Don't know if this applies to you, it does for ME so it is offered in case it may apply to you....

1. POSTURE. In many cases, mine in particular, POSTURE is very key. While I may have to maintain tighter scrutiny over my form/posture than the next guy and it may be unfair, that is MY reality. It sometimes is a matter of a millimeter off and all hell breaks loose. It is a constant reminder. Another reason I emphasize getting to know YOUR anatomy. When I got the diagnosis I wasn't surprised when they said my posture wasn't the greatest. And they assured me they'd seen people with worse posture with no problems. That is where the unfairness comes in. I'm predisposed. Lucky me. I have to work harder to maintain good posture and form in ALL things I do. Walking, sitting, standing etc. ALL things. It is a full time job. Do I slack off, yes...do I suffer every time I slack off...no, but usually. Get the know the route of the brachial plexus ...where it originates and where it ends. Know the structures it passes and how it passes. Know this and you start to see WHY posture in many can be key. Most, if not all of us, are bent forward a bit. From sitting and just general fatigue from time to time. The computer is one of the worst culprits for TOS because we all, or most of us sit in front of it sort of hunched over. Look at your "typical" posture when in front of the computer. Are your shoulders rolled/rounded forward? If so this is typically a big problem. If you do not know or understand what "rolled/rounded forward" is...ask someone who would. A rolled/rounded forward position creates LESS SPACE for the BP to pass in various structures as it exists the neck..one of the most vulnerable is the first rib/anterior scalene/clavicle area....compression of the nerve/arteries/veins here and you can have "chest pain" One of many symptoms. Not saying this is YOUR situation...again, I merely point out a possibility for YOU to look into. Ask if they know where your compression is...did you get an MRI/MRA/MRV? This is very expensive but generally will pinpoint exactly what is going on. IF this is the problem, what has helped me (and I'm still working on this and it's been over a year now so PATIENCE is also key) are exercises, stretches etc., that OPEN THE CHEST. Yoga has some poses that are literally called "chest openers" Yoga to that extent is one of many modalities that you may be able to use to help you get relief. IF this is your problem, as it is mine, you may also need, as I do, to strengthen certain muscles...the upper back muscles need to be stronger to hold BACK the shoulders which are being pulled forward and rounded by the tight pectorals. Try to see this in your mind...the upper back rhomboids are weak/stretched out...creating the roundedness in the back and the chest area is short/tight, exaggerating that roundness bringing the shoulders forward and concaving your chest...it is all scrunched in front...and hence less space for the BP to pass....open all of that up and keep it open...and you MAY (no guarantees) help yourself. Just one of many thoughts on the subject. Again this has helped ME...it may not apply to you...hence get to know YOUR anatomy