Maple Syrup Urine Disease


Ball WS: Metabolic, Congenital Neurodegenerative & Toxic Disorders, in Edelman RR, Hesselink JR, Zlatkin MB, Crues J (eds). Clinical Magnetic Resonance Imaging. 3rd Edition, Chapter 56, Elsevier-Saunders, Philadelphia, 2006, pp 1656-1704.


Maple Syrup Urine Disease (MSUD) is a branched-chain ketonuria resulting from a deficiency of alpha-keto acid decarboxylase. This results in the accumulation of the branched chain amino acids leucine, isoleucine, and valine in the urine or sweat, giving them a distinctive maple syrup or burnt odor from which this condition has derived its name. The classic form presents in the first weeks of life (75%) with hypotonia, lethargy, vomiting, failure to thrive, and seizures, with episodes of hypoglycemia in over half the patients. Death will follow within weeks if left untreated. Early treatment can prevent the consequences of brain injury through dietary restriction of the branched chain amino acids and administration of insulin or other compounds to allow the metabolism or utilization of excess branched chain amino acids.


CT and MR studies in the neonatal period are typically normal, but increased echogenicity may be seen in the periventricular white matter, basal ganglia, and thalami on transcranial ultrasound exams. After several days widespread edema becomes apparent on CT. The edema will soon evolve to characteristically affect the deep cerebellar white matter, dorsal brainstem, cerebral peduncles and posterior limb of the internal capsule. Eventually the edema may spread to involve the basal ganglia (globus pallidus) and cerebral hemispheres. Within the first two months the edema gives way to diffuse atrophy. The edema is hypodense on CT, hypointense on T1-weighted images, and hyperintense on T2-weighted MR studies. Cystic changes may be visible, corresponding to the spongiform changes seen pathologically in the cerebellar white matter, dorsal brain stem, cerebral peduncles and posterior limbs of the internal capsules. With early treatment, these findings may resolve.


Proton MRS can show a resonance at 0.9 ppm corresponding to the accumulated branched chain amino acids. The accumulation of abnormal branched-chain amino acids (BCAA) and branched-chain alpha-keto acids (BCKA) peak at 0.9 ppm accompanied by elevated lactate are manifested in patients. The presence of cytotoxic or intramyelinic edema as evidenced by restricted water diffusion on DWI, with the presence of lactate on spectroscopy, could imply cell death. However, in the context of metabolic decompensation in MSUD, it appears that changes in cell osmolarity and metabolism can reverse completely after metabolic correction. 

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