Musculocutaneous nerve injury: weak elbow flexion and lateral forearm numbness

The musculocutaneous nerve is named for its two jobs: muscle supply (musculo) and skin supply (cutaneous, meaning of the skin). It comes from the lateral cord, one of the three bundles that the brachial plexus (the network of nerves running from the neck into the arm) forms in the armpit. The nerve runs between biceps brachii and brachialis.

Its motor job is the three muscles of the anterior compartment (the front muscle group of the upper arm): biceps brachii, brachialis and coracobrachialis, the BBC muscles. Biceps brachii and brachialis bend (flex) the elbow, and biceps brachii also turns the palm upwards (supination). Coracobrachialis helps to flex the shoulder.

After supplying them, the nerve continues into the forearm as the lateral cutaneous nerve of the forearm, a skin nerve that supplies the lateral forearm.

An injury therefore produces weak elbow flexion, weak supination and lateral forearm numbness. Hand movements are largely preserved, because the small muscles of the hand are supplied by the median and ulnar nerves. Flexion is weakened rather than lost, because brachioradialis (a forearm muscle supplied by the radial nerve) can still bend the elbow.

Injury is relatively uncommon: typical causes are penetrating trauma, such as a stab wound to the axilla (armpit), and damage during shoulder surgery. To rule out the neighbours, note that preserved shoulder abduction (lifting the arm out to the side) argues against the axillary nerve (which supplies deltoid), and preserved wrist extension argues against the radial nerve.