Thoracic outlet syndrome will not cause change of consciousness. There is a very rare syndrome called “subclavian steal syndrome” where, if you use your arm too much, the capillaries open (which is typical) and “steal” your blood supply from your head (carotid artery). More commonly is that leaning forward can fill up the barro-ceptor (pressure switch) in your carotid which reduces cardiac output and can cause loss of consciousness. Cardiac arrythymias can do the same thing. Vaso-vagal symptoms can do the same thing. You need a cardiac workup.