Unlike relapsing forms of MS, primary-progressive MS (PPMS) is characterized by a fairly steady, gradual change in functional ability over time — most often related to walking — without any relapses. Due to this basic difference in the disease course, different criteria are used to make an accurate diagnosis of PPMS. The criteria for a diagnosis of PPMS are:
- One year of disease progression (worsening of neurological function without remission), AND
- Two of the following:
- A type of lesion in the brain that is recognized by experts in as being typical of MS
- Two or more lesions of a similar type in the spinal cord
- Evidence in the spinal fluid of oligoclonal band or an elevated IgG index, both of which are indicative of immune system activity in the central nervous system
Meeting these criteria can sometimes take a fairly long time, particularly if the person has only recently begun to experience neurologic symptoms. Several studies have suggested that the PPMS may take two to three years longer to diagnose than relapsing-remitting MS.