Invasive EEG investigation of the insular cortex has been performed with increasing frequency since the mid-nineties, in various forms of focal drug-resistant epilepsies. These include patients with a clear-cut intra-insular epileptogenic lesion, such as a focal cortical dysplasia, as well as patients whose non-invasive pre-surgical evaluation suggests perisylvian epilepsy, temporal plus epilepsy, sleep hypermotor epilepsy, MRI-negative frontal, and parietal lobe epilepsies. SEEG is currently the preferred method to investigate the insula, using orthogonal, oblique, or a combination of both trajectories, with no evidence of higher risk of intracranial bleeding than in other brain regions. Intra-insular ictal EEG patterns are often characterized by a prolonged focal discharge restricted to one of the five insular gyri, militating for a dense enough sampling of the insular cortex in suspected insular epilepsies. SEEG also offers the potential to perform thermolesion of insular epileptogenic zones which, together with MRI-guided laser ablation, represent a possibly safer alternative treatment to open-skull surgical resection of the insula.

Approximately 70% of patients with insular epilepsy require invasive investigation prior to resective or ablative surgery. Two broad techniques can be employed to invasively sample the insula and insular epileptogenic network. The first and most commonly used technique, stereo-electroencephalography (SEEG), involves the placement of intracerebral electrodes through drill-holes under stereotactic conditions (with or without robotic assistance) in the insula and relevant peri-insular network targets. The open technique involves the placement of depth electrodes within the insular cortex following the opening of the Sylvian fissure, in addition to peri-Sylvian grids over the cortical convexities or strips over the fronto-temporal-basal lobes/interhemispheric space. While there are no guidelines to decide which method to resort to, there are relative advantages, disadvantages, and scenarios where each may be beneficial. The vast majority of centers favor the SEEG technique, as it is minimally invasive, is associated with the lowest morbidity, and is particular adapted for investigating insular epileptic networks that usually involve widespread multi-lobar anatomical sites and deep structures (e.g., cingulate, mesial temporal structures, etc.). SEEG is also well-suited for bilateral cases and cases that involve reoperations, both of which are not infrequent in insular epilepsy. Finally, SEEG is an appealing option in patients in whom minimally invasive ablation (laser ablation or radiofrequency ablation) is being considered. In SEEG, insular electrodes can be placed through a trans-opercular orthogonal approach and/or through an oblique parasagittal approach (trans-frontally and/or trans-parietally). The open technique, on the other hand, is particularly suited for patients with superficial lesional epilepsy in whom the epileptogenic zone is clearly unilateral but requires invasive functional mapping, such as dominant hemisphere temporal lobe epilepsy with suspected insular involvement.