The diagnosis of MS is met when someone has symptoms suggesting lesions disseminated in space (multiple parts of the central nervous system or optic nerves) and time (either with two relapses separated by at least 30 days or slow progression for at least one year).
There should be evidence on the exam and/or MRI scans of lesions associated with the symptoms, and other causes should be ruled out. MRI scans of the brain (may also include orbits) and spine, typically with and without contrast when being diagnosed, should be performed. Spinal fluid examinations help establish dissemination in time. Typically, at least one clinical event must have occurred to confirm a diagnosis, so long as the MRIs and/or spinal fluid confirm dissemination in time.
The revised 2017 McDonald Criteria outlines specific guidelines, including MRI criteria distinguishing MS from other causes of white matter lesions in the brain and/or spine. The diagnostic criteria for MS changes on an irregular basis, and if MS is a consideration, it is best if the patient is referred to a neurologist.
There are two common errors in potentially diagnosing MS. The first is not thinking of MS in the differential diagnosis of MS, especially in younger individuals, resulting in delay of diagnosis. The second is that there are many causes of brain MRI abnormalities that may raise the concern for MS. In younger individuals, this is most commonly migraine, and in older persons this is most commonly vascular disease. This is confused further in that individuals may get a brain MRI for many reasons unrelated to MS (especially headache, or after a car accident or other trauma), and they may have findings highly suspicious for MS, despite having no overt symptoms typical of MS. This is referred to as “radiologically isolated syndrome” (RIS), a likely precursor to MS in at least some individuals.
It is important to remember that, at least in 2023, a formal diagnosis of MS is never made on MRI criteria alone.