Anatomy

The brachial plexus is formed by the anterior rami of the last four cervical and first thoracic nerves (Fig. 4-1). In general, the upper portion of the plexus innervates the shoulder abductors, external rotators, and the elbow flexors. It also provides sensation to the shoulder and radial side of the arm. The lower portion of the plexus primarily innervates the forearm and hand muscles and provides sensation to the ulnar side of the arm, forearm, and hand. The first thoracic ramus also communicates with the first thoracic ganglion, through which sympathetic fibers are carried to the face from the spinal cord. Thus, involvement of the lower portion of the plexus by disease or injury may produce Horner’s syndrome (ptosis, miosis, enophthalmos, and anhidrosis).

Fig. 4-1 The brachial plexus. Five rami normally combine to form three trunks, which divide to form three cords. The lower trunk (C8, T1) lies on the first rib and is most commonly involved in thoracic outlet syndromes. The posterior cord continues as the radial nerve (R). The medial cord contributes half of the median nerve and continues as the ulnar nerve (U). The lateral cord continues as the musculocutaneous (MU) nerve after contributing the other half of the median nerve (ME). G is the ganglion.

The plexus passes distally between the middle and anterior scalene muscles, which attach to the first rib (Fig. 4-2). Beneath the clavicle, it is joined by the subclavian artery. The subclavian vein passes anterior to the scalenus anticus muscle. Artery, vein, and plexus then enter the axilla beneath the pectoralis minor muscle, with the lower trunk of the plexus (C8, T1) lying on the first rib.

Fig. 4-2 Anatomy of the cervicobrachial region (the overlying clavicle has been partially removed). Note that the lower trunk of the brachial plexus (C8, T1) lies on the first rib (arrow).

Thoracic Outlet Syndromes

Thoracic outlet syndrome is a term used to describe a controversial condition thought to produce upper extremity symptoms as a result of neurovascular compression at the base of the neck. The thoracic outlet is the space through which the brachial plexus, subclavian artery, and vein pass on their way into the arm. The disorder is controversial because it is difficult to prove and symptoms are often vague and variable.

Three types of syndromes have been described based on the point of compression: (1) the cervical rib and scalenus anticus syndromes, in which a cervical rib or abnormal scalene muscle insertions may cause compression; (2) the costoclavicular syndrome, in which compression may occur under the clavicle; and (3) the hyperabduction syndrome, in which compression may develop in the subcoracoid region. These disorders all have very similar clinical features, and it is often impossible to differentiate among them on this basis alone. The symptoms and signs are related to the degree of involvement of each of the various structures at the level of the first rib. Primary neural involvement may lead to pain and numbness. Arterial involvement usually causes the extremity to feel “asleep.” This symptom may have a glove-type distribution and is occasionally associated with paresis. Venous involvement, such as that seen with acute thrombosis of the subclavian vein (effort thrombosis), may produce swelling. True arterial or venous compression or thrombosis is not common, but when it occurs, the diagnosis is easily made based on physical and radiographic abnormalities. Unfortunately, the diagnosis of thoracic outlet syndrome is most often used in the consideration of only pain affecting the arm, a situation that usually relies on the interpretation of highly subjective signs and symptoms.

CERVICAL RIB SYNDROME

The cervical rib usually arises from the seventh cervical vertebra and is the most common cause of neurovascular compression at the base of the neck (Fig. 4-3). The condition may be bilateral. The rib or its fibrous extension narrows the interval between the anterior and middle scalene muscles and produces a higher barrier over which the neurovascular structures must arch on their way into the arm. In older patients or those with muscular weakness, the shoulder may also sag more than normal, which further increases the tension on the neurovascular structures. The compression may also be increased by carrying a heavy object.

Fig. 4-3 A, Compression caused by a cervical rib (arrow). B, Abnormal scalene muscles that may cause compression at the cervicobrachial region (arrow).

In the absence of a cervical rib, compression can occur between the middle and anterior scalene muscles because of abnormal insertions or the presence of additional muscle slips in the interscalene interval (scalenus anticus syndrome).

The lowest components (C8, T1) of the plexus are most commonly involved because of their position against the rib. The symptoms therefore tend to be most noticeable in the hand and inner aspect of the forearm. Pain and paresthesias may develop along the distribution of the ulnar nerve. There may be weakness, numbness, or clumsiness in use of the hand. Coldness, Raynaud’s phenomenon, or even gangrene may be the initial symptom.

Clinically, the cervical rib may be palpable, and the brachial plexus is often tender. Weakness and atrophy of the muscles supplied by the lower trunk (interosseous, hypothenar muscles) are occasionally seen. Sensation may be diminished over the ulnar aspect of the forearm, arm, and the ulnar 1½ fingers. Swelling, coldness, cyanosis, trophic skin changes, and other signs of circulatory insufficiency are occasionally observed.

Provocative positioning tests are often described in the assessment of thoracic outlet syndromes. Adson’s test may be positive in cervical rib and scalenus anticus syndromes (Fig. 4-4). This test takes advantage of the fact that by tensing the scalene muscles the interval between them is decreased, and any existing compression of the subclavian artery is increased. The test is performed by having the patient breathe deeply, extend the neck, and turn the chin toward the affected side. When the test is positive, a decrease in the radial pulse is noted, and pain is reproduced. If the test is negative, it is repeated with the chin turned to the opposite side. Although a positive test result is suggestive of compression in the interscalene region, it is often positive in the normal population and is not necessarily diagnostic of cervical rib or scalenus anticus syndrome. Symptoms can also be reproduced occasionally by exercising the fingers with the arms elevated (Fig 4-5).

Fig. 4-4 Adson’s test. It should be repeated with the chin turned the opposite direction.

Fig. 4-5 Elevated arm stress test. With the arms held overhead, the hands are opened and closed. A positive test result is pain and fatigue within 30 seconds.

Roentgenographic examination may reveal an extra rib extending from the transverse process of the seventh cervical vertebra (Fig. 4-6). The rib may be fully developed or rudimentary, and it is often bilateral. Its presence does not necessarily imply that it is symptomatic, however, because cervical ribs are often present in asymptomatic individuals.