Types of MSUD

MSUD is a autosomal recessive metabolic disorder. As it is recessive, it is a very rare disease to be inherited through generations. What it means by recessive is that the parent of the child with MSUD can be a carrier or infected, carrying 1 normal gene and 1 mutated gene. The mutated gene is recessive and is usually dominated by the normal gene. Only when both mutated genes from the mother and the father will the mutated gene not be dominated and thus cause MSUD.

There are 4 subtypes of MSUD genetic disorders :

1. Classic MSUD
2. Intermediate MSUD
3. Intermittent MSUD
4. Thiamine-responsive MSUD

These 4 generally differ by the degree of enzyme activity(BCKDH), severity and age for MSUD to occur. 

Classic MSUD : Most common type of the 4 and has little to none enzyme activity occuring. Having little enzyme activity places classic MSUD as the most severe form. It is present in infants a few days after birth. 

Intermediate MSUD : It is an uncommon type of variant from the class MSUD. It has higher enzyme activity occuring hence have more tolerance towards BCAA. It is present ranging from infants to adults. 

Intermittent MSUD : A milder version of classic MSUD by having more enzyme activity occuring. It occurs much later in the childhood stage of about 1-2 years. 

Thiamine-responsive MSUD : Another uncommon type of MSUD that increases enzyme acitivity with response from thiamine doses. Thiamine can thus lower the severity of MSUD in the patient with the help of diatery restrictions too. It usually occurs after infancy.

Even though generally it is these 4 types, there are rare exceptions like dihydrolipoamide dehydrogenase(E3) deficiency and others.

Reference : 

Healthline, 2014. Maple Syrup Urine Disease (MSUD) [Online]
Available at : <http://www.healthline.com/health/maple-syrup-urine-disease>

UIC, 2014. Biochemical Background [Online]
Available at : <https://www.uic.edu/classes/phar/phar332/Clinical_Cases/aa%20metab%20cases/MSUD/MSUD%20biochem%20background.htm>

MSUD Family Support Group, 2010. MSUD Classifications [Online]
Available at : <http://www.msud-support.org/index.php/overview/msud-classifications>