Epilepsy Center - Pre-operative Evaluation
05 November 2004
Potential surgical candidates at the epilepsy surgery program undergo a detailed and involved pre-operative evaluation. The objective of these studies is to localize the seizure focus to a resectable area in the brain. The data collected are presented and discussed by a panel of epileptologists, neuropsychologists, neurosurgeons and neuroradiologists.
Continuous Scalp Audiovisual-EEG Telemetry
The purpose of this study is to record several seizures in order to characterize and localize them. It is important to establish at this stage that all of the patient's typical seizures are similar to one another and come from the same area of the brain. Patients are admitted to the hospital for one to two weeks. Electrodes are placed on the scalp, similar to a routine EEG. However, they are recorded continuously with concurrent video monitoring. During this period, anticonvulsant medications may have to be reduced or discontinued in order to bring on seizures. Once the evaluation is completed, patients are started on their medications again.
During the hospitalization, patients also undergo a battery of neuropsychological tests in order to determine the functional capacity of particular areas of the brain. These findings help localize the seizure focus further. They also allow the neuropsychologist to evaluate the psychological make-up of the patient, and allow the team to address any psychosocial issues that may interfere with a successful outcome.
All patients are evaluated with a head MRI scan in order to rule out structural abnormalities or lesions that may contribute to the epilepsy. This also provides more localizing information to the physicians. Some patients may also be evaluated with SPECT or PET for further localization.
If the seizures are localized to a resectable area, patients are advanced to the next stage of evaluation, angiogram-WADA. This study defines the cerebral vascular anatomy of the patient and allows us to determine language dominance and memory function of the patient.
This study involves the participation of the neuroradiologist and neuropsychologist; the procedure is performed in the radiology suite. Patients initially have an angiogram where radio-opaque dye is injected into the carotid and/or vertebral arteries. This allows visualization of the anterior and posterior circulation.
This procedure carries a small risk of embolic stroke, and bleeding. The neuropsychologist then tests the patient for language and memory function in each hemisphere separately after a short-acting barbiturate, amobarbital, is injected directly into one hemisphere, momentarily anesthetizing that hemisphere.
Intracranial Audiovisual EEG Telemetry
In a small group of patients, results of the initial studies are not straightforward and not wholly localized. The use of intracranial electrodes in EEG telemetry allows physicians to better localize a seizure focus, and enables them to offer epilepsy surgery to patients who otherwise would have been deemed non-surgical candidates. This procedure is in itself a surgical procedure and carries its own risks. There is a 1-2 percent risk of major complications, including hemorrhage, cerebral infarction, and infection.
The procedure involves placement of subdural electrodes and depth electrodes either directly on the brain surface or into the brain for the purpose of further EEG recording of seizures. It is otherwise similar to scalp EEG telemetry with the objective of recording several seizures for localizing purposes. Stimulation of various sites for functional mapping may also be performed. Anticonvulsant medications may need to be decreased or discontinued in order to induce seizures. Once the appropriate data are collected, the electrodes are removed by the neurosurgeon and the patient is observed in the hospital for a few additional days before discharge to home. Patients are restarted on their medications at the completion of the study.
After the pre-operative evaluations are completed, patients' data are reviewed to determine if they are surgical candidates. Patients should always bear in mind that the purpose of surgery is to relieve them of their seizures, but not put them at risk of having neurological deficits subsequent to the surgery. Therefore, it is possible that a patient may have undergone the entire series of tests and in the end is deemed a poor surgical candidate. These pre-operative tests are designed to localize seizures and determine the patient's eligibility only; they do not guarantee surgery or its successful outcome.