The Impact of Electrical Charge Delivery on Inhibition of Mechanical Hypersensitivity in Nerve-Injured Rats by Sub-Sensory Threshold Spinal Cord Stimulation

Abstract

Objectives: Spinal cord stimulation (SCS) represents an important neurostimulation therapy for pain. A new ultra-high frequency (10,000 Hz) SCS paradigm has shown improved pain relief without eliciting paresthesia. We aim to determine whether sub-sensory threshold SCS of lower frequencies also can inhibit mechanical hypersensitivity in nerve-injured rats and examine how electric charge delivery of stimulation may affect pain inhibition by different patterns of subthreshold SCS.

Materials and methods: We used a custom-made quadripolar electrode (Medtronic Inc., Minneapolis, MN, USA) to provide bipolar SCS epidurally at the T10 to T12 vertebral level. According to previous findings, SCS was tested at 40% of the motor threshold, which is considered to be a sub-sensory threshold intensity in rats. Paw withdrawal thresholds to punctate mechanical stimulation were measured before and after SCS in rats that received an L5 spinal nerve ligation.

Results: Both 10,000 Hz (10 kHz, 0.024 msec) and lower frequencies (200 Hz, 1 msec; 500 Hz, 0.5 msec; 1200 Hz; 0.2 msec) of subthreshold SCS (120 min) attenuated mechanical hypersensitivity, as indicated by increased paw withdrawal thresholds after stimulation in spinal nerve ligation rats. Pain inhibition from different patterns of subthreshold SCS was not governed by individual stimulation parameters. However, correlation analysis suggests that pain inhibition from 10 kHz subthreshold SCS in individual animals was positively correlated with the electric charges delivered per second (electrical dose).

Conclusions: Inhibition of neuropathic mechanical hypersensitivity can be achieved with low-frequency subthreshold SCS by optimizing the electric charge delivery, which may affect the effect of SCS in individual animals.

Keywords: Electrical charge; nerve injury; pain; rat; spinal cord stimulation.

Figure 1.. Experimental setup and protocol.

(A) Schematic diagram illustrating the experimental setup. The miniature SCS lead with four contacts (Medtronic, Minneapolis, MN) was implanted epidurally over the dorsal spinal cord at the T10–T12 vertebral level. Paw withdrawal threshold (PWT) was measured in response to mechanical stimuli (von Frey filaments, 0.38–13.1 g) applied to the mid-plantar area of the hind paw. (B) Illustration of the experimental procedure. In Week-1 SCS (days 14–16 post-injury), rats received one pattern of SCS or sham stimulation on three consecutive days (120 minutes, 1 session/day) at 40% motor threshold (MoT) intensity. In Week-2 SCS (days 21–23 post-injury), the pattern of SCS was changed.

Figure 2.. Time course of changes in paw withdrawal threshold (PWT) after different patterns of subthreshold SCS.

(A-F) Changes in PWT on the ipsilateral hind paw in nerve-injured rats (days 14–28 post-injury) before and after 50 Hz SCS (0.2 ms, 40% MoT, n=10, A), 200 Hz SCS (1 ms, 40% MoT, n=11, B), 500 Hz SCS (0.5 ms, 40% MoT, n=11, C), 1200 Hz SCS (0.2 ms, 40% MoT, n=11, D), 10 kHz SCS (0.024 ms, 40% MoT, n=11, E), or sham stimulation (n=10, F). SCS or sham stimulation was applied for 3 consecutive days (120 minutes/session). Data from animals that received sham stimulation in different studies were combined for analysis. *P < 0.05, **P < 0.01, ***P < 0.001 versus pre-SCS on day 1. One-way repeated measures analysis of variance with Bonferroni post hoc test. Data are expressed as mean ± SEM.

Figure 3.. Effects of different patterns of subthreshold SCS on mechanical hypersensitivity in rats after nerve injury.

(A-F) PWTs at pre-SCS, intra-SCS (averaged PWTs at 30, 60, and 90 minutes after initiating SCS), and post-SCS (averaged PWTs at 0, 30, and 60 minutes after cession of SCS) in rats treated with 50 Hz SCS (0.2 ms, n=10, A), 200 Hz SCS (1 ms, n=11, B), 500 Hz SCS (0.5 ms, n=11, C), 1200 Hz SCS (0.2 ms, n=11, D), 10 kHz SCS (0.024 ms, n=11, E), or sham stimulation (n=10, F). Data from animals that received sham stimulation in different studies were combined for analysis. *P < 0.05, **P < 0.01, ***P < 0.001 versus pre-SCS on day 1. One-way repeated measures analysis of variance with Bonferroni post hoc test. Data are expressed as mean ± SEM.

Figure 4.. Comparisons of pain inhibition and charge delivery by different patterns of subthreshold SCS.

(A) The maximum possible effect (MPE) of SCS was calculated at 30, 60, and 90 minutes after initiation on each treatment day (120 minutes, 1 session/day, 40% MoT). Then %MPE at each of these times was averaged for each group: 50 Hz (0.2 ms, n=10), 200 Hz (1 ms, n=11), 500 Hz (0.5 ms, n=11), 1200 Hz (0.2 ms, n=11), 10 kHz (0.024 ms, n=11), and sham stimulation (n=10). *P < 0.05, **P < 0.01, ***P < 0.001 versus sham stimulation. ##P < 0.01 versus 50 Hz. (B) Charge-per-pulse (pulse intensity) in each group was calculated as: pulse width × 40% MoT. (C) Charge-per-second (pulse density) was calculated as: charge-per-pulse × frequency. 1 mA is equal to the flow of 1 millicoulomb (mC) of electric charge in 1 second. *P < 0.05, ***P < 0.001 versus 10 kHz. One-way analysis of variance with Fisher’s LSD post hoc test. Data are expressed as mean ± SEM. MoT, motor threshold.

Figure 5.. Correlation between pain inhibition and charge delivery per second by subthreshold SCS.

The maximum possible effect (MPE) of SCS was calculated at 30, 60, and 90 minutes after initiation (intra-SCS %MPE, 120 minutes, 1 session/day, 40% MoT) on each treatment day. Scatterplots show the averaged %MPE across three treatment days plotted against the charge per second for each animal in four high-dose subthreshold SCS groups: (A) 200 Hz (1 ms, n=11), (B) 500 Hz (0.5 ms, n=11), (C) 1200 Hz (0.2 ms, n=11), and (D) 10 kHz (0.024 ms, n=11). (E) When data from these groups were combined, scatterplots showed a significant linear correlation between intra-SCS %MPE and charge per second. MoT, motor threshold; HD-SCS, high-dose spinal cord stimulation.