This nerve arises from the posterior cord of the brachial plexus (mainly from the C5 root, with a smaller contribution from C6) and supplies the teres minor and deltoid muscles. It may be involved in dislocations of the shoulder joint, fractures of the neck of the humerus, disc protrusion, and brachial neuritis; in other instances, no cause may be apparent. The anatomic diagnosis depends on recognition of paralysis of abduction of the arm (in testing this function, the angle between the side of the chest and the arm must be greater than 15 degrees and less than 90 degrees), wasting of the deltoid muscle, and slight impairment of sensation over the outer aspect of the shoulder.
The origin of this nerve is from the fifth and sixth cervical roots. It is a branch of the lateral cord of the brachial plexus and innervates the biceps brachii, brachialis, and coracobrachialis muscles. Lesions of the nerve result in wasting of these muscles and weakness of flexion of the supinated forearm. Sensation may be impaired along the radial and volar aspects of the forearm (lateral cutaneous nerve). Isolated lesions of this nerve are usually the result of fracture of the humerus.
This nerve is derived from the sixth to eighth (mainly the seventh) cervical roots and is the distal extension of the posterior cord of the brachial plexus. It innervates the triceps, brachioradialis, and supinator muscles, and continues below the elbow as the posterior interosseous nerve, which innervates the extensor muscles of the wrist and fingers, the main abductor of the thumb (the abductor pollicis longus, which is easier to isolate than the median nerve innervated abductor pollicis brevis), and the extensors of the fingers at both joints. A complete proximal radial nerve lesion results in paralysis of extension of the elbow, flexion of the elbow with the forearm midway between pronation and supination (a result of paralysis of the brachioradialis muscle), supination of the forearm, extension of the wrist and fingers, and extension and abduction of the thumb in the plane of the palm. If the lesion is confined to the posterior interosseous nerve, only the extensors of the wrist and fingers are affected. Sensation is impaired over the posterior aspects of the forearm and over a small area on the radial aspect of the dorsum of the hand.
The radial nerve may be compressed in the axilla (“crutch” palsy), but more frequently at a lower point, where the nerve winds around the humerus (see Table 43-8); pressure palsies incurred during an alcoholic stupor and fractures of the humerus commonly injure the nerve at the site of injury. It is susceptible to lead intoxication and is frequently involved as part of brachial neuritis and mononeuritis multiplex.