Untreated, MSUD causes abnormal brain development, mental retardation and death within a few months. Patients must be treated within ten days of birth to achieve normal intellect. Treatment is the dietary restriction of branched chain amino acids. In the most severe form, MSUD causes severe acidosis during the first week of life. This is characterized by progressively poorer feeding, vomiting , seizures , lethargy , and finally coma .
Untreated infants die in the first few weeks of life.
MSUD also occurs in an intermittent form and a mild form. Even the mildest form, MSUD results in retardation and bouts of acidosis precipitated by stresses such as illness. Infection or high protein intake may precede episodes of acidosis. Untreated acidosis progresses to coma followed by death. Large doses of vitamin B12 may be given during acute attacks.