Classic MSUD is the most common type of MSUD. In
classic MSUD, there’s little or no enzymatic activity taking place, usually
having 2% of the enzyme activity present only. Infants with classic MSUD will
show symptoms within the first several days of life. Generally, they will have
poor tolerance for BCAAs, so protein should be strictly restricted in their
diet.
Intermediate MSUD is a variation of the classic MSUD.
Those with intermediate MSUD have a higher level of enzyme activity which
approximately 3-8% from normal people. 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 there is much more enzyme activity compared to the previous types of
MSUD (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.
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