The Maple Syrup Urine Disease (MSUD) is classified into 5
different types, depending on the amount and type of enzyme present in the
affect individual. The 5 different types of MSUD consist of:
1) Classic
The Classic MSUD
is the most severe type amongst the others and defined by the neonatal onset
of encephalopathy. In classic MSUD, the branched-chain α-keto acid
dehydrogenase (BCKD) level is less than 2% of normal. Hence, affected
individuals are found to have elevated levels of the branched-chain amino acids
(BCAAs- leucine, isoleucine and valine), especially leucine, in the blood,
urine and cerebrospinal fluid. The presence of alloisoleucine (accumulated
amino acid due to the inability to break them down) in the fluids is the
diagnostic of MSUD.
Infants with the
classic MSUD appears to be normal at birth but symptoms start developing
rapidly by 4 to 7 days
after birth. Such infants generally have low tolerance for BCAAs, thus a strict
diet without protein
must be followed. The first characteristic symptoms are lethargy and low
interest in feeding. With
the progression of the disease, further signs include weight loss and
progressive neurological
deterioration. Neurological signs will alternate from hypo- to hypertonia, and
the extension of the
arms will start to resemble decerebate posturing (refer to Fig. 1). At this
time, the distinctive
sweet-smelling urine is apparent.
Fig. 1: Decerebate
Posturing
If the infants are
left untreated, they will develop seizures, lapse into a coma and die. The
prognosis of such untreated
infants is poor with death occurring within the first few months of birth due
to complications of
metabolic crisis and neurological deterioration.
2) Intermediate
The Intermediate
MSUD is a variant form of the classic MSUD. The difference between the 2
types is that for intermediate MSUD patients, they do not experience the
severity of classic MSUD in the neonatal period. Affected individuals have
a higher level of BCKD (3% to 30% of normal) and have a higher tolerance
for BCAAs. Even so, they are still troubled with elevated levels of BCAAs in
body fluids, and neurological deterioration. When sick or fasting,
intermediate MSUD patients will react just like a classic MSUD patient.
Treatment is similar for both classic and intermediate types of MSUD.
3) Intermittent
Individuals
affected with the intermittent MSUD have a BCKD level of 5% to 50% of normal. Initially, these
children will be asymptomatic and thus show normal early development with
normal intellectual
levels. During this period of time, the BCAA levels will remain normal.
However, when sick or fasting,
the BCAA levels increase, the distinctive maple syrup odour becomes apparent
and the child is at
risk for metabolic decompensation. Symptoms of the intermittent MSUD usually
start to develop between
5 months and 2 years of age, in response to an infection or sudden increase in the intake of
protein.
4)
Thiamine-responsive
The symptoms of
this more rare type of MSUD has a similar course of progress to that seen in
the intermediate MSUD
patients. The BCKD level is around 20% of normal and BCAAs levels are
around 5 times of normal. The presence of alloisoleucine is also apparent
in these patients. Following what the name implies, giving large doses of
thiamine together with a low protein diet will increase enzyme activity,
breaking down the BCAAs.
5) Dihydrolipoyl
dehydrogenase (E3)- deficient
This form of MSUD
is due to a deficient E3 component of the BCKD complex. The symptoms of
this rare type of MSUD are similar to that seen in the intermediate MSUD
patients. However, this form of MSUD is accompanied with severe lactic
acidosis. Initially, for the first few months of life, the affected
infants remain relatively normal. Between 2 and 6 months of age, persistent
lactic acidosis starts to develop.
References:
MSUD Family
Support Group, 2010. MSUD Classifications. [online]. Available at:
<http://www.msud-support.org/index.php?option=com_content&view=article&id=307&Itemid=88>
The Medical
Biochemistry Page, 2012. Introduction to Maple Syrup Urine Disease: MSUD.
[online]. Available at:<http://themedicalbiochemistrypage.org/msud.php>
Drugs Information
Online,. Diseases Reference Index «Decerebrate posture». [online] Available at:
<http://drugline.org/ail/pathography/943/>