Neurophysiologic monitoring of spinal nerve root function during instrumented posterior lumbar spine surgery

Retrospective review of 61 consecutive patients.

To determine the effectiveness of combining intraoperative monitoring of both spontaneous electromyographic activity and compound muscle action potential response to stimulation for detecting a perforation of the pedicle cortex irritation of nerve root during lumbar spine fusion surgery.

The complication rate from instrumentation used with lumbar spine fusion varies from 1 to 33%. To prevent neurologic complications, several monitoring techniques have been used to alert surgeons to possible neurologic damage being introduced during nerve decompression or placement of instrumentation with spine procedures. Because of different sensitivities, one monitoring technique may not be as effective for preventing complications as a combination of techniques.

Sixty-one consecutive patients who underwent instrumented posterior lumbar fusions received continuous electromyographic monitoring and stimulus-evoked electromyographic monitoring. A significant neurophysiologic event was signaled by sustained neurotonic electromyographic activity, prompting an alert and a pause in the surgical manipulations that precipitated the activity. After insertion of the transpedicular screws, the integrity of the pedicle cortex was tested by stimulating each screw head and recording compound muscle action potentials. In the presence of a pedicle breach, stimulus intensities below 7 mA were sufficient to evoke compound muscle action potentials from the muscle group innervated by the adjacent spinal nerve root, prompting a surgical alert and subsequent repositioning of the screw.

Fourteen significant neurophysiologic events occurred in 13 of 61 patients (21%). Sustained neurotonic electromyographic discharges occurred in 5 of 40 patients during placement of interbody fusion cages, in 2 patients during placement of transpedicular screws, and in 1 patient during tightening of rods. On pedicle screw stimulation, breaches of the pedicle cortex were detected in 6 patients. After surgery, no new neurologic deficits were found in 60 of the 61 patients. One patient who experienced temporary paraparesis had sustained neurotonic electromyographic discharges during retraction of the thecal sac and distraction of the disc space before placement of the cage.

These results suggest that intraoperative electromyographic monitoring provides a real-time measure of impending spinal nerve root injury during instrumented posterior lumbar fusion, allowing for timely intervention and minimization of negative postoperative sequela.