This is a rare brain malformation which is characterised by the enlargement and malformation of an entire cerebral hemisphere.


Medical text written December 2001 by Dr H Cross. Last updated September 2006 by Dr H Cross, Reader and Honorary Consultant in Paediatric Neurology, Institute of Child Health and Great Ormond Street Hospital, London, UK.

What are the symptoms?

Affected children usually suffer epileptic seizures (see entry Epilepsy) soon after birth (although these may be delayed until later in infancy) and display learning and psycho-motor difficulties. Later, children will display a hemiplegia, a weakness down one side. The condition may also be associated with facial enlargement and skin disorders. Other disorders may also involve malformation of one hemisphere, without the enlargement. Such children present with similar clinical features and require a similar course of management. The degree of developmental delay varies from mild to severe.

How is it treated?

Treatments may prevent seizures and improve the quality of life. The first line of treatment for seizures would be medication, although early consideration with regard to surgery should be given. Many children benefit from surgery, namely hemispherectomy, which is disconnection or removal of the abnormal hemisphere. It is unlikely this hemisphere is doing anything except causing seizures, therefore function is unlikely to be compromised over and above what would be inevitable from the malformation (e.g. hemiplegia).

Inheritance patterns and prenatal diagnosis

Inheritance patterns

Prenatal diagnosis
Ultrasound scanning may show up the different sizes of the cerebral hemispheres. Prenatal screening and genetic advice may be offered for future pregnancies. Ante-natal MR scan at 20 to 25 weeks in specialist units may also give further information.

Is there support?

There is no support group for hemimegalencephaly in the UK. Cross referrals to other medical information entries on our website are intended to provide relevant support for those particular features of the disorder. Organisations identified in those entries do not provide support specifically for hemimegalencephaly.

Families can use Contact’s freephone helpline for advice, information and, where possible, links to other families. To meet other families with disabled children, join Contact’s closed (private) Facebook group.

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