Cooled radiofrequency ablation produces enhanced thermal lesions and leads to beneficial neuroadaptions in vivo

Abstract

Purpose: Thermal radiofrequency ablation (RF) is a minimally invasive procedure used to denervate the pain-transmitting peripheral nerves in chronic pain patients. Standard RF (SRF) and cooled RF (CRF) are two commonly used thermal ablation technologies hypothesized to provide pain relief by temporarily blocking signaling of the pain-transmitting sensory nerves and eliminating perception of pain in the central nervous system (CNS). CRF overcomes the lesion size limitations inherent to SRF administered at 80°C by circulating fluid around a 60°C probe tip to remove heat from tissue adjacent to the electrode, thereby delivering energy within a larger radius. Although previous ex vivo studies have evaluated RF lesion size, the underlying central and/or peripheral mechanisms for RF-induced pain relief have not been characterized. Here, we present the first in vivo analysis of pathophysiological changes that occur in response to either SRF or CRF. We hypothesize that enhanced delivery of thermal energy provided by CRF correlates with sustained peripheral and/or CNS neuroadaptations. To test this hypothesis, we evaluated functional, structural, and biochemical responses in rodents exposed to sciatic nerve (SN) RF. Methods: As previously reported, sciatic nerve RF ablations can be performed in rats to mimic clinical applications allowing for rapid assessment of changes that occur over extended durations in humans. (Note: Two weeks of a rat lifespan corresponds to one human year.) In one group, male Lewis rats were exposed to recommended human clinical parameters of SRF (90s, 22-gauge, 5mm active tip, 80°C) or CRF (150s, 17-gauge, 2mm active tip, 60°) on the SN and assessed for immediate changes. Another group of Lewis rats were exposed to optimized levels of SRF (50s) or CRF (80s) to allow for long-term evaluation with minimal collateral damage to surrounding tissues and severe adverse events. Power output was recorded to calculate total energy delivered from the generator. Following ablation, rats were assessed for nerve function (electromyography, EMG) and lesion volume (9.4T Bruker MR scanner). EMG measurements were also taken repeatedly over the course of 12 weeks. At necropsy, nerves were harvested for histological analysis (H&E). A subsequent study exposed male Lewis rats to a pain phenotype in which local inflammation was induced via Freund’s complete adjuvant (FCA) injection in the left hind paw prior to ablation. Rats in the pain study were evaluated weekly via von-Frey for changes in left hind paw mechanosensitivity. At day 35 post-ablation necropsy, brains were collected for immunohistochemical analysis of CNS protein expression in brain regions known to be impacted by chronic pain. Results: Utilizing human clinical parameters, CRF delivered ∼3.5 times greater energy than SRF in vivo and remained consistent when optimized for the smaller rodent anatomy. In histological images of CRF nerves, a region of lesser thermal damage (pink) was flanked by regions of more severe damage (purple), paralleling the thermal profile of CRF lesions observed ex vivo. In images from SRF nerves, only a single thermal damage region was observed. High resolution MRI visualization of lesions immediately post-ablation (t=0) suggest larger volumetric lesions in CRF groups. Significantly greater attenuation in compound motor action potential (CMAP) amplitude was found immediately after ablation in and for up to 6 weeks post-ablation in CRF treated nerves, similar to the nerve-transection control group. A return of function was found as early as 4 weeks in SRF treated nerves. By week 8, both CRF and SRF treated groups show signs of return-to-function. A greater magnitude and duration of decreased mechanical sensitivity (up to 21 days) was found in rats treated with CRF compared to rats treated with SRF (7-14 days). Interestingly, CGRP expression in the central and basolateral amygdala was attenuated in CRF-treated rats 35 days post-ablation, relative both to SRF-treated and untreated FCA-Sham controls. Conclusions: Recent clinical results report CRF ablation provides pain relief that lasts more than twelve months. Enhanced lesion volume, a greater impact on nerve function and differential CNS neuroadaptions may elucidate the mechanisms underlying pain relief provided by CRF to chronic pain patients. Further work is warranted to the understand mechanistic differences between SRF and CRF procedures and their impact on pain related outcomes.