Thoarcic outlet syndrome (TOS) is a condition in which pressure on nerves and arteries from the neck to the arm causes numbness in the arm or hand. TOS is difficult to diagnose. There are at least nine different spots where the nerves or arteries could be pinched, and there are no simple tests that confirm TOS. A doctor considers the medical history, the physical exam, and the results of diagnostic tests in making a diagnosis of TOS.