The median nerve is formed by the combination of the medial (C8, T1) and the lateral cords (C6, C7) of the brachial plexus. The median sensory fibers innervate the thenar eminence; the thumb; and the index, middle, and lateral half of the ring finger. The motor fibers supply the proximal median forearm muscles and the muscles of the thenar eminence (opponens pollicis, abductor pollicis brevis, flexor pollicis brevis, and first and second lumbricals). The forearm muscles supplied by the median nerve are the pronator teres, flexor carpi radialis, palmaris longus, flexor pollicis longus, flexor digitorum sublimis, flexor digitorum profundus, and pronator quadratus (through the anterior interosseous nerve) see Fig. 13.1.

In the upper arm, the median nerve descends medial to the humerus and anterior to the medial epicondyle. However at the distal third of humeral shaft the median nerve is encased by the ligament of Struthers, which extends between the spur just superior to the medial epicondyle and the medial humeral epicondyle. In the antecubital fossa, the median nerve travels along and lateral to the brachial artery. As this nerve enters the forearm, it is inferior to a thick fibrous band (lacertus fibrosus) that extends from the medial aspect of the biceps tendon to the proximal forearm flexor musculature. The nerve then runs between the two heads of the pronator teres muscle to provide innervation to that muscle. The anterior interosseous nerve arises from the median nerve approximately 5–8 cm distal to the medial epicondyle. As the median nerve runs distally, it traverses deep to the flexor digitorum sublimis. It innervates the flexor pollicis longus, the medial head of the flexor digitorum profundus to the index and the middle fingers, and the pronator quadratus muscle in the distal forearm. This nerve is a pure motor nerve. However, some deep sensory fibers are carried through the anterior interosseous nerve to supply the wrist joint.

Before the median nerve enters the carpal tunnel, it gives off a palmar cutaneous sensory branch to supply sensation over the thenar eminence. The nerve then enters the carpal tunnel along the nine flexor tendons. The sides and the floor of the tunnel are formed by the carpal bones, and the thick transverse carpal ligament forms the roof.

In the palm, the median nerve divides into motor and sensory divisions. The motor division supplies the first and second lumbricals, and the recurrent thenar motor branch supplies the abductor pollicis brevis, the opponens pollicis, and the superficial head of the flexor pollicis brevis. The sensory fibers supply the medial thumb, the entire index finger, the entire middle finger, and the lateral half of the ring finger through digital lateral and medial branches. The clinical symptoms and signs of median nerve entrapment syndromes depend on the segment of the nerve that is entrapped.

Carpal tunnel syndrome (CTS) is considered to be the most common example of median nerve entrapment, the entrapment occurring within the carpal tunnel at the wrist. CTS involves all the sensory branches to the median-innervated digits, as well as the motor branch to the first two lumbricals. CTS also involves the recurrent thenar motor branch that supplies the abductor pollicis brevis and the flexor pollicis brevis muscles. The diagnosis of this syndrome is based on history, clinical findings, and electrophysiological studies.

In a multivariate analysis, the best combination of clinical tests was found to be the tourniquet test, the carpal compression test, and Phalen’s test, but the difference between these and the hand elevation test alone was negligible. Hence, the hand elevation test may be used alone, and it is superior to the questionnaires and other physical signs used in the clinical diagnosis of CTS.

The pathophysiology of CTS in the early stages is demyelination, accompanied by axonal loss in the advanced stages. The reduction in the number of axons can be estimated by the motor unit number estimation (MUNE) method.