Approximately one in 180 thousand Americans exudes a strange, though seldom unpleasant, smell. A genetic abnormality causing an inability to properly process the amino acids leucine, isoleucine, and valine imparts to these patients’ urine and sometimes earwax the scent of maple syrup. These proteins, called branched-chain amino acids, combine with urea to form a compound called sotolone, which is typically found in maple syrup and burnt sugar.
The condition is called maple syrup urine disease because of this unusual symptom, but despite the frivolousness of the name, the other symptoms can be quite serious. Patients with MSUD experience seizures and lethargy, and can become comatose. Children and adults suffering from the condition avoid food, and infants have difficulties feeding. However, undereating is one of the types of stress that can exacerbate the seizures or cause neurological damage in affected people. In fact, even a simple infection can cause this damage.
In some states, MSUD tests are part of mandatory infant screenings. Even when testing isn’t done automatically—or because the parents are carriers—symptoms of the most severe type of MSUD can appear as little as three days after birth. Infants who are fussy, are lethargic don’t feed readily or gain weight, have poor muscle tone, or have any of the usual symptoms of the disease are generally tested for MSUD. Children with milder forms of the condition may not show symptoms until later, sometimes into toddlerhood, but the characteristic urine smell is always an indication of the need for testing.
There is no cure for the condition, and treatment involves a restricted diet. The proteins involved are what are called essential amino acids, meaning they aren’t produced in the body, and so a diet low in proteins, and particularly branched-chain amino acids, is sufficient to prevent the build-up of these substances in the body. However, these dietary restrictions must be maintained for life, and a careful balance must be maintained between enough branched-chain amino acids for normal development without risking a flare of MSUD. In rare cases the vitamin thiamine can boost enzyme activity to allow a less restricted diet.