To the Editor:— The case report by Koff et al. 1illustrates that ultrasound guidance of peripheral nerve blockade will not eliminate the potential for nerve injury. The role of underlying nerve pathology, even if asymptomatic, is highlighted, and the accompanying editorial provides an excellent summary of the many factors that may impact nerve integrity in the perioperative period.2
One issue that bears additional discussion is intraneural (intraepineurial) injection. Many regionalists avoid this to the extent possible, for fear of injuring fascicles during needle insertion or injection, which may or may not produce symptoms3and which may or may not be visible on ultrasound.4Limited data provide evidence that intraepineurial injection may be safe,5but these data are far from conclusive.
At the level of interscalene block, distinguishing the epineurium of individual nerves with ultrasonography, or even the separation or grouping of nerve elements, is difficult. The article by Moayeri et al. 6in the same issue of Anesthesiology shows why this is so. The epineurium in the gross anatomy cross sections appears to be immediately adjacent to the fascial lining the interscalene groove, and barely distinguishable from it. On ultrasound, the largest nerve fascicles at interscalene levels appear as round or oval hypoechoic nodules, usually tightly embraced by the scalene muscles, with a small amount of nonneural tissue apparent between the muscle and nerve. Whether this tissue consists of the fascial lining of the muscle, frequently characterized as the “sheath” of the brachial plexus, or the epineurium of the roots or trunks, or a combination of both is unclear on ultrasound images. It is likely that, as we scan the interscalene region, we visualize fascicles grouped in ways that we cannot discern, and that moving the needle between what we perceive as roots may well result in piercing the inapparent epineurium. Because injectate frequently issues forth from a perforated nerve during injection,7this would not necessarily be apparent on ultrasound imaging. Likewise, the characteristic swelling of the nerve, expected during intraneural injection,5,7would be difficult to appreciate because the actual boundaries of the nerve are not apparent to us. What we perceive as expansion of the groove may, indeed, represent swelling of the nerve due to intraepineurial injection.
A study of the images in the article by Moayeri et al. makes another point clear. Although nerve trunks and roots of the plexus become oligofascicular as we proceed proximally,8they do not appear to be monofascicular at the level of interscalene block. We probably can see only the largest fascicles. The ultrasound image from the case report by Koff et al. shows only three, large fascicles, considered to be the C5, C6, and C7 nerve roots. However, the cross-sectional images provided by Moayeri et al. show no less than 14 fascicles at the interscalene level, many of which appear to be too small to appreciate with standard bedside ultrasound imaging units. Comparisons of ultrasound imaging and histologic sections reveal that only a portion of fascicles in peripheral nerves may be seen on ultrasound imaging.4In my experience, the image provided by Koff et al. is quite characteristic of ultrasound imaging in the interscalene region. My concern is that we may not be able to appreciate the boundary of the nerve elements (epineurium) and some of the contents (small fascicles).
The interscalene region differs from many other sites of peripheral nerve blockade. Because the fascial lining of the scalene muscles tightly invests the neural elements, as illustrated and exploited by Winnie when he described the interscalene block,9local anesthetic solution injected within the “groove” appears to be rather constrained. Although solution injected here may move both proximally and distally, and frequently escapes from the “sheath,”10it nevertheless appears to stay very much approximated to the nerves on both ultrasound and radiographic contrast studies.9,10The situation differs from a peripheral nerve coursing its way distally through an extremity, surrounded by adipose tissue or various muscle layers, but without a well-defined and constraining fascial envelope. The sciatic nerve in the popliteal fossa, or subgluteal space, for example, has little to “hold” the injected local anesthetic near it, and injection at one spot frequently fails to surround the nerve, leading to the possibility of partial or inadequate nerve blockade. In this setting, efforts to create a “halo” of local anesthetic surrounding the nerve seem appropriate. However, this is probably not necessary at interscalene levels. Before the use of ultrasound for guidance of interscalene block, peripheral nerve stimulation was used without direct visualization, and most authors recommended a single injection of local anesthetic when the motor stimulation was refined to a 0.2- to 0.5-mA range. Reported success rates with this technique were high.9,11If one single injection, without visualization, led to such efficacy, it is unclear why multiple injections would be necessary when ultrasound is used. An injection “in the groove,”i.e. , limited by the scalene muscles and their investing fascia, has a high likelihood of success.
Koff et al. note that they were careful to avoid penetrating the epineurium during the interscalene block. However, they also relate that the needle was moved to three separate sites to completely surround the three nerve roots with local anesthetic solution. Given the anatomical particulars described above, inserting the needle repeatedly around large fascicles within trunks or roots to ascertain that each is completely surrounded by local anesthetic may require repeated intraepineurial injections. I do not wish to criticize this practice by the authors, because I have done the same out of concern that I had not adequately “covered” the roots or trunks with local anesthetic. Further, any relation of the block technique to nerve injury in this case report is unclear. However, to avert injury to small, inapparent fascicles and to avoid repeated intraneural injections, it is probably safest to minimize needle manipulations and injections in the interscalene region.
As noted in the editorial by Hebl,2we cannot control all of the elements of nerve injury in the perioperative period and will never eliminate them. But the number of needle “sticks,” or insertions, in the vicinity of nerves is best held to the minimum that will be efficacious. One injection of local anesthetic solution into the interscalene groove, guided by ultrasound, should be highly successful in providing adequate blockade of the superior and middle trunks. Confirmation of local anesthetic solution surrounding the nerves need not occur at the level of local anesthetic injection: Examination of the supraclavicular area after a single site of interscalene injection usually shows highlighting of the superior elements of the plexus, evidence that solution freely moves distally, and amply bathes this portion of the plexus, as necessary for a successful interscalene block.9
University of Pittsburgh Medical Center–Southside, Pittsburgh, Pennsylvania. email@example.com