Ultrasound Detects Intraneural Injection

We read with interest the recent article by Paul Bigeleisen titled “Nerve Puncture and Apparent Intraneural Injection during Ultrasound-guided Axillary Block Do Not Invariably Result in Neurologic Injury.”1The author is to be commended for this small study, which supports the ability of ultrasound to detect intraneural injection during peripheral nerve blockade. Bigeleisen’s experience with ultrasound and low-volume intraneural injection complements our own in both the laboratory and clinical settings. In our recently completed study of ultrasound-detected intraneural injection, we inserted blunt-tipped insulated 22-gauge needles (Stimuplex®; B. Braun, Bethlehem, PA) directly into axillary brachial plexus nerves of anesthetized pigs and then injected dye-stained dextrose under ultrasound imaging.2,3After injecting 4 ml dye-stained dextrose, we visualized a 57% (median) increase in nerve diameter using real-time ultrasound imaging. We then harvested the injected nerves for histologic examination and found that dye had penetrated the epineurium in all 24 cases where nerve expansion was visualized on ultrasound. The dye had penetrated the perineurium in 2 of these cases, and none of the cases demonstrated fascicular dysplasia. Much like Bigeleisen, we concluded that ultrasound is a useful technique to detect intraneural injection. Unlike Bigeleisen, we know for certain that our needle was indeed intraneural at the time of nerve expansion on ultrasound. Subsequent to definitively characterizing the sonographic appearance of intraneural injection (pig study completed August 2005), we have performed more than 411 ultrasound-guided axillary brachial plexus blocks to date and have identified 12 patients in whom we accidentally performed one or more probable intraneural injections using a 50:50 mixture of 2% lidocaine:0.5% bupivacaine with 0.005 mg/ml epinephrine. We stopped the injection immediately after recognizing the pattern of nerve expansion, which was usually visible after injecting 1–3 ml local anesthetic. Bigeleisen reported that intraneural injection elicited paresthesiae or dysesthesiae with gross variability. By stark contrast, none of our 12 patients reported pain or dysesthesiae at the time of intraneural injection. We contacted each of these 12 patients on postoperative days 1 and 7 to find that none had any reports of pain, paresthesiae, dysesthesiae, or weakness associated with their recent axillary nerve block.

Our needle choice differs from that of Bigeleisen and may at least partially explain why we failed to demonstrate significant perineural penetration in our laboratory or elicit pain or dysesthesiae in our block room. We use a blunt-tipped insulated needle, whereas the needle used in Bigeleisen’s study was a sharp hypodermic B-bevel needle, which, as Bigeleisen suggests, may conceivably confer a greater risk of perineural puncture, intrafascicular injection, and consequent nerve damage.4Our clinical experience using a blunt-tipped needle is that the nerve floats away from the needle tip upon routine ultrasound-guided injection of local anesthetic. This seemingly protective phenomenon may be a function of needle choice, in addition to tissue displacement. Another reason why ultrasound-detected intraneural injection may not always result in nerve damage is because injectate tends to leak out of the nerve during injection. In our pig study, we directly observed dye-stained injectate leak out of the nerve along the needle tract after injecting as little as 1 ml.3 

In summary, we agree with Bigeleisen’s discussion and Borgeat’s accompanying insightful editorial commentary.5We believe that needle penetration and small-volume injection through the epineurium may be more common than anticipated in daily practice and most often benign in nature, and that the true danger zone for nerve damage likely lies beyond the perineurium. Unfortunately, current ultrasound technology does not allow the operator to visually differentiate the epineurium from perineurium. Nonetheless, ultrasound seems to be a useful tool to detect as little as 1–2 ml intraneural injectate and thus avoid presumably injurious high-volume local anesthetic intraneural injection. Whether ultrasound-detected intraneural injection culminates in clinical neurologic deficit is currently under investigation at our institution.

*University of Toronto, Toronto, Ontario, Canada. richard.brull@uhn.on.ca