What disease causes white spots on the brain and spine?

My mother in law has the following symptoms and has been misdiagnosed with multiple sclerosis.

Her MRI’s show white spots on her brain and spine
She has been losing mobility in her legs and getting weaker

She’s been tested for the following:
Lack of vitamins and minerals – results were negative
syphilis – negative
spinal infection – negative

If you don’t have an answer, a link to a website with additional information would be helpful.

thanks,