The role of the insula in temporal and temporal plus epilepsies has been poorly investigated. The insula is buried deep within the lateral sulcus and has dense connections with neighbored regions, thus limiting the usefulness of available non-invasive diagnostic tools, such as scalp EEG, SPECT, and 18FDG-PET. Also, clinical features related to the involvement of the insula might be misleading, thus making it mandatory to use invasive EEG recordings in most cases. In fact, only a primary involvement of the insula in the epileptogenic network may require to include this area in the resection to achieve seizure freedom. Using modern neurosurgical techniques, insular epilepsy surgery has proved to be safe and beneficial in carefully investigated cases; however, its efficacy remains to be fully elucidated.

The insula was described more than two centuries ago, in 1809, by the anatomist Johann Christian Reil. However, the functional anatomy of the insular cortex and the importance of the insula in psychiatric and neurologic pathological conditions remained obscure for a long period after the initial description of Reil. The location and the complex functional organization of the insula explain, for instance, the difficulty researchers and clinicians had in defining its contribution in temporal lobe epilepsy. This introductory chapter reviews the important steps that led to a better understanding of the role of the insula in human focal epilepsy.

Structural brain MRI is an indispensable tool in the diagnosis and management of patients with insular epilepsy. Similar to the pathology of other focal epilepsies MRI lesions span from the obvious to the occult cortical malformations, along with vascular malformations, neoplasms, encephaloclastic lesions (e.g., after stroke or trauma), and less frequent etiologies including Rasmussen encephalitis. The incidence of MRI-negative focal epilepsy arising from the insula is unknown. Appropriate epilepsy MRI protocol is a necessary, but not sufficient requirement for the structural exploration of the insula. Interpretation of the study strongly depends on the reader’s expertise in imaging of epilepsy. Advances in image processing and multimodality imaging have increased the diagnostic yield of MRI studies in nonlesional insular epilepsies and in particular in patients with occult focal cortical dysplasia, including bottom of the sulcus dysplasia. The utility of emerging approaches, such as 7-Tesla and quantitative MRI studies, in the evaluation of insular epilepsies is now being explored. Finally, the importance of re-interpreting each visually identifiable MRI lesion or postprocessing MRI finding in the context of the individual clinical presentation cannot be overemphasized.

The insular cortex is gaining attention in the epilepsy literature; however, characteristics of insular epilepsy on non-invasive electrophysiological studies are still being defined. In this chapter, we review this emerging literature with a focus on electroencephalography and present the ictal and interictal findings of 106 patients collected in the literature. Despite the location of the insula in the depth of the Sylvian fissure, a majority of patients have ictal and interictal epileptiform abnormalities on scalp EEG, which are most commonly found in the frontal or fronto-temporal electrodes. The distribution of these findings follows an antero-posterior gradient in keeping with the connectivity pathways of the insular cortex. Multifocal abnormalities are common in the pediatric population and should not preclude surgical evaluation. Evidence is increasingly favoring magnetoencephalography as a complementary modality to EEG and its use will likely be more prominent. Scarce data is available on the utility of high-density EEG, and more studies are thus needed.