Background/Objectives: Calcium pyrophosphate (CPP) and monosodium urate (MSU) crystals can both be present concomitantly in an arthritic joint. Epidemiology, response to treatment and outcome of such mixed crystal arthritides are not known. We therefore analyzed data from our prospective longitudinal gout cohort to compare MSU only to MSU and CPP crystal positive (mixed) arthritides.
Methods: We analyzed data of patients from our prospective gout cohort (n=49) with MSU crystals in the synovial aspirate at the time of inclusion (inclusion period 01/2013 – 12/2014). Multiple parameters were recorded at presentation, 3 months and 12 months thereafter. In 13 (26 %) of the 49 patients beside the MSU crystals, CPP crystals were detected in the synovial fluid .We compared 2 groups: Group 1 as gout (MSU crystals only), group 2 as mixed (MSU and CPP crystals) arthropathy according to synovial microscopy. Groups were compared regarding epidemiologic data, affected joints, laboratory parameters, treatment and outcome.
Results: Epidemiologic parameters were similar (group 1 vs. group 2) in both groups (median age 72 vs. 70 years, 86% vs. 85% men, median BMI 29.4 vs. 29.8 kg/m², similar co-morbidities). The majority of patients were hospitalized at inclusion (75 % vs. 92 %). All of the patients had active arthritis. The median number of arthritic joints was 2 vs. 1.5. 47 % vs. 54 % of patients had tophi. Laboratory parameters: Median GFR 60.5 vs. 45 ml/min, median serum uric acid level (SUA) 474 vs. 589 µmol/l, median CRP-level 29 vs. 61 mg/l. 24 (17 colchizine, 8 steroids, 5 NSAID, 6 combination treatment) vs. 5 patients were on anti-inflammatory treatment. 14 (13 allopurinol, 1 febuxostat) vs. 2 patients (allopurinol) were on uric acid lowering treatment. 1 or more flares were reported at 3 months FU; in 47% of the presenting 19 vs. 50 % of the presenting 8 patients and at 12 months FU in 62 % of the presenting 8 patients vs. 50 % of the presenting 4 patients. SUA target of 360 µmol was reached by 6 vs.4 patients at 3 months and by 4 vs. 2 patients at 12 months FU.
Significance and Conclusions: Prevalence of mixed CPP/MSU arthritis in above patients is higher as previously reported. Although there were no significant differences in epidemiology, treatment and outcome, mixed crystal arthritis patients had a trend to higher CRP-levels, worse kidney function and a higher SUA level compared to patients with MSU crystals only.