We thank Drs. Morfey and Brull for their response to our recent observations.1They raise some important questions on the ability of minimum stimulating current to detect intraneural needle placement and to predict neurologic injury after intraneural injection. Their most important question, however, concerned the reliability of our measurements: how sure are we that the needle tip was outside and inside the nerve during extraneural and intraneural measurements, respectively? Their question concerning what would be the outer layer of the supraclavicular brachial plexus is very reasonable. In their ultrasound-guided supraclavicular block procedure, accompanied by figures before and after injection, they describe that during the block this outer layer is intentionally breached, which is often felt as a loss of resistance or “pop.”
We have the same experience. At this site, the nerve fascicles are surrounded by epineurial layers, as shown in figure 4 of our original article. The configuration of epineurial layers may differ depending on the site of formation of the nerve trunks and cords of the brachial plexus. In addition, as stated in our discussion, adjacent to the epineurial layers, fascial layers that are continuous with the prevertebral and scalenic muscle fascias may be present. This can be better observed in an axial histologic cross-section (fig. 1) that evidently shows several layers surrounding the nerve fascicles, including an outer layer that cannot always be clearly separated from the adjacent epineurial layers. Both layers are very thin (≤ 0.2 mm). Thus, by intentionally breaching this layer, we believe that both layers are punctured, and the needle tip is inside the nerve, which we referred to as intraepineurial.
However, to objectively verify this position, we adopted two additional parameters, that is, the position of the needle tip adjacent to the hypoechoic (black) round to oval-shaped nodules combined with distention of the nervous structure after small volume injection. For that reason, it might have been more appropriate to define “inside the nerve” as parafascicular (next to the nerve fascicles). The outside location was verified by indentation of a hyperechoic layer by pressure from the needle tip and by the absence of nearby black nodules. This could have been described as nonparafascicular. Thus, we are confident that our measurements really represent intraneural and extraneural needle tip placement. In fact, the accompanying figures of Morfey and Brull show the same configuration of black, round to oval-shaped nodules. Unfortunately, the position of their needle during injection is not shown. Furthermore, they suggest that if they accept our description and conclusions, they may have performed intraneural injections of the supraclavicular fossa much of the time. Actually, figure 2 in their study can be considered as a confirming sign that shows what has actually happened during their blocks, but what always was difficult to interpret: the presence of local anesthetic fluid adjacent to nerve fascicles. Because their retrospective survey2did not reveal long-term neurologic injury, it underlines our previous statement that intraneural injection does not invariably result in neurologic injury.3,4
The relative amount of connective tissue in combination with the thinness of epineurial and outer layers may further explain this phenomenon.4Our findings may be generalizable to nerves at other anatomic sites. Recently, Robards et al. 5reported findings that are similar to those of ours for the popliteal sciatic nerve block. They observed intraneural injection in all cases with a motor response at a stimulation current of 0.2–0.4 mA. Therefore, our conclusion that stimulation thresholds more than 0.2 and less than or equal to 0.5 mA are not reliable to prevent intraneural needle tip position was verified at a second anatomical site.
In conclusion, a minimum stimulation threshold of less than or equal to 0.2 mA is reliable for parafascicular placement of the needle in ultrasound-guided supraclavicular block and possibly for other anatomic sites as well. Can this minimum current predict whether needle placement and local anesthetic injection will cause neurologic injury? No, it cannot. Are we convinced that our measurements inside and outside the nerve are reliable? Yes, we are convinced. Finally, are the ultrasound-guided supraclavicular blocks of Drs. Mofrey and Brull actually intraneural? Yes, that is our opinion, when anesthetic fluid is found adjacent to nerve fascicles.
We thank Drs. Mofrey and Brull for their interesting contribution to the continuing discussion on a possible relation between intraneural injection and neurologic injury.
*University of Pittsburgh, Presbyterian University Hospital, Pittsburgh, Pennsylvania. firstname.lastname@example.org