Designing Neuromodulation Devices for Feedback Control

History and Scientific Evidence

First approved by the United States Food and Drug Administration (FDA) in 1989, SCS has become a common intervention for patients with chronic pain in their back or limbs and represents nearly 70% of all neuromodulation treatments ( ). Despite the significant therapeutic benefit of the open-loop therapy, patients use their remote programmers to adjust for changes in pain intensity or body posture. Changes in body position are known to affect the perceived stimulation effect ( ). At the same stimulation settings, the paresthesia is usually perceived more intensely in the supine position than in an upright position. This position dependency is thought to be due to changes in the thickness of the cerebrospinal fluid layer around the spinal cord rather than to dislocation of the electrode or changes in electrode impedance ( ). Automatic adjustment of stimulation intensity and other parameters such as electrode configuration to account for spinal cord movement would help maintain consistent therapy and lower the burden associated with frequent use of the remote programmer.

At least two feedback signals have been identified for closed-loop control of stimulation adjustments related to spinal cord movement. For adapting stimulation based on body posture, motion sensors such as accelerometers can be used to provide a straightforward feedback signal for the control policy ( ). Feedback based on accelerometry, however, only addresses therapy needs when defined patient positions change; it does not address spinal cord movement in a static position or rotational spine or body movement. Electrophysiological biomarkers have been investigated as a mechanism to monitor these aspects of spinal cord stimulation objectively. Recordings of the evoked compound action potential (ECAP) from epidural electrodes in humans have shown some potential as an indicator of pain and anatomy ( ). A correlation between ECAP magnitude and thresholds for stimulation was demonstrated, showing that ECAPs may provide appropriate feedback on therapy parameters. Additionally, the magnitude of the ECAP was correlated with the degree of coverage of the painful area. ECAP localization may help in finding the best position for electrode implantation. This feedback during the lead placement procedure has the potential to provide clinical data in real time to enable optimal and objective placement of the electrode with respect to the spinal cord, without having to rely solely on patient cues.

Technology Landscape

Medtronic RestoreSensor

The RestoreSensor system uses a three-axis accelerometer to sense patient body position and automatically adjust the stimulation settings ( Fig. 23.2 ). An initial calibration is required, during which the patient is positioned in multiple orientations (e.g., standing, lying prone, lying supine) and appropriate stimulation settings are identified for each position. As the position changes, preferred stimulation settings are found by adjusting the amplitudes of multiple programs. By using multiple programs, the parameters of amplitude, pulse width, rate, and electrodes can be tailored to a patient’s therapy needs. In a clinical study involving 79 patients implanted with the RestoreSensor system, 86.5% of patients achieved the primary objective of improved pain relief with no loss of convenience or improved convenience with no loss of pain relief using automatic position-adaptive stimulation compared to using conventional manual programming adjustment alone ( ). Additionally, position-adaptive stimulation demonstrated a statistically significant 41% reduction in the daily average number of programming button presses for amplitude adjustment compared with manual programming. The adverse event profiles in both treatment groups were similar, and no unanticipated adverse device effects were reported. The study demonstrated that automatic position-adaptive stimulation is safe and effective in providing benefits in terms of patient-reported improved pain relief and convenience compared with using manual programming adjustment alone.