The Yield of Preoperative Sequential Routine Scalp EEGs in Patients Who Underwent Anterior Temporal Lobectomy for Mesial Temporal Sclerosis

Mesial temporal lobe epilepsy syndrome (MTLES) is the most common surgically remediable epileptic syndrome in adults. Its diagnosis is easy when clinical history is supported by positive laboratory findings. However, routine EEG may not be informative in some patients, thus delaying accurate diagnosis. Therefore, we sought to determine how often routine EEGs displayed epileptiform discharges pre-operatively in a group of patients who underwent surgery for MTLES.

Retrospectively, we reviewed the outpatient EEG records of MTLES patients who underwent surgery at our epilepsy center between 1997–2008 and had at least one routine pre-operative EEG recording in our outpatient laboratory. For each patient, serial EEGs were coded as normal, displaying nonspecific abnormalities or lateralized and localized interictal epileptiform discharges.

Seventy patients were included in the study. We reviewed 230 EEGs. In almost half of the patients (47.1%) all EEGs were normal or revealed nonspecific findings. In patients who had >1 EEG, almost 3 EEGs had to be recorded to detect the epileptiform discharges for the first time and 6.23 years were needed to accomplish this. Sleep deprivation considerably increased the yield.

In summary, patients who have a clinical history suggesting MTLES may need at least 3 routine scalp EEG recordings (with at least one of them after sleep deprivation) to detect epileptiform abnormalities and it may take much time. Therefore, a single prolonged outpatient video-EEG monitoring or an overnight inpatient monitoring might be reasonable alternatives to serial EEGs.