Abstracts

Rationale: Fixation-off sensitivity (FOS) is a rare but well-recognized phenomenon in epilepsy, where seizures and epileptiform paroxysms are elicited by conditions that eliminate central vision and fixation. FOS is observed in a wide spectrum of epilepsies that are associated with hyperexcitability of the occipital cortex, such as Gastaut syndrome and occipital epilepsies. We aim to delineate the neural underpinnings of FOS by studying the emergent pathological networks event locked to FOS evoked seizures. Prior studies evaluating FOS have mapped the symptomatogenic zone in the parieto-occipital cortices, but none have looked at the network dynamics in the posterior quadrant of the brain that occurs during FOS.

Methods: This research was carried out as a case investigation at a single primary care site where we analyzed stereoencephalography (SEEG) data from an 18-year-old male subject who underwent invasive EEG evaluation of a depth of sulcus dysplasia in the intraoccipital sulcus. FOS was tested with (1) alternating eye-closure and eye-opening and (2) with modified Ganzfeld stimulation (white-out protective goggles). We evaluated h2 non-linear correlation-based connectivity analysis during FOS and compared it with baseline followed by evaluation of inward vs outward flow of information (degree centrality).

Results: Increased functional coupling within regions in the posterior quadrant i.e., between lateral occipital cortex and precuneus and lateral occipital cortex to cuneus and occipital pole, was linked to the occurrence of FOS. Overall, the mean network h2 connectivity was increased during FOS compared to baseline (FOS: median:0.26, range 0.21-0.35 vs baseline: median: 0.24, range: 0.20-0.32, p=0.0003).

Conclusions: Our study highlights the advantage of studying pathological brain networks using a SEEG implantation. Such understanding could be potentially useful in the identification and characterization of epileptogenic networks. Network analysis with an understanding of the information flow (e.g., increased outdegree between functionally relevant areas such as that from visual recognition areas to central fixation and saccadic adaptation areas) can help in reasonably predicting functional outcomes, offer a potential to more accurate surgical planning and better post-surgical outcomes.

Since FOS phenomenon can be stereotypically and reliably induced by specific stimulation procedures (e.g., eye-closure) under a close observation in the epilepsy monitoring unit, the increased outflow of information to regions crucial to central fixation and saccadic adaptation can be interpreted as a primary underlying pathology of this clinical manifestation.

Funding: Please list any funding that was received in support of this abstract.: None.