Intraoperative functional mapping using direct electrical stimulations. Methodological considerations

The interindividual anatomo-functional variability of the central nervous system implies that brain surgery within eloquent regions may induce neurological sequelae. Consequently, several methods of functional mapping were developped, both preoperative non-invasive neuro-functional imaging and intraoperative cortico-subcortical electrical stimulations. While this technique was reported as safe, accurate, reliable and reproducible in the recent literature, a rigorous methodology is nevertheless mandatory in order to avoid any error in the detection of the so-called eloquent structures. Indeed, an erroneous mapping could lead to prematurely interrupting the resection (false positive), or to generate a postoperative permanent deficit due to the removal of critical areas interpreted as non-essential to function (false negative). The goal of the present review is to recall the electrophysiological principles of direct brain stimulations, and to consider the selection of stimulation parameters according to a theoretical approach, in order to adapt in practice the methods to each patient. The results reported in the literature are then analyzed, concerning the clinico-surgical contribution of intraoperative electrical mapping (in terms of extent of surgical indications, minimization of risk of permanent deficit, and quality of resection), their methodological interest (e.g. validation of neurofunctional imaging techniques), and their contribution to neurosciences (better understanding of the pathophysiology of brain structures, of the connectivity, and of the dynamic mechanisms underlying plasticity).