Before any operation for epilepsy can be performed, there has to be a period of careful testing and evaluation. These tests are done to make sure the surgery has a good chance of being successful and won't affect any of the important functions of the brain.
Most of the tests are used to pinpoint the area of the brain where seizures begin or to locate other areas, like speech and memory, that have to be avoided.
How many tests have to be done depends on the kind of operation that is being planned and how much information each test produces.
The following tests are most often used before a decision to operate is made:
- Electroencephalography (EEG) tests record electrical activity in the brain and identify areas of the brain where seizures occur
- Magnetic resonance imaging (MRI) scans take pictures of the inside of the brain. MRI scans may show tumors, abnormal blood vessels, cysts, and areas of brain cell loss or other brain damage.
- Simultaneous video (TV) monitoring and EEG recording help identify the type of seizure that is taking place.
- Neuropsychological tests, including IQ, memory, and speech tests, tell doctors more about where the seizures (or the brain damage which is causing the seizures) are located.
- An intracarotid sodium amobarbital test locates speech and memory centers. A drug is injected into an artery leading to the brain. It puts half of the brain to sleep for a short period of time. The doctors then check speech and memory on the side of the brain not put to sleep.
- Positron emission tomography (PET) scans may be used in certain cases to help identify where seizures are taking place. PET measures how intensely different parts of the brain use up glucose, oxygen, or other substances.
- Single photon emission computed tomography (SPECT) scans also help identify where seizures are taking place by measuring blood flow.
Tests Using Implants
Even after all the previously described tests are done, additional information may be needed to identify the epileptic area in the brain. This is because the area of seizure activity sometimes can't be found by electrodes attached to the surface of the head. To obtain that additional information, two separate operations may be required.
The first operation places electrodes in or on the brain itself. These special electrodes are called depth or subdural electrodes.
After they are placed, the patient remains in the hospital with the head wrapped in a large dressing, with wires attached to the electrodes coming out of the dressing. Seizures are then recorded directly from the brain, often on simultaneous video and EEG. This process is called electrocorticography.
Both kinds of recording instruments may be kept in place for some time while doctors monitor signals from within the brain during seizures.
The brain may be stimulated with mild electrical impulses via the electrodes to identify special areas controlling speech, movement and sensation. In addition, further electrical recording to map out the seizure focus (the exact area to be removed) may be done.
If the tests show that there is a single epileptic area and it can be removed safely, a second operation is performed to remove the affected area. If not, surgery is done only to remove the electrodes.
Sometimes all the tests and procedures rule out surgery as a suitable treatment. Other times the tests may fail to give enough information and the doctors may decide not to recommend surgery.
The Operation Successful epilepsy surgery depends on careful selection of patients and a skilled medical and surgical team.
The operation may take several hours to perform, as surgeons first locate and then remove the area of the brain identified in pre-testing as the source of the seizure activity, or carefully sever the nerve fibers between the two halves of the brain if a split brain operation is being performed, or make the incisions required by the MST procedure.
EEG recordings during the surgery help the physicians map out the exact area of brain to be removed.
The brain may be stimulated with mild electrical impulses during the operation itself to identify special areas controlling speech, movement and sensation.
Sometimes the whole operation is done with the patient awake but under local anesthetic. This is possible because brain tissue is not sensitive to pain.
Recovery
After the operation, the patient stays up to a week in the hospital and then goes home and continues to recuperate. After about three to eight weeks he or she can usually go back to normal activities.
Doctors usually recommend that surgery patients stay on antiepileptic medicines for up to two years after the operation. Some people may have to continue with medication indefinitely to retain seizure control.
Benefits and Risks
Lobectomies
While there are risks in all surgical procedures, including the placement of depth electrodes and grids, most brain surgery for epilepsy appears to be relatively safe. The success rate for epilepsy surgeries depends on the type of operation performed and can usually be predicted after all the test results are available.
For temporal lobectomies, 65 to 85% of patients will be seizure-free.
Complications occur in about 4 out of every 100 of these operations. Depending on the kind of surgery that's performed, possible complications include: partial losses of vision, motor ability, memory or speech. Infection or temporary swelling of the brain may also sometimes happen.
Corpus Callosotomies
Among patients having a corpus callosotomy (split brain operation), risks of major and minor complications after surgery are around 20 per 100 operations. Generalized seizures may stop or happen less often than before the operation. Partial seizures (that is, changes in movement, feeling or emotion without loss of consciousness) will probably continue and may even get worse. Still, the uncontrolled drop attacks and generalized tonic clonic seizures that the operation is designed to treat have risks of their own. Decisions to operate take all these possibilities into account.
Hemispherectomies
Excellent results for this operation, which involve removal of one half or almost one half of the brain, are being reported by the small number of very specialized centers doing these operations. However, there are more risks with hemispherectomies than with other types of epilepsy surgery.
Children who have hemispherectomy operations will continue to have loss of function on the side of the body opposite the side where the brain was removed.
