The first attempt to differentiate MS types was done by international expert consensus and was based on common clinical courses of the disease.e3 This classification was revised in 2013 by Lublin et al. as our understanding of MS increased owing to new insights into clinical relapse rate and imaging data (table 1). Briefly, this update removed progressive-relapsing MS (PRMS), introduced clinically isolated syndrome (CIS), and the concept of active and nonactive MS subtypes to all stratifications.In addition, the terms “worsening” and “disease progression” were differentiated, the former to describe advancing symptoms for any reason and the latter reserved specifically for “true progression” rather than relapse.Although the revised Lublin criteria were devised to help to diagnose progressive forms of MS and aimed to distinguish between clinical phenotypes, in our opinion some of the definitions are difficult to apply consistently in clinical practice, resulting in a lack of consensus on this method of differentiation. Furthermore, the phenotype definitions do not address a number of issues: most MS-related CNS inflammation occurs at the onset of MS6; over 80% of new brain MRI lesion formation detected by 0.6 T MRI is subclinical; in patients with radiologically isolated syndrome (RIS), thalamic atrophy (neuronal loss or neurodegeneration) has been identified8; and MS-related brain atrophy is already present at the CIS stage of MS.9 It should also be remembered that, after the age of 30–40 years, humans lose neurons at an increasing rate because of aging, a process which also consumes neurologic reserve. Once neurologic reserve is exhausted, aging will also contribute to slowly progressive neurologic disability.