The worldwide prevalence of multiple sclerosis.

Despite the wealth of epidemiological data deriving from the systematic studies of multiple sclerosis (MS) that have been carried out for over 70 years, any attempt at redefining the pattern of MS geQgraphic distribution is still a difficult task. In fact, comparing prevalence studies of different areas and at different times implies a number of problems: (a) the variability of the surveyed populations in terms of size, age structure, ethnic origin and composition [1]; (b) the difference when determining the numerator, i.e. the recognition of benign and very early cases [2]; (c) the extent to which complete case ascertainment is achieved based on geographic and time variables, access to medical care, local medical expertise, number of neurologists, availability of and accessibility to new diagnostic procedures, degree of public awareness about MS, and on the investigators' zeal and resources [2.3]; (d) the use of different diagnostic criteria and the interobserver variability when applying them [ I ]. A description of MS geography worldwide is tentatively presented (for detailed references, see review by Rosati [41).