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The severity of enthesitis in psoriatic arthritis is a potential marker for severe disease and joint damage, a cross-sectional study found.

For instance, a 10-unit increase on the Madrid Sonographic Enthesitis Index (MASEI) was associated with a 42% higher score on the modified Steinbrock Score (mSS) of peripheral joint damage, with an estimated exponential value (e^β) of 1.42 (95% CI 1.15-1.72, P<0.001), according to Lihi Eder, MD, PhD, of the University of Toronto, and colleagues.

In addition, a 10-unit increase on the MASEI was associated with an 89% higher score on the bone component of the mSS (e^β 1.89, 95% CI 1.33-2.69, P<0.001) and a 44% higher score on the soft tissue component (e^β 1.44, 95% CI 1.03-2.05, P=0.03), the researchers reported online in .

Enthesitis -- inflammation at sites where tendons or ligaments insert into bone -- is present in approximately one-third of patients with psoriatic arthritis and is most accurately assessed with ultrasound.

Some researchers have suggested that the entheses are the initial site of musculoskeletal inflammation in the spondyloarthropathies and that this type of inflammation is a primary factor in the pathogenesis of psoriatic arthritis. Some have also hypothesized that it may be directly involved in joint damage and may predict worse outcomes, although this has not been confirmed.

Therefore, to address this, Eder and colleagues analyzed data from the ongoing prospective Toronto Psoriatic Arthritis cohort, including 223 patients enrolled from 2011 to 2012 and 2014 to 2015.

Baseline ultrasound assessments were performed bilaterally for the quadriceps tendon insertion to the patella, the patellar tendon insertion to the patella and tibial tuberosity, Achilles tendon and plantar fascia insertions into the calcaneus, and the triceps tendon insertion to the olecranon process.

The bone component of the MASEI score rated calcific deposits, erosions, and enthesophytes, while the soft tissue component included structural changes at the tendon insertion, vascularization, and bursitis.

Participants' mean age was 56, and mean duration of psoriatic arthritis was 17 years. Mean tender and swollen joint counts were 2.5 and 1.1, respectively, while clinical enthesitis was present in 14%.

Mean baseline MASEI score was 15.6 and mean mSS was 18.7. The modified Stoke Ankylosing Spondylitis Spine Score (mSASSS) for spinal damage was 1.7, and sacroiliitis was present in 37% of patients.

Similar to what was seen on the mSS for joint damage, a 10-unit increase in the total MASEI significantly increased the likelihood of ankylosis (OR 1.93, 95% CI 1.37-2.72, P<0.001), arthritis mutilans (OR 1.77, 95% CI 1.23-2.54, P=0.002), and periostitis (OR 1.41, 95% CI 1.08-1.84, P=0.013).

Associations also were seen on negative binomial regression analyses for MASEI scores and measures of axial joint damage such as mSASSS (e^β 2.18, 95% CI 1.16-4.09, P=0.02).

"The study found that a higher MASEI score, which reflects more severe enthesitis, is associated with severity of peripheral radiographic joint damage as measured by mSS. Furthermore, the severity of enthesitis was associated with proliferative and erosive features of joint damage including joint ankylosis, arthritis mutilans, and periostitis," the researchers noted.

These findings also support the "" of psoriatic arthritis, in which inflammation of the enthesis is the primary event that triggers inflammation in the adjacent synovium. According to that hypothesis, the clinical differences seen in psoriatic arthritis compared with rheumatoid arthritis may relate to this specific pathophysiology.

Limitations of the study included the difficulties associated with detecting enthesitis in small joints and its cross-sectional design, which does not permit inferences about causation.

Nonetheless, "although the study cannot support a direct causal link between enthesitis and the development of peripheral or axial joint damage, the strong association observed between the extent of enthesitis and a number of features of joint damage in the peripheral and axial joints suggest that enthesitis may be a marker of more severe psoriatic arthritis phenotypes," Eder and colleagues wrote.

Additional longitudinal studies are needed to more fully elucidate the interplay between enthesitis, synovitis, and joint damage in psoriatic arthritis, they concluded.

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