Object: The aim of this observational clinical study was to analyze the impact of neurophysiological intraoperative monitoring (IOM) on the surgical procedure and to assess the benefits of such monitoring.
Methods: Data for 423 patients who underwent neurophysiological IOM with somatosensory evoked potentials and brainstem auditory evoked potentials during neurosurgical procedures were collected prospectively. The patients were classified into one of five groups according to the findings of IOM, the intervention following a monitoring alarm, and the patient's postoperative neurological condition. These groups were as follows: patients with true-positive findings with intervention (42 cases, 9.9%), those with true-positive findings without intervention (42 cases, 9.9%), those with false-positive findings (nine cases, 2.1%), those with false-negative findings (16 cases, 3.8%), and those with true-negative findings (314 cases, 74.2%). Different interventions followed an event identified with monitoring. These interventions were related to dissection in 17 cases, to perfusion pressure in 11, to a limitation of the surgical procedure in five, to vessel clipping in four, to vasospasm in three, and to retraction in one case. In one case the surgical procedure was abandoned. A critical analysis and cautious estimation of the interventions revealed that IOM was helpful in preventing a postoperative deficit in 5.2% of the monitored cases. CONCLUSIONS; For critical analysis of the benefits of IOM one must evaluate not only the findings of IOM and the patient's postoperative neurological condition but also the intraoperative findings and surgical interventions following a monitoring alarm. Evidence is presented that IOM is helpful in preventing a postoperative deficit.