I've heard of people getting faint when raising their symptomatic arm over their head, but that's not me:

If I let my left arm hang down at my side, I get lightheaded. If I do this long enough, I eventually get chest pains. My chiro had trouble finding my pulse when the arm hung down.

If I raise the arm above my head, it begins to lose circulation in less than 15 seconds. This does not exacerbate my lightheadedness, it might even lessen it.

If I prop my left arm up to be parallel to the floor, I get a nice balance which reduces symptoms in both the arm and the head. My chiro felt my pulse come in when he lifted the arm parallel.

What is this? Does anyone have the same pattern?

Have you seen a vascular surgeon? If not, it might be a good idea.

Hi Chroma

if I raise my arm up, the veins in my arm sink in (and leave an indent of where they were) and I was told that its because I am compressing the TO area on raising my arm - do you get that too?

If I lay with my arm propping my head up I get numb from my shoulder to my elbow.

I get dizzy/faint all the time so am not sure its due to position in my case

Down = vein compression

and up in my case - my veins swell up when down and sink in when up

YES!

Also the vein coming down my left neck is always indented in. More so if I tilt my head back to the right.

I need to get this TO area opened!

The dizziness leads me to believe this is vascular problem. You should see a specialist before considering anything else. If you get clean bill for circulation. The dizziness occurs in the head--well above the brachial plexus, so judging by your comments, I think it is unlikely to be TOS

Who told you that?

Also, my thing where my lightheadedness is a function of my arm position is not an instant or quick phenomenon. It took some experimenting to find that out.

Basically when I suspected I had TOS, I figured I should prop the arm up to open the TO area some, and it worked to alleviate over half of my symptoms.

Have you seen a vascular specialist like pixified and GaryA have indicated? What tests have you had?