Surface and Intracranial Electroencephalography

The goal of the presurgical evaluation is to define the epileptogenic zone, which is the region of cortex capable of generating seizures. Complete removal of the epileptogenic zone is necessary to produce a seizure-free state. y The clinical history and EEG are used to identify patients who may be candidates for surgery. The yield of surface EEG, however, is limited by the attenuating effects of the skull and intervening tissue, distance of the recording electrodes from the generator of ictal activity, and variable orientation of generator. Cerebral activity from mesial or basal cortical areas is particularly apt to escape detection. Modified electrode placements and semi-invasive techniques such as sphenoidal electrodes increase the yield of surface recordings. In general, the distribution of interictal abnormalities extends beyond the epileptogenic zone. Nevertheless, interictal epileptiform discharges generally provide more reliable localizing information than ictal recordings because of the tendency for seizures to spread rapidly to contiguous regions.

Intracranial EEG may be required when surface EEG and neuroimaging studies fail to adequately delineate the epileptogenic zone. Although more sensitive than surface EEG, intracranial EEG provides a limited view of cerebral activity, because recordings are obtained only from areas where electrodes are placed.

The successful use of intracranial EEG requires that noninvasive studies provide enough information to ensure appropriate electrode placement. Intracranial monitoring is performed with depth and subdural electrodes. Depth electrodes are multiple-contact wires placed stereotactically into the brain. Their primary indication is for the identification of the epileptogenic zone in patients with TLE and bitemporal abnormalities on surface EEG or MRI. Electrodes embedded in thin Silastic plates arranged in strips or grids can be placed through burr holes or craniotomy within the subdural space over cortical areas of interest. This technique allows for the recording of multiple regions over one or both hemispheres and mapping of eloquent cortex. Complications of depth and subdural electrodes include intraparenchymal hemorrhage, infection, and cerebral edema in 1 to 2 percent of patients.

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Figure 52-6 MRI showing atrophy (T1-weighted image on left) and increased signal (fluid-attenuated inversion-recovery sequence on right) in the right hippocampus in a patient with right mesial temporal lobe epilepsy.

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