Thoracic outlet syndrome (TOS) is complex clinical entity characterized by various neurovascular signs and symptoms of the upper limb.  It includes several different types of disorders, as follows:
- Classic (true) neurologic TOS
- Common (disputed) neurologic TOS
- Arterial TOS
- Venous TOS
Combined neurovascular TOS
TOS can involve various components of the brachial plexus, the blood vessels, or both at different sites between the base of the neck and the axilla.
The arterial form is caused by compression of the subclavian artery, the venous form is caused by compression of the subclavian vein, and the neurologic form is caused by brachial plexus compression. Combined neurovascular TOS is usually traumatic.
Most authors suggest that nonspecific neurologic TOS results from injury to the brachial plexus, by either traction or compression, at some point within the cervicoaxillary canal.
True (classic) neurologic TOS, which is rare, is caused by congenital anomalies. Usually these anomalies include a taut fibrous band or rudimentary cervical rib.
The exact prevalence of TOS is difficult to assess. True neurogenic or vascular TOS is considered rare, but common (nonspecific/disputed) neurogenic TOS is more prevalent.
TOS is more common in women than in men. The sex ratio varies depending on the type of TOS.
The female-to-male ratio in neurologic-type TOS is approximately 3.5:1.
The venous type is more common in males than in females, and the arterial type has no sexual predilection.
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