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As rheumatologists are well aware, psoriatic arthritis (PsA) is a prevalent comorbidity occurring in patients with psoriasis (PsO). However, findings from studies examining the epidemiology of PsA in patients with PsO have been inconsistent, according to the investigators of a newly published study on the annual prevalence and incidence of PsA among those with PsO and risk factors for PsA.¹

Particularly, the researchers suggest, studies examining the epidemiology of PsA among Asian populations have been limited. The research group, which is based in Korea, conducted a cohort study using national claims data recorded between January 1, 2008, and December 31, 2020, to understand PsA epidemiology in the Korean population.¹

Incidence and prevalence of PsA are rising

Patients who were enrolled in the study were 18 years of age or older and had at least 1 International Classification of Diseases, Tenth Edition (ICD-10) diagnosis code for PsO in their chart.¹ From these patients (n=965,063), the research team separated those who went on to develop PsA from those who did not (controls).

• After parsing the data, the investigators found that the prevalence of PsA in PsO patients increased between 2008 and 2020, with the largest jump noted between 2014 and 2020.
• In 2008, the prevalence of PsA per 1000 PsO patients was 6.17 (95% confidence interval [CI] 5.73 to 6.65), which increased to 8.33 (95% CI 7.90 to 8.79) in 2014 and to 19.03 (95% CI 18.39 to 19.70) in 2020. 
• The incidence of PsA per 1000 patients with PsO remained constant between 2008 and 2016: 3.35 (95% CI 3.01 to 3.72) in 2008 and 3.33 (95% CI 3.07 to 3.62) in 2016, rising to 5.01 (95% CI 4.68 to 5.36) in 2020.

These numbers are lower than those that had been reported previously in studies from other countries, but the authors speculate that this may be because the data likely included more patients with severe disease, including those who visited PsO clinics or tertiary hospitals. 

Clinical trials also tend to report higher prevalence estimates than observational or population-based studies, the authors noted. Increasing prevalence may be partially explained by accumulating numbers of PsA patients over time. Or it may be because there鈥檚 a growing understanding of PsA among patients, making them more likely to seek care.

Some types of psoriasis carry lower risk

Digging deeper into the data, the research team reported that, compared with plaque PsO, the risk of PsA was significantly lower among guttate PsO cases (adjusted odds ratio [aOR] 0.54, 95% CI 0.40 to 0.74; P=.0001). The investigators say this could be because guttate PsO is typically self-limiting (resolving within weeks or months), exposing patients to shorter periods of inflammation than those with plaque PsO.

Palmoplantar pustulosis cases also had a lower risk of PsA compared with plaque PsO (aOR 0.49, 95% CI 0.44 to 0.55; P<.0001), despite previously being identified as a potential risk factor for PsA. This could be due to limited comparisons with plaque PsO patients, or because many patients with palmoplantar pustulosis also suffer from plaque PsO, potentially leading to false associations of PsA development.

How does the type of therapy factor in?

The odds of developing PsA were reduced among patients who used topical therapy (aOR 0.49, 95% CI 0.43 to 0.55; P<.0001) but were higher among those prescribed oral systemic therapy (aOR 3.61, 95% CI 3.26 to 3.99; P<.0001) and phototherapy (aOR 2.13, 95% CI 1.93 to 2.35; P<.0001).

Patients who had never used a biologic had significantly lower odds of developing PsA than those who initiated biologics within a median of 3.8 years since their PsO diagnosis (aOR 0.39, 95% CI 0.25 to 0.60; P<.0001). However, those who initiated biologics after 3.8 years trended towards higher odds of developing PsA (aOR 1.90, 95% CI 0.97 to 3.72; P=.0619).

Previously, biologic treatment for PsO was shown to reduce the risk of PsA. The authors of the current study state that the inverse association reported in their analysis was likely because patients with moderate-to-severe PsO are more apt to use biologics before developing PsA. (The current findings are consistent with evolving evidence of the benefit of early biologics for preventing PsA or delaying its progression.)

Lastly, the risk of PsA was significantly higher among patients with fatty liver disease (aOR 1.30, 95% CI 1.12 to 1.50; P=.0006), uveitis (aOR 1.50, 95% CI 1.16 to 1.94; P=.0023), ankylosing spondylitis (aOR 5.82, 95% CI 4.17 to 8.13; P<.0001), and rheumatoid arthritis (aOR 3.10, 95% CI 2.74 to 3.51; P<.0001). These findings suggest that PsO patients with a higher burden of comorbid and associated autoimmune conditions, and presumably higher levels of systemic inflammation, have a much greater risk of developing PsA than individuals with PsO but without these other conditions.

Putting the study in perspective

The authors identified some potential limitations to their study. First, because the study used diagnosis codes, voluntary or involuntary miscoding may have led to misclassification. However, the authors noted that validation studies have reported that approximately 70% of diagnosis codes from the Korean insurance claims coincided with those from medical records. Second, because the diagnosis of PsA is often delayed, patients with undiagnosed PsA may initiate use of biologics before receiving a formal diagnosis, leading to an association of biologics with a future diagnosis of PsA. Third, delayed PsA diagnosis may have led to underestimated PsA prevalence and incidence. Lastly, unlike with randomized clinical studies, relevant patient- and disease-related clinical data, such as smoking or body mass index, were not always available.

Despite these potential limitations, this study presents important risk factors that should be considered in clinical practice to improve outcomes in patients with PsO at higher risk of developing PsA.

Published: December 18, 2024