To estimate the incidence of psoriatic arthritis (PsA) in patients with psoriasis, and to identify risk factors for its development.
The study was designed as a prospective cohort study involving psoriasis patients who did not have a diagnosis of arthritis at the time of study enrollment. Information was collected about lifestyle habits, comorbidities, psoriasis activity, and medications. Patients who developed inflammatory arthritis or spondylitis were classified as having PsA if they fulfilled the criteria of the Classification of Psoriatic Arthritis Study group. The annual incidence of PsA was estimated using an event per person-years analysis. Cox proportional hazards models, involving fixed and time-dependent explanatory variables, were fitted to obtain estimates of the relative risk (RR) of the onset of PsA, determined in multivariate models stratified by sex and controlled for age at onset of psoriasis.
The data obtained from the 464 patients who were followed up for 8 years were analyzed. A total of 51 patients developed PsA during the 8 years since enrollment. The annual incidence rate of PsA was 2.7 cases (95% confidence interval 2.1-3.6) per 100 psoriasis patients. The following baseline variables were associated with the development of PsA in multivariate analysis: severe psoriasis (RR 5.4, P = 0.006), low level of education (university/college versus high school incomplete RR 0.22, P = 0.005; high school graduate versus high school incomplete RR 0.30, P = 0.049), and use of retinoid medications (RR 3.4, P = 0.02). In multivariate models with time-dependent variables, psoriatic nail pitting (RR 2.5, P = 0.002) and uveitis (RR 31.5, P = 0.0002) were associated with the development of PsA.
The incidence of PsA in patients with psoriasis is higher than previously reported. A severe psoriasis phenotype, presence of nail pitting, low level of education, and uveitis are predictive of the development of PsA in patients with psoriasis.