The thoracic outlet is a space where blood vessels and nerves leave the torso on their way to the arm. It is formed by the first rib and the collar bone. The outlet is tightly packed, and there are a number of things that can cause compression to those blood vessels, or nerves, or both. This is Thoracic Outlet Syndrome.
Nerves are much more commonly affected than blood vessels, so the symptoms are generally quite uniform. Typically one sided pins and needles or numbness would be the main complaint. Most commonly this would be felt down the inside of the arm or into the hand or shoulder.
Often the cause is muscular. Rounded shoulders are associated with tight chest muscles- some of which can directly compress the nerves in question. In this case, we would consider why the shoulders are rounded in the first place. This might lead to an ergonomic assessment at work as a preventative measure. Treatment would then involve direct work to the tight muscles and exercise to improve posture. Sleeping with one arm overhead is a similar postural risk factor.
Exercise can also lead to Thoracic Outlet Syndrome; especially repetitive overhead movements such as throwing or swimming. Essentially this is still a muscular picture, with overdeveloped muscles around the small thoracic outlet impinging on the nerves or blood vessels running through.
Trauma to the neck or shoulder can be another trigger for the development of TOS. For example, a collar bone fracture may heal with a callus over the fracture site. Depending on location, this protrusion could take up space in the thoracic outlet.
Thoracic Outlet Syndrome is somewhat controversial, because accurate diagnosis is difficult. Similar symptoms can come from the neck, for example, and clinical tests are not 100% accurate at telling them apart. However, your osteopath can use a range of different tests, along with your account of your symptoms, to work out what is most likely the cause of your pain.