Summary:Purpose: EEG recording during sleep is widely used in the assessment of epilepsy, particularly in candidates for surgery, yet the diagnostic value of this procedure is not well established. We evaluated the predictive reliability of interictal epileptiform discharges (IEDs) for localization in presurgical patients with temporal lobe epilepsy (TLE) during non-REM sleep.Methods: Preoperative scalp EEG recordings with waking and sleep states were assessed in 83 patients with TLE in whom localization of the epileptogenic zone was subsequently confirmed by successful surgical treatment (patient seizure-free >1Results: The accuracy of EEG recordings for prediction of lateralization significantly changed from 5 1.8% during waking to 78.3% during sleep. After exclusion of patients who showed year). no discharges, the predictive value changed from 74.1 to 86.7%. However, in patients in whom the waking scalp EEG lateralized incorrectly, no improvement in reliability was achieved by sleep recording.Conclusions: Our results suggest that IEDs occurring in non-REM sleep provide more accurate information for lateralization of epileptogenesis than do those occurring during waking. This gain of diagnostic information was obtained in patients who showed either bilateral or no discharges in waking records, because unilateral discharges arising de novo in sleep were always correctly lateralizing. On the other hand, in patients who showed unilateral discharges in the awake state, whether ipsilateral or contralateral to the epileptogenic zone, the findings were generally unchanged during sleep. Key Words: Epilepsy-Electroencephalogram-Interictal discharge-Sleep.Sleep induction, achieved either by medication qr sleep deprivation, is a widely used method of EEG activation in clinical practice, because the frequency and extent of interictal epileptiform discharges (IEDs) .in the EEGs of people with epilepsy are generally increased in sleep (1-3). With a single waking EEG study, epileptiform discharges are detected in -50% of all patients with epilepsy (43). During sleep, epileptiform discharges appear in -80% (5).The region of IEDs does not always indicate the site of seizure onset (6). Patients with intractable epilepsy often show epileptiform discharges in regions more extensive than, and not necessarily contiguous to, the epileptogenic zone. Long duration of the illness and Iiigh seizure frequency may cause more complex pathophysiological phenomena, such as mirror foci or secondary epileptogenesis (7). Interictal or even ictal discharges in scalp recordings are variously estimated to have 6 6 9 5 % predictive reliability for the lateralization of epileptogenesis (8-12). Therefore, an increase in interictal discharges in sleep would not necessarily provide more reliable information for the localization and lateralization of epileptogenesis. However, few studies have addressed the localizing significance of epileptiform discharges in sleep.Some researchers believe that the localization of the epileptogenic zone by discharges in...