Maple syrup urine disease (MSUD) is a rare inherited metabolic disorder characterized by a deficiency of the branched-chain alpha-ketoacid dehydrogenase complex, causing an inability of the body to properly break down amino acids. There are 5 subtypes of MSUD including the most common classic form, second most common intermittent form, intermediate form, thiamine-responsive form, and E3-deficient form. Diagnosis can be made by measuring plasma amino acid concentrations. In newborns with the classic form, MSUD can be detected by screening with tandem mass spectrometry.

Classic MSUD symptoms can present within 48 hours following birth and include sweet-smelling urine that resembles maple syrup, poor feeding, irritability, vomiting, lethargy, dystonia, and apnea. Without treatment, the disease can cause neurologic damage or death following central respiratory failure. Treatment during acute episodes of metabolic crisis is aggressive and supportive. Long-term management involves limiting the consumption of branched-chain amino acids (BCAAs) and lifelong dietary restrictions. Prognosis can be normal with early intervention and prompt treatment of episodes.

Intermittent MSUD, the second most common subtype, does not present in infancy. Patients have normal intellectual development and growth and only experience symptoms in childhood and beyond following any catabolic stress such as infection, fever, dehydration, surgery, etc. The presentation of intermittent MSUD following catabolic stress includes ketoacidosis, anorexia, nausea, vomiting, fatigue, and signs of neurotoxicity including ataxia, cognitive impairment, hallucinations, sleep disturbance, dystonia, and seizures or coma. Treatment and management are similar to classic MSUD treatment and involve prompt and aggressive intervention during acute episodes and long-term dietary restriction of BCAAs.