Specific Imaging Findings

Hemimegalencephaly (HME) is characterized by gross brain asymmetry with enlargement of all or part of one hemisphere and possible concomitant enlargement of the ipsilateral half of the brainstem and cerebellar hemisphere. Midline shift can be present, either total or limited to the occipital area. The cerebral cortex is thickened and shows a variable spectrum of abnormalities, ranging from lissencephaly to polymicrogyria. The most severe end of this spectrum has been termed hemilissencephaly. In milder cases, the cortical mantle may be grossly normal. The affected white matter shows abnormal signal intensity that varies with patient age: in neonates, T1 and T2 shortening (T1 bright, T2 dark) reflects abnormally advanced myelination for the age, whereas in older children the white matter becomes T2 hyperintense due to gliosis and lack of myelin. The lateral ventricle in the affected hemisphere shows variable abnormalities, including a straightened or collapsed frontal horn, colpocephalic dilation of the occipital horns, and global enlargement. In some cases, only a portion of the cerebral hemisphere, usually corresponding to a single lobe, is enlarged. Serial imaging studies may show progressive atrophy of the involved hemisphere, so that it eventually becomes smaller than the unaffected one.

Pertinent Clinical Information

Affected patients have a variable clinical picture depending on the extent and severity of the malformation. Most present during the first days or weeks of life with medically intractable seizures, severe psychomotor delay, and contralateral hemiparesis. Management of these children is difficult. Seizure control may not be achieved by pharmacologic treatment, and adverse side effects may be severe. Hemispherectomy has been advocated for patients with frequent, refractory, intractable seizures. Less severe cases may present with mild to moderate psychomotor delay.