Interictal estimation of intracranial seizure onset in temporal lobe epilepsy

• Interictal intracranial tests such as focal slow activity (IFSA), single pulse electrical stimulation (SPES) and 18FDG PET can successfully complement ictal recordings.
• When lateralized to the same side, intracranial IFSA and SPES reliably predicted the side and site (mesial or lateral temporal) of seizure onset.
• IFSA and SPES are simple, fast and inexpensive. If performed early in the telemetry period they have the potential to reduce telemetry time, and the associated risks and costs.

ObjectivesTo evaluate the lateralizing and localizing values of interictal focal slow activity (IFSA), single pulse electrical stimulation (SPES) and 18FDG PET, in order to estimate their potential to complement ictal intracranial recordings and reduce prolonged monitoring in patients with temporal lobe epilepsy.MethodsThe study includes 30 consecutive patients with bilateral temporal subdural electrodes and focal seizure onset. IFSA, SPES and 18FDG PET when available, were visually assessed and their combined lateralization was based on the majority of the individual lateralizing tests.ResultsIn the 18 patients who had all three tests, lateralization was congruent with seizure onset areas in 15 (83%). When lateralized (15 patients), 18FDG PET was always congruent with intracranial seizure onset. In all 12 patients without 18FDG PET, lateralization combining IFSA and SPES was congruent with seizure onset, including two with bilateral independent seizure onset on subdural monitoring. 22 out of the 23 patients who had surgery enjoyed favorable outcome (Engel I or II).ConclusionIntracranial IFSA and SPES can reliably predict the side and site (mesial versus lateral temporal) of seizure onset when they lateralize to the same side.Significance18FDG PET can be useful in planning electrode implantation. During intracranial recordings, IFSA and SPES have the potential to reduce telemetry time, risks and costs.