(54) Spatially Focused Direct Current Allows Nociceptive-Selective Block in Peripheral Nerves

Introduction: Electrical nerve block has the potential to revolutionize chronic and acute pain treatment by providing gradable, rapidly reversible, on-demand pain relief that has no systemic side effects. There is minimal research in the area of electrical nerve block for sensory/pain applications. It has been claimed that direct current (DC) nerve block yields nociceptive fiber selectivity due to inter-neuronal differences in expression of voltage gated sodium channels. However, evidence for selectivity differs between studies. Here we introduce a hypothesis that the critical factor to nerve fiber size selectivity is due to the geometric properties of the DC field that is applied to the nerve. Specifically, creating a spatially focused depolarized region of a nerve can lead to nociceptive-selective block (figure 1).

Methods: Acute experiments were performed using isoflurane anesthetized adult Sprague-Dawley rats. Peripheral sensory axons were activated by applying electrical stimulation to either the plantar surface of the hindpaw or a sciatic nerve cuff electrode. Electrophysiological measurements (local field potential (LFP) and Wide Dynamic Range (WDR) single-neuron activity) of sensory activity were recorded with high-impedance electrodes inserted into L1 spinal cord. A carbon separated nerve interface electrode (CSINE) was used to block nerve conduction using DC in a monopolar configuration. The CSINE uses a tube filled with high-capacitance carbon powder in an electrically conductive gel to pass the current from the electrode to the nerve. In these experiments, the effect of the aperture sizes of the tube were compared.
In-silico studies were performed using NEURON to investigate tripolar CSINE configurations that may be more suitable for chronic implantation.

Results: Data from LFP and WDR electrophysiological recordings demonstrate that focused DC can selectively block nociceptive fibers (figure 2). While the broad field (1.75 mm diameter) selectively blocks large fibers that are involved in tactile sensation, the focused (0.26 mm diameter) yielded selective block of pain fibers.
In-silico studies suggest that a tripolar CSINE configuration can yield nociceptive-selective nerve block whilst also tolerating greater changes in position relative to the nerve compared to a monopolar configuration.

Conclusion: We have demonstrated that Focused DC is capable of selectively blocking pain in anaesthetized rats. Clinical applications using this technology could provide pain relief whilst allowing normal tactile sensation and voluntary movement.