Anne, none of us here are medical professionals. You need to be talking directly with your neurologist about this. It is not a good idea to have people without any background in the area try to make a diagnosis from small, low resolution versions of MRIs posted to a discussion board. Some have made wild guesses in other places and IMHO it is insane to be trying to get adequate or accurate medical advice this way.
As I said, you really need to consider first whether the paralysis is affecting her in any kind of problematical way. Unless it is interfering with her ability to eat or swallow, I really don't think this is something that you need to be concerned about right now *but you need to talk to your neurologist*
. The second consideration is–what are you planning to do about this if it turns out this is due to PSOM or SM? Would you wish to put your dog through further procedures at this point?
I will wager that a neurologist will say that this side effect is almost certainly another symptom of the condition she already has. That's again, why it is best for you to go make a decision about this after a conversation/appointment with your neurologist and an evaluation of whether whatever you are seeing actually has any effect on her quality of life. This has been worrying you for a month now so you really do need to go see a professional with Ella. It isn't fair to her to keep asking for guesses from all of us when it sounds like she needs a medical evaluation.
So I am going to ask you to please first have a discussion with a professional on this issue now. Once you get a response and informed medical recommendations perhaps you will want to return here to ask people for a response. But I really do not want this same conversation running in a loop as it has been for a while now, because I think it is risky for you to be taking decisions without any direct professional advice, and I also think it increases overall anxiety and distress because you are getting so many/too many different opinions here and elsewhere, as you've asked this on several forums. It sounds like other issues bother her a lot more and she is having more pain -- that sounds like maybe it needs re-assessment from your neurologist even more.
I am going to close this thread for now and when you have an informed response from Ella's neurologist, you will be able to start a fresh thread based on an actual diagnosis.