Primary and secondary progressive multiple sclerosis

The progressive phase of multiple sclerosis (MS) is the one most often associated with irreversible accumulation of disability. An important question remains about the place of primary progressive MS (PP-MS): does it form an integral part of the disease spectrum, or is it maybe a distinct entity? This question could apparently be very theoretical, but it is not, as patients with PP-MS remain orphans when regarding disease-modifying treatments. Thus, they are usually excluded from therapeutic trials. A clue to this question could be the comparison between the different MS subtypes with a progressive phase. We discuss here the clinical similarities and differences between secondary and primary progressive MS.