Introduction The incidence of epilepsy

Introduction The incidence of epilepsy ranges from 0.5% to 1.0% of the total population. Approximately one-third of epileptic patients have their seizures refractory to medical treatment. About half of the medically intractable focal epileptic patients are potential candidates for surgical treatment. Epilepsy surgery for intractable AL 8697 epilepsy (TLE) with or without structural lesions usually provides favorable surgical outcome. Structural lesions are found in about 30% of surgical specimens resected for intractable TLE. The proper evaluation of clinical semeiology, focal structural lesions, interictal and ictal electroencephalogram (EEG) abnormalities, and neuropsychological dysfunctions, especially when these factors are in concordance, may help to offer good seizure control postoperatively. The presence of a structural lesion in a patient with intractable TLE does not always localize the epileptogenic zone. The temporal lobe lesion can induce secondary epileptogenicity in the mesiotemporal structures. The incidence of an extrahippocampal lesion and mesiotemporal sclerosis ranges from 8% to 22%. Different surgical strategies, such as lesionectomy alone and lesionectomy with removal of mesiotemporal structures, have variable seizure control outcomes.
Preoperative investigations The preoperative investigations for intractable TLE consist of serial EEG recordings, long-term EEG/video monitoring with sphenoidal electrodes, magnetic resonance imaging (MRI), magnetic resonance spectroscopy (MRS), positron emission tomography with fluorodeoxyglucose (FDG-PET) and neuropsychological assessment. MRI is the only investigation that can distinguish between substrate-related and substrate-unrelated epilepsy. An MRI-identified lesion is a strong predictor of favorable seizure outcome following surgery. The sensitivity of MRI in detecting the structural lesions in refractory epileptic patients approaches 100%, with a reported specificity of 87%. In our reported series, MRI detected the lesion in all 12 patients in the vascular group, and in all 12 patients with neoplasms other than low grade astrocytomas in the neoplastic group. However, MRI detected the lesions in only 61% (14/23) of patients with low-grade astrocytomas. Won et al reported that among MRI, PET and single photon emission computerized tomography in lateralizing epileptogenic foci, PET was the most sensitive tool which correctly lateralized the focus in 85% of the patients. PET may be used as a complementary tool in cases of inconclusive lateralization with other presurgical evaluations. Detection of the function of the mesiotemporal structure before operation is mandatory in the surgical strategy for lesional intractable TLE. High resolution, thin-cut (3 mm) slices MRI with a typical epilepsy protocol is recommended for the recognition of hippocampal sclerosis. MRS can evaluate the biochemical and metabolic condition of the brain tissue. Reduction of the N-acetyl-aspartate (NAA) and decreased ratio of NAA/creatine plus choline complex indicates relative dysfunction of the measured area. MRS can provide evidence of hippocampal damage in a high percentage of patients with TLE. An extrahippocampal lesion in combination with MRS evidence of hippocampal sclerosis (dual pathology) can be found in up to 67% of patients with temporal neocortical epilepsy.
Surgical strategies The goal of epilepsy surgery in treating intractable lesional TLE is seizure freedom. It is necessary to remove the structural lesion as well as the epileptogenic cortex while preserving function. In Phase I preoperative investigations, the epileptogenic focus of epileptic activity may not correlate well with the abnormalities on MRI. Invasive Phase II studies using subdural grids, depth electrode implantation and intraoperative electrocorticography (ECoG) are mandatory to localize seizure onset. A meta-analysis of simple excision versus epilepsy surgery was performed by Weber et al in 1993. The authors concluded that in patients with lesional intractable seizures, epilepsy surgery had a better outcome of rendering the patient seizure free.