MR imaging has emerged as a powerful tool in the study of normal and
abnormal brain structure, function and biochemistry. Clinicians can
derive significant benefits from specific imaging findings in
Some of the neurometabolic disorders with typical neuroimaging
manifestations are phenylketonuria, maple syrup urine disease, glutaric
aciduria type I and methylmalonic acidemia. Although urea cycle
disorders are not associated with specific imaging findings,
hyperammonemia resulting from these disorders can lead to typical
findings and facilitate diagnosis.
In classical phenylketonuria, white matter abnormalities are frequently
observed. The earliest and most frequent abnormalities are high
signal-intensity lesions on T2-weighted images in the parieto-occipital
periventricular white matter (periatrial and peritrigonal regions)[Figure 6]
The white matter lesions are symmetrical and either band-like or patchy
and partly confluent in an irregular fashion. Frequently, the frontal,
and less often, the occipital white matter changes have a peculiar
configuration that extends like a small flame from the ventricular
border in line with the ventricle.
maple syrup urine disease, MRI shows diffuse symmetrical
hyperintensities involving the periventricular, deep white matter and
subcortical 'U' fibers[Figure 7]
The other areas involved are the globus pallidus, thalami and
brainstem. The globus pallidus showed diffuse involvement, but the
involvement of the thalami was limited mainly to the anterolateral
region, sparing the dorsomedial and dorsolateral regions. In the
cerebellum, the deep white matter as well as the dentate nuclei shows
T2 hyperintensities. In the intermediate form of MSUD, there may be
sparing of posterior limb of the internal capsule.
Proton MRS is particularly helpful in establishing a diagnosis during
metabolic decompensation. A peak is observed at 0.9 PPM, which results
from the resonance of the branched-chain amino acids and keto acids.
acidemias are another group of disorders with characteristic
neuroimaging findings, which aid in diagnosis. Widening of the sylvian
fissure, mesencephalic cistern and expansion of CSF spaces anterior to
the temporal lobes are cardinal signs of glutaric aciduria Type-1 [Figure 8]
. If combined with abnormalities of the basal ganglia and white matter, glutaric aciduria Type-1 should be strongly suspected.
In some patients, delayed myelination or white matter abnormalities are noted.,
The characteristic feature of methylmalonic acidemia is severe involvement of the globus pallidus,[Figure 9]
The bilateral pallidal necrosis as revealed by neuroimaging occurs due
to acute respiratory acidosis with cell necrosis. Involvement of other
ganglionic structures is usually not noted. There is no definite
explanation for the selective involvement of globus pallidus in
methylmalonic acidemia. This finding is noted both in individuals with
acute decompensation and in those without such episodes.