The goal of both intracranial and scalp electroencephalography (EEG) within the epilepsy surgery evaluation is to identify the brain region critical for seizure generation (epileptogenic zone) so that the patient may be rendered seizure-free by the removal of this region. For each, a focal ictal onset is sought, but spatial sampling issues of intracranial electrodes introduce a complexity to clinical decision making that scalp EEG does not have. This includes deciding the type of intracranial electrode and the placement locations. Depth EEG is the surgical placement of intraparenchymal electrode contacts that are along a thin, flexible, plastic tube. Grid and strip electrode contacts exist within a plastic sheet that is laid across the cerebral convexity and are discussed in Chapters 32 and 33.
Both depth and grid/strip EEG techniques are indicated for patients who are considering epilepsy surgery due to medication refractory seizures, whose noninvasive evaluation has not led to an adequate localization of the epileptogenic zone, yet provides sufficient evidence to produce a plausible hypothesis for the epileptogenic zone's location. The decision whether to use depth or subdural electrodes depends on which method best tests the hypothesis and provides the most useful information regarding the location and extent of the epileptogenic zone. In general, depth electrodes are more helpful when the potentially epileptogenic region is not across the lateral cerebral surface, extends across much of a hemisphere, or includes regions of both hemispheres. Regions that are not on the lateral cerebral surface include the medial surfaces and three-dimensionally complex abnormalities, such as schizencephalies. Subdural electrodes are usually better suited for evaluations of the cerebral surface across one or two lobes and when the evaluation must include an extraoperative mapping of cerebral function, as is possible by stimulating through the subdural electrodes. However, the simultaneous use of both techniques has become increasingly common.
When interpreting all intracranial EEG studies, the spatial sampling limitations always must be considered. Intracranial electrodes record from only immediately adjacent tissue, and ictal onsets outside the coverage region are manifested only when the epileptic rhythm propagates to include tissue adjacent to electrodes. Alternatively stated, the ictal onset depicted with intracranial EEG always shows an ictal onset zone, but the accuracy of this localization is entirely dependent on the accuracy of the electrode placement decision. Therefore, it is critical that intracranial electrodes are used only when the epilepsy surgery hypothesis is clear and the electrode anatomical coverage has the potential to confirm or reject an epileptogenic region's location and extent.
Comparison to Other Recording Methods
In general, the appearance of brain activity recorded by scalp EEG differs from that recorded by depth EEG in several ways.1,2 First, the scalp and skull preferentially filter high frequencies, and intracranial EEG depicts these frequencies clearly, which makes the background and occasional transients appear more sharply contoured. Second, ...