Lateralizing value of interictal spikes on overnight sleep-EEG studies in temporal lobe epilepsy

Purpose: To determine the lateralizing value of interictal epileptiform dis charges (IEDs) recorded during overnight sleep-EEG studies in temporal lobe epilepsy. Because IEDs are more prevalent in non-rapid eye movement (NREM) sleep than in wakefulness, overnight sleep-BEG recordings may contribute a dditional lateralizing information to the epilepsy surgery evaluation beyon d daytime EEGs. Methods: Twenty-four subjects with medically refractory temporal lobe epile psy underwent continuous overnight sleep-EEG recordings. Subjects were seiz ure free greater than or equal to 24 h before study and receiving stable do ses of medication. The IED foci recorded on overnight studies were compared with daytime EEGs, interictal samples, and ictal recordings during long-te rm monitoring, brain magnetic resonance images (MRIs), and surgical outcome . Results: (a) In all 24 subjects, including 13 without IEDs on daytime EEGs, temporal IEDs were present during NREM sleep and were exclusively or predo minantly (>95%) unilateral in 15 and bitemporal in nine. (b) Unilateral NRE M IEDs were concordant with surface or depth ictal-onset regions in 14 subj ects, even if MRIs were normal (three subjects) or surface ictal-onset regi ons were bilateral (five subjects). Eleven of 12 subjects with unilateral c oncordant NREM IEDs who have undergone surgery are seizure free. (c) Bitemp oral IEDs were associated with postoperative seizures in all subjects with normal MRIs or widespread MRI abnormalities. However, all subjects with bit emporal IEDs and MRI hippocampal abnormalities concordant with ictal-onset regions had good to excellent surgical outcomes. Conclusions: When combined with other investigations, IEDs recorded on over night studies add prognostic data to the epilepsy surgery evaluation not pr ovided by daytime EEGs.