Master's in Genetic Counseling (MGC)
Maple Syrup Urine Disease (MSUD)
Maple Syrup Urine Disease is an autosomal recessive disorder in which there is an inability to catabolize the branched chain amino acids leucine, isoleucine, and valine. Classic MSUD results from absent activity of the mitochondrial enzyme complex known as branched chain a-ketoacid dehydrogenase.
Clinical Features
The signs and symptoms of MSUD may present within the first few days of life. Infants may progress from poor feeding, irritability, vomiting and lethargy to seizures and coma within the first weeks of life. The biochemical abnormality may lead to severe metabolic acidosis and hypoglycemia. As the level of leucine continues to rise, ketoacids may appear in the urine. The buildup of the branch chain ketoacids results in a very characteristic odor of the urine resembling that of maple syrup. This odor may also be detected in the ear wax of some infants. (OMIM)
If not confirmed and treated properly, MSUD may result in severe, permanent neurologic damage or death.
Laboratory Tests
Screening for maple syrup urine disease is performed by determining the level of leucine in the dried filter paper blood spot. It is unusual to see even a transient elevation of leucine in the normal newborn. Exceptions may arise if the infant is premature or if the infant is receiving IV amino acid hyperalimentation.
MSUD is a disorder that may be rapidly fatal. Any abnormal newborn screening result for MSUD must be confirmed immediately with a quantitative amino acid profile. Immediate consultation with a metabolic disease specialist is recommended.
Treatment
Any infant with an abnormal newborn screen should be considered to have MSUD until proven otherwise. If the infant is experiencing any of the early signs or symptoms of MSUD, emergency intervention is warranted. Long term treatment for MSUD consists of a diet with low levels of the branched chain amino acids. This diet must be continued for life.
There are variant forms of MSUD that may require slightly different treatment and management, including intermediate, intermittent, and thiamine responsive forms of the disorder. For specific treatment guidelines, please consult with a genetic/metabolic specialist.
Screening Practice Considerations
Levels of plasma leucine begin to rise immediately after birth. Significant increases however, are dependent upon adequate protein intake. The sensitivity of testing for MSUD is at its greatest after 24 hours of protein feeding. Early discharge of infants may result in reduced sensitivity of the MSUD newborn screen. If not confirmed and treated properly, MSUD may result in severe, permanent neurologic damage or death. Therefore, it is critical that practitioners be aware of the clinical presentation of this disorder and arrange for appropriate testing despite a normal newborn screening result.
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