Anatomical Mechanisms Underlying Clinically Reported Complications of the Infraclavicular Brachial Plexus Block: A Narrative Review

<b>Background</b>: The infraclavicular brachial plexus block is a widely used regional anesthesia technique for surgery of the distal upper limb. Although generally considered safe-particularly with ultrasound guidance-a range of vascular, neurological, respiratory, and anesthetic-related complications continues to be reported. Understanding how anatomic factors can influence the occurrence of these events is essential for improving procedural safety. <b>Objective</b>: This narrative review aims to correlate clinically reported complications of the infraclavicular block with underlying anatomical mechanisms that may predispose to their development. <b>Methods</b>: A narrative review of the literature was conducted using PubMed, Scopus and Web of Science to identify clinical studies, observational series, and case reports published between 1995 and 2025 that documented complications associated with infraclavicular brachial plexus block in adults. Publications were selected based on relevance to vascular, neurological, respiratory, infectious, and local anesthetic systemic complications. Findings were synthesized descriptively, with emphasis on anatomical-clinical correlations rather than quantitative meta-analysis. <b>Results</b>: Reported complications include vascular puncture and hematoma formation, transient or persistent neurological deficits, Horner's syndrome, hemidiaphragmatic paralysis, pneumothorax, local anesthetic systemic toxicity, and infectious complications. The incidence of these events varies widely across studies, reflecting differences in block technique, use of ultrasound guidance, injected anesthetic volume, and operator experience. Anatomical factors-such as the close relationship of the cords of the brachial plexus to the axillary vessels and the continuity of fascial planes-provide plausible explanations for these variations. <b>Conclusions</b>: Most complications of the infraclavicular block can be understood and anticipated through careful consideration of regional anatomy. Integrating anatomical knowledge with ultrasound guidance and optimized injection strategies may substantially reduce the risk of adverse events. This review highlights key anatomical mechanisms underlying reported complications and outlines practical implications for clinical practice.