Ultrasound Guidance and Peripheral Nerve Injury: Is Our Vision as Sharp as We Think It Is?

F irst, please understand that the authors of this editorial were selected primarily on a self-deprecating criterionVthe presence of more than just a few gray hairsVthat, and an interest in neurologic injury associated with regional anesthesia, for which there are assuredly experts younger and brighter than ourselves, but who lack the perspective that we wish to convey. We began our anesthesiology training in the 1980s, shortly after the introduction of peripheral nerve stimulation (PNS) into clinical practice. This new nerve localization tool was exciting because it promised to end the BNeanderthal practice of no paresthesia, no anesthesia[ and in so doing put an end to regional anesthesiaYrelated nerve injury. Elegant diagrams depicted how emitted currents of electricity would identify neural targets but supposedly prevent the needle from actually touching the nerve. Alas, more than a quarter-century later, there are no convincing data that the frequency of nerve injury has been significantly reduced by PNS. Will the same hold true for ultrasound-guided regional anesthesia (UGRA), the newest nerve localization tool and one of the most exciting advances in regional anesthesia during our lifetimes? In less than 2 decades, ultrasound guidance has evolved from enabling the crude deposition of local anesthetic near a vascular structure known to be anatomically adjacent to a poorly imag(in)ed nerve, to a robust technical innovation that has become a major focus of anesthesia departmental budgetary planning and continuing medical education. Studies show that ultrasound guidance hastens block onset, reduces unintended vascular puncture, and lessens the volume of local anesthetic necessary to anesthetize a nerveVall worthy attainments that have not yet resulted in demonstrably higher success rates for surgical anesthesia or consistent reduction in serious local anesthetic systemic toxicity (or other complications). Ultrasound has clearly brought about renewed interest in regional anesthesia, perhaps increased its use, and has resulted in calmer attending anesthesiologists, who can now actually see just how far off-target a resident’s block needle has strayed. Real-time observation of nerves and other anatomy thus give us a sense of control that heretofore was missing in our practice and one that we suspect has led believers and nonbelievers alike to hope that UGRA might meaningfully reduce our patients’ risk of peripheral nerve injury. But actually proving that UGRA is safer than PNS or even blind paresthesia techniques is unlikely because permanent nerve injury is so rare that it precludes meaningful statistical analysis. For example, one might begin with the knowledge that a month or two after peripheral nerve block, about 3% of our patients manifest a new, persistent neurologic symptom that may or may not be block related. To prove that UGRA could reduce this incidence by 50% (as compared with PNS) would require approximately 3000 patients per group. Although this study may be possible in a busy center, the results might not be as helpful as we would wish for because most of early postoperative nerve symptoms are transient. Indeed, a reasonable estimate is that only 4 of 10,000 patients undergoing peripheral nerve block will have a needle placementYrelated deficit 12 months afterward. Using this number, we calculate that more than 70,000 patients per group would be required to prove a 50% reduction (> = 0.05, A = 0.8). Such a study is unlikely to be undertaken in the resource-limited world of anesthesiology research. In addition, because the frequency of nerve injury varies by block, a well-designed study would be exponentially more difficult if limited to a single block with idealized standardization of local anesthetic concentration and volume, adjuvant, needle type, and so on. Acknowledging the absent BHoly Grail[ of a definitive and sufficiently powered randomized controlled trial, what do we currently know about peripheral nerve injury and UGRA? Recent reviews of randomized controlled trials that compared UGRA to PNS found no difference in the incidence of unintended intraoperative paresthesia or postoperative neurologic symptoms. Two large case series have added further insight, but no definitive answers. Barrington et al collected data on EDITORIAL