Study: Axial AS With Psoriasis, Axial Psoriatic Arthritis 2 Distinct Diseases

Back pain more likely for patients with ankylosing spondylitis and psoriasis

Marisa Wexler MS avatar

by Marisa Wexler MS |

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People with ankylosing spondylitis (AS) and psoriasis — a skin disease causing red, scaly, itchy patches that often occur on the elbows, knees, trunk, and scalp — are more likely than those with psoriatic arthritis (PsA) to have back pain, but less likely to have nail lesions, according to a new study of people with axial spondyloarthritis alone.

Axial spondyloarthritis is a type of inflammatory arthritis affecting the joints of the spine, chest, and pelvis. The researchers had studied more than 3,000 people with AS or PsA living in Canada to learn more about these conditions.

Their findings, the researchers noted, lend support to the idea that axial AS with psoriasis and axial PsA are distinct clinical entities, even though symptoms can be very similar at first glance.

“Axial patients with PsA were older at diagnosis … more likely to have nail lesions … and less likely to have inflammatory back pain … compared with patients with … axial AS with psoriasis,” the team wrote.

The study, “Isolated axial disease in psoriatic arthritis and ankylosing spondylitis with psoriasis,” was published in the Annals of the Rheumatic Diseases.

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AS and PsA are both forms of spondyloarthritis, an umbrella term for a family of inflammatory diseases causing arthritis in areas where ligaments and tendons attach to bones.

Distinct diseases?

The hallmark of AS is inflammation in the joints at the base of the spine where it meets the pelvis. PsA, meanwhile, is defined by the simultaneous presence of psoriasis, a disease that causes non-contagious patches of thick and reddened skin and scales that are silvery.

While these two conditions often manifest distinctly, in some patients there is considerable overlap in symptoms. About 9% of AS patients have been found to have psoriasis, while less than 5% of all PsA patients have axial spondyloarthritis alone — a generally similar constellation of symptoms to what is seen in axial-only AS with psoriasis.

A previous study, published in 2019, found that “axial PsA appears to be distinct clinically from AS and is associated with worse peripheral arthritis and less back pain,” the researchers wrote. However, no study to date has compared axial-only AS patients with psoriasis with those with axial-only PsA.

Now, scientists in Canada analyzed the clinical data of 1,688 people with AS, who were followed at the Toronto Ankylosing Spondylitis Clinic. That data was compared with the medical information of 1,576 patients with PsA, who were seen at the University of Toronto Psoriatic Arthritis Clinic.

Among AS patients, 82 (4.86%) had axial-only disease and psoriasis. This was “the first study to exclusively study isolated axial AS with psoriasis,” the researchers wrote.

Of the PsA patients, 32 (2.03%) had only axial disease at the time of the study, with no peripheral joints, of those in the legs and arms, being affected. Data suggested that these patients generally had milder disease than those who showed both axial and peripheral involvement.

Compared with axial-only PsA patients, those with axial-only AS and psoriasis were significantly younger — with mean ages of 36.92 vs. 43.09 — and were diagnosed at a significantly younger age. The age at diagnosis was 29.65 versus 37.44.

Axial-only AS patients with psoriasis showed significantly more severe disease and lower quality of life than axial-only PsA patients. They also were more likely to have inflammatory back pain (77.33% vs. 50%), but less likely to have nail lesions (6.06% vs. 53.13%).

A genetic variation associated with AS, called HLA-B*27, was significantly more common among AS patients (75.95% vs. 34.62%).

Significantly fewer axial-only AS patients with psoriasis were being treated with disease-modifying anti-rheumatic therapies — a group of medicines commonly used for inflammatory arthritis. A total of 10.98% of the axial-only AS patients with psoriasis were given this treatment versus 28.13% of individuals with axial-only PsA.

However, more of them were on biologics, or therapies made using living cells, most commonly antibody-based medications (59.76% vs. 18.75%).

When adjusting for potential influencing factors simultaneously, significant group differences remained for age, and the likelihood of having inflammatory back pain and nail lesions.

These findings highlight that “isolated axial PsA appears to be a different clinical entity than isolated axial AS with psoriasis, with older age at diagnosis, a higher chance of nail lesions and lower odds of inflammatory back pain,” the researchers wrote.

“This study further solidifies the concept that axial PsA is indeed different from AS,” they added.

Further longer-term studies are needed, the researchers said.

“Given the paucity of studies focused on the uncommon clinical [form] of isolated axial disease within the [spondyloarthritis] family, more research is needed to further evaluate longitudinal clinical outcomes among those with isolated axial disease,” the team wrote.

This includes “the possible use of multistate models to evaluate the impact of clinical changes such as peripheral involvement over time,” they added.