I have never heard of brainstem compression causing such symptoms. Of course it is hard to assess what is the cause behind your problems - only a qualified medical professional would be able to do this. I just want to relate my personal experience. I have also experienced upper arm and shoulder pain. It was quite bad last winter, during the time that I was going through my consultations for surgery. I saw a physiotherapist recently and she explained that it was probably a combination of being very overworked and spending a lot of time on the computer, in addition to the stress. Specifically the neck muscles are very prone to tighten up when stressed out, causing all sorts of cascade symptoms. So my issues were what I would call "indirectly" related to the AN, and more specifically attributed to all the stress.
My guess is that you should explain in detail your problems to your primary care practitioner, and he/she should be able to direct you to the most appropriate specialist.
Marianna