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Hemimegalencephaly may occur in association with an established syndrome or may be an isolated finding. The most frequent syndrome associated with hemimegalencephaly is probably linear nevus sebaceous syndrome. Isolated hemimegalencephaly may occur with or without unilateral overgrowth of the body. Isolated hemimegalencephaly with unilateral overgrowth of the body may be associated with Wilms tumor or adrenal or hepatic tumors. Isolated hemimegalencephaly without unilateral overgrowth of the body often produces intractable seizures. Seizure control is important because even focal seizures in the large hemisphere may produce bilateral decreased cerebral oxygenation. Hemispherectomy may be needed to control the seizures. More about... 172

Neuronal Heterotopias
 Heterotopia results from an arrest of neuronal radial migration. Neuronal heterotopias are diagnosed by MRI of the brain. The MRI shows gray matter where there should be white matter (Figure 50.1). Heterotopias are classified based on the location of the heterotopic neurons as subependymal, subcortical, or diffuse.

Figure 50.1.— Heterotopia. T1-weighted coronal image demonstrates band heterotopia of the left hemisphere.

 
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pachygyria normal gyration, sulcation, white-gray interface of the right cerebral hemisphere ventriculus III lobus temporalis lobus temporalis sinus sagittalis superior falx cerebri band heterotopia right ventriculus lateralis left ventriculus lateralis right ventriculus lateralis pituitary gland cavernous sinus cavernous sinus sphenoid sinus vomer nasopharynx Donnai, 1995 Click on figure for animated labels. Pause pointer on different areas of the figure for labels. Figure must be centered. Alfonso, 2004