Spinal Cord Stimulation With "Conventional Clinical" and Higher Frequencies on Activity and Responses of Spinal Neurons to Noxious Stimuli: An Animal Study.

OBJECTIVES: Spinal cord stimulation (SCS) at both conventional and higher frequencies may effectively reduce pain, but optimal parameters need to be established. This study investigated how SCS at different frequencies and pulse widths acutely modulates nociceptive activity of wide dynamic range (WDR) and high threshold (HT) dorsal horn neurons in rats at a stimulus amplitude that influences both local circuits and dorsal column fibers.

MATERIALS AND METHODS: L2 -L3 and L6 -S2 spinal segments were exposed for SCS and spinal neuronal recordings, respectively. Responses to pinch of a hindpaw were recorded before and after SCS (40 or 200 µsec pulse width at 50, 500, 1 kHz and 10 kHz, amplitude: 90% of motor threshold) for 5 or 20 min. Pinch responses were tested within 30 s after SCS ceased (first pinch) and at ∼4 min intervals until response recovery.

RESULTS: 1) SCS for 5 min suppressed averaged first pinch responses, except for 40 µsec/50 Hz. 2) Only SCS with 40 µs/1 kHz suppressed more spinal neurons than 200 µsec/50 Hz. 3) All SCS parameters at 5 min increased pinch responses for a small population of cells, with the incidence being greater for WDR than for HT neurons. 4) SCS at 1 kHz (40 or 200 µsec) for 20 min reduced the response to the second pinch as compared with baseline responses. In addition, no neurons exhibited increased pinch responses.

CONCLUSIONS: Compared with a typical low frequency SCS (200 µs/50 Hz) or high-frequency SCS at 10 kHz, at an amplitude designed to influence both local spinal circuits and dorsal column fiber tracts, 1 kHz SCS suppressed nociceptive responses of more spinal neurons and/or demonstrated longer persisting suppressive effects. SCS at 1 kHz surpassed both low-frequency (50 Hz) and high-frequency (10 kHz) SCS application in this normal animal model.