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Xabier Michelena, Gabriele De Marco, Sayam Dubash, Dennis McGonagle, Helena Marzo-Ortega, Comment on: Is axial psoriatic arthritis distinct from ankylosing spondylitis with and without concomitant psoriasis?, Rheumatology, Volume 60, Issue 1, January 2021, Pages e24–e25, https://doi.org/10.1093/rheumatology/kez670
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Dear Sir, We read with interest the article by Feld et al., who report a retrospective analysis of 2069 patients from two real life, prospective parallel cohorts of AS and axial PsA (axPsA) attending the same tertiary centre in Canada [1]. The authors address an important question of significant clinical relevance, namely the characterization of axial SpA (axSpA) and axPsA and whether these may represent two distinct clinical entities with overlapping features rather than the same disease entity. The understanding, evaluation and timely therapy of axPsA indeed represents an area of unmet need in the study of SpA and poses many clinical challenges in real life where patients with psoriasis and back pain, in the absence of peripheral joint swelling, struggle to get access to appropriate treatment due to the lack of diagnostic criteria or even a consensus definition on what constitutes axPsA.
Axial involvement including sacroiliitis can occur in many musculoskeletal disorders, although it was its higher prevalence and distinct clinical presentation in PsA that led Moll and Wright to incorporate it as one of the main clinical sub-types of PsA [2]. The real prevalence of axial involvement in PsA, however, remains unclear, with reports varying from 25 to 70% [3], which reflects the methodological issues with the different studies and the lack of a standardized definition. Feld et al. [1] report an overall prevalence of 36% (477/1303) of axPsA in their overall PsA cohort, based on fulfilment of radiographic criteria for sacroiliitis (at least bilateral grade 2 or unilateral grade 3 or 4) not only at baseline, but at any point over the follow-up period with no requirement of reported back pain. Indeed, their study is the first longitudinal description of axPsA followed up for over a decade. Importantly, they also describe a contemporary AS cohort of which 12% (91/766) had concomitant psoriasis. However, by choosing to define axPsA based on the presence of radiographic sacroiliitis (bilateral grade 2 or unilateral grade 3 or 4) the authors may have missed those cases with isolated spondylitis including cervical spine involvement, estimated to be 35% [4], and who may never develop sacroiliac joint involvement, those presenting with milder asymmetrical or unilateral sacroiliitis [5] not fulfilling the specified radiographic criteria, or those without any radiographic findings that could have been identified by MRI.
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