Atypical Combination of Mixed Connective Tissue Disease and Multicentric Castleman Disease.

Castleman disease (CD) is a rare lymphoproliferative disorder with unicentric (UCD) and multicentric (MCD) forms, first detailed by Benjamin Castleman in 1956. It has three subtypes: hyaline vascular, plasma cell, and mixed. CD incidence is higher in HIV patients and is often associated with human herpes virus-8 (HHV-8). We report a 68-year-old woman with diabetes and mixed connective tissue disease (MCTD), which was diagnosed six months prior to presentation, who presented with lymphadenopathy, splenomegaly, and B symptoms. Imaging showed diffuse adenopathy. Biopsy confirmed the plasma cell subtype of MCD, with polyclonal plasmacytosis. The patient tested negative for HIV and HHV-8. Initial treatment with rituximab and corticosteroids resolved her symptoms. Six years later, she relapsed and was treated with an anti-IL-6 agent, which she could not complete due to adverse effects but still showed symptom improvement. This case is notable for the patient's age and the plasma cell subtype of MCD, as well as the concurrent diagnosis of MCTD. The patient's clinical presentation and histopathological findings underscore the importance of considering CD in the differential diagnosis of lymphadenopathy with systemic symptoms. Chronic inflammation and B lymphocyte proliferation, potentially linking MCTD and CD, were evident in this case. Despite extensive comorbidities, the patient remained clinically stable due to intensive multidisciplinary management. CD is a rare, heterogeneous disorder requiring a high index of suspicion. The potential link between CD and MCTD warrants further research. Effective management involves targeted therapies and close monitoring due to relapse risk. This case underscores the importance of individualized treatment plans considering comorbidities and treatment tolerability. Further research is needed to better understand CD's pathogenesis and develop effective treatments.