Hi Sally, I think Mick and gwa's comments are spot on: although I can understand why you're concerned, you may be worrying yourself into believing you have PPMS when the cause of your symptoms is much more prosaic. Especially the timing of your symptoms – coincident with your periods – seems clearly to point to something else… I hope that's the case, and that the MRI eases your fears.
If it doesn't, well for what it's worth I've been a member of that particular club for over 20 years now. You have many more weapons at your disposal in the form of diet, supplements and regimens – and of course huge amounts of information – that simply weren't around in 1986, and you can use those to take positive, preemptive action in the hope of gaining more time to a) enjoy your life and b) allow those scientists to come up with something.
Incidentally my first symptom was my left leg which began to drag reluctantly after I had walked for a few miles. Of course in the surgery my walking was always fine so one doctor took an interest in the wear patterns of my shoes to get some clues about my gait – still took them two years to diagnose me though!
I had a few very occasional visual disturbances more like migraine auras than anything else, and a phenomenon which might just be poor distance vision. If I try to focus on something small in the distance the object looks somehow " rippling" or lively. Or if I try to look at an insect on a plain wall a few feet away it appears to be wriggling even if it's a squashed fly. My glasses help, though, so it's probably just my eyesight,