Could it be some sort of repetitive motion , desk, computer use , posture related?
I suggest you explore our Thoracic outlet syndrome forum (TOS) for short.
http://neurotalk.psychcentral.com/forum24.html
helpful sticky threads-
http://neurotalk.psychcentral.com/thread125577.html
http://neurotalk.psychcentral.com/thread84.html
Even if you don't feel you fit the symptoms fully , you could be pre TOS or a mild version..so the posture /self care ideas are a good thing to try.
I'm assuming MRI and other tests showed no issues as a cause?