7.2.1 Purpose

To identify the seizure focus as a target for resection, patients are admitted to an epilepsy monitoring unit (EMU) for continuous video electroencephalography (vEEG). The goal of vEEG monitoring is to record at least three to four of the patient’s typical seizures, ¹³ to determine the seizure onset based on scalp EEG.

7.2.2 Procedure

Patients are admitted to the EMU for 5 to 8 days, and electrodes are placed on the head according to a standardized montage. Often, the patient’s ASDs are lowered to facilitate recording seizures in a week-long admission. ^{14 ,​ 15} Epileptologists identify seizures from the patient’s symptoms and from the EEG, and use visual inspection of the seizures to determine the location of onset. In some cases, particularly if the patient fits into a defined epilepsy syndrome and has a known lesion on imaging, the information obtained on scalp EEG may be sufficient to proceed with surgery.

7.2.3 Additional Testing

Focal findings on MRI or PET that correspond to the patient’s seizure onset on scalp EEG are predictive of a better outcome after surgery. ¹⁶ Additional testing, such as magnetoencephalography (MEG) or ictal single-photon emission computed tomography (SPECT) can be supportive if no lesion is found on MRI or PET. ^{17 ,​ 18} Neuropsychology testing revealing deficits corresponding to the suspected seizure onset zone is also supportive, and is predictive of less cognitive decline after surgery. ^{19 ,​ 20}

If the seizure onset zone is suspected to be close to eloquent cortex or in the dominant hemisphere, additional brain “mapping” with Wada, functional MRI (fMRI), or ECoG may be required prior to surgery to ensure that the resection would not result in major deficits.

7.3.1 Technique

Prior to a Wada test, the patient has EEG electrodes placed on the head (so that any seizures during the procedure can be detected). During the test, after local anesthesia to the groin for introduction of the angiography catheter, the neurosurgeon introduces the catheter into the internal carotid artery (generally beginning with the side under consideration for surgery). ²⁵ Contrast is injected, and placement confirmed radiographically. The patient raises his or her arms and may be asked to count. Then the anesthetic is administered through the catheter at a rate of 25 mg every 5 seconds (typical dose 75–150 mg sodium amytal, though occasionally more is needed) until contralateral motor hemiparesis is observed. Typically, an epileptologist or neurologist in the case assesses motor strength to determine when the hemiparesis develops. EEG slowing of the affected hemisphere can be observed at the same time. Excess anesthetic can result in oversedation of the patient, making test results invalid.

Once the hemisphere has been anesthetized based on motor findings, the catheter is partially withdrawn and the neuropsychologist or neuropsychometrist uses a standard battery of tests to assess language production and reception, and gives the patient verbal lists and nonverbal items to remember. Throughout the test, motor function is assessed intermittently to ensure the hemisphere remains anesthetized.

After the effects of the anesthetic wear off (typically after 10–15 minutes), the neuropsychologist tests the patient for free recall and for recognition of the verbal and nonverbal items. The same procedure is then repeated on the opposite hemisphere. ^{21 ,​ 25}