Abstracts

RATIONALE: Previous reports on insular cortex stimulation in humans have revealed a rostro-caudal organization of responses with visceral and somesthetic symptoms elicited anteriorly and posteriorly, respectively. Based on numerous anatomical and microelectrodes studies in monkeys, the involvement of the insular cortex in processing painful inputs has been increasingly contemplated. Functional imaging studies have confirmed that the insular cortex is involved in pain integration in humans. Except in the first two patients of this series, included in a previous study, no pain sensation has been reported during insular cortex stimulation as well as during stimulation of the convexity of the human cortex.
METHODS: Fourty-three patients with temporal lobe epilepsy (24 right TLE, 19 left TLE), undergoing an intracranial investigation, were included in this study. Language lateralization was assessed by intracarotid amobarbital test in all patients. The insular cortex was explored through trans-opercular electrodes implanted orthogonally to the mid-saggital plane (1 to 3 per patient). Square pulse of constant polarity were applied between the two deepest adjacent contact of the transopercular electrodes, 49 electrodes exploring the right insular cortex and 44 the left insular cortex. Fifty-two low frequency (LFS : 1 Hz) and 135 high frequency (HFS : 50 Hz) stimulations were performed.
RESULTS: Painful sensations, from mild to intolerable, were evoked in 14 patients, mainly during HFS. Painful sensations were evoked in the upper posterior part of the insular cortex, mostly in the non-dominant insular cortex. Responses showed a somatotopic organization within the posterior insular cortex.
CONCLUSIONS: Our data are coherent with connectivity and microelectrode studies in monkeys which show that the posterior part of the insular cortex is involved in processing noxious stimuli. Insular sites where painful sensations were elicited in our study are caudal to mean peaks of activation reported during PET studies in humans using painful somesthetic stimuli. The non dominant lateralization of sites where painful sensations were evoked is coherent with the phylogenetically known role of the right hemisphere in species survival. Our data suggest that painful symptoms during seizures may sometimes reflect the propagation of the epileptic discharge to the posterior part of the insular cortex. Finally, one should be cautious when stimulating the posterior part of the non dominant insular cortex, pain sensations occurring in one third of patients.