Incidence and prevalence of ankylosing spondylitis in Northern Norway

Objective. To determine the incidence and prevalence of ankylosing spondylitis (AS) over a prolonged period in the 2 northernmost counties of Norway, where HLA-B27 has a high prevalence in the population. Methods. We conducted a cohort study of all patients registered with a diagnosis of AS between 1960 and 1993 at the University Hospital of Northern Norway, which is the sole rheumatology department serving these counties. We registered demographics, year of disease onset (clinical disease), and year of diagnosis (radiograph confirmation) for all patients. The date of onset of clinical disease in patients with AS was used in the calculation of incidence rates. Annual incidence and point/period prevalence rates were expressed per 100,000 adults. Primary AS was defined as AS in the absence of psoriasis or inflammatory bowel disease (IBD). Results. A total of 534 patients (75.1% male, mean age at clinical diagnosis 24.2 years, 93.0% HLA-B27 positive) had a confirmed diagnosis of AS (by the modified New York criteria). Median time from disease onset to radiologic confirmation was 8.0 years. Annual incidence of primary AS (n = 417) was 7.26, while estimated point prevalence rose from 0.036% in 1970 to 0.10% in 1980 and to 0.21% in 1990 with a period prevalence of 0.26%. AS was secondary to psoriasis or IBD in 117 patients (18.1%), with a diagnostic delay similar to that in primary AS. Annual incidence (14.1) and period prevalence in 1982-1993 (0.41%) were significantly higher in the town of Tromso than in the surrounding rural region (5.21 and 0.22%, respectively). Mortality in patients with AS was low. Conclusion. The incidence of AS was relatively stable in the northern part of Norway over a 34-year period. Incidence and prevalence are higher than reported in similar studies from Finland and Minnesota, possibly due to a higher population prevalence of HLA-B27.