of maple syrup urine disease.

Prevention

Genetic counseling is suggested for people who want to have children and who have a family history of maple syrup urine disease. Many states now screen all newborns with blood testing for MSUD.

If a screening test shows that your baby may have MSUD, a follow-up blood test for amino acid levels should be done right away to confirm the disease.

References

Gallagher RC, Enns GM, Cowan TM, Mendelsohn B, Packman S. Aminoacidemias and organic acidemias. In: Swaiman KF, Ashwal S, Ferriero DM, et al, eds. Swaiman's Pediatric Neurology: Principles and Practice. 6th ed. Philadelphia, PA: Elsevier; 2017:chap 37.

Kliegman RM, St. Geme JW, Blum NJ, Shah SS, Tasker RC, Wilson KM. Defects in metabolism of amino acids. In: Kliegman RM, St. Geme JW, Blum NJ, Shah SS, Tasker RC, Wilson KM, eds. Nelson Textbook of Pediatrics. 21st ed. Philadelphia, PA: Elsevier; 2020:chap 103.

Merritt JL, Gallagher RC. Inborn errors of carbohydrate, ammonia, amino acid, and organic acid metabolism. In: Gleason CA, Sawyer T, eds. Avery's Diseases of the Newborn. 11th ed. Philadelphia, PA: Elsevier; 2024:chap 29.

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