Epilepsy surgery failure is not uncommon, with several explanations having been proposed for its occurrence. Not recognizing an insular focus has been suggested to cause a non-negligible number of epilepsy surgery failures but remains to be adequately described.

We aim to detail cases of epilepsy surgery failure subsequently attributed to insular involvement.

We retrospectively identified patients investigated between 2004 and 2020 at the epilepsy monitoring units of two Canadian academic tertiary care centers. Included patients were evaluated at 18 years of age or over, had previously undergone one or multiple epilepsy surgeries with persistence/recurrence of epileptic activity post-operatively, and were subsequently determined to have an insular epileptogenic focus. Clinical, electrophysiological, neuroimaging, and surgical data were retrieved and synthesized.

We present 14 patients who were found to have insular epileptic activity post-surgery-failure as detected by intracranial EEG, MEG, or seizure improvement after insular resection. Half of the patients had manifestations evoking possible insular involvement prior to their first epilepsy surgery. Eight patients had initial surgeries targeting the temporal lobe, five had surgeries in the frontal lobe, and one had a parietal operculum resection. Seizure recurrence ranged from the immediate post-operative period to one year. The main modality used to confirm insular involvement was MEG (8/14). Eight patients underwent re-operations that included insular resection; five achieved a favorable post-operative outcome (Engel I or II) while three did not. In hindsight, eight patients likely presented temporal-plus epilepsy, whereas the remaining six likely had operculo-insular epilepsy.

Our series suggests that lowering our threshold for suspecting insular epilepsy may be necessary to diminish epilepsy surgery failures. Furthermore, uncovering insular involvement post-surgery-failure may allow for re-operations which may lead to good outcomes. Our findings support MEG as being the most promising non-invasive method for detecting insular involvement after surgery failure.