The musculocutaneous nerve is a relatively flat, straight nerve that can be seen in long axis when it passes through the coracobrachialis muscle. The coracobrachialis muscle is most reliably identified by sliding the transducer from distal to proximal starting at the mid-humeral level with the brachial artery and humerus in the imaging field of view. The coracobrachialis has a “doming” appearance when viewed in short axis.

In some arms (8-30%) the musculocutaneous nerve will pass over the coracobrachialis muscle rather than passing through it. Interestingly, in these arms the musculocutaneous nerve undergoes a similar shape change, probably because the short head of the biceps plays a similar mechanical role as the coracobrachialis muscle in shaping the nerve.

References

Schafhalter-Zoppoth I, Gray AT. The musculocutaneous nerve: ultrasound appearance for peripheral nerve block. Reg Anesth Pain Med. 2005 Jul-Aug;30(4):385-90. PMID: 16032591