The prevalence of rheumatoid arthritis (RA) is approximately 1% of the population. Efforts to find a genetic cause for the development of RA have yielded results that offer only a partial explanation. Changes over time in the incidence of RA suggest the involvement of environmental factors in its etiology. Doran and colleagues have reported in the past on the trends of RA incidence in Rochester, Minnesota. In this study (Arthritis & Rheumatism 46:625-631, 2002) they extend their observations for ten more years, resulting in a 40-year population-based history of RA.
Comprehensive medical records are available for all residents of Rochester, Minnesota via a record linkage system that connects the medical records of all the health care providers for the local population. Members of the inception study cohort were at least 18 years of age and met the America College of Rheumatology 1987 criteria for RA. They were entered from 1955 to 1994 and followed up to January 1, 2000. Incidence rates were estimated and age- and sex-adjusted to the 1990 white population of the US. Birth cohort analysis was performed by grouping all of the incident cases into 10-year calendar year groups. Survival curves from the RA incidence date were estimated using the Kaplan-Meier method.
609 were enrolled in the incidence cohort, 445 of whom (73.1%) were female. The mean age at incidence was 58.0 years. The overall age- and sex-adjusted annual incidence of RA among residents of Rochester, Minnesota was 44.6/100,000 population (95% confidence interval 41.0-48.2). Incidence rates fell over time. The incidence rates of RA were 61.2, 47.3, 46.0, and 32.7 per 100,000 population in 1955-64, 1965-74, 1975-84, and 1985-94, respectively. However, there were indications of cyclical trends over time. Birth cohort analysis revealed diminishing incidence rates beyond the 1880-1890 birth cohort. Incidence rates increased with age up until age 85, but peaked earlier in women than in men. The survival rate was lower for RA patients compared with the general population (P<0.001) and no improvement was observed over the forty year period.
The decline in the incidence of RA over the past 40 years suggests that a change in exposure to an environmental factor may contribute to the etiology of RA.
The increasing exposure of women since the 1960s to exogenous estrogens for contraception and hormonal replacement is one hypothesis that has been considered but not yet resolved. Decreasing exposure over time to an infectious agent could also explain the decline in incidence but numerous investigations to date on this subject have not identified an agent. The absence of a decline in mortality over time is disturbing since others have reported that the advent of the use of Methotrexate since the early 80s has been associated with a decline in mortality. More work is needed in this area, particularly in view of the new treatments for RA that are now available.