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The right cerebral hemisphere is diffusely enlarged with entire thickening and disorganization of the cortical grey matter (migration arrest). Abnormal high signal on FLAIR and T2WI throughout the white matter of the affected hemisphere. The ipsilateral lateral ventricle is dilated with abnormal morphology, abnormal structure and orientation of the right basal ganglia, dysgenesis of the corpus callosum, expanded right calvarium, contralateral displacement of the anterior falx, and extension across the midline of the right occipital lobe. The right cerebellar hemisphere is also enlarged with an abnormal cortex. The right half of the brainstem appears also mildly enlarged.
The left hemisphere appears normal.
MRI features of a total hemimegalencephaly which is considered as the rarest type
Hemimegalencephaly is an uncommon brain anomaly characterized by an enlarged and dysplastic cerebral hemisphere with a variable degree of tissue impairment. The contralateral cerebral hemisphere usually is normal, except for the mass effect.
The brain anomaly may be isolated or associated with syndromes such as NF1, Bourneville's disease, Proteus syndrome, epidermal nevus syndrome, and Klippel-Trenaunay syndrome.
The etiology remains unknown but generally assumed that it results due to an abnormal neural and glial proliferation or apoptosis.