Effects of Kilohertz Frequency on Paresthesia Perception Thresholds in Spinal Cord Stimulation.

Abstract

Background: Paresthesia-based spinal cord stimulation (SCS) depends upon dorsal column (DC) fiber activation to engage pain-relieving neural mechanisms. However, the mechanisms for 10-kHz paresthesia-free SCS have not been fully elucidated. Preclinical work has shown selective drive of inhibitory dorsal horn neurons, while other hypotheses suggest that DC fibers may be activated. To provide clinical data for guiding mechanism work, we analyzed paresthesia perception thresholds (PPT) over a range of low to high kHz frequency and compared those values to the stimulation parameters from the therapeutic 10-kHz SCS programs used by patients.

Objective: The goal of this study was to provide clinically relevant stimulation parameters for translational mechanism work.

Study design: Retrospective chart review of technical data collected during baseline evaluation from two prospective clinical studies.

Setting: Acute outpatient follow-up.

Methods: Data were extracted from study files of enrolled patients who had used fully implanted SCS for at least 3 months with leads positioned in the epidural space at the T8-T11 vertebral levels to treat their chronic intractable back and/or leg pain. PPTs had been measured using a bipole program at 10 kHz at pulse width (PW) = 30 µs, and at 50 Hz, 500 Hz, 1 kHz, and 5 kHz at PW = 80 µs. Therapeutic stimulation amplitudes for 10 kHz at 30 µs were obtained from patients' IPG log files at the time of study enrollment.

Results: PPTs were obtained from 23 patients with failed back surgery syndrome (11 M/ 12 F; 60 ± 15 years old). PPTs at PW = 80 µs were PPT (50 Hz) = 7.9 (5.7 - 9.7) mA, PPT (500 Hz) = 7.0 (5.2 - 9.1) mA, PPT(1 kHz) = 7.0 (5.5 --8.5) mA, and PPT (5 kHz) = 6.1 (4.1- 7.9) mA; all higher frequencies had statistically significantly lower PPTs than PPT(50 Hz at 80 µs). For 10 kHz at 30 µs, the PPT was higher than 15 mA for 13 (56%) of the subjects; for the remaining 44%, the PPT =  8.3  ± 4.0 mA was statistically significantly larger than the therapeutic stimulation pulse amplitudes = 2.4 ± 0.4 mA.

Limitations: Retrospective chart review, small number of patients.

Conclusions: Therapeutic 10-kHz SCS uses stimulation amplitudes far lower than the PPT, providing evidence that therapeutic 10-kHz SCS does not activate dorsal column axons.  Additionally, the PPT decreases with increasing kHz frequency, suggesting that a presumed asynchronous pattern of activation from kHz stimulation does not raise the threshold at which sensation occurs.