What is a severe axial spondyloarthritis?

While now widely accepted, the concept of axial spondyloarthritis (axSpA) is relatively recent as it was formally established in 2009 with the publication of the Assessment of SpondyloArthritis international Society (ASAS) classification criteria [1]. Over the years, the description of axSpA has been reshaped and additional definitions were established for “active axSpA” or “early axSpA” [2], [3]. However, there is currently no formal definition for “severe axSpA”. Generally, the severity of a disease is best defined by an excess of mortality or life-threatening complications. However, in chronic musculoskeletal conditions such as axSpA, physical function may be a more relevant outcome. Severity can also encompass quality of life, work participation, cost-of-illness or treatment requirements (Fig. 1). In this broader context, the term ‘disease burden’ is sometimes preferred over ‘severity’.