Psoriasis in ankylosing spondylitis (AS) is not related to axial psoriatic arthritis (PsA), a type of psoriasis that causes lower back inflammation and pain similar to AS, according to a study published in Rheumatology.

The study included approximately 1303 patients with PsA and 766 patients with AS from 2 Canadian clinics. Of the patients with PsA, 477 had axial PsA and 826 had peripheral PsA. In the AS group, 91 participants had psoriasis and 675 did not.

Patients at both clinics were followed and assessed using a variety of diagnostic tools, with visits every 6 to 12 months for an average of 12.6 years for patients with axial PsA and 6.7 years for the patients with peripheral PsA. The average follow-up for AS with psoriasis was 5.4 years and 3.5 years for those without psoriasis.

The analysis showed that overall, patients with AS were younger at diagnosis, with an average age of 21.3 years in patients with psoriasis and 22.9 years in those without psoriasis compared with axial patients with PsA who had an average age at diagnosis of 34.4 years. There were also more males in both AS groups compared with the axial PsA group.

Spondyloarthritis is a family of inflammatory diseases causing arthritis and involves areas where ligaments and tendons attach to bones. AS is the most common form of spondyloarthritis and mainly affects the joints at the base of the spine where it meets the pelvis.

More patients in both AS groups tested positive for HLA-B27, a genetic marker for inflammatory arthritis of the spine and joints. Of the AS patients with psoriasis, 82% tested positive, while 75% of those without psoriasis tested positive. In the axial PsA group, only 19% tested positive.

Approximately 90% of patients with AS with psoriasis and 92% without psoriasis reported back pain compared with only 21% of patients with axial PsA. Furthermore, compared with patients with axial PsA, patients with AS and with and without psoriasis had a higher grade of sacroiliitis, or inflammation of the joints connecting the lower spine to the pelvis.

When the data were analyzed over time, there was also an increase in joint swelling in both patients with axial PsA and patients with peripheral PsA, whereas patients with AS with or without psoriasis were more likely to have back pain and a higher BASMI, or Bath Ankylosing Spondylitis Metrology Index, a measure of AS disease severity.

The study reveals that axial PsA and AS with psoriasis are 2 different diseases with different genetics, demographics, and disease expression, according to the study authors. 

Reference
  1. Feld J, Yang Ye J, Chandran V, et al. Is axial psoriatic arthritis distinct from ankylosing spondylitis with and without concomitant psoriasis?. Rheumatology. Published October 08, 2019. https://doi.org/10.1093/rheumatology/kez457. Accessed October 15, 2019.