Role of intraoperative neurophysiologic monitoring in lumbosacral spine fusion and instrumentation: a retrospective study.

Background: This study was carried out to determine the efficacy of IONM in detecting iatrogenic neural injury during pedicle screw insertion, in comparison to the efficacy of computerized tomography (CT) and direct visual inspection of adjacent nerve roots.

Methods: We reviewed the records of 86 patients, who had had 414 titanium pedicle screws inserted for posterior lumbar instrumentation. A standardized multimodality technique under total intravenous anesthesia was used. A relevant neurophysiologic change (surgical alert) was defined as a reduction in amplitude of at least 50% for somatosensory evoked potentials or at least 65% for transcranial electric motor evoked potentials (tcMEPs) compared with baseline. A stimulation threshold of 8 mA or less indicated that the screw was too close to the nerve root.

Results: Immediate feedback via evoked electromyography (EMG) using stimulating pedicle probes in appropriate muscle groups was suggestive of pedicle cortical bone compromise in 28 screws (6.7%). Twenty-one screws were removed and redirected. Four false-positive evoked EMGs in 4 patients were detected by direct visual inspection of the nerve roots and the pedicles, and the surgeon elected not to reposition the screws. None of those patients had postoperative neurologic deficit, and the postoperative CT confirmed the integrity of pedicles. Three false-negative EMGs in 3 patients were detected postoperatively by new neurologic deficits and abnormal CT (3.48%).

Conclusion: Intraoperative neurophysiologic monitoring is a valuable tool to add to the surgical skill and intraoperative fluoroscopy to protect neural tissue during pedicle screw instrumentation. However, postoperative CT is the ultimate test to determine the accuracy of positioning of the titanium screws. We propose a wake-up test in the operating room after extubation and urgent CT if the patient develops a new neurologic deficit to determine whether to reposition the screws in the same setting.