Local anesthetic does not spread circumferentially around the sciatic nerve after a single injection. GMM = gluteus maximus muscle;; QFM = quadratus femoris muscle; LA = local anesthetic. Arrows = sciatic nerve. (A) Short-axis and (B) long-axis views showing noncircumferential spread of local anesthetic around the sciatic nerve at the subgluteal level. (Image source: Anesthesia & Analgesia)

Regional anesthesia should come on more quickly when the local anesthetic spreads circumferentially around the nerve. There is evidence for this with popliteal blocks, but not for blocks of the proximal sciatic nerve. The proximal sciatic nerve is the largest peripheral nerve. It is deeper and harder to visualize in the subgluteal region than the more distal sciatic nerve blocked in the popliteal fossa. Does circumferential spread of local anesthetic for a subgluteal sciatic nerve block increase the rate of onset of regional anesthesia?

Dr. Shinichi Sakura, Department of Anesthesiology, Shimane University School of Medicine, Izumo City, Japan, and colleagues compared 86 patients in whom the sciatic nerve was clearly visualized with ultrasound who received either single (group S) or multiple injections (group M) around the sciatic nerve. The results of this study are summarized in the article titled “A Prospective, Randomized Comparison Between Single- and Multiple-Injection Techniques for Ultrasound-Guided Subgluteal Sciatic Nerve Block,†which was published in this month’s issue of Anesthesia & Analgesia.

The authors performed sciatic nerve blocks using a subgluteal approach for patients undergoing minor knee surgery, including meniscectomy, meniscal repair, or synovectomy. Using ultrasound for visualization, a needle connected to a nerve stimulator was advanced until foot plantarflexion, foot inversion, dorsiflexion, or foot eversion was elicited. The current was then gradually reduced and a minimal-evoked current eliciting the motor response was obtained. Next, each patient received an injection of 20 mL 1.5% mepivacaine with 1:400,000 epinephrine. In Group S (“singleâ€) the drug was only injected at the initial position of the needle. In group M (“multipleâ€) the drug was injected in multiple small boluses circumferentially via repositioning of the needle’s tip.

The rate of intraneural injection was similar in the two study groups: 5 (11.6%) patients in group S and 6 (14.0%) patients in group M. It took slightly longer (about 1 minute) to place the block in group M. At 30 minutes after block completion, the rate of sensory blockade of all sciatic components was greater after multiple injections compared with a single injection (41.9%% vs. 16.3%). There was also a greater rate of motor blockade after multiple injections. No patient had prolonged postoperative sensory or motor dysfunction.

The use of sciatic nerve blocks for “minor†knee surgery, as described by the authors, may be a more aggressive use of regional anesthesia than practiced in the United States. Nonetheless, the authors have shown that multiple injections of the proximal sciatic nerve, and the resulting circumferential spread of local anesthetic, produces faster onset of regional anesthesia.