INTRODUCTION AND BACKGROUND
The brachial plexus is a network of nerve fusions and divisions that originate from cervical and upper thoracic nerve roots and terminate as named nerves that innervate muscles and skin of the shoulder and arm. Although detailed knowledge of the elements of the network is important for distinguishing between radiculopathy and mononeuropathy, a syndromic approach is more useful for diagnosing lesions involving the plexus itself.
This topic will briefly review the underlying anatomy, pathogenesis, and general clinical features of brachial plexopathies, and then discuss a number of specific brachial plexopathies, classified for convenience by clinical setting into traumatic, nontraumatic, iatrogenic, and neonatal types.
Anatomy — Nerve roots from C5 through T1 contribute to the brachial plexus (figure 1). The plexus can be divided into regions that include (from proximal to distal) trunks, divisions, cords, branches, and nerves. Trunks and divisions are further subdivided with a nomenclature based on overall relationships with other upper extremity anatomic structures and include upper, lower, and middle trunks, and posterior and anterior cords.
- C5 and C6 roots merge to form the upper trunk. The C7 root forms the middle trunk. C8 and T1 roots merge to form the lower trunk.
- The upper trunk divides and gives branches to the lateral and posterior cords. The middle trunk divides and gives branches to the lateral and posterior cords. The lower trunk divides and gives branches to the posterior and medial cord.
- The lateral cord branches and gives rise to the musculocutaneous nerve and contributes to the median nerve. The posterior cord branches and gives rise to the axillary nerve and then becomes the radial nerve. The medial cord branches and contributes to the median nerve and then becomes the ulnar nerve.
- Other nerves arise from various elements of the plexus. The dorsal scapular nerve arises from the C5 root. The long thoracic nerve arises from C5, C6, and C7 roots. The suprascapular nerve arises from the upper trunk.
- Contributions to motor and sensory function vary within the brachial plexus. The largest percentages of motor fibers are from C5 and C6 roots, and the least from C7 and T1 roots. The greatest number of sensory fibers is from the C7 root, with lesser amounts from C5, C6, C8, and T1 roots . Postganglionic sympathetic nerve fibers from vertebral ganglia course through the brachial plexus.
Pathogenesis — The pathologic basis and histologic changes seen with brachial plexus lesions vary with the underlying cause, which include compression, transection, ischemia, inflammation, metabolic abnormalities, neoplasia, and radiation therapy. Because the brachial plexus is relatively inaccessible to direct investigation, most pathologic processes are deduced.