A 53-year-old man complained of right arm weakness.

Evaluation and management of brachial plexopathies require a thorough knowledge of neuroanatomy. The brachial plexus is formed from the ventral primary rami (spinal nerves or roots) of C5 through T1. A prefixed plexus (when C4 contributes a branch to the brachial plexus) is seen in approximately two-thirds of cases. The brachial plexus is divided into five major components: (a) roots, (b) trunks (upper, middle, and lower), (c) divisions (anterior and posterior), (d) cords (lateral, posterior, and medial), and (e) branches. Typically, the brachial plexus is composed of five roots, three trunks, six divisions (two for each trunk), and three cords (see Fig. 131.1).

Brachial plexus injuries may be complete or incomplete. Injuries may be preganglionic (proximal to the spinal ganglion) or postganglionic. Plexus injuries can result in muscle weakness, neck and shoulder pain, paresthesias or dysesthesias, absent muscle stretch reflexes, and sensory loss. Despite some clinical variations, application of full pressure sensation to the thumb evaluates the corresponding C6 spinal nerve, median nerve, and lateral cord; application of deep pressure to the middle finger evaluates the corresponding C7 spinal nerve, median nerve, and lateral cord; and application of deep pressure to the little finger evaluates the corresponding C8 spinal nerve, ulnar nerve, and the medial cord.