Abstract Introduction Invasive electroencephalogram (EEG) monitoring is a critical tool for identifying ictal onset zones in patients undergoing evaluation for epilepsy surgery. Beyond seizure localization, intracranial electrodes can reveal physiologic brainwave patterns not typically accessible through scalp EEG alone. In this case, depth electrodes placed in the insular region demonstrated clear sleep spindle activity occurring simultaneously with spindles recorded on scalp channels. To our knowledge, sleep spindle activity directly recorded from the insula has not previously been reported. This observation provides a novel window into the physiology of thalamocortical oscillations during sleep. Report of case(s) A 36-year-old male with medically refractory epilepsy, experiencing multiple seizures monthly that started with an altered awareness and with occasional progression to generalized tonic-clonic convulsions, was admitted for invasive EEG monitoring to localize the ictal onset zone and guide further therapies. He had failed multiple antiepileptic medications and was considered at high risk of sudden unexplained death in epilepsy (SUDEP) since his seizures were not controlled. A 3T brain MRI was non-lesional except for a subtle finding of relatively increased FLAIR signal in the left insula compared with the right. He underwent invasive EEG monitoring with depth electrode implantation that included insular contacts. Ultimately, seizures were observed arising from the anterior polar and neocortical electrode contacts. Of note, during stage N2 sleep, characteristic spindle activity, waxing-waning bursts within the 11-16 Hz frequency range, was observed on the insular electrodes in addition to the usual expected localizations. The spindle activity recorded on the insula contacts temporally aligned with scalp-recorded spindles and showed similar durations and morphologies. The presence of spindle activity in the insula suggests potential involvement of this region in the propagation or modulation of thalamocortical sleep rhythms. Conclusion This case highlights a novel finding: sleep spindle activity recorded directly within the insula during invasive EEG monitoring. While sleep spindles are traditionally associated with thalamocortical circuits involving frontal and parietal cortices, this observation raises important questions regarding the broader network dynamics underlying spindle generation. Further study is warranted to determine the prevalence, distribution, and mechanistic significance of insular spindle involvement in sleep physiology. Support (if any)
Oster et al. (2026) studied this question.