A classification study of clinical subsets in an inception cohort of early psoriatic peripheral arthritis--'DIP or not DIP revisited'.

Introduction: Multiple psoriatic arthritis (PsA) classification criteria exist, but these are based on established PsA when pre-existing joint damage and the effect of medication may confound their validity. This study examined the application of the Veale classification criteria in early PsA to determine the effect of disease progression and treatment on classification and to determine the effect of the number of involved joints and the presence of distal interphalangeal (DIP) joint involvement at initial presentation on clinical and radiological outcome.

Methods: A total of 129 patients presenting with PsA to an Irish early synovitis clinic were assessed at presentation and at 1- and 2-yr follow-up. The Veale criteria were used for PsA classification and the Sharp score of hands and feet was used to quantify radiological outcome.

Results: At presentation, 52 (40%) had oligoarticular PsA and 77 (60%) had polyarticular PsA. Patients with polyarticular PsA were administered disease-modifying anti-rheumatic drugs (DMARDs) more frequently than patients with oligoarticular PsA and this resulted in a significant number of polyarticular PsA patients being reclassified as oligoarticular PsA at 1- [27/70 (39%)] and 2-yr [26/53 (49%)] follow-up. Fewer patients initially classified with oligoarticular PsA were reclassified as polyarticular PsA. More patients with oligoarticular PsA at baseline were in DMARD-free remission and there was less radiological damage at 2-yr follow-up. DIP disease was associated with other classic seronegative disease features-enthesopathy and nail dystrophy-but did not influence clinical or radiological outcome and the separation of DIP disease as a distinct subgroup in classification criteria was not supported. Synovitis-acne-pustulosis-hyperostosis (SAPHO) syndrome was not observed as a separate subgroup.

Conclusion: This study confirms that the application of classification criteria of PsA based on the pattern and number of involved joints may be confounded in established PsA by the effects of DMARDs. The application of classification criteria based on disease pattern prior to treatment may be more useful in studies of pathogenesis and long-term outcome in PsA.