Evolution of spinal evoked compound action potential thresholds, visual motor thresholds, and impedances in a rodent spared nerve injury model

Introduction: The mechanisms of spinal cord stimulation (SCS) on neuropathic pain are commonly studied using the spared nerve injury (SNI) model, with stimulation amplitudes typically programed relative to the visual motor threshold (vMT). Recent work explored the relationship between vMTs and spinal evoked compound action potential thresholds (ECAPTs)—a sensed measure of neural activation—in SNI rodents to better translate towards clinical dosing. However, changes across chronic healing beyond two days and pain states is unknown. Methods: This study tracked ECAPs through a traditional SNI-SCS approach, where nine rats were implanted with an SCS lead to evaluate effects of acute healing (days 0 to 1), chronic healing (days 1 to 7), nerve injury (days 7 to 14), and continuous SCS (days 14 to 16) using differential target multiplexed programing (DTMP). Results: ECAPT:vMT ratios significantly increased on subsequent recordings from day 0 through day 14 (i.e., post-injury), but not between days 14 and 16 (after SCS), across anesthesia states, or SCS pulse widths. On average, ECAPT:vMT increased from 35 ± 2% (mean ± S.E.) on implantation day to 54 ± 1% on day 16. Discussion: Future studies may use this approach to further elucidate the effects of chronic pain and SCS on the spinal ECAP.