Pick the top 5 most annoying/troubling symptoms (issues) and concentrate on those, mention there are others, but these are the top 5.
You should have a talk about your symptoms/medical history etc., a few physical checks along the lines of what your balance is like, how strong your grip/legs are, reflexes, they may look at your eyes for signs of nystagmus - you follow the tip of their finger/pen with your eyes and they can see if your eyes are tracking from side to side, up and down smoothly. Stuff like that.
Do you have a copy of your MRI results/scan? If not, they may want their own MRI done.
Just focus on the key symptoms and changes you’ve noticed, and make sure to bring that list of experience keep it short and sweet to maximize your 40 minutes.