A challenging diagnosis of idiopathic multicentric Castleman disease with complex systemic presentation: A case report

KEY CLINICAL MESSAGE: Idiopathic multicentric Castleman disease (iMCD) is challenging to diagnose due to clinical similarities with other conditions, such as Still's disease. Once diagnosed, iMCD may be effectively managed with the anti‐interleukin‐6 antibody siltuximab. ABSTRACT: Here, we present the case of a 19‐year‐old Polish woman with persistent fever and enlarged lymph nodes and whose diagnosis remained inconclusive following initial clinical assessments and extensive laboratory analyses. The patient had subsequent complaints of joint pain and erythema which were suspicious of Still's disease and resolved with treatment with tocilizumab. Later, the progression of symptoms, such as lymphadenopathy, and elevated interleukin‐6 levels were consistent with Castleman disease, leading to the diagnosis of idiopathic multicentric Castleman disease seven years after the patient first reported symptoms. Treatment with the anti‐interleukin‐6 antibody siltuximab resulted in complete symptom resolution and normalization of inflammatory parameters. No adverse events were reported due to treatment with siltuximab.