Hemimegalencephaly (HME) is a rare neurological condition in which one-half of the brain, or one side of the brain, is abnormally larger than the other. The structure of the brain on the affected side may be markedly abnormal or show only subtle changes. In either case, as a consequence of this size and structural differences, the enlarged brain tissue causes frequent seizures, often associated with cognitive or behavioral disabilities. Seizures in association with HME often begin in early infant life including an association with infantile spasms. Hemimegalencephaly may occur as an isolated or sporadic brain malformation or it may be associated with other neurodevelopmental syndromes. Thus, when detected, HME should prompt a search for other syndromic diagnoses.
Anti-seizure medications typically are not effective in controlling seizures in HME and thus, surgery is often recommended to control the seizures. If the affected side is surgically removed (anatomic hemispherectomy) or disconnected from the other brain structures (functional hemispherectomy), the remaining side of the brain may gradually take over the functions normally performed by the affected side.
Any combination of altered mental status, seizures, enlarged head, and /or altered skin pigmentation should prompt consideration of HME. In general, the presence of HME is definitively diagnosed by brain MRI. With the evolution of more widespread fetal imaging including ultrasound and MRI, a number of HME cases are detected prenatally.
HME typically is identified in the neonatal period when the baby develops presents seizures. The seizures usually do not decline in severity or number with medical treatment and in some cases they may exceed 50 or more per day. On physical examination, a child with HME may presents with enlarged head circumference or an asymmetrical head shape. There may be movement or motor deficits on the side opposite to the HME. When these signs are present, the neurologist may suspect the presence of HME and order magnetic resonance imaging (MRI) examination.
HME may occur in association with other syndromes, such as Proteus syndrome, epidermal nevus syndrome, tuberous sclerosis complex (TSC), linear sebaceous nevus syndrome, neurofibromatosis, and Sturge-Weber syndrome which are associated with abnormalities of skin pigmentation that can be detected on physical examination. Any child with these skin markings and seizures should be further evaluated for HME as well as other brain malformations. Hemimegalencephaly may also occur in association with Sotos syndrome and Alexander disease. These syndromes arise as a result of complex genetic activities such as single or multiple gene mutations. The mutations causing these disorders can be inherited or occur randomly during fetal development.
The basic cause(s) of HME is not well understood. The disorder occurs because the cells of one hemisphere of the brain grow much more rapidly than do the corresponding cells of the other half of the brain (hamartomatous overgrowth of one hemisphere). It is widely believed that a single or multiple gene mutations contribute to this process. As might be expected, the cortex of the enlarged brain is malformed (dysplastic) and the white matter is abnormal. One of the common, empty spaces of the brain (lateral ventricle) in the enlarged hemisphere is enlarged in proportion to the lateral ventricle of the smaller hemisphere.
Some clinicians believe that HME occurs as a result of damage to the fetal brain during the first or second trimester of pregnancy that affect the genetically programmed process that establishes symmetry as well as the development of different classes of brain cells.
Hemimegalencephaly is a very rare disorder for which prevalence estimates are not available.
Examination by MRI is usually sufficient to confirm a suspected case of HME. Thus, an MRI examination should be performed as soon as HME is suspected. Seizures are diagnosed and defined by electroencephalography (EEG).
Persistent, intractable seizures are seldom brought under control by means of anti-epileptic medications. Most patients undergo surgery to separate one hemisphere of the brain from the other. The surgical procedure may involve "functional hemispherectomy" in which the nerves and tissue connecting one side of the brain to the other are severed, but the abnormal hemisphere remains within the skull. Complete or anatomic hemispherectomy involves disconnecting one side of the brain from the other and extracting the abnormal hemisphere. These surgeries are typically performed by a neurosurgeon trained in epilepsy surgery.
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Contact for additional information about hemimegalencephaly:
Peter B. Crino MD, PhD
Director, PENN Epilepsy Center
3400 Spruce Street
Philadelphia, PA 19104
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