These studies raise several questions about how different the monoarticular/oligoarticular and polyarticular PsA patients truly are from one another and if the distinction between these groups continues to be as clinically relevant as once believed.
The questions are further underscored by work from more than 20 years ago, which demonstrated that, over time, patients can move from the monoarticular/oligoarticular subtype to polyarticular subtype—and vice versa, in rarer cases. Investigators found that, among patients who started with monoarticular PsA, 33% evolved into oligoarticular and 56% into polyarticular disease over 10 years. Among those patients starting with oligoarticular disease, 78% evolved into polyarticular disease over the same duration of follow-up.5
It should be noted that recent use of ultrasound technology has further indicated the distinction in groups may be different than what seems to be clinically apparent. In a study of 80 patients with early oligoarthritis, 644 joints were evaluated by clinical exam and by ultrasound. The ultrasound evaluation detected synovitis in 33% of joints that were not found to have synovitis with physical exam alone. Moreover, in almost two-thirds of patients, evidence of subclinical disease was seen on ultrasound, and one-third of patients could be reclassified as polyarticular based on ultrasound findings, compared with findings from clinical assessment only.6
Clinical Implications
Dr. Coates concluded the session with the question: Can clinicians extrapolate findings from clinical trials in PsA to both subtypes of patients—those with polyarticular disease, as well as those with monoarticular/oligoarticular disease?
The question is important given the characteristics of patients in the large phase 3 drug trials used to understand the efficacy of various treatment modalities for PsA. Dr. Coates demonstrated the mean number of tender joint counts for inclusion in the trials was around 20 joints and the mean number of swollen joints was 10–15. This clearly indicates these trials are enrolling polyarticular PsA patients at a higher rate than those with monoarticular/oligoarticular disease. A study in Iceland indicated that two-thirds of PsA patients in that country who are treated with tumor necrosis factor inhibitor therapy would not have qualified for the randomized clinical trials performed on these medications because these patients had too few tender/swollen joints to meet the inclusion criteria.7
Although more recent drug trials have begun enrolling higher numbers of patients with fewer tender and/or swollen joints, the issue remains that the study populations used to test old and new PsA medications do not represent the entire population of patients. Thus, clinicians are left with the challenge of selecting an appropriate therapy for a patient in the absence of data specific to that patient. Although this issue is not completely unique to PsA, it’s perhaps more prevalent when making PsA treatment decisions than when making RA treatment decisions.
