Selecting Appropriate Candidates for Epilepsy Surgery

While roughly 2/3 of patients with epilepsy are successfully treated with anti-seizure medications, the remaining continue to have seizures and may be candidates for surgical treatments. For selecting appropriate candidates for epilepsy surgery an extensive presurgical evaluations is mandatory. At least five different specialties should be involved in the epilepsy presurgical assessment including epileptologist, radiologist, neuropsychiatrist, neurosurgeon and nuclear medicine. During the presurgical evaluation, five different brain areas are determined including irritative zone, symptomatogenic zone, seizure-onset zone, functional deficit zone and epileptogenic lesion. These five zones may not be exactly identical but are overlapped. The aim of presurgical evaluation is finding epileptogenic zone defined as the minimum amount of cortex that must be resected surgically to produce seizure freedom. Since the epileptogenic zone (EZ) could not be determined directly, it should be found indirectly by defining the other zones discussed above. If the patient is seizure-free after surgery, we conclude that the epileptogenic zone must have been included in the resected cortex. Irritative zone is area of cortex that generates interictal epileptiform discharges. Seizure onset zone is area of cortex that initiates clinical seizures. Symptomatogenic zone is area of cortex that produce seizure semiology. Usually the ictal semiology is due to spread of the discharge from an EZ located in a symptomatically silent area to a distant area of eloquent cortex that is outside the epileptogenic zone. Functional deficit zone is the area of cortex that is not functioning normally in the interictal period. This area could be assessed by neuropsychiatric test, ictal SPECT and interictal PET. The purpose of electrocorticography recording is to refine the presumed physical boundaries of following zones: the eloquent cortex and the irritative zone. Ictal SPECT has a higher sensitivity of interictal SPECT and if the isotope is injected within the first 20 seconds after seizure onset, it could be significantly correlated with localization of EZ. In patients with epilepsy FDG-PET scan is typically performed in the interictal state with the goal of detecting focal areas of decreased metabolism, that are presumed to reflect focal functional disturbances of cerebral activity associated with epileptogenic tissue. PET is more valuable in temporal lobe epilepsy rather than extratemporal ones.