Abstracts

Rationale: Startle epilepsy is associated with seizures reported to closely involve the supplementary sensorimotor area (SSMA). Epilepsy surgery in such cases were reported when structural lesions were identified by magnetic resonance imaging (MRI). In a patient with debilitating, MRI-negative startle epilepsy, we sought to define the epileptogenic zones by subdural mapping to allow for epilepsy surgery. Methods: A 23 year-old man with a history of sound-induced startle epilepsy resistant to anti-epileptic drugs and VNS, and who showed no MRI abnormalities, was evaluated with long term video EEG (LTM), PET, ictal SPECT and magneto-encephalography (MEG). LTM and MEG evaluation pointed towards epileptogenc activity at the right frontal and fronto-mesial regions. Workup included subdural grids with a total of 128 electrodes. Sampling rate was available at 250 Hz. Review of the subdural EEG was performed with referential and bipolar montages, with a display of up to 5 sec per page. Guided by subdural EEG recordings and cortical mapping, a resection of an area of the superior right frontal cortex was performed. Results: Startle and spontaneous seizures were identified. Startle seizures revealed widespread ictal onset at the right frontal mesial electrodes, almost simultaneously with electrodes at the right superior frontal region. There was extreme fast spread to other frontal electrodes. About half of the seizures occurred spontaneously with ictal EEG activity, originating at the right frontal electrodes, with spreading within 200-msec to the right frontal mesial electrodes. High-frequency oscillations (HFO) bursts were also seen interictally, predominantly over 2 frontal electrodes, and allowed localization of a definite epileptogenic area. The HFO pattern was readily identified when a bipolar montage with a limited amount of frontal electrodes was analyzed, and was difficult to appreciate on referential montages. Due to the proximity of an identified frontal eye field zone to a presumed mesial frontal seizure onset region, only resection of areas containing and surrounding the HFO pattern on the right superior frontal electrodes was performed. After resection, the patient's daily tonic seizures subsided, while subtle startle seizures remained, but without tonic progression. The patient reported significant improvement in his quality of life. The pathology report findings were consistent with focal cortical dysplasia. Conclusions: Previous reports have indicated that epilepsy surgery is contemplated in patients with startle epilepsy when a discernible lesion is identified on MRI. Our case report reveals successful identification of the epileptogenic zone in a case of MRI-negative startle epilepsy, and that HFO could be a useful biomarker for the epileptogenic region. To our knowledge, this is the first case of a patient with MRI-negative startle epilepsy in which the epileptogenic zone was localized with HFO, leading to successful epilepsy surgery.