THE INCIDENCE OF GIANT-CELL ARTERITIS IN OLMSTED COUNTY, MINNESOTA - APPARENT FLUCTUATIONS IN A CYCLIC PATTERN

Objective: To investigate trends in the incidence of giant cell arteritis over Design: Population-based incidence study. Setting: Olmsted County, Minnesota. Methods: All incidence cases of giant cell arteritis first diagnosed between 1950 and 1991 were identified using the unified record system at Mayo Clinic. Age- and sex-specific incidence rates were calculated using the number of incidence cases as the numerator and population estimates as the denominator. Overall rates were age- and sex-adjusted to the 1980 United States. white population. The annual incidence rates were graphically illustrated using a 3-year centered moving average. Results: Between 1950 and 1998, 125 Olmsted County residents (103 women and 22 men) were diagnosed with giant cell arteritis. The age- and sex-adjusted incidence per 100 000 persons 50 years of age or older was 17.8 (95% CI, 14.1 to 21.0); incidence was significantly higher in women (24.2 [CI, 19.5 to 28.9]) than in men (8.2 [CI, 4.8 to 11.6]). Age-specific incidence rates increased with age (P < 0.0001). The annual incidence rates increased significantly over the study period (P = 0.002) and appear to have clustered in five peak periods, which occurred about every 7 years. A significant calendar-time effect was identified; it predicted an increase in incidence of 2.6% (CI, 0.9% to 4.3%) every 5 years. Conclusions: Our observation of a regular cyclic pattern in incidence rates over time supports the hypothesis of an infectious cause for giant cell arteritis. Similar studies in other populations are needed to confirm our findings.