Background and Aims: 
Spinal cord stimulation (SCS) is an established treatment for chronic pain; however, long-term success remains suboptimal.1,2 Current SCS therapies are fixed-output and do not account for large variation in electrical field strength due to changes in distance between the electrode and SC.3 We report data from two prospective studies: Evoke and Avalon.

Methods: 
In Avalon, 50 subjects were implanted and programmed in closed-loop; in Evoke, 134 subjects were randomized into open-loop (OL-SCS) or closed-loop (CL-SCS). ECAPs, a measure of SC activation, are recorded following each stimulation pulse in both groups (Figure 1). Each subject’s therapeutic window (TW) is determined individually as the ECAP amplitude range between sensation perception threshold and discomfort. Without measure of SC activation (eg, ECAPs), TW can only be based on perception of intensity; however, stimulation can produce variable SC activation (ECAP amplitude) as the electrode to SC distance varies with movement (Figure 2).

Results:
In Evoke, each subjects’ TW was determined in the clinic, along with the clinician prescribed level. There was no statistical difference between the two groups’ TWs (Figure 3); however, CL-SCS subjects spent significantly more time in the TW despite having equivalent therapeutic ranges (Figure 4). Long-term data showed similar percentages of stimuli in the TW (83%-97%; Figure 5).

Conclusion:
TW can be individually defined by ECAP amplitudes (measure of SC activation), removing the need to rely on subjective reports of intensity, which can vary over time and with movement.

DEFINING THE THERAPEUTIC WINDOW (TW) FOR SPINAL CORD STIMULATION (SCS) USING EVOKED COMPOUND ACTION POTENTIAL (ECAP) RECORDINGS - RESULTS FROM THE EVOKE STUDY