Suggested Technique

Ultrasound imaging of the musculocutaneous nerve in the axilla can be used to facilitate regional block.² This directly addresses one of the primary weaknesses of traditional axillary block.

The lateral course of the nerve and its changes in shape as it passes through the coracobrachialis muscle are characteristic features that allow ultrasound identification of the nerve.³ The nerve typically has a flat shape within the coracobrachialis muscle; therefore, this is a desirable location for regional block. The relatively high surface area–to–volume ratio may improve onset kinetics of the block.

The block is performed with the arm abducted. The needle approaches from the lateral side of the arm. The choices of transducer and block needle are not critical. The needle tip is positioned at the lateral corner of the musculocutaneous nerve within the fascial plane of the coracobrachialis muscle that contains the nerve in the axilla.

The lateral cutaneous branch of the musculocutaneous nerve can be more selectively blocked distal to the nerve exit from the coracobrachialis muscle. The point of exit is usually easy to identify because the nerve has a triangular shape in that location. Blockade of the more proximal motor fibers of the musculocutaneous nerve is unlikely with this approach. However, the nerve to the biceps brachii continues for about 4 cm after the exit from the coracobrachialis before branching off.⁴

The estimated incidence of pass-over musculocutaneous nerve (musculocutaneous nerve path over the coracobrachialis muscle rather than through it) ranges from 8% to 30%.⁵ This usually does not present a problem because the nerve can be directly imaged.

Fusion of the median nerve and musculocutaneous nerve is another common anomaly (a low-lying lateral cord). In these fusion products, the median contribution is typically larger and more superficial than the musculocutaneous contribution.⁶ Small muscular branches can sometimes be identified that course medial to lateral from the musculocutaneous contribution.

In some patients the lateral cutaneous nerve of the forearm extends to the dorsal aspect of the thumb (musculocutaneous dominance of the dorsum of the hand). Local anesthetic infiltration through the anatomic snuffbox blocks both superficial radial and musculocutaneous contributions to the dorsum of the hand.

FIGURE 33-1 External photograph showing the approach to musculocutaneous nerve block in the axilla (A). An in-plane approach from the lateral aspect of the forearm is shown. The corresponding sonogram before needle placement is shown (B).

FIGURE 33-2 Division of the musculocutaneous nerve into anterior and posterior branches in the distal arm. Sonograms are shown proximal (A) and distal (B) to the division. Musculocutaneous nerve blocks are usually performed proximally for more complete anesthesia.

FIGURE 33-3 Image sequence showing musculocutaneous nerve block in the axilla. An in-plane approach is demonstrated whereby the needle tip is placed under the lateral aspect of the musculocutaneous nerve (A). After injection, local anesthetic is distributed around the musculocutaneous nerve (B).

FIGURE 33-4 Distribution of local anesthetic after musculocutaneous nerve block in the axilla. After injection, local anesthetic is distributed around the musculocutaneous nerve as seen in short-axis view (A) and tracks along the nerve as seen in long-axis view (B).

FIGURE 33-5 Musculocutaneous-median nerve fusion (low-lying lateral cord). In this variation, the two nerves are together in the axilla. Muscular and cutaneous branches may split off directly from the fusion product more distally in the arm.

FIGURE 33-6 Musculocutaneous-median nerve fusion (low-lying lateral cord). After regional block, local anesthetic separates this fusion product.

FIGURE 33-7 Pass-over musculocutaneous nerve. In most individuals the musculocutaneous nerve passes through the coracobrachialis muscle. However, in some individuals the musculocutaneous nerve passes over the muscle. Under most circumstances the nerve can be directly imaged, thereby facilitating regional block.

FIGURE 33-8 In some individuals a large cutaneous branch of the musculocutaneous nerve can be identified that travels to the skin of the lateral arm (A and B). When present, this branch may be important for arm tourniquet tolerance.