Epidemiology of rheumatoid arthritis

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Rheumatoid arthritis (RA), the most common form of chronic inflammatory polyarthritis, represents a significant health burden in the developed world. The damage and deformity of the synovial joints characteristic of RA most commonly develops in the sixth decade but can occur at any age and will usually require treatments and interventions for the rest of an individual's life [1]. It is diagnosed and distinguished from other arthritic diseases on the basis of criteria defined in 1987 [2]. The criteria are based on factors such as the presence of the autoantibody, rheumatoid factor, erosion of joints visible upon x-ray, stiffness, swelling and symmetry of affected joints. Interestingly, with improved treatments, many patients will display some of these features only transiently and would strictly speaking only satisfy criteria if they were applied cumulatively, as has been proposed when the criteria are used in epidemiological studies [3] (Tab. 1).

Rheumatoid arthritis has a worldwide prevalence of approximately 1% (prevalence is the number of cases occurring in a population at a given time) and is consistently observed to affect women 2-3 times more frequently than men. The occurrence of RA is not, however, the same throughout the world [4]. Prevalence rates are low in the less developed and rural parts of the world and it has been suggested that RA is a modern disease, its appearance seeming to coincide with industrialisation or urbanisation. A study in South Africa found a low frequency of RA among Bhantu-speaking people in their traditional rural environment but higher rates in the same ethnic group living in the modern urban townships of Soweto, similar in fact to Caucasians living in nearby Johannesburg [5, 6]. This apparent influence of urbanisation was not however observed in a study comparing rural Chinese with those living in the highly industrialised society of Hong Kong. The frequency of RA was low in both the Chinese populations studied. Other factors such as diet and a lower or different genetic susceptibility, may explain these apparently contradictory findings.

In common with other autoimmune or chronic inflammatory conditions, most notably multiple sclerosis, there appears to be a latitude related gradient for the

The Hereditary Basis of Rheumatic Diseases, edited by Rikard Holmdahl © 2006 Birkhäuser Verlag Basel/Switzerland

Table 1 - Modification to 1987 ARA criteria to improve ascertainment of inactive cases. (With permission by the Journal of Rheumatology, from [3])




1. Morning stiffness in and around the around the joints lasting at least 1 h before maximal improvement

2. Arthritis of three or more joint areas

3. Arthritis of hand joints: Involvement of at least one area in a wrist MCP or PIP joint

4. Symmetrical arthritis: Simultaneous involvement of the same joint areas on both sides of the body

5. Rheumatoid nodules: Subcutaneous nodules

6. Serum rheumatoid factor

7. Radiographic changes

Reported in the 6 weeks prior to interview At any time in the disease course

With soft tissue swelling or fluid at current examination

With soft tissue swelling or fluid at current examination

With soft tissue swelling or fluid at current examination

Present at current examination

Detected at current examination

With swelling at current examination or deformity and a documented history of swelling

With swelling at current examination or deformity and a documented history of swelling

With swelling at current examination or deformity and a documented history of swelling

Present at current examination or documented in the past

Present at current examination or documented to have been positive in the past by any assay method

Erosive changes typical of RA on postero- Erosive changes typical of RA on postero-anterior hand and wrist radiographs anterior hand and wrist radiographs

Individuals satisfying four or more criteria classified as having RA

prevalence of RA with rates being highest at more northerly latitudes. In Finland, France and Italy rates for men and women are; 0.6% and 1%, 0.32% and 0.86%, 0.13% and 0.51% respectively [7]. Many factors, both genetic and environmental, such as such as climate, ultraviolet (UV) exposure, diet could contribute to this observation.

There is some evidence that prevalence rates, particularly in women, are declining but monitoring of patterns of incidence of disease (i.e., the number of new cases in a given time period) can be more sensitive to changing patterns and may give clues as to the aetiology of disease. A number of studies have shown a fall in the incidence of RA over the last four decades [8-10]. In the US (Rochester, Minnesota) the incidence rate in women decreased from 83/100,000 in 1955-1964 to 40/100,000 in 1985-1994 [8]. One explanation put forward for this fairly rapid change is the protective effect of the oral contraceptive pill (OCP). This hypothesis provides an appealing explanation for the gender differences observed and data collected over the next couple of decades should be revealing, as one would expect that, as OCP use reaches saturation, incidence rates should stabilise.

In contrast to the idea that RA is a modern disease is the observation of skeletons found in North America dating back several thousand years, showing evidence of RA [11]. This finding coupled with the fact that some of the highest prevalence rates for RA are observed in Native American peoples has led to the suggestion that RA may have been introduced to The Old World by explorers returning to Europe from the New World. The incidence and prevalence of RA in Pima Native Americans has fallen substantially in recent years and it will be interesting to observe future rates in developing populations in which RA seems to have arrived more recently than in Europe [12]. A number of studies also suggest that RA is becoming less severe although teasing this out in the context of ever improving therapies and patient management could prove difficult [13].

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