Disease phenotypes in adult patients with suspected undifferentiated autoinflammatory diseases and PFAPA syndrome: Clinical and therapeutic implications

Background: Undifferentiated autoinflammatory diseases are characterized by recurrent or persistent fever, usually combined with other inflammatory manifestations, and negative or inconclusive genetic studies for monogenic autoinflammatory disorders. Aims: To define and characterize disease phenotypes in adult patients diagnosed in an adult reference center with undifferentiated autoinflammatory diseases, and to analyze the efficacy of the drugs used in order to provide practical diagnostic and therapeutic recommendations. Methods: Retrospective study (2015-2022) of patients with undifferentiated autoinflammatory diseases among all patients visited in our reference center. Demographic, clinical, laboratory features and detailed therapeutic information was collected. Results: Of the 334 patients with a suspected autoinflammatory disease, 134 (40%) patients (61% women) were initially diagnosed with undifferentiated autoinflammatory diseases. Mean age at disease onset and at diagnosis was 28.7 and 37.7 years, respectively. In 90 (67.2%) patients, symptoms started during adulthood. Forty-four (32.8%) patients met diagnostic/classification criteria for adult periodic fever with aphthous stomatitis, pharyngitis and cervical adenitis (PFAPA) syndrome. In the remaining patients, four additional phenotypes were differentiated according to the predominant manifestations: a) Predominantly fever phenotype (n = 18; 13.4%); b) Predominantly abdominal/pleuritic pain phenotype (n = 9; 6.7%); c) Predominantly pericarditis phenotype (n = 18; 13.4%), and d) Complex syndrome phenotype (n = 45; 33.6%). Prednisone (mainly on demand), colchicine and anakinra were the drugs commonly used. Overall, complete responses were achieved with prednisone in 41.3%, colchicine in 40.2%, and anakinra in 58.3% of patients in whom they were used. By phenotypes, prednisone on demand was more effective in adult PFAPA syndrome and colchicine in patients with the abdominal/pleuritic pain pattern and PFAPA syndrome. Patients with complex syndrome achieved complete responses with prednisone (21.9%), colchicine (25.7%) and anakinra (44.4%), and were the group more often requiring additional immunosuppressive drugs. Conclusions: The analysis of the largest single-center series of adult patients with undifferentiated autoinflammatory diseases identified and characterized different disease phenotypes and their therapeutic approaches. This study is expected to contribute to increase the awareness of physicians for an early identification of these conditions, and to provide the best known therapeutic options.