Acute joint swelling in psoriatic arthritis: Flare or "psout"-A 10-year-monocentric study on synovial fluid

Psoriatic arthritis (PsA) is a multifaceted inflammatory disease associated with psoriasis that can affect peripheral joints, entheses, and the axial skeleton with a variable clinical course. Acute episodes of joint swelling in PsA patients can have different causes and require specific treatments. We aimed to describe the acute joint swelling in PsA patients via synovial fluid (SF) analyses, assessing in particular the presence of pathogenic crystals, to determine whether it is a flare or an acute episode of gout ("psout") during the course of the disease. This retrospective study was based on the results of SF analysis of samples collected from unselected adult PsA patients referred to our clinic for acute joint swelling. Demographic characteristics, disease involvement, laboratory findings on SF, and treatment options were recorded and reviewed. Among 5,478 SF samples analyzed in a 10-year time span, 213 complete SF records from PsA patients were evaluated. Overall, after adjustment for the degree of synovial inflammation, significant differences were observed in term of sex (p = 0.0017) and ongoing therapy (p = 0.0246). Non-inflammatory SFs, indeed, were mainly described for female PsA patients under therapy. Regarding serum uric acid levels, there were 19/213 (8.9%) PsA with hyperuricemia (HU), who were older, mostly male, patients with mild articular involvement and rare pathogenic crystals in their SF. Although it is known that the risk of gout is higher among patients with PsA ("psout"), monosodium urate crystals were reported only in 5/213 SFs (2.4%) of our cohort and in 2/19 SFs (10.5%) of HU PsA patients. Moreover, hyperuricemia seems not to modify the SF features in PsA patients. This study results seem to suggest that the convergence of gout and PsA, involving the role of urate crystals, is a more intricate relationship, which needs further insights to be unraveled.