Median Nerve Palsy

The median nerve is a continuation of the middle and lateral cords of the brachial plexus that receives innervation from all roots of the brachial plexus (C5-T1). After leaving the shoulder, it travels with the brachial artery under the ligament of Struthers, the bicipital aponeurosis, and the two heads of pronator teres into the anterior compartment of the forearm. Compression at this point in the course of the median nerve results in pronator syndrome. Just distal to the elbow joint, the median nerve gives off its first terminal branch: the anterior interosseous nerve (AIN).The AIN travels in the deep flexor compartment of the forearm between the flexor digitorum profundus (FDP) and the flexor pollicis longus (FPL) until it terminates in the pronator quadratus (PQ). The AIN provides motor innervation to the FPL, the FDP to the index and middle fingers, and the PQ. Note that the ulnar half of FDP to the little and ring fingers is innervated by the ulnar nerve.The palmar cutaneous branch of the median nerve branches from the median nerve in the forearm, and travels to the hand to innervate the lateral (radial) aspect of the palm. Importantly the palmar cutaneous nerve does not travel through the carpal tunnel, which explains why sensation to the lateral palm is typically spared in carpal tunnel syndrome.After giving off the AIN and the palmar cutaneous branch, the median nerve travels distally between the flexor digitorum profundus (FDP) and the flexor digitorum superficialis (FDS). In the forearm, the median nerve provides motor innervation to all superficial flexors of the forearm, while the AIN provides motor innervation to the deep flexors. The exception to this is the flexor carpi ulnaris (FCU), which is innervated by the ulnar nerve. The FCU is the strongest flexor of the wrist and receives its innervation from the ulnar nerve. The median nerve then enters the hand through the carpal tunnel. There are nine other anatomic structures in addition to the median nerve passing through the carpal tunnel: the four tendons of the flexor digitorum profundus (FDP), the four tendons of the flexor digitorum superficialis (FDS), and the flexor pollicis longus (FPL) tendon. The carpal tunnel is defined by the proximal carpal row dorsally and the transverse carpal ligament palmarly. Incision and release of the transverse carpal ligament is the treatment for carpal tunnel syndrome. In the hand, the median nerve divides into two branches: the recurrent motor branch of the median nerve and the digital cutaneous branch of the median nerve. The recurrent motor branch of the median nerve is a radial structure at the level of the carpal tunnel and provides motor innervation to the muscles of the thenar eminence, including abductor pollicis brevis, opponens pollicis, and the superficial head of the flexor pollicis brevis. Of note, the deep head of flexor pollicis brevis and the adductor pollicis brevis receive their innervation from the ulnar nerve.The digital cutaneous branch of the median nerve further branches into the proper palmar digital branch and the common palmar digital branch. The proper palmar digital branch is more radial and provides the digital nerves to the thumb and the radial digital nerve of the index finger, which also supplies the first lumbrical. The common digital branch then branches into the ulnar digital branch of the index finger, both digital nerves of the middle finger and the radial digital nerve of the ring finger, which also provides innervation to the second lumbrical. Note that other than the aforementioned muscles of the thenar eminence and the radial two lumbricals, all other intrinsic muscles of the hand receive motor innervation from the ulnar nerve.While detailed, an understanding of the anatomy of the median nerve and its branches can help practitioners to accurately diagnose the cause of median nerve pathology and determine the appropriate treatment. Palsies of the median nerve may be acute, requiring urgent intervention or chronic, indicating a more conservative approach. Therefore, a history and physical exam are the most important tools a practitioner can employ for appropriate management of such conditions.