Abstracts

Rationale: Patients with suspected mesial temporal lobe epilepsy (MTLE) typically undergo video-EEG monitoring for less than 2 weeks to establish localization and lateralization of the seizure focus or foci. Longer EEG monitoring may be required to confirm that seizure onsets arise from one or both mesial temporal lobes (MTL). The intent of this analysis is to assess whether MTL seizures can be reliably lateralized within a 30-day period. Methods: Long-term ambulatory electrocorticography (ECoG) was analyzed in subjects with MTLE who were implanted with a responsive neurostimulator and leads while participating in a double blinded randomized investigational trial of the RNS System (NeuroPace , Inc.) as an adjunctive treatment for partial onset seizures. Subjects with MTLE were implanted with quadripolar depth leads placed stereotactically along the longitudinal axis of each hippocampus (Hc) or with subtemporal quadripolar cortical strip leads. Electrographic seizures (ES) were defined as episodes of low-voltage fast-activity or rhythmic sharp activity, distinct from background, and longer than 25 seconds. The electrocorticograms for ES were stored by the neurostimulator and then uploaded to a secure data management system for review. Data were collected during an evaluation period (EP) after implant but before therapy randomization. Additionally, in those subjects who had only unilateral events during the EP, ES were evaluated after the EP to determine if events were ever recorded in the contralateral side. Antiepileptic medications (AEDs) remained stable during the EP. The side of each ES onset was determined as well as the longest interval that ES onsets remained on one side before switching to the contralateral side.Results: 82 of the 191 subjects in the trial had MTLE. 78/82 (95%) had Hc depth leads and 4/82 (5%) had subtemporal cortical strip leads. All subjects had failed at least 5 AEDs. Subject demographics are presented in the table 1.:Based on scalp and/or intracranial EEG recording, 69 subjects (84%) had presumed bilateral MTLE and 13 subjects (16%) had presumed unilateral MTLE but were not considered to be candidates for temporal lobectomy because there was bilateral Hc atrophy (3), onsets contralateral to the Hc atrophy (1), failed WADA testing (4), failed dominant lobe resection (3), discordant EEG and PET lateralization (1) or patient preference (1). The EP for subjects was an average of 30 days (17 42 days). The average number of days before onsets switched from one MTL to the other was 16.5 days (4 39 days). The table below describes the onsets observed during the EP based on the subject's presumed lateralization. Conclusions: Many weeks of EEG monitoring with scalp or intracranial electrodes may be required in order to confidently lateralize epilepsy of MTL origin and to ensure that seizures do not arise bilaterally. Ambulatory EEG or ECoG monitoring can provide prolonged data in a natural environment and could provide more confident localization and lateralization than short term inpatient monitoring.