Incidence and prevalence of Behçet's disease in Oslo, Norway: a two-decade population-based analysis

To estimate the incidence and prevalence of Behçet's disease (BD) in Oslo, Norway from 1999 to 2021.

First, we identified BD cases from the Oslo hospital administrative databases and Norwegian connective tissue and vasculitis registry (NOSVAR) using the code for BD (M35.2) of the 10th revised edition of the International Classification of Diseases. Next, all identified cases were manually chart-reviewed and classified by the International Study Group (ISG) criteria and International Criteria for Behçet's Disease (ICBD). Cases meeting at least one of the two criteria were included.

In total 58 patients were included, 43 of them incidental; 52% were female and 57% with non-Norwegian ancestry. The mean age at symptom onset was 24 years and at diagnosis 33 years. The median diagnostic delay was 5 (interquartile range 2-12) years. Incidence was 0.29 (95% CI: 0.18, 0.41) per 100 000 person-years, higher in non-Norwegians (0.44 vs 0.22). Among Norwegians, the incidence was higher in females (0.27 vs 0.08). The prevalence was 2.36 per 100 000 inhabitants in 1999 and 7.14 in 2021, higher for non-Norwegians (12.68 vs 4.32 in 2021). Incidence and prevalence appeared to increase, but reclassification by ISG criteria adjusted the trend downward. This trend was significantly reduced when accounting for the proportion of non-Norwegians.

BD is a rare disease in Oslo, Norway, with female predominance in Norwegian descendants, but higher prevalence overall in non-Norwegians. The observed increases in incidence and prevalence are likely due to changes in population genetics and introduction of the more sensitive ICBD criteria. Diagnostic delays are common, a significant concern, in Behçet's disease.