Surgery for Epileptic Foci

It has long been recognized that seizures can begin with an anatomic focus of electrical synchrony that spreads to include the remainder of the cortex. Precise intraoperative localization of such a focus is an important part of surgery for drug-resistant seizures. This localization is performed by electrocorticography in the awake patient or by provocative techniques (discussed later), if necessary, during general anesthesia. Alternatively, a grid of subdural electrodes or depth electrodes may be placed in the anesthetized patient, with their locations based on preoperative surface electroencephalographic recordings and imaging studies. Subsequent detailed mapping is done in the awake patient in the electroencephalography laboratory over a period of several days to a week. The patient then returns to the operating room for resection of the excitable focus under local or general anesthesia. When intraoperative localization is desired, the anesthetic level is minimized, and a provocative technique such as hyperventilation or small-dose barbiturate (most commonly methohexital) administration can be employed to trigger the focus, aiding in its localization. If the cortex is excessively depressed pharmacologically, seizure activity cannot be provoked. Intraoperative seizure mapping requires the involvement of an electroencephalographer familiar with this technique. Rarely would an anesthesiologist possess the expertise required for intraoperative seizure focus localization.