Is the epidemiology of psoriatic arthritis (PsA) truly changing? Is the incidence indeed rising? If so, what are the possible reasons? Is it because psoriasis is becoming more prevalent? Clearly, genetic factors do not change over a few decades; therefore, how are environmental factors influencing the disease? These are the questions that spring to mind on reading the article by Wilson, et al in this issue of The Journal1.
PsA is a form of seronegative spondyloarthritis associated with psoriasis2. Although the occurrence of arthritis associated with psoriasis was probably recognized as early as 1818, it was as recently as 1964 that PsA was recognized by the American Rheumatism Association (American College of Rheumatology) as a distinct clinical entity3. And it was as late as 1996 that studies on prevalence and incidence of PsA were published4. A recent review of studies undertaken to December 2006 has shown widely varying estimates of incidence and prevalence4. While estimates obtained from studies conducted within Europe and North America vary significantly, the most striking difference is between Europe and Japan. The incidence in Europe and North America ranged between 3 and 23.1 cases/105, whereas that in Japan was only 0.1 cases/105. Similarly, the prevalence in Europe and North America ranged between 20 and 420 cases/105, but in Japan it was only 1/105. This large difference is most likely due to differences in ethnicity, since low prevalence of other spondyloarthropathies in Japan has also been reported5.
The challenges in conducting epidemiological studies in PsA neatly elucidated in 1994 by O’Neill and Silman are still relevant3. The most important problem identified was lack of validated classification criteria. It should be noted, however, that although a …
Address reprint requests to Dr. Chandran, Centre for Prognosis Studies in the Rheumatic Diseases, Toronto Western Hospital, 1E 412, 399 Bathurst Street, Toronto, ON M5T 2S8, Canada. E-mail: vinod.chandran{at}uhnres.utoronto.ca
