What Saluda Medical’s successful Evoke System trials means for Closed Loop Spinal Cord Stimulation

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On March 2021 by Brooker at al., the publication “ECAP-Controlled Closed-Loop Spinal Cord Stimulation Efficacy and Opioid Reduction Over 24-Months: Final Results of the Prospective, Multicenter, Open-Label Avalon Study'' in Pain Practice journal described the outcomes of a 50 patient trial using Saluda Medical’s closed-loop Spinal Cord Stimulation (SCS), also called the “Avalon” study. Patients with chronic back and/or leg pain (with or without previous back surgery) were recruited from 5 clinical sites in Australia. The prospective, multicenter, single-arm study was positive, reporting “responder rates (≥ 50% pain reduction) and high responder rates (≥ 80% pain reduction) for overall pain were 89.5% and 68.4%, respectively.” The authors conclude “Over a 24-month period, the Evoke closed-loop SCS maintained its therapeutic efficacy despite a marked reduction in opioid use and steady decrease in the need for reprogramming.” What does this result mean for the SCS field in general?

Saluda Medical is a pioneer in closed-loop SCS, but many companies including Medtronic are expected to develop their own systems. To understand what closed-loop SCS does, its useful to first understand how systems work without closed-loop. Conventional SCS is open loop (not closed loop): that means the device is programmed by a doctor (or adjusted by a patient) but then this program runs in a fixed and continuous way, until the doctor or patient makes the next adjustment. The program governs at what dose electrical stimulations are delivered to the spinal cord, these stimulation work by masking pain. With open-loop SCS the system is always adjusted to the patient, but the adjustment always involves a person (doctor, or patient). This conventional form of SCS is FDA approved for use in the USA (see list of approved devices). At this time, no closed-loop SCS device, included by Saluda Medical, is approved for use. One can be confident Saluda Medical is in discussion with the FDA on this matter.

With closed-loop SCS the device programming is changed not only by the doctor or patient, but by the SCS device itself. How does the SCS device decide how to change programming? In this case using a probe-and-sense technology called “ECAP '' which stands for Evoked-Compound-Action-Potential. All SCS devices implant a lead with electrodes on it near the spinal cord. The leads are long so can extend up and down along the spinal cord. This is true for closed-loop SCS devices as well, except some electrodes are used for stimulation and some are used for recording. A ECAP starts with one part of the SCS device delivering a small electrical stimulation to the spinal cord, that electrical stimulation activates (hence Evoked) the Action Potential in axons of the spinal cord (a bunch of them hence the Compound). The ECAP is then detected by a sensor on another end of the SCS lead. Now here’s the thing: the simulations that are provided to generate the ECAP are the same simulations that are provided for pain control. The only difference is that now the SCS device can also sense the ECAPS produced by that stimulation. The final explanatory piece is that in closed loop SCS the devices adjust the stimulation dose to produce an ECAP of an “ideal” size. Ideal means optimized pain control with minimal side effects. The trick is that, while a doctor or patient can only make adjustments every now and then, the closed-loop SCS device can make adjustments 100 times per second (about the rate of the electrical stimulation pulses).

Here is how the Brooker at all paper explained closed-loop SCS works:

“This system uses the measured ECAP (i.e., the patient’s neural response to electrical stimulation), in a feedback mechanism to provide consistent spinal cord activation. The feedback mechanism adjusts stimulation current continuously, millions of times per day and automatically to maintain a target ECAP amplitude during physiological changes and movement. By maintaining the neural response within a narrow range, abrupt changes in stimulation (over or under stimulation) resulting from the movement of the electrode with respect to the spinal cord during physiological changes and movement are minimized.”

So, what does the Booker et al. study mean for closed loop SCS? In their press release Saluda Medical emphasizes this is “the first peer reviewed SCS study to show responder and high responder rate improvement over time” with “~90% overall back and leg responder rate, ~68% high responder rate” and “~83% of patients reduced or eliminated opioid use”. This trial clearly provides evidence for closed loop SCS efficacy. And may contribute to eventual FDA clearance. But prior clinical trials on efficacy had already been published including in the prestigious Lancet Neurology : a multi-center, double-blind, parallel-arm, randomized controlled trial concluding “ECAP-controlled closed-loop stimulation provided significantly greater and more clinically meaningful pain relief up to 12 months than open-loop spinal cord stimulation.” And the neuromodulation community as a whole can already be considered convinced of both the principles of closed-loop SCS and that it provides clinical benefit (it “works”)- including multiple presentations at the 2022 NANS meeting. Indeed, other SCS companies, like Medtronic, are expected to develop their own systems.

With SCS (open and closed loop) ongoing research and development is not about “does it work” but how to increase efficacy and patient satisfaction. For example, a common side effect of SCS is paresthesia, a mild tingling that overlaps with the pain area. Some forms of SCS now allow “paresthesia free” SCS, which means no tingling. Closed-loop SCS, as currently proposed by Saluda Medical, is with paresthesia (essentially one cannot get the ECAPS without paresthesia). A key question driving doctor and patient preference (market share of FDA cleared devices) may be the relative benefits of conventional SCS with paresthesia, SCS without paresthesia, and (once FDA approved) closed loop SCS with paresthesia. Also, one might expect future developments that combine closed loop with paresthesia free or other new SCS technologies. So, in studying the SCS development closely (and these developments are coming faster and faster) it is doctor and patient preference that will determine relative success. And ongoing clinical trials (sponsored by companies) will try and establish these relative benefits.

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