"Temporal" intermittent rhythmic delta activity: the true localizing nature of TIRDA

Temporal lobe epilepsy is the most common form of focal epilepsy and is frequently resistant to antiseizure medication. Non-invasive biomarkers are crucial when resective epilepsy surgery is considered in order to guide diagnostic work-up and management. Interictal epileptiform discharges, when concordant with ictal EEG recording and a focal abnormality on functional imaging or anatomic MRI in patients with temporal lobe epilepsy, portend a favorable outcome with resective or ablative surgery. An interictal non-epileptiform feature on EEG believed to have the same localizing potential as epileptiform discharges is temporal intermittent rhythmic delta activity (TIRDA). The precise localization of TIRDA has been a subject of debate, but has been associated with seizures that arise from the temporal region. We report a 64-year-old female who underwent unsuccessful right anterior temporal lobectomy for drug-resistant focal epilepsy, suspected to originate from the right temporal lobe. Subsequent video-EEG monitoring revealed right, greater than left, TIRDA and interictal epileptiform discharges arising from the temporal regions bilaterally, despite a generous temporal lobectomy demonstrated by brain MRI. Further, using EEG source localization, we identified TIRDA using scalp EEG in sensor space, localized to the ipsilateral orbitofrontal region. We discuss the proposed localization of TIRDA in this case and address the importance of characterizing TIRDA in the presurgical evaluation of patients with epilepsy.