We read with great interest the article by Cohen et al. ,1and we complement the authors on such a well-designed randomized study. Cohen et al.  1highlight the importance of improving diagnostic efficacy of cervical medial branch blocks by injecting two different volumes (0.25 and 0.5 ml) of injectate. We note from the results that three nerves were missed in each  treatment group, indicating a 93% accuracy rate. Thus, missing the nerve is more of a “technical challenge” rather than being due to the volume of local anesthetic injected (0.25 vs.  0.5 ml). As shown in table 1 of their article, the authors note that six (54.5%) of the medial branch blocks with 0.25 ml versus  three (25%) of the medial branch blocks with 0.5 ml received greater than 50% pain relief. Decreasing the volume of local anesthetic by 50% led to doubling (from 25 to 54.5%) of pain relief. Cohen et al.  1then comment in the discussion section: “it is interesting to note that the higher incidence of inadvertent spread to untargeted nervous tissue did not translate to reduced pain scores in the 0.5 ml.” This leads us to conclude that if this is truly the case, that decreasing  the volume of injectate to improve the diagnostic accuracy led to increased  prevalence of pain relief, then the volume injected does not really matter in improved outcomes. That is to say, using volumes of 0.25 or 0.5 ml will produce similar outcomes in diagnostic cervical medial branch blocks with comparable true positive rates (93%).

*Vanderbilt University Medical Center, Nashville, Tennessee. sdattamd@gmail.com

Cohen SP, Strassels SA, Kurihara C, Forsythe A, Buckenmaier CC 3rd, McLean B, Riedy G, Seltzer S: Randomized study assessing the accuracy of cervical facet joint nerve (medial branch) blocks using different injectate volumes. Anesthesiology 2010; 112:144–52