Thoracic Outlet Syndrome
(illustration from The Nicholas Institute of Sports Medicine and Athletic
Trauma's webpage on TOS. Copyright by NISMAT)
Thoracic Outlet Syndrome (TOS) is a the name given to a variety of clinical conditions in which the nerves and/or the arteries (collectively called the "neurovascular bundle") of the arm are constricted in the region of the base of the neck and the upper shoulder. The constriction of, or pressure on, the neurovascular bundle can be from bony, ligamentous, or muscular structures, or a combination of structures. These structures are usually congenital (present from birth) but slowly become symptomatic (cause symptoms) as the patient gets older. The nature, treatment, and even the existence of the syndrome is an area of debate and research. Luckily, most patients can be treated (ie, not "cured" but improved to the point that they do not need surgery) by appropriate patient education, activity modification, and therapy.
I would recommend a paper by a highly respected physical therapist, Peter Edgelow, who I know personally and whose approach has been shown to be very effective in my practice. You can read what he and co-authors have written here (page is no longer there). Another page on TOS is by The Nicholas Institute of Sports Medicine and Athletic Trauma. (Please note: I usually only link to websites when I personally know the authors. I do not know anyone at the NISMAT, but feel that their information was of good quality when I first reviewed their content. I do not necessarily agree with all of the content, but do feel that it is a quality, responsible site.)