The Histologic Effects of Pulsed and Continuous Radiofrequency Lesions at 42°C to Rat Dorsal Root Ganglion and Sciatic Nerve

In Brief Study Design. Experimental histologic study of the effects of radiofrequency (RF) or convective heating of the rat dorsal root ganglion or sciatic nerve to 42°C. Objective. To determine whether treatment causes neuropathologic changes in an effort to explore the mechanisms and safety of pulsed RF pain therapy. Summary of Background Data. Clinical data suggest that low temperature pulsed RF energy delivered to the DRG is a safe and effective form of therapy for low back pain. However, the mechanism by which this treatment modifies pain is unclear. Methods. A total of 118 Sprague-Dawley rats were divided into five groups for different RF and thermal treatments. All treatments increased tissue temperature to 42°C. Treatments of the DRG included pulsed RF, continuous RF, and conductive heat. The generator output was increased until 42°C was obtained in the tissue and was then maintained for 120 seconds. As a positive control, some rat sciatic nerves were treated with continuous RF lesions at 80°C. Animals were killed for histologic study at 2, 7, or 21 days after treatment. Tissue was fixed in gluteraldehyde and embedded in plastic resin for detailed light microscopic neuropathologic evaluation. Results. The methods used to heat the tissue to 42°C caused no significant difference in pathology. However, subclinical changes included endoneurial edema caused by alterations in the function of the blood-nerve barrier, fibroblast activation, and collagen deposition. Tissue returned to normal conditions by 7 days in nerve and 21 days in the DRG. These minor structural changes observed at the light microscopic level in normal animals do not exclude the possibility that there would be nonstructural changes in gene expression or cytokine upregulation in injured tissue. Lesions at 80°C caused consistent thermal injury characterized by Wallerian degeneration of nerve fibers. Conclusions. The data support the hypothesis that pulsed RF treatment does not rely on thermal injury of neurologic tissue to achieve its effect. Neuropathologic methods were used to compare the consequences of heating rat dorsal root ganglions and sciatic nerve to 42°C with pulsed radiofrequency energy, continuous radiofrequency energy, or thermal (heat probe) energy for 120 seconds. This was done to mimic a new clinical technique being used to treat low back and other forms of spinal pain, to test the safety of the therapy, and to determine whether tissue damage is a part of the mechanism of pain relief. The results indicate that, while there are transient changes in the hydration of the nerve and activation of fibroblasts that result in the deposition of epineurial and endoneurial collagen, there is no significant injury to nerve fibers, DRG neurons, or their support cells.