Epidemiology of progressive multiple sclerosis

Since the earliest clinical descriptions of multiple sclerosis (MS), it has been recognised that a minority of patients experience slow accumulation, or progression of disability from onset of disease [1]: this phenotype is now commonly termed primary progressive MS (PPMS). However, more commonly patients go on to develop progression only following an initial period of relapses; this is referred to as secondary progressive disease (SPMS) [2]. These two distinct phases of relapses and progression are now thought to reflect the dual pathological processes of neuroinflammation and neurodegeneration which, in part, give rise to the wide clinical phenotype of MS [3]. Although our understanding of MS has made significant advances in recent years, there remains no single definitive diagnostic test or biomarker which accurately reflects the underlying disease process or biology, and as a result our ability to identify and quantify disease progression remains limited [2]. As we enter a new treatment era for MS, including the advent of clinical trials in progressive disease, epidemiological studies remain important in providing insights into the biology and evolution of disability in primary and secondary MS and are also key to the planning of successful interventional studies