Peripheral nerve damage affecting the upper extremities can vary widely in cause and extent. Many disorders, ranging from mild carpal tunnel syndrome to severe brachial plexopathy, need to be considered in a patient presenting with pain, sensory loss, or weakness involving the shoulder, arm, or hand. Peripheral nerve syndromes involving the lower extremities are discussed separately. (See "Overview of lower extremity peripheral nerve syndromes".)
Nerve roots emerge from the spinal cord from the C2 level and below, each exiting through its own individual foramen. The C2-C4 roots merge close to the spinal column into a conglomeration known as the cervical plexus. Nerves emerging from this group include the phrenic and greater auricular. Although generally uninvolved in neurologic disease, lesions can occasionally affect this region and cause head extensor weakness or diaphragm paralysis.
The C5 through T1 roots also emerge from the spinal column and give off small branches proximally that contribute to form the long thoracic nerve, which innervates the serratus anterior muscle. The roots then merge in a complex region known as the brachial plexus. A number of regions within the plexus have been defined by anatomists, including trunks, divisions, cords, branches, and proximal nerve; however, for practical purposes an understanding of the individual trunks, cords, and nerves is all that is necessary to correctly classify a problem affecting this region (figure 1):
- The C5 and C6 roots merge to form the upper trunk; the C7 root alone makes up the middle trunk; the C8 and T1 roots form the lower trunk.
- The upper trunk becomes the lateral cord after giving off branches that contribute to the posterior cord; the lower trunk becomes the medial cord after giving off branches that contribute to the posterior cord; the middle trunk becomes the posterior cord after giving off branches which join both the upper and lower trunks.
- A number of nerves emerge from the lateral region of the plexus, including, from proximal to distal, the dorsal scapular nerve, the suprascapular nerve, the musculocutaneous nerve, and, in part, the median nerve. The ulnar nerve is the major nerve from the medial region of the plexus, while the posterior cord gives off a number of significant nerves including the axillary, subscapular, thoracodorsal, and most importantly, the radial nerve.
- As the major nerves descend down the arm, the radial and median give off two important branches, the posterior and anterior interosseous nerves, respectively. All of the nerves give off multiple branches even more distally, although most are not clinically relevant except in very specific situations. These smaller branches will be discussed where clinically distinct syndromes exist.
A number of different processes may affect normal nerve function in a focal fashion, as compared to a disorder such as polyneuropathy in which more diffuse neuronal dysfunction occurs. (See "Overview of polyneuropathy".)