Multimorbidity in psoriasis as a risk factor for psoriatic arthritis: a population-based study.

Objectives: To examine multimorbidity in psoriasis and its association with the development of psoriatic arthritis (PsA).

Methods: A retrospective cohort study was performed using the Rochester Epidemiology Project. Population-based incidence (2000-09) and prevalence (1 January 2010) cohorts of psoriasis were identified by manual chart review. A cohort of individuals without psoriasis (comparators) were identified (1:1 matched on age, sex and county). Morbidities were defined using two or more Clinical Classification Software codes ≥30 days apart within prior 5 years. PsA was defined using ClASsification of Psoriatic ARthritis (CASPAR) criteria. χ2 and rank-sum tests were used to compare morbidities, and age-, sex- and race-adjusted Cox models to examine the association of baseline morbidities in psoriasis with development of PsA.

Results: Among 817 incident psoriasis patients, the mean age was 45.2 years with 52.0% females, and 82.0% moderate/severe psoriasis. No multimorbidity differences were found between incident psoriasis patients and comparators. However, in the 1088 prevalent psoriasis patients, multimorbidity was significantly more common compared with 1086 comparators (odds ratio 1.35 and 1.48 for two or more and five or more morbidities, respectively). Over a median 13.3-year follow-up, 23 patients (cumulative incidence: 2.9% by 15 years) developed PsA. Multimorbidity (two or more morbidities) was associated with a 3-fold higher risk of developing PsA.

Conclusion: Multimorbidity was more common in the prevalent but not incident cohort of psoriasis compared with the general population, suggesting that patients with psoriasis may experience accelerated development of multimorbidity. Moreover, multimorbidity at psoriasis onset significantly increased the risk of developing PsA, highlighting the importance of monitoring multimorbid psoriasis patients for the development of PsA.