Epidemiology of axial spondyloarthritis

Axial spondyloarthritis (axSpA) is an inflammatory arthritis of the sacroiliac joints and spine. The prototype is ankylosing spondylitis, the radiographic form of the disease; however, more recently, an earlier or less-differentiated presentation has been described termed non-radiographic axial spondyloarthritis (nr-axSpA). Extra-articular manifestations commonly include anterior uveitis, inflammatory bowel disease, and psoriasis. There is a strong association with the human leukocyte antigen B27 (HLA-B27) allele, and the prevalence of the disease tends to follow the frequency of the allele. Epidemiological studies in axSpA are relevant in the population studied and therefore have limited external validity. This chapter describes the epidemiology of axSpA.