Identification of somatosensory pathways by focal-cooling-induced changes of somatosensory evoked potentials and EEG-activity - an experimental study

Object. Function-preserving neurosurgery requires methods to identify funct ionally important CNS-areas intraoperatively. We investigated whether a com bination of focal cerebro-cortical cooling and monitoring of somatosensory evoked potentials (SEP) is suited for this task, i.e. whether it is able to outline structures belonging to the somatosensory pathway. Methods. In 17 Wistar rats the somatosensory cortex was focally cooled by 2 0 degrees C below the initial tissue temperature for periods of five minute s. A cryoprobe with a tip diameter of 3 mm was used and tissue temperatures were measured below and at different distances to the cryoprobe. Tibial ne rve evoked SEPs and EEG-spectra were recorded continuously. Results. During cortical cooling the SEP-responses showed a marked delay an d amplitude increase of the cortically generated components P13 and N18 and a small latency increase of the subcortically generated wave III. EEG-spec tra were depressed mainly in the low frequency range. All cooling effects w ere reversible and in light- as well as electron-microscopic examinations n o tissue damage was found. Conclusions. Focal cooling of the cortex induces easily recognizable and re versible changes of the bio-electrical activity without causing any histolo gical damage. Therefore the method seems suitable for identifying eloquent areas. It can be expected that clinical application of the cooling techniqu e in combination with intraoperative electrophysiological monitoring will b e helpful to further lower the risk of neurosurgical operations. We propose that cooling mainly interferes with the synaptic transmission wi thin the somatosensory cortex, because the observed amplitude increase can be explained by cold-induced depression of inhibitory cortical activity (di sinhibition).