Evaluation for Epilepsy Surgery
The treatment options that are available for a patient depends on many factors and the process for determining these options is individualized for each patient. The evaluation can include all noninvasive testing performed on epilepsy patients, but can also require other more advanced but invasive techniques for patients with medically intractable epilepsy being evaluated for surgery.
Phase I Monitoring (Presurgical Evaluation Admission to the EMU):
The presurgical evaluation admission to the EMU procedure is the same as for a Diagnostic Admission, with some small changes. To learn more about and admission to the EMU please refer to the Our Services page.
SPECT (single-photon emission computed tomography)
SPECT may be performed during a video EEG. Like PET, this test involves the injection of a radioactive solution. It must be injected near the very beginning of a seizure, and it is only active in the body for a brief period of time. By comparing the results for an injection performed during a seizure (ictal scan) and one not during a seizure (interictal scan), it may be possible to identify the area in the brain responsible for seizures. SPECT scans typically require admission the Epilepsy Monitoring Unit.
fMRI (functional MRI)
fMRI is used to identify areas of the brain involved in certain functions. Using special MRI protocols, changes in blood flow are measured while the patient is asked to perform certain tasks. Most commonly, this is used to identify areas of the brain involved in controlling movement or language.
Neuropsychological (Neurocognitive) Testing: Neuropsychological testing used to get a detailed pictures of a patients cognitive (including memory, attention and language) functions. It consists of a set of written and oral questions and tasks administer by a neuropsychologist and their assistants. Results are compared to the typical performance of people who are the same age and have the same amount of formal education as the patient. The results help to identify both areas of relative strengths and weaknesses. The pattern may suggest particular parts of the brain that are functioning as expected and those that are not. In many patients, parts of the brain that are not functioning well may correspond to areas involved in seizures.
Phase II Monitoring (Intracranial EEG):
Intracranial EEG recording is used to better define the seizure focus and its relationship to critical areas of the brain, such as movement and language center. Intracranial EEG can be performed in a variety of ways including using subdural electrode (SDE; called electrocorticography or ECoG) or depth electrodes (usually called stereo EEG, or SEEG). The types of electrodes used and where they will be placed is carefully planned out by the neurosurgeons and epileptologists based on the results of the above testing.
Subdural electrodes (used in ECoG)
Consist of thin plastic sheets containing metal disks. SDEs are placed through an opening in the skull, usually requiring temporarily removing part of the skull, which is then replaced. SEEG electrode are placed through small holes (about 2-3 mm) drilled into the skull, often using a surgical robot assistant. Beyond the types of electrodes used and the
The general steps involved in Phase II monitoring are as follows:
- Admission to hospital, usually the night before planned procedure
- If needed, MRI brain on night of admission used during surgery (typically needed for stereo EEG)
- Placement of electrodes by neurosurgeon in the operating room
- CT head to confirm location of placed electrodes
- Close observation for 12-24 hours after electrode placement, potentially in ICU
- Connection of implanted electrodes to EEG recording equipment
- Admission to Epilepsy Monitoring Unit for medication adjustment and recording of seizures (typically 5-7 days, but depends on how quickly the needed data is obtained)
- Electrical stimulation mapping using implanted electrodes – for localization of critical functions such as movement or language and/or for confirmation of seizure focus
- For SDE, discussion of results of Phase II monitoring at Epilepsy Surgery Conference, followed by removal of electrodes and resection of seizure focus, if appropriate
- For SEEG, removal of electrodes followed by later discussion at Epilepsy Surgery Conference (usually 1-2 weeks after electrode removal)
- Observation in hospital for one to three days after electrode removal
Electrical Stimulation Mapping
In addition to recording seizures, the implanted electrodes can also be used to stimulate the brain. Electrical current is passed between to electrodes to stimulate the surrounding brain tissue. Because the brain itself does not have any sensation, this process is not painful. Instead the stimulation may produce certain movements or sensations or may temporarily disrupt certain function. The main goal is to identify parts of the brain that are important for specific tasks such as movement, sensation, language, or vision. Another goal may be to better identify parts of the brain involved in the seizure by stimulating them to see if typical seizure symptoms or discharges are produced. Depending on the number of electrode sites to be tested, this can take from one to several hours.