Last revised by Arlene Campos on 10 Jun 2024

Hemimegalencephaly is a rare congenital disorder of cortical formation characterized by hamartomatous overgrowth of all or part of a cerebral hemisphere. This overgrowth results from either increased proliferation or decreased apoptosis (or both) of developing neurons 2.

Hemimegalencephaly is a cryptogenic congenital disorder and does not have a recognized racial or gender predilection 3. It is rare, accounting for only ~0.2% of cases of childhood epilepsy 3.

The majority (90%) of patients present with focal and generalized infantile spasms. Developmental delay, hemiparesis, and hemianopia are usually also evident 1-3.f

Although normocephaly is often present, macrocephaly may be pronounced, and normal vaginal delivery may be impossible. Despite the markedly enlarged head, there are no symptoms of raised intracranial pressure 1,3.

Hemimegalencephaly is divided into three forms 1-3:

It is a congenital malformation in which defective cellular organization and neuronal migration result in hamartomatous overgrowth of a hemisphere. Macroscopically the affected hemisphere demonstrates, in addition to overgrowth, abnormal cortical development is present with areas of lissencephaly, agyria, pachygyria and polymicrogyria in varying proportions, with other areas appearing normal 3.  

All modalities able to image the brain (ultrasound, CT and MRI) will be able to more or less identify the key features, although MRI will give the best imaging. Typical features of the affected hemisphere include 1:

The cortex of the involved hemisphere may be normal, but often demonstrates 2:

Brain SPECT and PET both demonstrate hypometabolism in the affected hemisphere 3.

Treatment is targeted to the control of epilepsy, which can be difficult to manage medically. In refractory cases hemispherectomy is the treatment of choice and results in seizure control in at least 60% of cases when carefully selected; patients with contralateral malformations have a poorer surgical outcome 3

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