What Is Maple Syrup Urine Disease?

Maple syrup urine disease (MSUD) is a rare metabolic disorder that some babies are born with. It’s caused by a defect in the enzymes that break down some amino acids. 

Most newborn babies in the United States have their blood tested for MSUD as part of newborn screening. This lets doctors start treatment, usually with a special diet, right away to help prevent problems.

What Are the Signs & Symptoms of Maple Syrup Urine Disease?

Newborns with MSUD have pee or earwax that smells sweet, like maple syrup. They also may:

• have problems with feeding
• vomit
• be fussy or irritable
• have a high-pitched cry
• have a low energy level and be very sleepy

A baby or child with MSUD also might have: 

• trouble growing and gaining weight
• unusual muscle movements and muscle tightness
• developmental delays
• seizures

What Causes Maple Syrup Urine Disease?

Amino acids help kids grow and develop. The body uses them to make proteins. In MSUD, three amino acids — leucine, isoleucine, and valine — are not broken down. These amino acids and their byproducts then build up in the blood, damaging the brain and other organs.

MSUD happens when there’s a change (mutation) in the genes that make the enzymes that break down these three amino acids, which are found in food.

To have MSUD, a child must inherit two changed MSUD genes, one from each parent. Parents who have one changed gene do not have any symptoms, but can pass the gene to their children. In the U.S., MSUD is seen most often in some Mennonite communities.

How Is Maple Syrup Urine Disease Diagnosed?

In the U.S., doctors do a blood test to check babies for MSUD as part of newborn screening.

Children with milder forms of MSUD may not be diagnosed until they are older and start to show symptoms. If doctors suspect MSUD, they will check the amino acid levels in the blood and other acids in the urine.

The doctor may recommend genetic testing if a baby is born into a family with a history of MSUD.

How Is Maple Syrup Urine Disease Treated?

The main treatment for MSUD is a low-protein diet with low levels of the three amino acids. Babies with MSUD must be on a special formula as soon as possible. Then, they’ll follow the special diet for the rest of their lives. Some also need to take nutritional supplements. Following this diet will help prevent the problems linked to MSUD.

Your doctor will do regular checks of the amino acid levels in your child's blood. Working with the health care team, including a dietitian who specializes in metabolic disorders like MSUD, can help keep these levels in an acceptable range.

Doctors may recommend a liver transplant for some children with severe MSUD.

When Should I Call the Doctor?

Maple syrup urine disease is treatable when found early. Doctors will closely watch kids who have it. 

Some people with MSUD may have episodes of metabolic crisis. These are triggered by stress, such as:

• illness
• injury
• exercise
• going a long time without food

Call your doctor right away if your child is sick, has a fever, is not eating well, or is very sleepy, or if you notice balance problems, trouble walking, or changes in behavior.

What Else Should I Know?

Because MSUD is a genetic condition, you may want to speak to a genetic counselor about testing other family members and to learn how MUSD runs in families. You also can find more information and support online at:

• MSUD Family Support Group 
• Medical Nutrition Therapy for Prevention Program (MNT4P)