Sustained Long-Term Outcomes With Closed-Loop Spinal Cord Stimulation: 12-Month Results of the Prospective, Multicenter, Open-Label Avalon Study

Abstract

Background: Spinal cord stimulation (SCS) activates the dorsal column fibers using electrical stimuli. Current SCS systems function in fixed-output mode, delivering the same stimulus regardless of spinal cord (SC) activation.

Objective: To present long-term outcomes of a novel closed-loop SCS system that aims to maintain the SC activation near a set target level and within a therapeutic window for each patient. SC activation is measured through the evoked compound action potential (ECAP) generated by each stimulus pulse.

Methods: Fifty patients with lower back and/or leg pain who were successfully trialed received a permanent system (Evoke; Saluda Medical, Sydney, Australia). Ratings of pain (visual analog scale), quality of life, function, sleep, and medication use were collected at baseline and at each visit. SC activation levels were reported in summary statistics. The therapeutic window for each individual patient was defined as the range of ECAP amplitudes between sensation threshold and uncomfortably strong stimulation.

Results: At 12 mo, the proportion of patients with ≥50% relief was 76.9% (back), 79.3% (leg), and 81.4% (overall), and the proportion with ≥80% pain relief was 56.4% (back), 58.6% (leg), and 53.5% (overall). Patients spent a median of 84.9% of their time with stimulation in their therapeutic window, and 68.8% (22/32) eliminated or reduced their opioid intake. Statistically significant improvements in secondary outcomes were observed.

Conclusion: The majority of patients experienced more than 80% pain relief with stable SC activation, as measured by ECAP amplitude at 12 mo, providing evidence for the long-term effectiveness of the Evoke closed-loop SCS system.

Keywords: Action potentials; Back pain; Chronic pain; Electric stimulation; Feedback; Pain management; Spinal cord stimulation.

Graphical Abstract

FIGURE 1.

Schematic representation of an ECAP. ECAPs have a well-defined shape with 3 peaks: 2 positive and 1 negative, labeled P1, N1, and P2, in order of appearance. In simple terms, they are the combined electrical field generated by the action potentials of stimulated fibers at the recording site. The first P1 peak stems from capacitive coupling between the inside and outside of the fibers and is caused by the incoming action potential. The N1 and P2 peaks result from ionic flow (sodium [Na+] and potassium [K+]) in and out of the fibers that form the well-known action potential. As the measurement is done outside of the fibers, the polarity of the peaks is reversed compared with intracellular recordings often depicted in textbooks. ECAP, evoked compound action potential.

FIGURE 2.

Patient progression through the study.

FIGURE 3.

Individual patient responses for back pain VAS reduction at 12 mo. VAS, visual analog scale.

FIGURE 4.

Mean VAS ratings over time for A, low back pain, B, leg pain, and C, overall pain for permanently implanted patients. *P < .0001. Error bars represent the standard error of the mean. VAS, visual analog scale.

FIGURE 5.

Responder and high responder rates for overall pain over time.

FIGURE 6.

Proportion of patients reporting each level of disability on the Oswestry Disability Index over time for permanently implanted patients.

FIGURE 7.

A, Mean daily use (MME/d) at baseline and 12 mo for all patients on opioids at baseline. B, Proportion of patients (who were taking opioids at baseline) reducing or eliminating opioid medications over time. C, Mean daily use (MME/d) at baseline and 12 mo of all patients on high-dose opioids (>50 MME/d) at baseline. MME, morphine milligram equivalent.

FIGURE 8.

Median percent stimuli below, within, and above the therapeutic window from the 3- to 12-mo visits. TW, therapeutic window. The reader will note that, unlike the average values, median values do not add up to 100%.

FIGURE 9.

Number of unscheduled programming visits per patient per month between each study time point up to 12 mo.