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 electricalactivity 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 effectivetreatment. 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 generalanesthesia, 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.
Aftercare
Immediately after the operation, the patient may be on a
mechanical ventilator for up to 24 hours. Patients remain in the hospital for at
least one week. Physical and occupational therapy are part of the rehabilitation
program to improve strength and motor function.
Risks
Hemorrhage during or after surgery is a risk for hemispherectomy.
Disseminated intravascular coagulation, or blood clotting within the circulatory
system, is a risk that may be managed with anticoagulant drugs. “Aseptic
meningitis,” an inflammation of the brain’s covering without infection, may
occur. Hydrocephalus, or increased fluid pressure within the remaining brain,
may occur in 20–30% of patients. Death from surgery is a risk that has decreased
as surgical techniques have improved, but it still occurs in approximately 2% of
patients. The patient will lose any remaining sensation or muscle control in the
extremities on the side opposite the removed hemisphere. However, upper arm and
thigh movements may be retained, allowing adapted function with these parts of
the body.
Normal results
Seizures are eliminated in 70–85% of patients, and reduced
by 80% in another 10–20% of patients. Patients with Rasmussen disease, which is
progressive, will not benefit as much. Medications may be reduced, and some
improvement in intellectual function may occur.
Morbidity and mortality rates
Death may occur in 1–2% of patients
undergoing hemispherectomy. Serious but treatable complications may occur in
10–20% of patients.
Alternatives
Corpus callosotomy may be an alternative for some patients,
although its ability to eliminate seizures completely is much less. Multiple
subpial transection, in which several bundles of nerve fibers are cut, is also
an alternative for some patients.
See also Corpus callosotomy; Vagal nerve stimulation.
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