Applied Anatomy

The median nerve is one of the main terminal nerves of the brachial plexus, formed by contributions from the lateral and medial cords (Figure C14-1). The lateral cord component, comprised of C6–C7 fibers, provides sensory fibers to the thumb and thenar eminence (C6), index finger (C6–C7), and middle finger (C7) and motor fibers to the proximal median innervated forearm muscles. The medial cord component, comprised of C8–T1 fibers, provides sensory fibers to the lateral half of the ring finger (C8) and motor fibers to the hand and distal median innervated forearm muscles.

Figure C14-1 Median nerve course and branches in the forearm and hand. 1 = the palmar cutaneous branch, and 2 = the terminal median sensory nerve.

(From Haymaker W, Woodhall B. Peripheral nerve injuries. Philadelphia, PA: WB Saunders, 1953, with permission.)

The median nerve descends with no branches in the arm. In the antecubital fossa, it passes between the two heads of the pronator teres and sends muscular branches to the pronator teres, flexor carpi radialis, flexor digitorum sublimis, and palmaris longus muscles. Soon after and while in the proximal forearm, the median nerve gives off the anterior interosseous nerve which is a pure motor nerve that innervates the flexor pollicis longus, medial head of the flexor digitorum profundus and the pronator quadratus muscles.

Right before entering the wrist, the median nerve gives off its first cutaneous branch, the palmar cutaneous branch, which runs subcutaneously (does not pass through the carpal tunnel) and innervates a small patch of skin over the base of the thumb and the thenar eminence (see Figure C14-1). Then, the main trunk of the median nerve, along with nine finger flexor tendons, enters the wrist through the carpal tunnel. The carpal bones form the floor and sides of the tunnel while the carpal transverse ligament, which is attached to the scaphoid, trapezoid, and hamate bones, forms its roof (Figure C14-2). The carpal tunnel cross-section is variable but is approximately 2.0 to 2.5 cm at its narrowest point in most individuals.

Figure C14-2 A cross-section of the wrist revealing the carpal tunnel and its contents.

(From Hollinshead WH. Anatomy for surgeons: the back and limbs, 3rd ed., vol 3. Philadelphia, PA: Harper and Row, 1982, with permission.)

Right after exiting the tunnel, the median nerve branches into motor and sensory branches. The motor branch innervates the first and second lumbricals and gives off the recurrent motor branch, which innervates the thenar muscles (abductor pollicis brevis, opponens pollicis, and half of the flexor pollicis brevis). The sensory branch divides into terminal digital sensory branches to innervate three and one-half fingers (thumb, index, middle finger, and lateral half of the ring finger) with the corresponding palm.

Clinical Features

Carpal tunnel syndrome (CTS) is the most common entrapment neuropathy. It is slightly more common in women and usually involves the dominant hand first. It is most prevalent after 50 years of age, but it may occur in younger patients, especially in association with pregnancy and certain occupations or medical conditions. Most cases of CTS are idiopathic, but many are associated with disorders that decrease the carpal tunnel space or increase the susceptibility of the nerve to pressure. Among the medical conditions with a high risk for CTS are pregnancy, diabetes mellitus, hypothyroidism, acromegaly, rheumatoid arthritis, sarcoidosis, and amyloidosis. Some patients have congenitally small carpal tunnels, while others have anomalous muscles, wrist fractures (Colles or carpal bone), or space occupying lesions (ganglia, lipoma, schwannoma). Occupational CTS, which has reached a near-epidemic level in the industrial world, is seen in patients whose jobs involve repetitive movements of the wrists and fingers. Although most cases of CTS are subacute or chronic in nature, it occasionally may be acute, such as after crush injury of the hand, fracture (Colles or carpal bone), or acute tenosynovitis.

The most common symptoms of CTS are episodic numbness and pain in the affected hand, mostly at night. A characteristic of CTS is frequent awakening at night because of hand paresthesias, hence the name, nocturnal acroparesthesia. Symptoms usually are relieved by shaking the affected hand. In addition, these symptoms are often exacerbated by certain activities, such as driving, holding a book, or knitting. There is wrist and hand pain, which may radiate proximally to the forearm and, less commonly, to the arm or shoulder. Weakness of the hand and loss of dexterity are common in more advanced cases.

Phalen sign (reproduction of paresthesias in a median nerve distribution after passive flexion of the hand at the wrist) is extremely sensitive, present in 80–90% of patients with CTS with rare false positives. Tinel sign (paresthesias in a median nerve distribution after percussion of the median nerve at the wrist) is less common sign, present in about 50% of patients and may be false positive. On examination, there is often relative hypesthesia throughout the median nerve distribution, particularly in the fingertips and excluding the skin over the thenar eminence. Sometimes, the sensory loss is more selective to one or two fingers. Fasciculations or myokymia of the thenar muscles is not uncommon. Atrophy of the thenar muscles with weakness of thumb abduction may be evident in advanced cases. Less common associated conditions include vasomotor skin changes and Raynaud phenomenon.

The differential diagnoses of CTS include:

The treatment for CTS includes correcting the offending occupational factor or medical illness, wrist splinting at night, and the use of oral nonsteroidal anti-inflammatory agents or corticosteroids. Corticosteroid injection into the carpal tunnel area also is helpful to alleviate sensory symptoms and pain in patients with mild to moderate compression. Surgical decompression is indicated in patients with:

Electrodiagnosis

Carpal tunnel syndrome (CTS) is the most common reason for referral to the EMG laboratory. Aims of the EDX studies are to confirm the diagnosis by assessing the status of the median sensory and motor fibers across the carpal tunnel, and to exclude other possible causes of the symptomatology, such as a cervical radiculopathy.

The main underlying pathophysiology in CTS early in the course is primarily paranodal demyelination. Hence, the electrophysiologic hallmark of CTS is focal slowing of conduction at the wrist, resulting in prolongation of the latencies of both motor and sensory fibers. In severe and advanced CTS, axonal loss dominates the picture.