Thanks Lysha. Are the symptoms pretty severe by the time there are lesions?
I'm not Lysha, but no, the symptoms don't have to be "pretty severe" by the time there are lesions. Lesions can appear on an MRI for all sorts of reasons (MS, migraine, brain injury, viral disease, bacterial disease, brain tumor, seizure disorder, vasculitis -- these are just some of them)
or for no reason at all. Some folks upon death have been autopsied, and lesions have appeared on the brain, yet they showed no symptoms while alive that were attributed to the lesions.
But if lesions show up on an MRI, and show up in places that are specific to MS, and if the doctor, upon examining you in his office, finds sufficient clinical signs (not just symptoms you report that he cannot test for, but signs that he can find through testing, that are repeatable) it could be enough for a diagnosis.
But diagnosing MS (or many other diseases!) can take a long time, as there are lots of disorders and diseases -- some of them neurological in nature, many of them metabolic (meaning not neurological, things like diabetes, thyroid problems, infectious disease, bacterial diseases, etc.) that might look like MS from the outside, but in fact are something else.
So if this is one of your first doctor visits to explore what is wrong with you, expect that you might not get answers immediately, but have to undergo lots of tests to rule out other disorders, before the doctor will finally diagnose MS.
...I am not a doctor, nor health professional, and don't pretend to be one, here.....