Safety Versus Efficacy of Spinal Cord Stimulation for the Generation of Motor-Evoked Potentials in the Rat

The relative safety and efficacy of direct versus indirect methods of spinal cord stimulation for the production of descending motor-evoked responses was studied in pentobarbital-anesthetized rats (n = 39). Electrical stimuli were delivered for 1 h, either directly to the cord dorsum using silver ball electrodes or indirectly through jeweler's screws implanted in the intact laminae. Compound muscle action potentials (CMAPs) were recorded differentially in the quadriceps and evaluated for their morphology and reproducibility. The traumatic effects of stimulation were assessed using intraoperative somatosensory-evoked potentials, blinded neurological examinations for 2 weeks postoperatively, and histopathological and neurochemical analyses in postmortem spinal tissues. In separate experiments, the neural substrates of the muscle-evoked response to indirect cord stimulation were examined. Direct, epidural stimulation of the spinal cord at intensities sufficient to elicit reproducible CMAPs consistently resulted in mild behavioral deficits (13 of 18 animals) that were accompanied by postmortem changes in spinal histology and chemistry. Some of these behavioral deficits (5 of 13 animals) were resolved at 2 weeks. There was rarely an early sign of motor or sensory conduction derangement in these animals. In 2 animals with severe behavioral dysfunction, the somatosensory-evoked response was abolished immediately after spinal stimulation. However, CMAP responses were unaltered. Examination of the strength–duration relationship for the production of threshold responses to translaminar constant current stimulation, as well as experiments using selective transection of the dorsal columns, revealed the CMAP responses to be neurally mediated and conducted through the cord independent of the ascending sensory tracts that mediate the rat's somatosensory-evoked response. Data are discussed in terms of the potential experimental usefulness of CMAPs elicited by indirect dorsal spinal stimulation.