Most likely if something obvious was the culprit it would have shown up on the tests. I hope anyway...
like - an extra cervical rib, large ligaments or abnormal bony structures?

You could have nerve compressions and /or vascular compressions/impingement. more in the link below

Oh, a simple but basic test to give more clues is the provocative head/arm potions & pulse check compare both sides. more in the link below

my MD only did one side so she knew a small bit about it but not enough and she only did one version/position.

The advanced PT guys did much better and more of the head/arm positions, but the MDs didn't want to change their notes or report on that basis. { mainly because these aren't a concrete way to diagnose and a small percentage of the normal population will have blood flow issues also with these tests.}
But it is still a possible indicator.

My chiro did these tests on my first visit after hearing all of my symtpoms, I happened to be in severe spasms at the time.

If not anomalies or spine problems have shown up on testing, I would look into the myofascial, soft tissue aspect & posture/bodywork.


Is one shoulder higher or lower than the other? most of us w/TOS have noticed that.

Did the chiro you went to only do adjustments and no other PT type modalities? like- ultrasound, low level laser, IF stim, NIMMO & upper cervical tx?
mine does all of it as needed for me.

here's the link to the useful sticky thread - it has valuable info that we've collected over the years - kind of a crash course of info & links for TOS - there's also info on testing as well as videos for self care and posture.
http://neurotalk.psychcentral.com/thread84.html

If you think it might be postural or repetitive forward arm/head caused a simple thing to try might be these - many x a day but only if no symptom or pain increase.

a firmly rolled up towel will work fine