Hemispherectomy Health Article

Definition

Hemispherectomy is a surgical treatment for epilepsy in which one of the two cerebral hemispheres, which together make up the majority of the brain, is removed.

Purpose

Hemispherectomy is used to treat epilepsy when it cannot be sufficiently controlled by medications.

The cerebral cortex is the wrinkled outer portion of the brain. It is divided into left and right hemispheres, which communicate with each other through a bundle of nerve fibers called the corpus callosum, located at the base of the hemispheres.

The seizures of epilepsy are due to unregulated electrical activity in the brain. This activity often begins in a discrete brain region called the focus of the seizure, and then spreads to other regions. Removing or disconnecting the focus from the rest of the brain can reduce seizure frequency and intensity.

In some people with epilepsy, there is no single focus. If there are multiple focal points within one hemisphere, or if the focus is undefined but restricted to one hemisphere, hemispherectomy may be indicated for treatment.

Removing an entire hemisphere of the brain is an effective treatment. The hemisphere that is removed is usually quite damaged by the effects of multiple seizures, and the other side of the brain has already assumed many of the functions of the damaged side. In addition, the brain has many "redundant systems," which allow healthy regions to make up for the loss of the damaged side.

Children who are candidates for hemispherectomy usually have significant impairments due to their epilepsy, including partial or complete paralysis and partial or complete loss of sensation on the side of the body opposite to the affected brain region.

Demographics

Epilepsy affects up to 1% of all people. Approximately 40% of patients are inadequately treated by medications, and so may be surgery candidates. Hemispherectomy is a relatively rare type of epilepsy surgery. The number performed per year in the United States is likely less than 100. Hemispherectomy is most often considered in children, whose brains are better able to adapt to the loss of brain matter than adults.

Description

Hemispherectomy may be "anatomic" or "functional." In an anatomic hemispherectomy, a hemisphere is removed, while in a functional hemispherectomy, some tissue is left in place, but its connections to other brain centers are cut so that it no longer functions.

Several variations of the anatomic hemispherectomy exist, which are designed to minimize complications. Lower portions of the brain may be left relatively intact, or muscle tissue may be transplanted in order to protect the brain's ventricles (fluid-filled cavities) and prevent leakage of cerebrospinal fluid from them.

Most surgical centers perform functional hemispherectomy. In this procedure, the temporal lobe (that region closest to the temple) and the part of the central portion of the cortex are removed. Additionally, numerous connecting fibers within the remaining brain are severed, as is the corpus callosum, which connects the two hemispheres.

During either procedure, the patient is under general anesthesia, lying on the back. The head is shaved and a portion of the skull is removed for access to the brain. After all tissue has been cut and removed and all bleeding is stopped, the underlying tissues are sutured and the skull and scalp are replaced and sutured.

Diagnosis/Preparation

The candidate for hemispherectomy has epilepsy untreatable by medications, with seizure focal points that are numerous or ill defined, but localized to one hemisphere. Such patients may have one of a wide variety of disorders that have caused seizures, including:

• neonatal brain injury
• Rasmussen disease
• Hemimegalencephaly
• Sturge-Weber syndrome

The candidate for any type of epilepsy surgery will have had a wide range of tests prior to surgery. These include electroencephalography (EEG), in which electrodes are placed on the scalp, on the brain surface, or within the brain to record electrical activity. EEG is used to attempt to locate the focal point(s) of the seizure activity.

Several neuroimaging procedures are used to obtain images of the brain. These may reveal structural abnormalities that the neurosurgeon must be aware of. These procedures will include magnetic resonance imaging (MRI), x rays, computed tomography (CT) scans, or positron emission tomography (PET) imaging.

Neuropsychological tests may be done to provide a baseline against which the results of the surgery are measured. A Wada test may also be performed, in which a drug is injected into the artery leading to one half of the brain, putting it to sleep. This allows the neurologist to determine where in the brain language and other functions are localized, and may also be useful for predicting the result of the surgery.