Objective: The use of ultrasound (US) for localization of neural structures allows real-time visualization of anatomy; however, variability in the arrangement of structures has been observed. The impact of these variations on the performance and outcome of regional anesthetic techniques remains unclear. We discuss possible anatomic explanations and correlation with clinical observations.

Case Report: We report limited spread of local anesthetic observed during the performance of a US-guided interscalene block. This was associated with an anomalous vessel arising from the subclavian artery, which effectively divided the brachial plexus into 2 compartments. Spread of local anesthetic was restricted to the upper compartment around the C5 through C7 nerve roots. There was no anesthetic fluid visualized in the lower compartment. This produced a block that provided surgical anesthesia for shoulder arthroscopy as well as excellent postoperative analgesia, although the medial and distal aspects of the arm remained unaffected.

Conclusions: This case illustrates the ability of US to identify anatomic variations and their relevance to the performance of regional anesthetic techniques.