Thoracic Outlet Syndrome: Pathophysiology and Diagnostic Evaluation

Nerves

In addition to the brachial plexus, the phrenic and long thoracic nerves are seen during supraclavicular dissections. Very close to the area but rarely seen are the dorsal scapular nerve and cervical sympathetic chain.

Variations of the Scalene Muscle

A significant number of anatomic variations in scalene muscles have been observed both in the normal population and in patients with nTOS. They are too common to be called anomalies. These variations might be regarded as anatomic and developmental variations predisposing to TOS, but are not regarded as causes.

• Splitting of the anterior scalene around C5 and C6. Such splitting was noted in 45% of cadavers and in only 21% of nTOS patients.

• Scalene minimus muscle. This muscle arises from the transverse processes of the lower cervical vertebrae, runs in front of C8 and T1 and behind the subclavian artery, and inserts on the first rib or Sibson fascia. Its incidence is 25% to 55%.

• Interdigitating muscle fibers. Such fibers commonly run between the anterior and middle scalene muscles. They were noted in 75% of nTOS patients and in only 40% of cadavers.

• Scalene triangle width. The width of the scalene triangle varies from very narrow to quite wide, with the distance between the two scalene muscles at their insertion on the first rib ranging from 0.3 to 2.0 cm. The nerve roots emerge lower in a wide triangle compared with their pathway in a narrow triangle ( Fig. 123.4 ). Observations during surgery have revealed that most nTOS patients have the narrower type of triangle.

  

• Congenital bands and ligaments. These structures are frequently observed in the normal population, with an incidence as high as 63%. Although bands have been observed in almost every area of the scalene triangle and have been nicely classified, their role is as a predisposing factor rather than a direct cause.

• Relationship of the subclavian vein to the phrenic nerve. The phrenic nerve usually runs posterior to the subclavian vein. However, in 5% to 7% of individuals, the phrenic nerve runs anterior to the vein and in this position can be a rare cause of subclavian vein obstruction.

• Relationship of the subclavian vein and costoclavicular ligament. The subclavian vein passes cephalad to the first rib just before it is joined by the jugular vein. It is bounded medially by the costoclavicular ligament and superiorly by the subclavius tendon. A variation in the position of the vein is its position more medially on the first rib so that it is partially compressed by the costoclavicular ligament. Variations in the thickness of the subclavius tendon can also result in compression of the vein from above. These variations in position between the three structures predispose to the development of subclavian vein obstruction. Obstruction is then caused by excessive abduction of the upper extremity, which results in compression of the vein against the dynamic outlet created by the costoclavicular ligament, subclavius tendon, and the first rib.

Skeletal Abnormalities

As many as 29% of patients with TOS have been reported to have skeletal anomalies, including cervical ribs, clavicular anomalies, and isolated first rib abnormalities; the majority of which are congenital. Anomalous or rudimentary first ribs are often overlooked on routine chest radiographs. In one study conducted in 1939, 5000 chest radiographs were reviewed. It was found that the incidence of anomalous first ribs and cervical ribs in these subjects was 0.76% and 0.74%, respectively. Anomalous ribs occurred equally in women and men. In contrast, the female–male ratio of cervical ribs was found to be 7:3. Cervical ribs and anomalous first ribs are both narrower and are positioned more cephalad than a normal first rib. The only difference between the two is that a cervical rib arises from the transverse process of C7, whereas an anomalous first rib arises from the transverse process of T1 (see Fig. 123.3 ).

About 30% of cervical ribs are complete ribs fused to the first rib by a true joint or a fibrous attachment. The other 70% are incomplete cervical ribs with no direct attachment to the first rib. However, most of these incomplete ribs have a tight fascial band extending from the tip of the cervical rib to the first rib. The cervical rib and attached band lie within the middle scalene muscle, thereby narrowing the space within the scalene triangle through which the nerve roots of the brachial plexus course. Thus an incomplete cervical rib with associated band can behave like a complete rib. Cervical and anomalous first ribs can compress the subclavian artery and cause stenosis with or without poststenotic aneurysm formation. More commonly, these anomalous ribs compress the lower trunk of the brachial plexus and cause neuropathy. However, most cervical and anomalous first ribs remain asymptomatic throughout life.

When symptoms do develop, they are usually neurogenic. In a recent review of 47 nTOS operations involving abnormal ribs, 15% were performed in the setting of anomalous first ribs and 85% were performed in the setting of cervical ribs. Spontaneous onset of symptoms occurred in 50% of patients with complete cervical ribs versus only 20% with incomplete ribs. Neck trauma preceded the onset of symptoms in the other 80% with incomplete ribs and in the 50% with complete ribs. All patients with cervical ribs are predisposed to the development of some form of TOS, most commonly nTOS. Cervical ribs or anomalous first ribs are almost always present in patients with aTOS. On the other hand, symptomatic cervical ribs are more likely to produce symptoms of nTOS than aTOS. Over a 28-year period during which more than 1000 nTOS operations were performed, there were 39 cervical ribs and 7 anomalous first ribs treated in this group of patients, with an incidence of 4%. Among the same group of TOS patients there were only nine with aTOS, five with complete cervical ribs and four with anomalous first ribs, with an incidence of less than 1%.

Contributions of the Pectoralis Minor

The PM can also cause compression of the structures of the thoracic outlet, leading to nTOS, vTOS, and aTOS in rare circumstances. Pectoralis minor syndrome (PMS) results when this muscle causes compressive symptoms in the subcoracoid or PM space. The PM space is actually outside the thoracic outlet area, but compression of the upper extremity neurovascular bundle can occur in this space between the PM muscle and ribs of the chest wall. Although this might better be termed “axillary compression syndrome,” it is appropriate to consider the PM space as an extension of the thoracic outlet area, because compression here is common and the axillary and the thoracic outlet neurovascular bundles contain the same elements (see Fig. 123.1 ). The PM syndrome results from excessive trauma or exercise of the shoulder girdle caused by hyperabduction. The PM muscle becomes tight and hypertrophied and can compress the brachial plexus, axillary vein, and rarely the axillary artery. PMS is frequently responsible for recurrent nTOS after thoracic outlet decompression, being responsible for up to 75% of cases of recurrent nTOS. It is this PM space that should be evaluated more carefully, particularly in patients with recurrent or persistent symptoms, after initial operation for nTOS.