There are "Specific Criteria" that The Neurology Association has established. A certain amount of sclerotic lesions in the brain and if there are a smaller amount in the brain too meet the definition, then spinal cord lesions are also added to the mix.

That's why an MRI of the Brain & Cervical Spinal Cord is the gold standard baseline study to start making the differential diagnosis ... If there's a good likelihood that you're having a active MS Flare-Up, they'll also do the "MRI with Contrast" to see if there are lesions that "light up" when "soaked" with the Contrast infusion. Sometimes they'll look even further along the Spinal Cord with Thoracic (chest) and Lumbar (lower back) MRI studies.

Those tests along with symptoms, evoked potential (nerve electricity) of arms, legs & eyes AND the Spinal Fluid analysis (lumbar puncture) are all like "little tiles of a Mosaic" that gives the picture of the disease ... All are clues that help point out the culprit !!!

@trip10 ... Not to be nit-picky ...
A Lumbar Puncture doesn't show Lesions ... the spinal fluid collected is analyzed in a lab to measure the presence debris of the Myelin Sheath present when it's being "chewed up" by cells of the Immune System.
Those results plus lesions seen on the MRI provide conclusive evidence that the diagnosis is definitely Multiple Sclerosis.

In some cases, like mine, LP's are Negative but there are a sufficient number of sclerotic lesions in the brain & spinal cord present on the MRI along with the clinical symptoms to meet the "official" definition and confirms the diagnosis.

In most cases, "If it walks like a duck and talks like a duck ..."

@PharmacyGirl ... That "doctor" was an idiot ! Maybe a good clinician but an awfully, horrible, terrible physician. So sad that guy had NO interpersonal skills and you had to suffer needlessly :-( I hope there was a Nurse or a Chaplain or Social Worker that came to you with a better bed-side manner able to provide some emotional support !!! <3 <3

I kept telling my doc to consider MS. It was the final deciding factor when she ordered an MRI after a CT scan. She only ordered an MRI to rule out a tumor cause of my recent migraines. Imagine her surprise to find out there they were; the lestions typical of MS.

Quack, quack, I'm a duck. An MRI + Lumbar Puncture confirmed my INTERNIST'S suspicions. It took 2 weeks to get my diagnosis.