31. Intraoperative neurophysiological monitoring for lesions in infratentorial surgery with particular emphasis on long tract functions

Abstract

During the surgical removal of infratentorial lesions intraoperative neuromonitoring is mostly focused on cranial nerves and brainstem auditory potentials. Despite the known risk of perforating vessel injury during microdissection within the vicinity of the brainstem, there are few reports about intraoperative neuromonitoring with somatosensory (SEP) and motor (MEP) evoked potentials assessing the medial lemniscus and corticospinal tract. This study analyses the occurrence of intraoperative changes in MEP and SEP with regard to lesion location and postoperative neurological outcome. 210 patients (49 ± 13 years, 109 female) undergoing surgeries within the vicinity of the cerebello-pontine angle (77), cerebellum (59), brainstem (40), skull base tumours (26) and others (5) were analysed. 171/210 (81.4%) surgeries were uneventful for long-tract monitoring. 9/39 SEP and MEP-alterations (23%) were transient and without neurological long-term sequelae. Permanent deterioration was seen in 19/39 patients (49%), of which 4 were related to tumour dissection and followed by slight permanent neurological deficit. 11 patients (28%) encountered losses of at least one modality, which were related to surgical microdissection within the vicinity of the brainstem in 9 patients. 4/9 patients suffered a severe long-term deficit. Overall, positive predictive value for long tract neuromonitoring was 0.394, negative predictive value 0.994, sensitivity 0.934 and specificity 0.886. 28/39 (71%) SEP- and MEP-alterations occurred in 66 brainstem respective skull base tumours compared to 11/39 (29%) alterations in all other 144 locations. Tumour location and alterations in intraoperative neuromonitoring significantly correlated with patient outcome (p < 0.001, chi-square test). In summary, long tract monitoring with SEPs and MEPs in infratentorial surgeries has a high sensitivity and negative predictive value indicating postoperative neurological status. It is recommended especially in those surgeries where microdissection within the vicinity of the brainstem might lead to injury of perforating vessels and subsequent perfusion deficit within the brainstem.