Introduction: The neural elements of the brachial plexus between the anterior and middle scalene muscles are readily visible by ultrasound. However, the epineurium of these nerve structures is difficult to discern on ultrasound imaging because of the proximity of the scalene muscles to the nerve elements, and this may lead to unintentional subepineurial injection (SEI). To evaluate whether typical needle tip placement under ultrasound guidance results in SEI, as opposed to extraneural injection, we undertook this cadaver study.

Materials and Methods: Six nonpreserved cadavers served as subjects for 10 injections. After imaging revealed the hypoechoic fascicles of the brachial plexus at the interscalene level, the tip of a 22-gauge, 5-cm short-bevel needle was inserted into a position adjacent to one of the fascicles by ultrasound guidance. At this point, 0.1 to 0.2 mL of india ink solution was injected. The brachial plexus at this level was then dissected and removed. The nerve elements discolored by the ink were removed, fixed, and stained for histologic analysis and were then examined for evidence of subepineurial ink deposition. Four nerve segments, which were unaffected by the injected ink, served as controls. These were subjected to topical india ink application for a 60-min period and were then washed, fixed, and stained for histologic analysis.

Results: In all 10 interscalene sites, ultrasonography revealed multiple hypoechoic nodules that could be traced proximally to the spine, as in live subjects. On gross analysis after dissection, the superficial nerve elements of the brachial plexus appeared to be stained by the ink. On histologic examination, 5 of 10 nerve specimens revealed ink within the epineurium (subepineurial), whereas in the other 5, it had not penetrated this barrier. The india ink did not penetrate the perineurium in any of the specimens. Among control specimens, none had evidence of subepineurial ink.

Conclusions: In a cadaver model of needle tip placement for ultrasound-guided interscalene block, we found that SEI occurred more frequently than expected.