Maple syrup urine disease (MSUD) is an autosomal recessive metabolic disorder affecting branched-chain amino acids. It is one type of organic acidemia. The condition gets its name from the distinctive sweet odor of affected infants urine and earwax, particularly prior to diagnosis and during times of acute illness. Signs and symptoms
The disease is named for the presence of sweet-smelling urine, similar to maple syrup, when the person goes into metabolic crisis. The smell is also detected in ear wax of an affected individual during metabolic crisis. In populations to whom maple syrup is unfamiliar, the aroma can be likened to fenugreek, and fenugreek ingestion may impart the aroma to urine. Symptoms of MSUD varies between patients and is greatly related to the amount of residual enzyme activity. Classic MSUD
Infants with classic MSUD will display subtle symptoms within the first 24–48 hours. Subtle symptoms include poor feeding, either bottle or breast, lethargy, and irritability. The infant will then experience increased focal neurologic signs. These neurologic signs include athetoid, hypertonia, spasticity, and opisthotonus that lead to convulsions and coma. If MSUD is left untreated, central neurologic function and respiratory failure will occur and lead to death. Although MSUD can be stabilized, there are still threats of metabolic decompensation and loss of bone mass that can lead to osteoporosis, pancreatitis, and intracranial hypertension. Additional signs and symptoms that can be associated with classic MSUD include intellectual limitation and behavioral issues. Intermediate MSUD
Intermediate MSUD has greater levels of residual enzyme activity than classic MSUD.
The second step involves the conversion of α-ketoacids into acetoacetate, acetyl-CoA, and succinyl-CoA through oxidative decarboxylation of α-ketoacids. The BCKAD complex consists of four subunits designated E1α, E1β, E2, and E3. The E3 subunit is also a component of pyruvate dehydrogenase complex and oxoglutarate dehydrogenase complex. MSUD can result from mutations in any of the genes that code for these enzyme subunits, E1α, E1β, E2, and E3. Mutations of these enzyme subunits will lead to the BCKAD complex unable to break down leucine, isoleucine, and valine. The levels of these branched chain amino acids will become elevated and lead to the symptoms associated with MSUD. Glutamate levels are maintained in the brain by BCAA metabolism functions and if not properly maintained can lead to neurological problems that are seen in MSUD individuals. High levels of leucine has also been shown to affect water homeostasis within subcortical gray matter leading to cerebral edema, which occurs in MSUD patients if left untreated. Diagnosis
Prior to the easy availability of plasma amino acid measurement, diagnosis was commonly made based on suggestive symptoms and odor. Affected individuals are now often identified with characteristic elevations on plasma amino acids which do not have the characteristic odor. The compound responsible for the odor is sotolon (sometimes spelled sotolone).On 9 May 2014, the UK National Screening Committee (UK NSC) announced its recommendation to screen every newborn baby in the UK for four further genetic disorders as part of its NHS Newborn Blood Spot Screening programme, including maple syrup urine disease.