Monitoring epilepsy patients with scalp-recorded electroencephalography (EEG) is the mainstay in the evaluation for potential curative surgery. Inpatient combined video-EEG monitoring (VEM) to record seizures can be used to help delineate the general region of the brain in which seizures start in a specific patient, as well as determine the type of seizure and exclude the diagnosis of nonepileptic seizures. In addition to the information gained by EEG recording, other noninvasive tests can be used to determine the ictal onset zone and underlying pathology. However, in a select group of patients, intracranial EEG monitoring is needed for determining eligibility for epilepsy surgery and for planning that surgery. The evaluations of these patients identify whether a resectable focus is present when noninvasive methods have not yielded the concordant findings needed to proceed to resective surgery.
Intracranial VEM has no role with patients who have been excluded as candidates for surgery. This could be for as simple a reason as patient choice. Some patients would not consider brain surgery of any kind, either resection of brain tissue or implantation of a stimulation device, and would therefore not be appropriate for intracranial VEM, given that there are associated risks and that the study could provide no useful information for these patients. Other patients can be excluded as candidates for epilepsy surgery based on scalp EEG and other noninvasive diagnostic modalities. Such patients include those who clearly have multiple ictal onset zones or have generalized onset seizures. Some of these patients may be candidates for corpus callosotomy, but this surgery does not require intracranial VEM.
Other patients are clearly candidates for epilepsy surgery, and this determination and the planning for the surgery can be completed using noninvasive techniques. High-resolution magnetic resonance imaging (MRI) has been the most effective scanning technology in expanding the group of surgery candidates who do not require intracranial VEM. Patients with focal abnormalities, such as tumors, vascular abnormalities, and hippocampal sclerosis, and have corresponding scalp EEG recordings demonstrating ictal onset in the region of the structural lesion may be able to proceed to surgery without electrocorticography based on the anatomical localization from the MRI.1 Other imaging modalities, including positron emission tomography (PET), magnetoencephalography (MEG), and single-photon emission computed tomography (SPECT) sometimes are used to confirm the localization. Since the newer noninvasive technologies have become available, fewer patients who are potential candidates for epilepsy surgery now require intracranial electrode recording.
There are several types of intracranial electrodes. Seldom-used epidural peg electrodes provide more focused information about the region of seizure onset than scalp electrodes, but they do not provide the spatial resolution necessary for surgical intervention and very rarely add sufficient information to justify their use over noninvasive scalp EEG (Figure 30-1). The two types of intracranial electrodes most commonly used for preoperative evaluation are subdural and depth electrodes. Subdural electrodes come in multiple arrangements, from strip electrodes that can be ...