Axial Psoriatic Arthritis and Ankylosing Spondylitis: Comparing Clinical, Radiologic Characteristics

It is unclear whether ankylosing spondylitis and axial psoriatic arthritis are part of a spectrum of the same disease, or whether they are separate diseases with overlapping features.

It remains a matter of ongoing debate whether axial-dominant psoriatic arthritis (PsA) and ankylosing spondylitis (AS) are separate entities with overlapping characteristics, or whether they represent different clinical presentations of the same disease. Although it does not definitively resolve the controversy, a recent literature review published in Nature Reviews Rheumatology highlights the clinical, radiologic, and genetic similarities and differences between axial disease in PsA and AS.1

Inflammatory back disease was first identified as a feature of PsA in 1956.2 Estimates of the prevalence of axial involvement in PsA range from 25% to 70%.1 The wide range of reported prevalence is likely a result of divergent definitions of axial involvement in PsA, which vary in the literature from isolated radiographic features to the New York or Assessment of Spondyloarthritis International Society (ASAS) criteria for the diagnosis of AS.3 

It is not unusual for patients with PsA to have concomitant peripheral arthritis and axial disease, but less than 5% of patients with PsA have isolated spondylitis.1 In these patients, the clinical picture of inflammatory axial disease in PsA can be indistinguishable from that of AS.3 Results from a prospective, single-center, cross-sectional observational study that compared the prevalence and clinical and radiographic characteristics of psoriatic spondyloarthritis in PsA vs AS showed that 24% of enrolled patients met the diagnostic criteria for both conditions.4 In other patients with axial PsA, spinal disease can be clinically silent. Only 45% of patients with PsA and radiographic evidence of axial involvement experience inflammatory back or neck symptoms.5 In contrast, the presence of inflammatory back pain or limitation of motion are necessary to make the diagnosis of AS.6

Axial PsA and AS are both associated with high degrees of hereditability and familial clustering. The hallmark genetic marker of AS is the HLA-B27 allele, carried by more than 80% of patients diagnosed with AS.1 In contrast, the HLA-B27 allele occurs in patients with PsA with a prevalence rate of only 20%. Its presence in patients with PsA is associated with a severe phenotype whose features include an AS-like axial involvement, earlier age of arthritis onset, and progression of joint damage.7 A number of additional susceptibility loci have been separately identified for AS and PsA; however, the HLA-B27 allele is the only genetic variant that is common in both diseases.1

Patients with AS are more likely to be male, to be of younger age, and to have more severe inflammatory back pain and limitation of spinal range of motion than patients with axial PsA. Patients with axial PsA are more likely to be affected by dactylitis, enthesitis, and peripheral arthritis and to have a greater prevalence of spondylitis without sacroiliitis. Sacroiliac joint involvement is more likely to be asymmetrical in axial PsA compared with AS.8 AS and axial PsA have an equivalent clinical effect, with patients reporting similarly high scores for measures of disease activity, disability, and quality of life.1,9

For additional clinical perspective, Rheumatology Advisor interviewed Dafna D. Gladman, MD, FRCPC, coauthor of the literature review published in the Nature Reviews Rheumatology. Dr Gladman is a professor of medicine at the University of Toronto, senior scientist at Krembil Research Institute, and deputy director at the Centre for Prognosis Studies in the Rheumatic Diseases in Toronto, Ontario.

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Rheumatology Advisor: What challenges do rheumatologists face in distinguishing axial PsA from axial AS?

Dr Gladman: The question we face as rheumatologists when a patient with axial disease has psoriasis is whether they have AS with psoriasis or whether they have psoriatic arthritis. This distinction is not very challenging if the patient responds to nonsteroidal anti-inflammatory medications. However, if they do not, do you treat them as a patient with AS? Until recently, we only had anti-[tumor necrosis factor] agents for AS, whereas for PsA there are a number of other agents. Not all agents work for all conditions.

Rheumatology Advisor: What further research on this topic is necessary?

Dr Gladman: We need more research comparing the patients who are clearly diagnosed with AS who happen to have psoriasis and those with axial PsA. The research should include longitudinal observations, as assessments made at 1 point in time make it difficult to interpret the results. Both clinical and laboratory comparisons should be made, and genetic factors should be examined.

Rheumatology Advisor: Please feel free to add anything else about this topic that may be of interest to rheumatologists.

Dr Gladman: Moll and Wright introduced the concept of spondyloarthritis, which includes AS and PsA as major prototypes.10 They recognized a group of patients with predominantly axial disease among patients with PsA. The increased prevalence of psoriasis among patients with AS has been recognized as well. It may very well be that some patients with AS do have coexisting psoriasis. But it is likely that these patients are different from patients with PsA, as the latter are more likely to also have peripheral arthritis, and their axial symptoms are not as severe as those of patients with AS. I think it is important for rheumatologists to make the right diagnosis and follow and treat patients accordingly.

This interview was lightly edited for clarity.

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