Criteria for electrical nerve stimulation and outcome of radiofrequency treatment of the dorsal root ganglion in the lower back for chronic pain

Abstract

Aim of investigation: To investigate the influence of electrical nerve stimulation (ENS) threshold value on outcome of radiofrequency treatment (RF) of the dorsal root ganglion (DRG) in the lower back for chronic segmental pain radiating to the leg. Methods: Sensory and motor ENS threshold values were determined in 279 patients during RF-DRG procedure. Sensory ENS value limits between 0.5 and 1.0 V were pursued. The motor ENS value was required to be at least 1.5 times the sensory ENS value. RF-DRG was done at 67°C during 90 seconds. Results were determined after 2 months, using a 4-point verbal pain scale. More than 50% pain relief was considered a success. Results: Sensory ENS values ranged between 0.3 and 2.5 V (mean 0.66 V, SD 0.24). Motor ENS values ranged between 0.75 and 4.00 V (mean 1.54 V, SD 0.54). In 59% of patients RF-DRG was successful. No relation was found between sensory ENS threshold value and outcome of RF-DRG (chi-square p = 0.68). Logistic regression analysis did not show any difference between patients with or without prior low back surgery. No unwanted side effects were reported. Discussion: Possible explanations for the findings are presented: (a) procedure of ENS; (b) distance to nerve and type of nervous structure; (c) difference between RF-DRG and other types of radiofrequency treatment; (d) local anatomical and neurophysiological factors; (e) diameter, myelinisation and conduction velocity of nerve fibres; (f) mode of effect of RF; and (g) inaccuracies in the actual practice of ENS. Conclusions: Sensory ENS threshold value does not influence outcome of RF-DRG. The role of ENS in preventing unwanted sensory or motor side effects could not be established.