The axillary (circumflex humeral) (C5, 6) and radial (C6, 7) nerves are the primary derivatives of the posterior cord of the brachial plexus. Descending behind the axillary vessels, the axillary nerve curves posteriorly and below the subscapularis (C5, 6) muscle. It next passes through a quadrangular space, bounded above by the teres minor (C5), below by the teres major (C5-7), medially by the triceps brachii long head, and laterally by the humerus. An anterior branch passes to innervate the deltoid muscle. The posterior branch innervates both the deltoid and the teres minor muscle. The axillary nerve terminates as the superior lateral cutaneous nerve of the arm, supplying the most upper portion of the arm immediately below the shoulder.
Axillary neuropathies are characterized by shoulder abduction weakness and diminished cutaneous sensation of the lateral shoulder, an area having C5 dermatome representation. Acute axillary neuropathies most typically result from blunt trauma, anterior shoulder dislocations, and/or humerus fractures, or perhaps from an autoimmune disorder, such as a forme fruste of brachial plexus neuritis. These primarily require differentiation from C5 radiculopathies. Electromyography is particularly helpful because the deltoid and teres minor are the only two muscles innervated by this nerve. Denervation confined to these muscles is diagnostic of a primary axillary nerve lesion, whereas the concomitant finding of infraspinatus/supraspinatus and/or rhomboid denervation favors a C5 radiculopathy.
The musculocutaneous nerve originates directly from the lateral cord of the brachial plexus innervating the biceps brachii, brachialis, and coracobrachialis (C5, 6) muscles. It terminates as the lateral antebrachial cutaneous nerve, supplying sensation to the forearm from immediately below the elbow to just proximal to the thumb. Isolated musculocutaneous neuropathies are rare. These may occur as a forme fruste of an acute brachial plexus neuritis. Other settings predisposing to such a condition include weight lifting, postsurgical procedures, and prolonged pressure during sleep. Patients present with weakness of forearm flexion and supination, with sensory loss of the lateral dorsovolar forearm. More distal lesions, primarily affecting the lateral antebrachial cutaneous nerve, may result from attempted cannulation of the basilic vein in the antecubital fossa.
The thoracodorsal nerve is derived from the posterior cord of the brachial plexus and innervates the latissimus dorsi (C6, 7, 8). Isolated lesions are rare; latissimus dorsi atrophy very rarely develops subsequent to chest tube insertion.