What is Thoracic Outlet Syndrome?

Thoracic outlet syndrome is a complex disorder characterized by a constellation of signs and symptoms resulting from the compression of blood vessels and nerves (neurovascular bundle) in the thoracic outlet region where they exit the chest. The thoracic outlet is a space located between the thorax (rib cage) and the clavicle (collar bone) which contains major blood vessels (subclavian artery and vein) and nerves (brachial plexus). The thoracic outlet is the area through which nerves and blood vessels travel to and from the arm.

Thoracic outlet syndrome is considered a “syndrome” since it involves multiple systems, including:

Neural (nerve) complexes
Vascular structures
Musculoskeletal system

Neural Complex

The nerves that travel through the thoracic outlet originate at the level of the cervical and/or thoracic spine (C5-C8 and T1). These nerves are bundled together into the brachial plexus. The brachial plexus passes through a notch in the bone at the base of the neck and then passes under the scalene muscles and continues under the collarbone and across the front of the shoulders. In the area of the axilla (arm pit) the brachial plexus divides into the three major nerves of the arm, ulnar, median, and radial nerves. Thoracic outlet syndrome involves the brachial plexus before it divides and typically affects the ulnar and radial nerves.

Vascular Structures

The blood vessels that travel through the thoracic outlet include:

Subclavian artery which supplies oxygenated blood to the arm from the aorta
Subclavian vein which returns the deoxygenated blood from the arm to the heart

Musculoskeletal System

The bones and muscles form the backdrop for proper passage and support for the nerves and blood vessels that traverse the upper body. The components involved in this process include:

Proper bone alignment – the neck vertebrae, first rib and collarbone must be aligned properly to allow enough space for the brachial plexus and subclavian blood vessels to pass through the correct path without obstruction or interference.
Proper muscle alignment – muscles of the upper body must be aligned in proper form, particularly the scalene muscles. The scalene muscles consist of three powerful muscles on each side of the neck that bend and rotate the neck, and assist in breathing by raising the first two ribs during inspiration(breathing in). The ideal posture which promotes the most appropriate muscle alignment is when the head sits directly atop the shoulders which we identify as erect posture.

Anatomy of the Thoracic Outlet

There are actually three spaces where compression can occur causing the symptoms of thoracic outlet syndrome , including:

The thoracic outlet (also called the scalene triangle) – is located at the base of the neck above the first rib and behind the clavicle. This space lies closest to the neck. The boundaries of the thoracic outlet consist of:
- anterior scalene muscle which forms the front of the thoracic outlet
- middle scalene muscle which forms the back of the thoracic outlet
- first rib which forms the bottom of the thoracic outlet.

The subclavian artery and the brachial plexus are located between the first rib and the scalene muscle while the subclavian vein sits outside the scalene muscle.

Costoclavicular space – this space is located adjacent to the scalene triangle and lies between the following structures:CLavicle, first rib,costoclavicular ligament,edge of the middle scalene muscle.

The subclavian artery, vein, and brachial plexus all pass through the costoclavicular space.

Subcoracoid space which lies adjacent to the costoclavicular space and is closest to the arm. The location of this space is:
- under the pectoralis muscle
- under the coracoid process
- in front of the ribs

If there are any anomalous bony structures (e.g., cervical rib) touching upon these small spaces, they have the potential to compress the neurovascular structures which pass through them. Approximately 0.17 to 0.74% of the general population has an anomalous rib and only 10% of people with such a cervical rib actually develop thoracic outlet syndrome. Thoracic outlet syndrome typically occurs following trauma to the cervical spine.

Because thoracic outlet syndrome is thought by some experts to be underdiagnosed and in some cases misdiagnosed, it is difficult to estimate with any degree of accuracy how many people suffer from this condition. The incidence of thoracic outlet syndrome in the U.S. population has been broadly estimated to range from 0.3% to 8%. The most common age range is 25 to 40 years and women are affected about 4 times more frequently than men.

Knowledge is Critical when Dealing with a Life-Altering Condition such as Thoracic Outlet Syndrome

The theories regarding the underlying causes of thoracic outlet syndrome.
The risk factors that can increase a person’s chances for developing thoracic outlet syndrome.
A detailed overview of the three major types of thoracic outlet syndrome, which include:
- Neurogenic thoracic outlet syndrome
- Vascular thoracic outlet syndrome
- Neurovascular thoracic outlet syndrome

The signs and symptoms of thoracic outlet syndrome.
How thoracic outlet syndrome is diagnosed based on factors such as signs/symptoms, patient history, physical examination, provocative compression maneuvers, and imaging studies. *Other underlying medical conditions that may be confused with thoracic outlet syndrome and must be considered in the differential diagnosis of thoracic outlet syndrome.

Understanding the standard treatments – and the treatment options – is critical for successfully achieving the goals of treatment for thoracic outlet syndrome. As you read through the section of the Guidebook that focuses on the treatments for thoracic outlet syndrome, you will specifically learn about:

The role of conservative treatments as the first-line approach for the treatment of thoracic outlet syndrome, including:
- Physical therapy
- Ultrasound therapy
- Transcutaneous electrical nerve stimulation (TENS)
- Postural training
The role of lifestyle modifications in helping patients to better control the symptoms of thoracic outlet syndrome.

Radiography

Plain images, such as chest radiographs and upper thoracic and cervical spine studies, can effectively depict congenital or acquired bony anomalies (eg, cervical ribs, healed fractures). Radiography is important if prior images are not available. Such findings may help focus subsequent more complex and more invasive radiologic studies of a particular region. In addition, unsuspected findings such as a Pancoast tumor of the lung may be identified on these initial studies.

False positives/negatives

Although plain radiography is relatively inexpensive as a radiologic screening test, it is highly limited in its ability to depict the fine anatomic details that contribute to the symptoms of thoracic outlet syndrome. Plain radiography has a lower sensitivity for these findings than other modalities, such as CT.

Computed Tomography

CT is most helpful when plain radiographs show abnormal findings involving the thoracic outlet. CT performed both before and after intravenous administration of contrast agent and CT angiography are useful in identifying lesions (eg, cervical disks, neoplasm, bony spurs) that encroach on the structures of the thoracic outlet.

Investigators have reported the use of various sophisticated protocols for spiral CT angiography. After a prospective analysis, Remy-Jardin et al concluded that reconstructed volume-rendering images have the highest sensitivity (95%) and specificity (100%), compared with cross-sectional imaging and image reconstruction with multiplanar and 3-dimensional-shaded surface display techniques. In another article, Remy-Jardin et al describe changes in the functional anatomy of the thoracic outlet in 79 patients with symptomatic thoracic outlet syndrome who underwent CT angiography.

Degree of confidence

Most clinicians consider arteriography or venography to be the definitive tests for identifying vascular lesions and their associated complications. Both CT and MR angiography are relatively new and promising techniques for the vascular evaluation of the thoracic inlet that continue to evolve.

Magnetic Resonance Imaging

Multiple MR angiographic sequences and protocols can be used to obtain images of the arterial and venous anatomy of the body, including 2-dimensional (2D) and 3-dimensional (3D) time-of-flight, pulse-echo, and phase-contrast imaging. Compared with conventional angiography, MR imaging combined with MR angiography has the added advantage of enabling visualization of adjacent soft tissue abnormalities; it is particularly useful in evaluating fibro muscular causes of narrowing. Additionally, the specific application of MR angiography to the thoracic outlet has been reported.

Dymarkowski et al reported the use of 3D time-resolved contrast-enhanced MR angiography and T1-weighted spin-echo MR imaging for the evaluation of vascular causes of thoracic outlet syndrome. Five patients with symptoms and clinical examination findings suggestive of arterial or venous thoracic outlet syndrome were studied during ipsilateral arm adduction and hyper abduction. In all patients, 3D MR angiography during hyper abduction revealed the location of the vascular compression, while the images obtained during adduction showed normal vessel patency. The stenosis were precisely located with the maximum intensity projection images.

In 3 patients in the Dymarkowski study, the results of conventional angiography performed within a 2-day interval confirmed the location. The T1-weighted images showed the anatomic etiology of the vascular compression, which was confirmed during surgery. The findings of this small study demonstrate the potential of MR angiography in the diagnosis of vascular causes of thoracic outlet syndrome.

Demondion et al also described the normal MR anatomy of the thoracic outlet and its alteration with positional maneuvers by correlating the gross anatomic images with the corresponding MR images.

Degree of confidence

As discussed above, MRI is a promising noninvasive technique for the diagnosis of vascular causes of thoracic outlet syndrome, allowing the evaluation of both the vascular and soft-tissue anatomy of the thoracic inlet. Potential drawbacks of MR imaging include factors that may limit the image quality and the relatively time-consuming process of obtaining images. Dymarkowski et al noted that the need for a time interval between the acquisitions of the 2 series of images was a potential drawback to their approach. Additional larger trials are needed to compare the accuracy of MR imaging with that of conventional angiography, as well as to confirm the clinical utility of MR angiography.

False positives/negatives

Several factors may limit the image quality of MR imaging and contribute to artifacts that reduce its sensitivity. These factors include abrupt changes in the path of a vessel, turbulent flow, changes in the direction of flow relative to the plane of imaging, and patient motion. As MR angiography continues to evolve, newer techniques may be able to overcome many of these pitfalls.

The need to study the patient in different positions (eg, in abduction or in adduction) is important for the physician performing the MR imaging study because a simple image obtained in the anatomic position may obscure unprovoked vascular compression.

Ultrasonography

The arterial system and the venous system of both upper extremities are usually studied during duplex ultrasonography. The patient lies supine with his or her head turned to the side opposite to that being examined. The arm is initially examined in a neutral position and then at varying degrees of abduction, such as 90°, 135°, and 180°. The subclavian artery and the entire venous circulation, including the internal jugular vein, subclavian vein, axillary vein, and innominate vein (in which the portion just above the superior vena cava may not be visualized), are examined.

The criteria for hemodynamically significant venous compression include the obliteration of flow through the subclavian vein or the loss of normal cardiac pulsatility or respiratory phasicity. The criteria for hemodynamically significant arterial narrowing include a 2-fold or greater increase in the peak systolic velocity compared with that measured with the arm in the neutral position or the obliteration of flow. Duplex ultrasonography has been useful in the follow-up care of patients after surgical or radiologic intervention, and a baseline post procedural examination is routinely performed.

False positives/negatives

Although ultrasonography is a useful noninvasive test, the false-positive rate with the arterial criteria is 20%, and the false-positive rate with the venous criteria is 10%. Another limitation of duplex ultrasonography is the incorrect identification of a large collateral vein as the subclavian vein in subclavian vein thrombosis. Careful delineation of the entire course of the vessel and its relationship to the artery, which normally is posterior to the subclavian vein, is necessary in order to avoid this mistake. At the present time, another study (eg, conventional angiography) is usually performed to confirm abnormalities identified with ultrasonography.

Arteriography

Arteriography is the most specific diagnostic examination for thoracic outlet syndrome, and it is indicated in a patient with ischemic upper extremity symptoms. The entire arterial circulation of the upper extremity is examined from the aortic arch to the distal arteries of the fingers. Access through the common femoral artery is preferred to evaluate both upper extremities, if necessary, and to evaluate various stress positions. In addition, in the presence of subclavian artery occlusion, delayed imaging after injection may be useful to demonstrate antegrade collateral circulation, with distal reconstitution of the artery beyond the point of obstruction (see the image below).

An angiogram in a 35-year-old woman with right arm ischemia that demonstrates right subclavian artery occlusion from the medial margin of the first rib to the axillary artery at the level of the humeral head. The patient was successfully treated with right first rib resection.

Examination in a minimum of 3 positions may be required to demonstrate findings that may not be present in 1 position but are evident in another. In many patients, smooth extrinsic arterial narrowing is seen only on images obtained during stress.

The examination is begun with the patient in the completely adducted neutral position. Although the findings are often limited to minimal narrowing at most (or no findings are seen at all), the neutral position is best for evaluating intrinsic arterial diseases, arterial thrombosis, and poststenotic dilatation or aneurysms.

The Lang maneuver is then performed, with the arm abducted to 90° and with the patient lifting a 2-lb weight 2 cm above a tabletop. Images are obtained during deep inspiration and with the patient’s head turned sharply to the opposite side. The Lang maneuver, also called the modified Allen maneuver, elicits sites of compression in the costoclavicular space, the scalene triangle, and the costocoracoid space inferior to the pectoralis minor tendon, a site of compression of the axillary artery; however, evidence of minor compression with this exaggerated stress position has also been shown in persons without thoracic outlet syndrome.

The Adson maneuver consists of depression of the patient’s shoulder with his or her head turned to the symptomatic side.

Other positions for imaging include hyper abduction of the arm and the costoclavicular maneuver; the clinical examinations with these positions are equivalent (see the images below).

A venogram that was obtained in a 28-year-old man complaining of intermittent right arm swelling, taken with his right arm in the anatomic (neutral) position. A venogram of a 28-year-old man complaining of intermittent right arm swelling, obtained with the right arm in abduction. High-grade mid-subclavian venous stenosis in the region of the thoracic inlet is depicted (same patient as in the previous image).

If technically feasible, an evaluation of the patient under conditions that produce the symptoms should be considered. These conditions may include various sitting positions with the arm abducted.

Frequently, findings are readily apparent, with severe narrowing of the subclavian artery accompanied by poststenotic dilatation or a subclavian artery aneurysm. Such findings are strong indicators of a hemodynamically significant lesion. Other findings that may indicate extrinsic compression of the subclavian artery include a ridge like defect of the inferior margin, which indicates compression against the first rib; a similar defect combined with an impression along its superior margin, which indicates narrowing of both the scalene triangle and the costoclavicular space; or a tapered cutoff of the artery as it emerges from the scalene tunnel, which indicates compression within the scalene triangle and the scalene tunnel.

Another finding that is not accepted as a strong indicator of a hemodynamically significant lesion includes an oblique compression defect or a twist of the subclavian artery as it passes through the scalene tunnel.

Venography

Ascending brachial venography is the preferred and most definitive examination for diagnosing venous thrombosis. Collateral veins are more evident with venography than with ultrasonography, and therapeutic techniques, such as thrombolysis, can be readily performed if necessary. Venography is indicated in patients with definite symptoms of venous obstruction or when a duplex ultrasonography finding suggests venous abnormalities.

During the procedure, the patient’s arm is placed in slight abduction to prevent artificial venous occlusion. Because the cephalic vein may lead to collateral vessels that bypass the site of thrombosis in the subclavian or axillary veins, the basilic vein is injected to ensure complete opacification around the venous thrombosis. One potential drawback of venography is that the proximal extent of the thrombus may be difficult to accurately identify.

Two major findings of upper extremity venous thrombosis are described. The first finding is a short area of obstruction at the subclavian-axillary vein junction between the first rib and the clavicle at the thoracic inlet. A variable number of adjacent collateral veins are visualized. This finding is often seen with acute obstruction. The second finding is a long area of obstruction extending distally from the axillary vein and possibly into the brachial vein. Approximately 75% of cases also involve the subclavian vein, though innominate vein involvement is rare (see the image below). Chronic and intermittent venous obstruction with or without a thrombus may be seen as an area of scarring or narrowing at the subclavian-axillary vein junction.

Degree of confidence

At the present time, most clinicians consider catheter-based arteriography or venography to be the criterion standard for evaluating the vascular anatomy of the thoracic outlet; however, MR angiography and CT angiography are 2 promising and relatively new techniques that may be able to replace many diagnostic conventional angiographic examinations. Further clinical trials are required to compare the accuracy of these new techniques with that of conventional angiography before they can be considered acceptable alternatives in clinical decision-making.

The diagnosis of TOS is clinically based. Imaging may be helpful in informing the clinician as to the anatomic structures undergoing compression, the location of that compression, and the anatomic structures responsible for it (whether normal or abnormal). Indeed, all these features may influence the treatment. Radiographs may demonstrate predisposing bone abnormalities (elongated C7 transverse process, cervical rib). To the best of our knowledge, the respective advantages of the complementary imaging techniques have not been assessed in comparative studies. In the case of neurogenic or neurovascular symptoms, MR imaging in association with postural maneuvers has proved useful, especially in demonstrating brachial plexus compression and the existence of fibrous bands. CT with contrast medium injection (if not contraindicated) and postural maneuvers appears effective in demonstrating vascular compression by means of volume-rendered images, which allow analysis of the relations with bony structures. Color duplex sonographic examination and B-mode scanning, in association with postural maneuvers, is a valuable supplementary tool to CT and MR imaging when the results of the latter prove negative.