Background and Aims:
The Evoke Compound Action Potential (ECAP) is the measured sum of action potentials from multiple nerve fibers, elicited by stimulation of the spinal cord (SC). Normal movement and physiological activity, with fixed-output stimulation of the SC, lead to inconsistent fiber activation and, thus, variable inhibition of pain processing pathways. ECAPs, a measure of SC activation, can be utilized to control this variability during SC stimulation (SCS). Results of a multicenter, prospective, randomized, double-blinded study comparing closed-loop SCS (CL-SCS) with fixed-output, open-loop SCS (OL-SCS) show differences in SC activation and clinical outcomes (clinicaltrials.gov #NCT02924129).

Methods:
134 subjects with chronic back and leg pain were enrolled, with the primary composite endpoint as percentage of responders in overall pain without an increase in medications. ECAPs were recorded and used to characterize electrophysiological differences between CL-SCS and OL-SCS.

Results:
At the primary endpoint, CL-SCS is superior to OL-SCS in overall pain responders (Figure 1). Additionally, the CL-SCS group clearly showed higher activation levels during daily-use (Figure 2) and less variability during physiological changes and movement (Figure 3). The variability in perceived changes in intensity was also significantly different between the cohorts (Figure 4).

Conclusions:
Subjects with CL-SCS show more-consistent SC activation and sustained clinical outcomes, with minimal changes in sensation. Improved outcomes when utilizing detection and compensation for physiological changes illustrate the importance of closed-loop SCS. This may lead to future developments in activity/physiological monitoring, furthering understanding and improving long-term outcomes.

EVOKE COMPOUND ACTION POTENTIAL (ECAP) RECORDINGS PROVIDE INSIGHT INTO THE VARIABILITY OF SPINAL CORD (SC) ACTIVATION DURING SPINAL CORD STIMULATION (SCS)