MSUD Fact File

Prevalence 1:116,000 (UK data)
Defect type Autosomal recessive condition caused by defects in the branched chain 2-keto acid dehydrogenase (BCKAD) complex.
Typical age at clinical diagnosis 1 weeks – 5 years


Clinical Effects:

  • There are three forms of MSUD: the classical (severe) form of the disease, the intermittent (mild) form and an intermediate form.
  • Most patients have the classical (severe) form of the disease. 
  • In MSUD, people are unable to breakdown three different branched chain amino acids called leucine, isoleucine and valine.  The build up of these in the body can lead to harmful effects.
  • Typically patients first experience problems as newborn babies. The first signs are poor feeding, vomiting and excessive sleepiness. Without treatment, this leads to a coma and permanent brain damage.


  • Treatment aims to control the levels of branched chain aminoacid (leucine, isoleucine and valine) because high levels are harmful to the body.
  • If patients become severely unwell, they may need dialysis on an intensive care unit. This is sometimes needed in newborn babies but it is seldom needed in older children. 
  • Life-long management is required to stay healthy. This involves a special diet, low in branched chain aminoacids, and regular monitoring.
  • During illness, it is important to stop normal food and to start the Emergency Regimen (special drinks). If the drinks are vomited or refused, the child should be taken to hospital.

Effectiveness of treatment:

With early diagnosis and treatment, most children with MSUD develop normally. However, patients need to stay on their special diet throughout life,  and use their emergency regimen during illnesses.

For people with the milder ‘intermittent’ form of the disease, the diet can be more relaxed. These patients may only need to avoid high protein foods and to use an emergency regimen during illnesses.


Positive predictive value 50% (UK data)
Sensitivity Approx 100% (classical form)
Screening index metabolite Leucine
Confirmatory test Urinary organic and plasma aminoacids