I'm so so sorry you have had this confirmed- you must be in bits and I understand exactly how you feel. Until you see the pictures and talk to Alison you can't know the full extent of the damage.
You will find that you discuss Medication vs Surgery. Medication can be very effective, I can't remember how old Misty is but have a gut feeling she is a little bit older??
A long thin Syrinx can be better for a dog than a short wide one.
I would get all the information you can while you are there regarding how far the disease has progressed so far and what Allison feels her best options are.
All you need to do is get her comfortable, pain free and mantain a good quality of life.
Dont go alone- I did and it was one of the most horrendous experiences of my life. I couldn't drive when I picked Ruby up as I was in such a state (this was back before I joined this forum) I was lost and had no clue what I was dealing with.
I sat in my car and cried for an hour before I could turn the engine on.
One of the best things I can say to you is that Misty doesn't know what has just happened and never will- her life will be back to sniffs, walks, food and sleep which is no different to what it was two days ago.
All you can do is get all the info, take it home and discuss it with your family, friends and people close to you and make a desision that suits you, Misty and your life.