Thoracic outlet syndrome (TOS) is a condition in which the nerves or blood vessels are squeezed tightly in a small space behind the collarbone (clavicle). This space is called the thoracic outlet. TOS can cause pain, numbness, or weakness in your shoulder, arm, or hand. Sometimes it causes swelling.

Symptoms typically occur with lifting the arms to shoulder level or other positions that put pressure on the nerves and vessels behind the collarbone. Treatment includes stretching and strengthening exercises as well as avoiding activities that cause symptoms. In rare cases, surgery is needed to relieve the pressure. If blood vessels are affected, medicines or procedures may be needed to treat blood clots.