Figure 4. Device Performance, Dose-Response Relationship, and Therapeutic Window Differences Between Closed- and Open-Loop SCS.

A, Device performance is defined as the ability of the spinal cord stimulation (SCS) device to adhere to the prescribed neural response. Owing to the ever-changing coupling of the electrical field to the target neural tissue, open-loop systems are not capable of continuously adhering to the prescribed neural response. A consistent neural response at the prescribed level may only be achieved with a physiological closed-loop control system that continually adjusts on every stimulation pulse. This figure depicts the neural response of an Evoke study patient performing the same series of postures in clinic in both the open-loop and closed-loop stimulation modes and demonstrates how closed-loop minimizes the error between the prescribed target neural response and the observed elicited neural response pulse by pulse. In this patient, the elicited neural response deviated 22.1 times more from the target neural response with open-loop compared with closed-loop (66.4 μV deviation vs 3.0 μV deviation, respectively). B, In pharmacology, the importance of characterizing the dose-response relationship has long been understood because the concentration of a drug at its site of action controls its effect. In SCS, the charge (stimulation dose [μC/pulse]) produces a neural response (evoked compound action potentials [ECAP]; μV), which results in a clinical effect. ECAP amplitude (ie, a measure of the number of fibers activated) increases with increasing charge.⁹ In this study, dose-response data (ie, ECAP amplitude and charge) were collected during scheduled programming visits in a sitting position at patients’ perception threshold, comfort, and maximum. Patient perception threshold to maximum defined the patients’ therapeutic window. The mean (standard error) dose-response for the Evoke study patients at 24 months is presented. C, On average, closed-loop patients were nearly always within the therapeutic window (median [IQR], 93.9% [48.0%-98.9%]), while open-loop patients were only with the window half of the time (median [IQR], 46.1% [25.9%-71.9%]), and half of the time below threshold (median [IQR], 49.3% [22.7%-74.1%]); the differences in time spent with, below, and above the therapeutic window were significant between groups (P < .05).

^aSignificant difference between treatment groups (P < .05).