Non-Classical Complications of Adult-Onset Still's Disease: A Multicenter Spanish Study

Objective: To investigate the prevalence and clinical spectrum of atypical or non-classical complications in adult-onset Still's disease (AOSD) beyond macrophage activation syndrome (MAS) and to identify factors linked to their occurrence. Methods: Multicenter cross-sectional study of AODS cases included in the Spanish registry on Still's disease. Results: This study included 107 patients (67% women), of whom 64 (59.8%) developed non-classical complications. These include macrophage activation syndrome in 9.5%, atypical skin manifestations in 38.8%, cardiac involvement in 22.7% (comprising pericarditis, myocarditis, pulmonary arterial hypertension, and noninfectious endocarditis), pleuritis in 28.9%, transient pulmonary infiltrates in 4%, significant headache in 14.1%, lower abdominal pain with evidence of peritonitis in 8.4%, and secondary amyloidosis in 0.9%. In the multivariate logistic regression analysis, lymphadenopathy (OR 2.85, 95% CI 1.03-7.91, p = 0.044) and the systemic score system (SSC) index (OR 1.86, 95% CI 1.29-2.69, p = 0.001) were independently associated with the development of non-classical clinical manifestations. In contrast, typical exanthema was associated with a reduced risk of these complications (OR 0.32, 95% CI 0.11-0.95, p = 0.041). Conclusions: In addition to the typical clinical manifestations and MAS, a significant proportion of patients with AOSD develop uncommon complications, some of which can be potentially life-threatening. These should be considered in the evaluation and follow-up of patients. Early recognition and prompt management are crucial to significantly reduce morbidity and mortality.