Anomalous Brachial Plexus Anatomy in the Supraclavicular Region Detected by Ultrasound

Abstract

To the Editor: Increasingly, ultrasound is being used to assist performance of regional anesthesia. We report a previously undescribed anomaly of the superior trunk of the brachial plexus detected during performance of an ultrasound-guided supraclavicular block. A 57-yr-old man presented for elective tenolysis and reconstruction of the left hand following a previous crush injury with open fractures several months earlier. After a discussion with the patient, we chose to provide anesthesia with an ultrasound-guided supraclavicular brachial plexus block. Upon scanning the brachial plexus in the left supraclavicular fossa, we discovered that at the level of the first rib, the superior trunk was medial to the subclavian artery whereas the middle and inferior trunk was in the usual location lateral to the subclavian artery (Fig. 1). A more comprehensive scan revealed an anomalous course of the C5/C6 roots and superior trunk of the brachial plexus. The C5 root was located just lateral to the internal jugular vein, and medial to the anterior scalene muscle (ASM); the C6 root lay slightly more lateral, but still inferomedial to the ASM (Fig. 2). The C5/C6 roots remained medial to the ASM as they descended distally and coalesced into the superior trunk (Fig. 3). A similar anatomic pattern was seen on the right side.Figure 1.: Brachial plexus in the left supraclavicular region. The plexus is divided into 2 portions (arrows), one on either side of the subclavian artery (SA). The medial portion (solid arrow) was identified as the superior trunk by electrostimulation, which elicited deltoid and biceps contractions. MED = medial, LAT = lateral.Figure 2.: Brachial plexus (arrows) in the left interscalene region. The C5 and C6 nerve roots (dark circles) are clearly visible medial to the anterior scalene muscle (ASM). MED = medial, LAT = lateral, IJV = internal jugular vein, ICA = internal carotid artery.Figure 3.: Brachial plexus in the left root of the neck. The superior trunk (C5/6 nerve roots) (solid arrows) is medial to the anterior scalene muscle (ASM). The lower trunks (outlined arrow) are in their usual location in the interscalene groove. MED = medial, LAT = lateral, MSM = medial scalene muscle.We performed a supraclavicular block at the level of the first rib (Fig. 1) using an in plane approach as described by Chan et al.1 However, we modified this by performing two separate injections; first directing the needle in a medial-to-lateral, and then lateral-to-medial direction, to reach the superior trunk and the middle/inferior trunks respectively. Neurostimulation was used to confirm the identity of the superior trunk. A total of 25 mL of a 0.5% bupivacaine and 2% lidocaine admixture resulted in complete sensory and motor block. Anomalies of the upper nerve roots of the brachial plexus have been reported to occur with a frequency of 13% to 35% in the interscalene region.2–4 These usually involve an abnormal relationship to the ASM, with the C5 and C6 nerve roots either coursing through or anterior to the ASM, before assuming their normal position in the interscalene groove posterolateral to the subclavian artery. In this case, the superior trunk descended medial to the ASM throughout the posterior triangle of the neck, and remained medial to the subclavian artery even at the level of the first rib. We were unable to find a similar description of this anatomical anomaly at the supraclavicular level in the literature, although location of the inferior trunk inferior to the subclavian artery has been reported.5 Performance of either an interscalene block or a supraclavicular block in this patient using an anatomical surface landmark-guided technique would almost certainly have resulted in sparing of the C5 and C6 dermatomes. Ultrasound-guided brachial plexus blockade above the clavicle is relatively easy because the relevant structures are superficial and easily visualized. Given the prevalence of brachial plexus anomalies in the root of the neck, and the proximity of important neurovascular structures to each other, we recommend that all blocks above the clavicle should be guided by ultrasound whenever possible. Ki Jinn Chin, MBBS, FANZCA, FRCPC Ahtsham Niazi, MD, FCARCSI Vincent Chan, MD, FRCPC Department of Anesthesia Toronto Western Hospital University Health Network Toronto, Ontario, Canada [email protected]