Non-radiographic axial spondyloarthritis and ankylosing spondylitis: what are the similarities and differences?

The development of the axial spondyloarthritis and ankylosing spondylitis (ASAS) classification criteria has had several implications for our understanding of the entire spectrum of spondyloarthritides (SpA). Going beyond the modified New York criteria, which concentrate on conventional radiographs of the sacroiliac joints (SIJ) for the classification of ankylosing spondylitis, the ASAS criteria add active inflammation of the SIJ as obtained by MRI and human leucocyte antigen (HLA) B27 to classify patients with chronic back pain starting at a young age as axial SpA (axSpA). AxSpA should be considered as one disease that includes AS, the radiographic form, as well as the non-radiographic (nr-axSpA) form. Similarities and differences between these subgroups have been described in 3 studies: 1 local study, 1 national study (German SpA Inception Cohort) and 1 international study mainly conducted to test the efficacy of a tumour necrosis factor α blocker. Most clinical features and assessments of axSpA showed the same prevalence in patients with and without radiographic changes. However, some differences have been observed: the male:female ratio, the proportion of patients with objective signs of inflammation such as bone marrow oedema as detected by MRI, and the proportion of patients with increased levels of C reactive protein were higher in patients with AS. Importantly, these factors have also been identified as prognostic factors for more severe disease in terms of new bone formation. Thus, nr-axSpA may represent an early stage of AS but may also just be an abortive form of a disease which does cause much pain but which may also never lead to structural changes of the axial skeleton. Since the cut-off between nr-axSpA and AS is artificial and unreliable, we think that the term nr-axSpA should not be used for diagnosis but only for classification for historical reasons.