Thoracic outlet syndrome (TOS) is a compression of the neurovascular bundle as it exits the area between your collar bone and first rib called the thoracic outlet. Within the neurovascular bundle of the thoracic outlet are the brachial plexus (nerves), subclavian artery, and subclavian vein. Identifying TOS can be complex because there are many conditions that resemble TOS. There are three types of TOS: neurogenic, venous, and arterial. Symptoms vary depending on which tissues are affected. Compression of nerves can cause numbness, tingling, pain and weakness. Compression of the blood vessels can cause swelling, pain, arm fatigue, discoloration in the hand and arm and altered temperature sensations. Causes of TOS include poor posture, being overweight, repetitive activities, and injuries to the structures around the thoracic outlet such as the collar bone (clavicle). Congenital anomalies may also be a contributing factor such as having an extra rib.