This nerve originates from the fifth cervical to the first thoracic roots but mainly from the sixth cervical root and is formed by the union of the medial and lateral cords of the brachial plexus. It innervates the pronators of the forearm, long finger flexors, and abductor and opponens muscles of the thumb and is a sensory nerve to the palmar aspect of the hand. Complete interruption of the median nerve results in inability to pronate the forearm or flex the hand in a radial direction, paralysis of flexion of the index finger and terminal phalanx of the thumb, weakness of flexion of the remaining fingers, weakness of opposition and abduction of the thumb in the plane at a right angle to the palm (abductor and flexor pollicis brevis), and sensory impairment over the radial two-thirds of the palm and dorsum of the distal phalanges of the index and third fingers. The nerve may be injured in the axilla by dislocation of the shoulder and in any part of its course by stab, gunshot, or other types of wounds, and like the radial nerve, is often a component of the mononeuritis multiplex syndrome. Incomplete lesions of the median nerve between the axilla and wrist may result in causalgia (see further on).
Carpal tunnel syndrome (See also Chap. 10.) Compression of the median nerve at the wrist (carpal tunnel syndrome) is the most common disorder affecting the median nerve and the most frequent nerve entrapment syndrome. The problem arises usually as a result of excessive use of the hands and occupational microtrauma. Infiltration of the transverse carpal ligament with amyloid (as occurs in multiple myeloma and amyloidosis) or thickening of connective tissue in rheumatoid arthritis, acromegaly, mucopolysaccharidosis, and hypothyroidism are less commonly identified causes. It is also common for the condition to make its appearance during pregnancy. In elderly individuals, the cause of the carpal tunnel syndrome is often not apparent. According to Kremer and colleagues, it was McArdle, in 1949, who first suggested that the cause of this syndrome was compression of the median nerve at the wrist and that the symptoms would be relieved by division of the flexor retinaculum forming the ventral wall of the carpal tunnel. Dysesthesias and pain in the fingers, referred to for many years as “acroparesthesia” came to be recognized as a syndrome of median nerve compression only in the early 1950s.
The syndrome is essentially a sensory one; the loss or impairment of superficial sensation affects the palmar aspect of the thumb and the index and middle fingers (especially the index finger) and may or may not split the ring finger (splitting does not occur with a plexus or root lesion). The paresthesias are characteristically worse during the night. As pointed out in Chap. 10, the pain in carpal tunnel syndrome may radiate into the forearm and even into the region of the biceps and rarely, to the shoulder. Weakness and atrophy of the abductor pollicis brevis and other median-innervated muscles occur in only advanced cases of compression. Electrophysiologic testing confirms the diagnosis by demonstrating prolonged sensory conduction across the wrist and explains cases in which operation has failed (see the review by Stevens).
Several provocative tests are useful. The Phalen maneuver consists of hyperflexion of the wrist for 30 to 60 s—usually performed by opposing the outer surfaces of the hands with the wrists flexed. The Tinel sign is elicited by lightly tapping the volar aspect of the wrist at the transverse carpal ligament (distal to the first wrist crease). Both of these tests are meant to elicit pain or paresthesias over the digits innervated by the median nerve. The sensitivity of these tests is close to 50 percent, but their specificity is considerably higher. Other tests involving prolonged pressure over the median nerve have been devised, but they are of uncertain value, for example, Durken’s test of the Phalen maneuver combined with digital compression of the nerve.