Hemimegalencephaly

What Is It?

Hemimegalencephaly is a rare, severe congenital malformation of the brain which is usually associated with epilepsy. As the name implies, an entire hemisphere of cortex, half of the brain, is involved. Like other forms of cortical dysplasia, there is an abnormality during development of the nervous system which leads to abnormal formation of the cerebral cortex, the outer layer of the brain, a form of grey matter. Whereas in typical cortical dysplasia this malformation of the cortex is localized to a restricted area of the brain, in hemimegalencephaly an entire hemisphere is involved.

The involved hemisphere is larger than normal. In addition to being large, it is also profoundly dysfunctional as the brain in that area has not developed properly. The unaffected side typically has normal cortical developement but is abnormally small, because the large abnormal side restricts the normal growth of the unaffected side.

The exact cause of this developmental disease is unknown and is probably multifactorial. While it can occur as an isolated finding, it can also occur along with other developmental syndromes.

What Types of Symptoms Are Typical?

The most common presenting symptom of this disease is seizures. The abnormally developed hemimegalencephalic brain is prone to seizures which occur frequently, usually starting at or shortly after birth, and often spread to involve the whole brain. In addition, the patient generally has abnormal function of the involved hemisphere and can have sensory and motor dysfunction on the opposite side of the body.

The presence of the abnormal brain, along with the chronic damage done by frequent seizures, often results in developmental delay and if left untreated these patients often deteriorate significantly to severe mental retardation.

How Is The Diagnosis Typically Made?

In a young patient, generally an infant, who presents with neurological symptoms and epilepsy, a thorough neurological examination and brain MRI usually reveal the presence of hemimegalencephaly. The appearance of this condition on MRI is quite characteristic and is usually not confused with other diagnoses. However, biopsy of the abnormal tissue at the time of surgery can help confirm the diagnosis.

What Are Some Common Treatments?

As with other forms of epilepsy, hemimegalencephaly may be treated by both medical and surgical means. Generally, patients are initially treated with anti-seizure medications. In this disease, these medications are rarely sufficient to control seizures completely. Additionally, the presence of the large, abnormal hemisphere may put pressure on and limit the normal growth and development of the other side of the brain.

In order to minimize seizures, to minimize further developmental delay and to allow the other side of the brain the best chance to preserve function and develop normally, surgery is often recommended. Although each case is different and must be considered separately, one of the most common surgical treatments is removal and/or disconnection of the affected hemisphere. This, so called, hemispherectomy is a dramatic surgical treatment but may be the best option to improve the outcome of these unfortunate children. While this procedure removes half the brain, it is brain that was abnormal to begin with. Additionally, if done in very young patients, the opposite side, assuming it is fairly normal, can gradually take over some of the function. These patients do typically have some degree of hemiparesis or hemiplegia, weakness or paralysis, of the other side of the body.

This dramatic developmental disease is difficult to treat and is usually done at centers that specialize in complex pediatric diseases of the nervous system. Each patient is different so each patient and their family should consult their own treating physicians about the most appropriate treatment options in their unique case.

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Important Note: This site is not intended to offer medical advice. Every patient is different, and only your personal physician can help to counsel you about what is best for your situation. What we offer is general reference information about various disorders and treatments for your education.

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