LAUSR

The axial spondyloarthritis (axSpA) disease concept has undergone substantial change from when the entity ankylosing spondylitis was defined by the modified New York criteria in 1984. Developments in imaging, therapy and genetics have all contributed to changing the concept of axSpA from one of erosions in the sacroiliac joints to a spectrum of disease with and without changes evident on plain radiographs. Changes to the previously held concept and construct of the disease have also necessitated new classification criteria. The use of MRI, primarily of the sacroiliac joints, has substantially altered the diagnosis and differential diagnosis of axSpA. Many in the axSpA community believe that the current classification criteria lack specificity, and the CLASSIC study is underway to examine this area. Although much about the evolving axSpA disease concept is universally agreed, there remains disagreement about operationalizing aspects of it, such as the requirement for the objective demonstration of axial inflammation for the classification of axSpA. New imaging technologies, biomarkers and genetics data will probably necessitate ongoing revision of axSpA classification criteria. Advances in our knowledge of the biology of axSpA will settle some differences in opinion as to how the disease concept is applied to the classification and diagnosis of patients. In this Review, the authors examine how the definition and description of axial spondyloarthritis (axSpA) has evolved, most notably with the introduction of the concept of non-radiographic axSpA, and discuss the implications of these changes for diagnosis, classification and therapy. The concept of axial spondyloarthritis (axSpA) has expanded from ankylosing spondylitis with evidence of erosions to a spectrum of disease encompassing non-radiographic axSpA and radiographic axSpA. The current classification criteria capture the entire spectrum of axSpA, but many in the field believe they lack specificity; the CLASSIC study is underway to further assess this issue. The concept of axSpA is largely agreed upon in the research community, but opinion still diverges about some aspects, for example, the demonstration of objective axial inflammation for axSpA classification. The current definition of a positive sacroiliac joint MRI scan lacks specificity for axSpA, as demonstrated in imaging studies of individuals with and without back pain and post-partum women. Concepts such as the theory of natural kinds and latent class analysis enable us to further examine the crucial features of the axSpA concept, with sacroiliitis being the core feature. Advances in our understanding of the biology of axSpA via novel imaging, genetic and biomarker studies will probably enable the resolution of many current issues in axSpA diagnosis and classification.