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- PATIENTS AND METHODS
Ankylosing spondylitis (AS) is a chronic inflammatory disease (1, 2) that primarily affects the sacroiliac joints (SI joints) and spine. The prevalence of AS shows considerable variation, with a 6% prevalence among the male population of the Haida Indians in Canada (3) and the virtual nonexistence of AS in several African populations (4–6). Although its cause remains elusive, an association with the HLA class I molecule B27 has been firmly established (7). The prevalence of AS correlates roughly with the population frequency of HLA–B27, supporting a role for HLA–B27-mediated antigen presentation in the pathogenesis of AS (8).
Studies on the incidence of AS are sparse and indicate an annual incidence of 6–7 per 100,000 persons in caucasian populations (9). Most studies on AS prevalence and incidence include patients with AS secondary to psoriasis, inflammatory bowel disease (IBD), and/or reactive arthritis. Because epidemiologic studies on primary AS in a region with a high population background of HLA–B27-positive individuals are rare, we performed a study based on hospital records to estimate the minimum incidence and prevalence of primary AS in the 2 northernmost counties of Norway.
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- PATIENTS AND METHODS
In this region, the estimated annual incidence for primary AS in the period 1960–1993 was 7.3 (95% CI 5.3–9.2), and for combined primary and secondary AS it was 8.7 (95% CI 6.4–11.0) per 100,000 adults. There was no clear change in incidence estimates over the 34-year study period. The period prevalence of AS was 0.26% for primary AS and 0.31% for primary and secondary AS, whereas the point prevalence 6-doubled during the study period. Incidence and point prevalence were higher in the town of Tromsø than in the surrounding rural region.
Considering that AS is not an uncommon inflammatory disease, there are surprisingly few published data on the incidence of AS. A population-based study from Finland found an AS incidence rate of 6.9 per 100,000 person-years (2), and a survey based on hospital records in Rochester, Minnesota for the period 1934–1989 found the overall age- and sex-adjusted incidence rate of primary AS to be 6.3 per 100,000 person-years, and that of primary and secondary AS together to be 7.3 per 100,000 person-years (9). Except for the estimates in the town of Tromsø, our results are largely in agreement with these findings, despite some methodologic differences. The Finnish study excluded patients with psoriasis and reactive arthritis, but included patients with IBD under the assumption that patients with AS frequently have inflammatory gut lesions. However, we categorized patients with AS and psoriasis, Crohn's disease, or ulcerous colitis as having secondary AS, and included patients with AS secondary to reactive arthritis in the group with primary AS. The Finnish study (2) was based upon a national registry of patients entitled to receive specially reimbursed medication for AS, but no such registry exists in Norway. However, patients with AS who do not require or no longer require disease-specific pharmacologic treatment are not registered in this material, which could reflect a potential underestimate of AS incidence. However, the same group has conducted another survey on the incidence of rheumatic diseases, where the diagnosis of AS was confirmed by radiology, and found a similar figure for AS incidence (6.3 per 100,000 person-years) (17). Therefore, in spite of these differences in approach, the incidence of primary AS in Northern Norway based on hospitalized patients is comparable with the incidence of primary and secondary AS in Rochester, Minnesota and with the reported incidence of AS in Finland. In light of the higher prevalence of HLA–B27 and a previous estimate of AS prevalence in the region (11, 18), it is rather surprising that the incidence of AS calculated for the county of Tromsø was only 2 times higher than that reported in the US (9). The incidence of AS in northwestern Greece was estimated to be 1.5 per 100,000 person-years (19), and the estimated incidence of all types of spondylarthropathies in the Japanese population was 0.48 per 100,000 person years (20). These figures are considerably lower than the estimates from Finland, Minnesota, and our population, and could reflect the lower prevalence of HLA–B27.
The period prevalence of AS rose relatively steeply after 1978 in this study. Because this increase coincided with the establishment of the sole department of rheumatology in this region, the increased AS prevalence is most likely explained by the ensuing increased awareness of AS. This assumption is supported by the decrease in the median diagnostic delay from 11.0 years (range 0–33 years) in the period 1960–1970 to 5.0 years (range 0–16 years) in the period 1982–1993. Similarly, the data (Figure 2) also indicate a peak in the incidence rate of AS in the period around 1978, with lower rates in both the prior and ensuing periods. Rather than actual declining incidence rates, we believe this finding also represents the establishment of the rheumatology department. However, it is also possible that the knowledge about AS among the general practitioners in the region subsequently increased and that less severe cases were not referred to our department.
All of our estimates are based on data from the hospital registry and do not necessarily reflect all population cases; selection bias may have occurred. The extent of selection bias, however, is likely limited because there are only 3 additional local hospitals in this region, which refer most patients with rheumatic disease to our department, as do the majority of general practitioners in both counties. Although patients had free access to primary and secondary care, county authorities were responsible for all health care expenditures and required an evaluation by a specialist in the region before patients were allowed to receive medical care in other counties. Therefore, for financial reasons, most patients with active rheumatic diseases who were in need of specialist evaluation in this period have been registered at our department. In contrast, there was discrepancy between the incidence and prevalence rates in the town of Tromsø compared with the surrounding rural region, although patients from Tromsø and the rural region did not differ with regards to age at symptom onset, diagnostic delay, or sex distribution (data not shown). The fact that incidence rates were almost 3 times higher in the city of Tromsø could indicate that not all patients with AS in the more remote parts of the counties have been diagnosed and referred to our department. Increased awareness and knowledge of rheumatic diseases after the foundation of our department may have contributed to this, but in general the threshold for seeking medical attention has been found to be higher in rural areas, resulting in lower estimates of incidence and prevalence as seen with rheumatoid arthritis (21).
To validate our findings, we compared the number of patients being diagnosed at our clinic with the number of patients examined at the department of radiology with radiologic sacroiliitis. In 1990, 51 patients were diagnosed with arthritis after radiograph examination of the SI joints. In the same year, 26 patients received the diagnosis at our clinic. The department of radiology's statistical database does not allow us to examine all images individually, and among the 51 patients there could possibly be individuals already registered with the diagnosis of AS at an earlier stage. Also, the local AS society (personal communication) provided a rough estimate of 350 current patients with AS, which is in agreement with our prevalence estimates. Our data are based on the use of the modified New York criteria (13) because these were most relevant to the current study period. In contrast to more recent European Spondylarthropathy Study Group classification criteria (22), the New York criteria do not encompass patients with unspecified spondylarthropathies and therefore our results provide no more than a lowest common denominator for AS. In comparison with the population-based study by Gran et al (11) who estimated the prevalence of AS to be 1.1–1.4%, our estimates of incidence and prevalence lie at the lower end of this spectrum and should therefore be considered minimum incidence and prevalence rates in the region.
The survival rate in this AS cohort was 89% after 35 years, with the survival curve indicating a marked reduction in survival rates after 20–30 years of disease. Standardized mortality ratios for patients with AS (Table 2) indicated lower death rates in all age and sex subgroups of patients with AS compared with controls.
In conclusion, primary AS is a common disease, which currently affects at least 0.4% of the adult population in Northern Norway. Because these figures are hospital based, the factual prevalence might even be higher if rigorous population-based studies could be realized. A rather stable AS incidence rate combined with low mortality during the 20 years after disease onset resulted in a steadily increasing prevalence of AS.