Dynamic Detection of Spinal Cord Position During Postural Changes Using Near-Infrared Reflectometry

Introduction Motion of the spinal cord relative to a spinal cord stimulator epidural electrode array can cause suboptimal stimulation: either noxious, inefficient, or insufficient. Adaptive stimulation attempts to mitigate these effects by modulating stimulation parameters in a position‐dependent fashion. Near‐infrared (NIR) reflectometry is demonstrated to provide real‐time direct measurement of spinal cord position at the site of stimulation, which can facilitate closed‐loop adaptive stimulation during static and dynamic motion states. Methods A miniature sensor array consisting of an NIR light emitting diode flanked by phototransistors potted in epoxy was placed in the dorsal epidural space of a human cadaver at the T8 level via laminotomy. Turgor of the subarachnoid space was maintained by intrathecal infusion of saline. NIR reflectance was measured as the cadaver was rotated about its longitudinal axis on a gantry. NIR reflectance was correlated with gantry position and velocity. Results NIR reflectometry suggests gravitational force is the primary determinant of cord position in static, ordinal positions. Under dynamic motion conditions, there was statistically significant cross‐correlation between reflectometry data and the tangential velocity squared, suggesting that centripetal force was the primary determinant of cord position as the gantry was rotated. Reflectometry data strongly correlated with a simple geometric model of anticipated spinal cord precession within the spinal canal. Conclusions Spinal cord position during dynamic motion has been shown to differ from static predictions due to additional influences such as centripetal force. These findings underscore limitations in extrapolating spinal cord position from surrogates such as body position or body acceleration at sites remote from the stimulating electrodes. NIR reflectometry offers a real‐time direct measure of spinal cord position in both static and dynamic motion states, which may facilitate closed‐loop adaptive stimulation applications.