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Imaging in Classic Form of Maple Syrup Urine Disease: A Rare Metabolic Central Nervous System

Maple syrup urine disease (MSUD) is a rare autosomal recessive disorder of branched-chain amino acid metabolism. The condition gets its name from the distinctive sweet odour of affected infants’ urine. MSUD is caused by a deficiency of the branched-chain α-ketoacid dehydrogenase enzyme complex, leading to accumulation of the branched-chain amino acids (leucine, isoleucine, and valine) and their toxic by-products (ketoacids) in the blood and urine. Imaging is characterestized by MSUD oedema affecting the myelinated white matter. We present a neonate with classic type of MSUD and its imaging features on computed tomography, conventional magnetic resonance imaging, diffusion-weighted imaging, and magnetic resonance spectroscopy.


CASE REPORT
Maple syrup urine disease (MSUD) is a rare inherited autosomal recessive disorder of branched-chain amino acid (BCAA) metabolism presenting with life-threatening cerebral edema and dysmyelination in affected individuals. MSUD affects approximately 1 out of 180,000 infants [1] and has a much higher prevalence in children of Amish, Mennonite, and Jewish descent. [1,2] We present a case report of a baby boy born to consanguineous married parents, who presented on the 8 th postnatal day with history of poor feeding, lethargy, and tonic-clonic seizures, since 2 days. The child was born at term (39 weeks) after an uneventful pregnancy and normal vaginal delivery.
Non-enhanced computed tomography (NECT) scan of the brain showed bilateral symmetrical white matter hypodensities in the posterior limb of internal capsule, thalami, midbrain, and cerebellar white matter [ Figure 1].
Conventional magnetic resonance imaging (MRI) showed bilateral symmetric white matter hyperintensites in the posterior limb of internal capsule, thalami, midbrain, corticospinal tracts, and cerebellar white matter on T2-weighted magnetic resonance (MR) sequence [ Figure 2a and b]. Turbo inversion recovery magnitude sequence showed similar findings as T2-weighted images. No abnormalities were detected on T1-weighted images.
Diffusion-weighted imaging (DWI) showed characteristic pattern of bilateral symmetrical restricted diffusion within the myelinated areas in the posterior limb of the internal capsule, centrum semiovale, corona radiata, corticospinal tract, thalami, posterior aspect of the mid brain, pons, middle cerebellar peduncle, medulla, and cerebellar white matter, attributed to intramyelinic edema [3] [ Figure 3a and b].

MR spectroscopy
Multivoxel proton MR spectroscopy showed the presence of BCAAs and branched-chain α-keto acids resonating at 0.9-1.0 ppm, which are seen especially during a metabolic crisis [4,5]

[Figures 4 and 5].
Three days later tandem mass spectroscopy revealed increased levels of BCAAs (valine, leucine, and isoleucine), confirming the diagnosis of MSUD.

MSUD
It is a rare autosomal recessive disorder, associated with defects in the branched-chain � ketoacid dehydrogenase complex. It is divided into four major categories: (1) Classic, (2) intermediate, (3) intermittent, and (4) thiamine responsive, which carry differing symptoms and prognostic factors. [7] The exact cause for brain injury is not clearly understood. According to a study by Zinnanti et al., [8] they suggest two converging mechanisms of brain injury in MSUD including: (i) Neurotransmitter deficiencies and growth restriction associated with BCAA accumulation and (ii) energy deprivation through Krebs cycle disruption associated with branched-chain ketoacid accumulation.
This disease leads to accumulation of BCAA and metabolites (neurotoxic). The rapid accumulation of leucine in particular causes neurological symptoms. Increased plasma isoleucine is associated with maple syrup odour.
Neonates will be normal at birth, presents after disease-free interval, usually within the 4-7 days of life with poor feeding, vomiting, poor weight gain, and increasing lethargy. In crisis, patient's urine smells like maple syrup, secondary to the large accumulation of isoleucine. Maple syrup odour may be difficult to identify in first few days of life.
Imaging features are diagnostic in the early weeks of life. Classic appearing MSUD edema involving: Cerebellar white matter, brain stem, globus pallidus, thalamus, cerebral peduncles, and corticospinal tracts.
NECT of brain shows diffuse bilaterally symmetrical edema not sparing brainstem and cerebellum. [6] DWI shows marked restriction and decreased apparent diffusion coefficient (ADC) which indicates MSUD edema is an intracellular oedema (cytotoxic oedema). DWI is more sensitive than conventional MRI in detecting MSUD brain alterations and it can become a useful tool for early diagnosis and follow-up of metabolic diseases in neonates. [9] Kilicarlsan et al., [3] reported six cases with DWI in which the changes in all patients were reversed with treatment without evidence of volume loss or persistent tissue damage.
Acute "metabolic rescue" to reverse cerebral edema may require hemodialysis during acute crisis to limit neurotoxicity/  The changes in cell osmolarity and metabolism can reverse completely after metabolic correction in metabolic decompensated MSUD with clinical neurological improvement. [10,11] CONCLUSION MSUD is a rare autosomal recessive disorder of BCAA metabolism. Early imaging diagnosis of this condition can prevent the progress of neurological deficits and help in appropriate management of the disease.