Page 39 - Recognizing axial spondyloarthritis - Janneke de Winter

BACKGROUND
Spondyloarthritis (SpA) is a prevalent and potentially disabling form of chronic inflammatory arthritis, affecting 0.5-1.5% of the Western population (1,2). SpA has classically been subdivided into several subtypes, including ankylosing spondylitis (AS), psoriatic arthritis (PsA), reactive arthritis, arthritis/spondylitis associated with inflammatory bowel disease (IBD), and undifferentiated SpA. Classification criteria for SpA have been developed by the Assessment of SpondyloArthritis international Society (ASAS), which classify SpA as axial or peripheral SpA (3–5). The axial SpA disease spectrum classifies patients as having either radiographic axial SpA (AS) whether the modified New York criteria (mNYc) are fulfilled, or as having non-radiographic axial SpA (nr-axSpA) in the absence of definite SI joint changes on plain radiograph.
Whether nr-axSpA is a different form (6,7) of AS, an early form (8–10) of AS or two manifestations in the same disease continuum (11–13) is still subject to debate. There are several reasons to assume that AS and nr-axSpA should be considered as the same disease. First, AS and nr-axSpA have in general similar clinical characteristics, especially when related to disease activity (9–11). Patients with AS and nr-axSpA not only show similar levels of disease activity, they also show a similar clinical disease course in the absence of tumor necrosis factor (TNF) α inhibiting treatment, as shown by recent longitudinal results from the German Spondyloarthritis Inception Cohort (GESPIC) (14). Second, patients with nr-axSpA respond similarly to TNF α inhibiting treatment (15–18). Third, radiographic changes only appear after several years; therefore the requirement of radiographic changes clearly reduces the sensitivity of the mNYc. Not only is sensitivity of the mNYc rather limited; several studies have shown that scoring of radiographs is subject to considerable inter- and intra-reader variability. Scoring by both trained readers and local rheumatologists/radiologists not only yield modest sensitivity and specificity at best, but also show moderate agreement with respect to the recognition of radiographic sacroiliitis (19,20). These limitations challenge the crucial role of radiographic scoring in the process of diagnosing AS. Magnetic resonance imaging (MRI) is increasingly used to visualize inflammation in the SI joints, since active inflammatory lesions are present on MRI before radiographic lesions are detected (13). However, MRI also has limitations in terms of scoring agreement, sensitivity, specificity and costs.
On the other hand, AS is characterized by a male predominance and a higher level of C-reactive protein (CRP) in comparison to nr-axSpA (9–11). Other studies suggest that AS and nr-axSpA differ in their genetics (21), since in some studies human leukocyte antigen (HLA)-B27 carriage is higher in AS than in nr-axSpA (11,15,22), whilst other studies suggest no difference in HLA-B27 carriage between both groups (9,23,24).
Even though spinal inflammation and structural damage are the main features of axial SpA, many patients have concomitant peripheral (arthritis, enthesitis,
DISEASE MANIFESTATIONS IN AXIAL SPA
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