Hemimegalencephaly

Hemimegalencephaly is a rare congenital disorder of cortical formation with hamartomatous overgrowth all or a part of a cerebral hemisphere. This 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 recognised 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 generalised infantile spasms. Developmental delay, hemiparesis, and hemianopia are usually also evident 1-3.

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.

Classification

Hemimegalencephaly is divided into three forms 1-3:

It is a congenital malformation in which defective cellular organisation and neuronal migration results 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:

Nuclear medicine

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

Malformations of the central nervous system
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Article information

rID: 1266
Section: Pathology
Synonyms or Alternate Spellings:
  • Unilateral megalencephaly

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