New Perspectives on Neurogenic Thoracic Outlet Syndrome
The definition, incidence, diagnosis, and treatment of thoracic outlet syndrome (TOS) are somewhat controversial. Originally coined in 1956, the term TOS indicated a “compression of the neurovascular structures in the interscalene triangle corresponding to the possible etiology of the symptoms.”1 The controversy is centered on the fact that TOS refers to the anatomy or location of the problem without identifying the cause—either vascular or neurogenic.
Therefore, TOS generally is defined as a group of disorders caused by compression of the brachial plexus, subclavian artery, or subclavian vein in the thoracic outlet, the area between the clavicle (collarbone) at the base of the neck and the first rib, including the front of the shoulders and chest. TOS is a progressive condition marked by the impingement of the nerves and blood vessels that feed the thoracic outlet. Compression of the subclavian muscle, which is known to compress the subclavian artery, reduces bloodflow to the carotid arteries and the vertebral arteries. This may lead to intractable migraine, one of the first symptoms of TOS.
Neurogenic TOS (NTOS), the most common form of TOC, can result from inadequate space caused by scalene hypertrophy, fibrosis, or congenital abnormalities, such as the occurrence of a cervical rib. Other causes include repetitive motions that can enlarge or change the tissue in or near the thoracic outlet (similar to carpel tunnel syndrome). These repetitive activities include assembly line work, typing, and other movements; hyperextension-flexion injuries; neck injuries from motor vehicle accidents (whiplash); and sports-related injuries, particularly from swimming, baseball (pitching), weightlifting, and volleyball.
Frequent symptoms of NTOS include numbness; tingling in the fingers; pain in the neck, shoulder or arm; muscle spasms around the scapula; headaches; and weakness in the upper extremities (Table 1).2
In many patients, the etiology of NTOS involves a combination of a “double hit” of a congenital predisposition and an injury to the area that compromises the outlet. The narrowed space affects the scalene muscles, the brachial plexus, the long thoracic and suprascapular nerves, and the stellate ganglion (Figure 1).
Although the notion of NTOS as a complex spectrum disorder provokes some controversy in the field, its impact on patients is beyond dispute. Data indicate that the quality of life for a patient with untreated TOS is as impaired as that of someone with chronic heart failure.3
TOS has been divided into 3 forms:
- Neurogenic TOS (brachial plexus compression)
- True neurogenic TOS
- Common neurogenic TOS
- Arterial (subclavian artery compression)
- Venous (subclavian vein compression)
As noted, nearly all cases of TOS (95%) are neurogenic in origin. NTOS is an underappreciated and often overlooked cause of shoulder and neck pain and numbness. Like patients with other chronic pain conditions, patients with untreated neurogenic TOS experience a diminished quality of life, reduced financial well-being, functional limitations, and an increased risk for depression and anxiety.4-6
True NTOS, which is confirmed with objective findings, accounts for only 1% of cases, whereas common NTOS, which has symptoms suggestive of brachial plexus compromise but no objective findings, makes up 99% of neurogenic cases of TOS.7,8 The remaining cases of TOS are arterial (1%) and venous (3%-5%).1
Neurogenic TOS occurs in an estimated 3 to 80 per 1,000 individuals, the wide range reflecting the lack of confirmation in many patients with signs and symptoms indicative of the condition. Women with NTOS outnumber men by 3 to 4:1. The syndrome is particularly common in people who perform repetitive tasks with their upper extremities, such as violinists, data entry personnel, and workers on assembly lines. Athletes with repetitive overhead arm motion, including volleyball players, swimmers, baseball pitchers, and weightlifters, also are at increased risk, as are people who have experienced neck trauma.9
Histologic studies suggest that injury to either the anterior scalene muscle (ASM) or the middle scalene muscle are the main causative factors of NTOS. Muscle fibrosis is a prime finding on examination of excised scalene muscles, with NTOS patients having 3 times as much scar tissue as unaffected subjects.8-10
The ASM derives from the transverse processes of the C3-C6 vertebrae. The muscle, which attaches to the first rib, serves as an accessory muscle of respiration, and also rotates the neck slightly. Spasm of the ASM puts traction on the brachial plexus and causes edema of the muscle and nerves, which, in turn, limits the space of the outlet. Development of scar tissue and fibrosis of the ASM further worsen neural compromise and perpetuate pain.8,11
Targeting treatment to relieve tension and spasm of the ASM can interrupt the chain of events that leads to NTOS.
There is no one standard for the diagnosis of TOS. The diagnosis of NTOS can be difficult because it often has a nonspecific clinical presentation. In a classic case, the patient will complain of pain originating in the area of the shoulder and radiating along the inner aspect of the arm. Other common symptoms involve pain in the neck; the trapezius, mastoid, and anterior chest wall muscles—all from upper plexus compression (C5-C7). Physical examination will reveal tenderness in the scalene muscles, trapezius, and chest wall. Patients may have a positive Tinel sign over the brachial plexus in the neck, reduced sensation in the fingers to light touch, and positive provocative maneuvers.9
Complicating the differential diagnosis, however, is that the entire arm often is involved without dermatomal preference. The clinician must distinguish cervical radiculopathy from disk herniation or stenosis and rule out carpal tunnel syndrome.
A thorough history and physical examination are key to accurate diagnosis of NTOS. Testing for NTOS is unreliable. Ancillary testing lacks sensitivity and specificity. Similarly, provocative testing, including the Adson maneuver,12 has unknown reliability and specificity. The Adson maneuver, in particular, produces many false positive results and no longer is considered useful for identifying patients with NTOS.13
Provocative maneuvers, nerve tension tests, and thumb pressure over the brachial plexus can assist in the determination of NTOS, but the elevated arm stress test, or Roos stress test, is perhaps the most reliable indicator.13 Another potentially useful diagnostic test includes the Spurling test to identify cervical disk disease.8,14,15