Hemimegalencephaly

Hemimegalencephaly is the hamartomatous overgrowth of all or part of a cerebral hemisphere with defects of cellular organization and neuronal proliferation, migration, and organization. Hemimegalencephaly may be associated with pachygyria, polymicrogyria, heterotopia, and balloon cells.There is usually an inverse relationship between the size of the affected hemisphere and the severity of its malformation.

It has been suggested that hemimegalencephaly is a disorder of the entire brain that is more evident on one side.

Clinical Presentation and Classification

The classical triad is psychomotor retardation, contralateral motor deficit, and epilepsy. Prognosis depends on severity of epilepsy and associated neurological deficits.

Three clinical types have been described:

• Isolated form: The classic and most common type. Sporadic without evidence of cutaneous or systemic involvement
• Systemic form: Associated with partial or total hemigigantism and/or several neurocutaneous syndromes, including epidermal nevus syndrome (sporadic spectrum of disorders that includes linear sebaceous nevus syndrome and the Proteus syndrome)
• Total hemimegalencephaly: The least common form, in which there is involvement of the ipsilateral cerebellum and brainstem.

Imaging

MRI will show the obvious enlargement of one hemisphere, as well as disorders of cortical migration and development, such as agyria, pachygyria, micropolygyria, and lissencephaly. Because the treatment for seizures is hemispherectomy, it is important to also make note of any cortical abnormalities on the contralateral side, as hemispherectomy in such cases would likely not be effective.

MRA may show anomalies of vascular structures, including hypoplasia of the deep venous system, hypertrophy of the veins draining into the sagittal sinus, abnormal extension of the sylvian veins up to longitudinal superior sinus, and arterial hypervascularization of the affected hemisphere

Differential Diagnosis

• Disorders of neuronal migration
• Tuberous sclerosis
• Hemiatrophy
• Gliomatosis cerebri
• Unilateral cortical edema

References

Di Rocco C, Battaglia D, Pietrini D, Piastra M, Massimi L. Hemimegalencephaly: clinical implications and surgical treatment. Childs Nerv Syst. 2006 Aug;22(8):852-66.