Intraoperative evoked compound action potential measurements in the dorsal column to guide spinal cord stimulation lead placement

Abstract

Introduction: The positioning of spinal cord stimulation (SCS) leads in the epidural space to deliver therapy for chronic pain currently relies on intraoperative feedback from the patient. This feedback is not always reliable due to sedation, discomfort, and prone positioning of the patient. Excluding the need for patient feedback by performing objective intraoperative measurements to guide lead placement could be possible using the recording capabilities of closed loop SCS leads.
Objective: Assess the feasibility of performing intraoperative evoked compound action potential (ECAP) measurements in the spinal cord using stimulation on the dorsal root ganglion (DRG) corresponding to the painful area of the patient to guide SCS lead placement.
Methods: Intraoperative measurements were performed during placement of the SCS leads. The L3, L4 or L5 DRG was stimulated with a RF-needle and a recording electrode on the lead measured the neural activity in the dorsal column. The reference electrode was either on a subdermal needle or on one of the electrodes on the lead. The DRG was stimulated with a frequency of 10 Hz, a pulse width of 300 μs and a biphasic pulse type with negative polarity. By increasing the stimulation amplitude until ECAPs are visible, the activation threshold was determined, and by recording on different electrodes on the lead, propagation of the signal was validated. All data gathered during the measurements was processed and analyzed in MATLAB, and conduction velocities were calculated.
Results: We included eight patients, and in two out of eight patients it was possible to measure ECAPs in the epidural space when stimulating on the DRG. A measured signal was deemed an ECAP if certain characteristics were met. The morphology should match that of an ECAP. Also, an increase in the stimulation amplitude on the DRG should lead to a linear increase in the ECAP amplitude. Lastly, an ECAP propagates along the dorsal column. In many patients, the stimulation on the DRG did not activate enough fibers to create an ECAP that was measurable in the epidural space.
Discussion: There is no clear common denominator in the patients or stimulation parameters in the measurements where ECAPs were successfully measured in the epidural space, making specific improvements to the measurement protocol difficult. Age, gender or medication use seemed unrelated to whether the measurements succeeded. The experimental set-up could be improved by validating the distance between RF-needle and the DRG. Also, the use of sedation allows for an increased stimulation amplitude use, which could increase the amount of fibers activated to elicit an ECAP.
Conclusion: Stimulating the DRG with a RF-needle can lead to measurable ECAPs in the dorsal column, which could help guide lead placement without the need for feedback. However, the measurement protocol as it is now does not produce predictable results and should be improved if to continue with this study. Also, the need for this intraoperative procedure should be reevaluated since experienced surgeons estimate the lead localization quite accurately. In combination with 12 electrode leads covering 3 vertebrae, the rest of the coverage overlap can often be achieved during postoperative programming of the stimulation.