Differences and similarities between ankylosing spondylitis and rheumatoid arthritis: epidemiology.

Ankylosing spondylitis (AS) and rheumatoid arthritis (RA) are among the most common rheumatic diseases. The epidemiology of these diseases highlights both similarities and differences. Prevalence rates of approximately 0.2-1% have been reported for the diseases, but the rate for AS is increasing while RA is declining. Geographical variations exist in the incidence and prevalence of the diseases, although the majority of studies have been conducted in northern Europe and North America. AS is a predominantly a male disorder, whereas more females are affected by RA. Both diseases result in increased disability, reduced work productivity, and increased mortality rates. These similarities and differences may give us important clues as to the aetiology of both diseases.