Abstracts

Rationale: The phenomenon of seizures with bilateral symmetrical electrographic onset evolving to focal electrographic and clinical semiology has been recognized for years. However, the underlying mechanisms of this rare epilepsy type remain poorly understood. We aimed to clarify the pathophysiology through stereotactic EEG (sEEG) implantation, recording, stimulation, and high-frequency oscillation (HFO) analysis.

Methods: We identified patients with seizures with bilateral electrographic onset evolving to a focal ictal rhythm, who had undergone sEEG implantation. Patients had pre-surgical epilepsy work-up, including PET, ictal/interictal SPECT, MEG, EEG-fMRI, and prolonged video scalp EEG prior to sEEG implantation.

Results: Two patients were identified. The first was an 17-year-old woman with drug-resistant seizures from age 12 years, involving behavioural arrest, left versive head and eye deviation, evolving to bilateral tonic-clonic convulsions. The second was a 10-year-old girl with seizures from age 9 years. Her events involved both behavioural arrests and bilateral tonic-clonic convulsions preceded by left head and eye deviation. She often had left post-ictal paresis following the convulsive events.

In both patients, prolonged scalp video EEG recording identified 3-Hz bilateral spike-wave bursts typically lasting 5-10 seconds. During these events the patients sometimes had increased blinking and behavioral arrest, but often no clear clinical change. With habitual seizures with focal-to-bilateral tonic-clonic semiology, a bilateral burst of epileptiform activity was seen initially, evolving into a focal ictal rhythm, which then spread bilaterally (Figure 1).

Both patients had subtle imaging findings that raised concern for focal lesions, and underwent bilateral sEEG implantation. In both patients, interictal patterns suggested a diffuse/multifocal process, albeit with one focus appearing more active than the others. Both patients had many absence events during sEEG, but each had only one event with bilateral tonic-clonic semiology. In both, a definitive region of ictal onset for the tonic-clonic seizure was not determined from sEEG recording (Figure 2 shows event for patient #2). In both patients, electrode stimulation did not trigger habitual convulsive seizures, but diffuse bilateral epileptiform discharges with subtle clinical correlate were provoked by stimulation of both frontal regions. HFO analysis in both showed bilateral focal regions with very high rates of fast ripples, well beyond what has been seen in healthy brain tissue.

Conclusions: Both patients had near identical presentations with stereotyped convulsive seizures with consistent focal semiology. Surprisingly, events were electrographically preceded by diffuse, bilateral activity resembling absence seizures. Our data suggest this bilateral-to-focal presentation occurs due to a diffuse, bilateral network. However, both patients had a single dominant focus which may explain why the focal aspect of their seizures had a consistent clinical semiology. Determining whether this presentation reflects a truly generalized epilepsy, or whether there are focal/multifocal cerebral abnormalities will require further study.

Funding: Please list any funding that was received in support of this abstract.: N/A.