Abstracts

Rationale: In the presurgical evaluation of epilepsy, accurate localization and lateralization of the seizure onset zone are critical for achieving optimal outcomes. Ictal electroencephalogram (EEG) findings from a long-term video scalp EEG remain the cornerstone of noninvasive seizure monitoring for seizure onset zone localization. Rarely, seizures may be falsely lateralized to the wrong hemisphere based on ictal scalp EEG recording. Although the exact mechanism of false lateralization remains unknown, theories include rapid seizure propagation, volume conduction, severe tissue damage leading to a reduction in the voltage on the side of the lesion, or obscured activity from deep sources such as the mesial or posterior temporal lobes

Methods: We present a case report of a 36-year-old male with long-standing drug-resistant epilepsy since 2011. Seizures were characterized by no warning, loss of awareness, language disturbance, and postictal paraphasias. MRI showed encephalomalacia in the right inferior posterior temporal region with unclear etiology. In 2019, he underwent responsive neurostimulation (RNS) implantation based on stereo EEG (sEEG) findings of bilateral independent hippocampal onset seizures, with a left subtemporal strip and right mesial temporal depth electrode targeting the hippocampus and the area of encephalomalacia, with minimal improvement in seizure frequency over 5 years.

Results: Despite prior recording of left hippocampal seizures during sEEG, over 5 years of long-term RNS recording, there were no left hippocampal onset seizures. For consideration of further surgery, the patient had a repeat EMU stay in 2024, where he underwent scalp EEG recording with concurrent RNS recording. A typical electroclinical seizure was captured, which localized to the left temporal chain on scalp EEG. However, a time-synced RNS long episode showed the right posterior hippocampal ictal onset. The scalp EEG recording was therefore considered a false lateralization. Given the long-term intracranial RNS recordings, structural imaging findings, and suboptimal response to RNS, the patient proceeded with a right anterior temporal lobectomy (ATL) in 2024. He remains seizure-free postoperatively, supporting the conclusion that scalp EEG had falsely lateralized seizure onset.

Conclusions: Misinterpretation of seizure lateralization can have serious clinical consequences, potentially leading to ineffective or misguided surgical intervention. This case highlights the potential for false lateralization of seizures on scalp EEG, especially when the focus lies in deep or posterior temporal regions. In such cases, chronic intracranial recordings such as those from RNS can provide critical localization data, particularly when scalp EEG and imaging are discordant

Funding: No funding was received in support of this abstract.