Epidemiology and outcome of adult-onset Still's disease in Northern Norway

Background: Adult-onset Still's disease (AOSD) is considered a rare disease, but few data exist on the incidence and prevalence of AOSD. This study has analysed the epidemiology, disease presentation, and outcome of AOSD in a stable homogeneous population in Northern Norway. Methods: A retrospective cohort study of all AOSD patients registered in 1990 - 2000 at the only hospital in the region with a Rheumatology Service. Clinical diagnosis and exclusion of patients were directed by the Yamaguchi criteria for AOSD. Demographic and clinical data at baseline were extracted from patient records and supplemented with data gathered at control visits. Data were analysed with nonparametric methods. Results: AOSD was ascertained in 13 patients; the estimated annual AOSD incidence was 0.4/100 000 adults (95% CI 0.11 - 0.97), while point prevalence of AOSD increased from 3.4/100 000 ( 95% CI 0.8 - 9.4) in 1990 to 6.9/ 100 000 in 2000 ( 95% CI 2.7 - 14.2). Mean diagnostic delay was 5.2 months ( range 0.5 - 18). Serum ferritin >5 times the normal upper level had 63% diagnostic sensitivity. During 69 months' follow-up, one patient died, 6/13 patients achieved sustained remission, while six patients developed a chronic progressive (n=3) or a relapsing/remitting disease course (n=3). Four of these six patents had to enter social security programmes. Conclusion: The annual incidence of AOSD in Northern Norway is at least 0.4/100 000 adults. AOSD in this region is more prevalent than in France or Japan, affects more males, and approximates to the prevalence of juvenile Still's disease. Half of all patients have a monocyclic disease course, while mortality and invalidity occur in patients with chronic disease.