Incidence of spondyloarthritis subtypes: a systematic review.

Objectives: Several epidemiologic studies of spondylarthritis (SpA) and its subtypes have been reported during the last decades. The majority of these studies provided prevalence estimates and showed a considerable variation in the reported frequency of SpA subtypes. Most systematic reviews published in this field aimed to summarise the results of prevalence studies, however, incidence studies are important for an accurate picture of a disease occurrence in a defined population. We conducted a systematic review regarding the incidence of SpA subtypes on studies published during the last 25 years, to compare their methodology and summarise their results.
Methods: A systematic literature search of PubMed was performed to identify all published studies on the incidence of SpA subtypes between 1/1/1995 and 31/12/2019. Studies were considered eligible if the incidence of one or more SpA subtypes was measured in the general population, and met concrete inclusion criteria. Incidence rates (IR) were summarised using a random effect model.
Results: A total of 24 publications fulfilled the inclusion criteria. Most of them included results for two or more SpA subtypes. Sixteen studies presented the incidence of psoriatic arthritis, which gave an overall IR estimate of 9.7 cases per 100.000 person-years. Thirteen studies presented the incidence of ankylosing spondylitis with an overall IR estimate of 4.8, and eight studies presented reactive arthritis incidence with an overall IR estimate of 3.4. A small number of studies referred to the incidence of enteropathic arthritis or undifferentiated spondyloarthritis.
Conclusions: Incidence studies of SpAs differ considerably in their methods, and result in a wide variation of the IRs for all SpA subtypes. Methodological differences may only partly explain the differences in disease occurrence observed among studies. More studies from different populations based on specific classification criteria are needed for a more accurate picture of SpA epidemiology.