Ultrasound Evidence of Intraneural Injection

To the Editor: Schafhalter-Zoppoth et al. (1) reported an interesting case of delayed sensory recovery purportedly as a result of inadvertent femoral nerve impalement and subtotal intraneural injection visualized by ultrasound. A short video clip they provided showing withdrawal movement of the block needle that has initially traversed the femoral nerve is presumably evidence of nerve impalement. A second video clip shows presumably a subtotal intraneural injection represented by a hypoechoic fluid collection underneath the hyperechoic nerve shadow. These ultrasound images and videos invite some comments. The elliptical hyperechoic density shown in the article (Fig. 1A) is indeed a typical ultrasonographic image of the femoral nerve in cross-section. Generally, nerve fascicles are not readily seen in the femoral nerve location, and it is difficult to tell if the hyperechoic density represents a single nerve structure. One may visualize divisions of the femoral nerve at the time of local anesthetic injection when the hyperechoic nerve structure is split into two, highlighted by surrounding hypoechoic fluid collection within an expanding femoral sheath compartment. Without knowledge of prolonged anesthesia in this case, I would have considered the image in Figure 1B a picture of an extraneural injection within the sheath compartment.Figure 1.: A, Transverse sonogram of the brachial plexus of a pig using a 12 MHz probe before direct intraneural saline injection. A = axillary artery, n = brachial plexus, arrows indicate size of nerve. B, Transverse sonogram of the brachial plexus after an intraneural 10-mL saline injection. A = axillary artery, n = brachial plexus, arrows indicate an increase in nerve size.The authors suggest that an intraneural injection diminishes nerve diameter as the result of compression and increased intraneural pressure (Fig. 1). To the contrary, I suggest a true intraneural injection will show on ultrasound an obvious expansion of the hyperechoic nerve structure as the nerve increases in size. Our preliminary study in a pig model shows that a purposeful direct intraneural injection into the brachial plexus with a 22-gauge long bevel needle and 10 mL of saline dramatically increases nerve dimension on gross examination and the ultrasonographic appearance of the hyperechoic nerve structure (Fig. 1). We speculate that ultrasound imaging may add safety to the practice of peripheral nerve block by showing early evidence of an inadvertent intraneural injection, but further imaging studies are warranted. Vincent W. S. Chan, MD, FRCPC Department of Anesthesia and Pain Management University of Toronto University Health Network Toronto, Ontario, Canada [email protected]