Four general classifications are used to identify the types of MSUD: classic, intermediate, intermittent and thiamine-responsive. These terms refer to the amount and type of enzyme activity present in the affected child, which can vary considerably within each classification.

Classic is the most common type of MSUD. In classic MSUD, little or no enzyme activity (usually less than 2% of normal) is present. Infants with classic MSUD will show symptoms within the first several days of life. They generally have poor tolerance for the BCAAs, so protein must be severely restricted in their diet.

Intermediate MSUD is a variant of the classic type of the disease. Those with intermediate MSUD have a higher level of enzyme activity (approximately 3-8% of normal). They can usually tolerate a greater amount of leucine. However, when ill or fasting, the child with intermediate MSUD reacts just like a child with classic MSUD. Management is similar for the intermediate and classic types of MSUD.

Intermittent MSUD is a milder form of the disease because of the greater enzyme activity present (approximately 8-15% of normal). Often the child does not have symptoms until 12 to 24 months of age, usually in response to an illness or surge in protein intake. During episodes of illness or fasting, the BCAA levels elevate, the characteristic maple syrup (or burnt sugar) odor becomes evident, and the child can go into a metabolic crisis.

Thiamine-responsive MSUD is basically just what the name implies. Giving large doses of thiamine to the thiamine-responsive child will increase the enzyme activity which breaks down leucine, isoleucine and valine. In most cases only moderate protein restriction is needed for this more rare type of MSUD.