Thoracic Outlet Sndrome (TOS) is a syndrome involving compression at the superior thoracic outlet resulting from excess pressure placed on a neurovascular bundle passing between the anterior scalene and middle scalene muscles. It can affect one or more of the nerves that innervate the upper limb and/or blood vessels as they pass between the chest and upper extremity, specifically in the brachial plexus, the subclavian artery, and, rarely, the subclavian vein.

Source:  Wikipedia

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