EDITORIAL - Epidemiology of psoriatic arthritis

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Journal of Rheumatology

Editorial

Feb 2009

Epidemiology of Psoriatic Arthritis

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 , 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 number of

proposed classification criteria were available, until now most

epidemiological studies have used the co-occurrence of psoriasis and

arthritis or the European Spondylarthropathy Study Group (ESSG)

criteria to identify cases of PsA4,6. Use of these criteria may not be

appropriate because even if patients with inflammatory arthritis were

correctly identified, not all patients with psoriasis and inflammatory

arthritis have PsA. Moreover, the ESSG criteria have poor

sensitivity7. The original criteria for PsA proposed by Moll and

in 1973 were meant to be diagnostic rather than classification

criteria8. A number of classification criteria have since been

proposed but none have been universally accepted7. Recently, the

Classification criteria for Psoriatic ARthritis (CASPAR) Study Group

compared the performance characteristics of these criteria and

developed a new set7. These new classification criteria were developed

in patients with long-standing disease and in the original study had

specificity of 98.7% and sensitivity of 91.4%. Subsequently, the

criteria were shown to have excellent sensitivity in both early and

late disease9. The CASPAR criteria were found to have been developed

using sound measurement principles although they remain to be fully

validated10. In fact, modifications of these criteria for

epidemiologic studies have already been proposed11. One major issue

yet unresolved is the definition of “inflammatory musculoskeletal

disease.” The CASPAR criteria can only be applied to those fulfilling

this mandatory criterion. Identifying inflammatory musculoskeletal

disease is usually not difficult for rheumatologists, but may be a

stumbling block for wider application of criteria by physicians,

whether in the community or in dermatology practices. In spite of this

drawback, CASPAR criteria have been recognized to be simple and easy

to apply to data collected retrospectively9,12. Moreover, using these

criteria it is possible to classify patients as having PsA even when

they do not have current, past, or family history of psoriasis.

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Read the entire editorial here:

http://jrheum.org/content/36/2/213.full

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