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

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.