Intraoperative Neurophysiologic Methods for Spinal Cord Stimulator Placement Under General Anesthesia

Abstract

To demonstrate that spinal cord stimulators (SCSs) may be placed safely and accurately under general anesthesia (GA) and that the proposed evaluation method activates structures predominantly in the dorsal columns. Data were retrospectively analyzed from 172 electrodes implanted with spinal cord SCSs at the Lahey Clinic between September 2008 and July 2011. All patients had their SCS placed under GA. Electromyography was recorded from upper or lower limb muscle groups related to the placement of the stimulator electrode. Lateralization was performed based on electromyographic responses and electrode pairs stimulated. In a select group of patients, standard neurophysiologic tests, paired pulse, and collision studies were performed to demonstrate that the pain stimuli were activating the dorsal columns. One hundred fifty-five patients had standard thoracic or cervical SCS placement. Preoperatively this cohort of patients had a visual analog score (VAS) of 7.51 ± 1.93, while postoperatively the VAS was 3.63 ± 2.43 (a reduction of 52.11%). Based on the electromyographic recording technique, the electrodes were repositioned intraoperatively in 15.9% of patients. The recovery time (initial approximately 70 msec and complete approximately 150-300 msec) in both the paired-pulse tests and the collision studies showed that the stimulation used to elicit the compound muscle action potentials came from antidromic activation of the dorsal columns and not from the corticospinal tract. GA SCS is safe and appears to be at least as accurate and efficacious as using the awake SCS placement technique based on a 50% improvement in the VAS. In addition, the technique presented herein demonstrates that the test stimuli activate the same fiber tracts as that of the therapeutic stimulation.