Your description makes me think of thoracic outlet syndrome {TOS}.
I'm not up on all the MS symptoms so i don't know if that is a usual thing with MS or not.
But I do have TOS from multiple RSI injures and that's why the thought jumped out at me.
Any repetitive work, car accident , sport injuries?
Often previous arm,neck, shoulder injuries can be a precursor to TOS.
We do have a TOS forum here if you would like to read more about it-
http://neurotalk.psychcentral.com/forum24.html