Thoracic outlet syndrome (TOS) has been an important clinical entity for more than a century. In 1821, Cooper described axillary-subclavian artery symptoms due to compression from a cervical rib. In 1875, Paget described clinical symptoms resulting from subclavian vein thrombosis (eg, arm swelling, pain). In 1884, von Schroetter correctly attributed these upper-extremity symptoms to thrombosis or compression of the subclavian vein at the thoracic outlet. Consequently, venous thrombosis at the thoracic outlet is known as venous TOS or Paget-Schroetter syndrome.
TOS has distinct clinical pictures (ie, neurogenic, arterial, and venous) caused by compression of the neurovascular structures at the thoracic outlet. The classification is based on which structure is primarily involved.
The relevant anatomy of TOS focuses on the scalene triangle and the costoclavicular space. Reports of compression of the neurovascular bundle at the area of the pectoralis minor space exist, but this is very uncommon and will not be discussed further.
Neurogenic and arterial TOS result from compression that occurs in the scalene triangle, which is defined by the first rib, the anterior scalene muscle, and the middle scalene muscle. The subclavian artery and the branches of the brachial plexus pass through the borders of this triangle (see the image below).
Venous TOS occurs secondary to compression that occurs in the costoclavicular space. The borders of the costoclavicular space are the first rib, the costoclavicular ligament, the subclavius, and the anterior scalene muscle. As the subclavian vein passes through this space, it is susceptible to compression by these structures, as depicted in the image below.
Other important local structures are the phrenic nerve, the lateral thoracic nerve, and the thoracic duct. The phrenic nerve passes from lateral to medial along the anterior border of the anterior scalene muscle, and the lateral thoracic nerve passes through the body of the middle scalene muscle. The thoracic duct joins cervical lymphatics and drains into the superior aspect of the jugulosubclavian vein confluence behind the left sternocleidomastoid muscle. Be careful to avoid injury to these structures during surgery.
Search for cervical, rudimentary, or broad first ribs. Resect these structures during surgical therapy. Rudimentary ribs usually arise higher in the neck than normal first ribs and typically articulate with the second rib rather than with the sternum. Cervical ribs and rudimentary first ribs occur in less than 0.5% of the population.
Abnormal scalene muscle anatomy also has been identified and may be a cause of some symptoms. For example, these muscles have been noted to interdigitate around the cords of the brachial plexus and, thus, have been implicated in the irritation of the cords of the brachial plexus.
In a study of 98 meticulously dissected cadavers, the authors noted a number of abnormalities of the thoracic outlet fibrous bands and cervical ribs, and other abnormalities were found in most of the patients. Only 10% of the dissected cadavers were found to have normal anatomy bilaterally.
Neurogenic TOS most commonly is associated with a history of neck trauma. Swollen and scarred muscles or aberrant scalene anatomy can irritate cords of the brachial plexus locally and lead to the neurologic symptoms.
Arterial TOS often is associated with cervical ribs or a rudimentary first rib. [1, 2] This aberrant anatomy leads to repeated intermittent arterial compression coinciding with arm movement. This repetitive localized trauma leads to intimal lesions, focal arterial stenosis, poststenotic dilatation, aneurysmal change, and subsequent thromboembolic complications. The second portion of the subclavian artery, which has a retroscalene position, often is the site of positional compression and stricture.
Venous TOS usually results from compression of the subclavian vein by the subclavius and the costoclavicular ligament.  When local structures are placed in abnormal or unaccustomed positions by extremes of activity or injury, vein compression and subsequent vein thrombosis can result. Venous TOS tends to occur in the more active dominant extremity.
Arterial and venous TOS usually are associated with certain predisposing anatomic abnormalities, whereas neurogenic TOS is more likely to result after traumatic injury.
The potential for either neurologic or vascular compression exists at the thoracic outlet. When compression occurs, one of the TOSs may develop. Neurogenic and arterial TOS result from compression that occurs in the scalene triangle (see the first image below), and venous TOS results from compression in the costoclavicular space (see the second image below). 
Neurogenic TOS is the most common presentation, occurring in approximately 95% of patients. Arterial TOS is the next most common presentation and occurs in about 2-3% of patients who are affected. Venous TOS is the least common presentation, representing only 1-2% of patients with TOS.
Approximately 70% of patients with neurogenic TOS are females aged 20-50 years. Venous TOS occurs with a male-to-female ratio of 2:1.
Most patients do not experience full relief of symptoms after surgery for neurogenic TOS. A good result occurs when partial relief is achieved and symptoms do not progress. Approximately 40-80% of patients who undergo surgical treatment have some relief of symptoms. Approximately 10-15% of patients who initially experience symptomatic relief have recurrence of symptoms.
If only first-rib resection was performed, these patients may benefit from anterior and middle scalenectomy along with neurolysis. If only scalenectomy was performed, a second operation with first-rib resection and neurolysis may be considered. Only 40% of patients experience long-term symptomatic relief after reoperation for TOS.
Patients with arterial and venous TOS usually do well if workup and treatment have been appropriate.