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Thoracic outlet syndrome (TOS) is the general term for a group of disorders that affect the nerves running from neck to the arms (called the brachial plexus) and the nerves and blood vessels running between the base of the neck and the armpit (called the axilla).
TOS is a controversial condition, and some physicians doubt whether some or all types of TOS actually exist. In many cases, TOS cannot be definitively confirmed by laboratory tests. Because of this, it is not known if the condition is underdiagnosed or overdiagnosed. Recent data indicate that it occurs in 3 to 80 people out of every 1,000.
TOS is described as neurogenic (affecting the nerves) or vascular (affecting the blood vessels). However, it is rare that someone would be affected with either one or the other. In most cases, the condition is caused by external factors, such as aging, poor musculature or injury, that affect both the blood vessels and the nerves in the region. TOS is divided into five general categories by the National Institute of Neurological Disorders and Stroke:
- Traumatic TOS. This is the most common type of TOS. It is often caused by hyperextension neck injuries (e.g., whiplash) and repetitive stress injuries, in which pressure is exerted on the nerves. Symptoms include pain, tenderness and paresthesias (abnormal burning or prickling sensation in the hands, arms, legs or feet). Other symptoms may include numbness, fatigue and weakness.
- True neurologic TOS. A rare, usually painless, condition caused by the compression of nerves by an abnormal bony growth in the neck. This type of TOS generally occurs in middle–aged women and almost always on one side of the body. Symptoms include weakness and numbness in the hands, and a wasting of the hand muscles.
- Arterial TOS. Like true neurologic TOS, this type is rare, occurring in about 2 to 3 percent of cases, and is often related to a congenital defect. Symptoms may include sensitivity to cold in the hands and fingers, numbness or pain in the fingers, finger ulcers, or inadequate blood circulation in the fingers (severe limb ischemia).
- Venous TOS. This is the rarest type of TOS, affecting only 1 to 2 percent of people with TOS. Venous TOS (also called Paget–Schroetter syndrome) can develop suddenly and usually results from the compression of the subclavian vein (located under the collar bone, or clavicle) by the surrounding muscles and ligaments. Symptoms include pain and arm swelling.
- Disputed TOS. Also called common or nonspecific TOS, disputed TOS is highly controversial. The exact cause in unknown. Some physicians do not believe it exists, and others believe it is very common. Symptoms include numbness or tingling in the fingers; pain in the arm, shoulder or neck; headaches; and weakness in the arm or hand.
Although both are rare, prolonged arterial and venous TOS have been linked to potentially dangerous cardiovascular conditions. Arterial TOS may cause aneurysm within the subclavian artery. Pulmonary embolism has been reported in some patients with venous TOS. | 
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