Psoriasis in ankylosing spondylitis (AS) patients is not related to axial psoriatic arthritis (PsA), a type of psoriasis causing lower back inflammation and pain similar to AS, a new study has concluded.
The results of the study, “Is axial psoriatic arthritis distinct from ankylosing spondylitis with and without concomitant psoriasis?” were published in the journal Rheumatology.
Spondyloarthritis is an umbrella term for a family of inflammatory diseases causing arthritis. It is different from other types of arthritis because it involves areas where ligaments and tendons attach to bones.
The most common form of spondyloarthritis is ankylosing spondylitis (AS), which mainly affects the joints at the base of the spine where it meets the pelvis.
Psoriatic arthritis (PsA) is another form of spondyloarthritis that typically occurs in people with skin psoriasis. PsA mainly affects the joints of the toes, ankles, and fingers (peripheral). It can also affect the joints of the lower spine — similar to AS — in 20–75% of PsA patients (axial PsA).
About 10% of patients with AS also have skin psoriasis.
This prompted researchers to ask whether AS with psoriasis and axial PsA are related or two different diseases.
To find an answer, 1,303 PsA patients and 766 AS patients were recruited from two clinics in Toronto, Ontario, Canada. Of the PsA patients, 477 had axial PsA and 826 had peripheral PsA. In the AS group, 91 had psoriasis, while 675 did not.
Patients at both clinics were followed and assessed using a variety of diagnostic tools, with visits every 6–12 months for an average of 12.6 years for the axial PsA patients and 6.7 years for the peripheral PsA patients. 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, AS patients were younger at diagnosis, with an average age of 21.3 years in patients with psoriasis and 22.9 years in those without psoriasis. This is compared to axial PsA patients who had an average age at diagnosis of 34.4 years. There were also more males in both AS groups compared to the axial PsA group.
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.
Significantly more AS patients reported back pain — 90% and 92%, with and without psoriasis, respectively — compared to 21% of axial PsA patients.
All AS patients had higher scores using the Bath Ankylosing Spondylitis Disease Activity Indexes (BASDAI) and the Bath Ankylosing Spondylitis Metrology Index (BASMI). Both are measures of AS disease severity. Higher scores reflect more severe disease.
AS patients also fared worse following global disease assessments by a physician, and more AS patients were treated with biological medications, typically used when other medications are ineffective.
Compared to axial PsA patients, AS patients with and without psoriasis had a higher grade of sacroiliitis — inflammation of the joints connecting the lower spine to the pelvis.
When the data were analyzed over time, there was an increase in joint swelling in both axial PsA and peripheral PsA patients, while AS patients with or without psoriasis were more likely to have back pain and a higher BASMI.
The authors wrote, “In conclusion, our study suggests that axial PsA and AS with psoriasis seem to be two different diseases with different genetics, demographics, and disease expression.“