Page 57 - THE EVOLUTION OF EARLY ARTHRITIS AND CARDIOVASCULAR RISK Samina A. Turk

HOW DOES ESTABLISHED RHEUMATOID ARTHRITIS DEVELOP
More recent studies in Denmark(56) and Minnesota(57) suggest that the incidence
may be rising again, with annual increases of RA incidence of 6% (1995-2001) and 2.5% (1995-2007), respectively, remarkably only in women. However, in Finland, a further
annual decline of 2% was seen for RF-positive RA over the period 1980-2000(55). It was 3 speculated that a combination of environmental changes leading to either increased
risk or loss of protection plays a role in the increasing RA incidence found in the above- mentioned countries. As alluded to in the earlier text, obesity seems to be an important emerging risk factor of RA development. Crowson et al. linked the recent increase of obesity in the population to the higher incidence of RA(58). It was calculated that an increase in obesity could explain 52% of the increase in the incidence of RA among women in the period 1995-2007. Also, other factors may play a role, for example, lower doses of hormones in the oral contraceptives over the years, slower decline of smoking rates in women compared with men, and more vitamin D deficiency(57).
Another important note about changes in incidence rates over time is that the timing of the measurement and used RA criteria can vary between studies, and it also depends on the duration of the study period, mode of presentation, awareness of the disease by general practitioners, and the delay of referral after symptom onset. In the following, we describe two of these factors in more detail. First, the new ACR/European League Against Rheumatism (EULAR) 2010 criteria for RA (see subsequent discussion) are more sensitive than the earlier criteria, which will probably lead to earlier detection (and treatment) and thereby affect the measurement of incidence rates in the coming years(59). Second, within Europe, the variation in the delay of first assessment of RA patients is substantial, with a median range of 16-38 weeks per center and a difference at its highest of 34% in seeing patients within 12 weeks of symptom onset(60). This could partly explain differences of changes in incidence rates across European countries, and even less is known about such a variation outside Europe.
In conclusion, relevant trends are a steady decrease of worldwide RA incidence during the period 1955-1995, followed by a recent increase in at least Denmark and the USA, probably explained in part by changing environmental factors. Furthermore, factors such as differences in the use of RA criteria and differences in the awareness of RA across countries can affect the incidence rates over time.
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