Adult-Onset Still's Disease and Secondary Hemophagocytic Lymphohistiocytosis: Diagnostic Pitfalls in Lymph Node Histology.

Background & Objective: Adult-onset Still's disease (AOSD) is a rare systemic autoinflammatory disorder characterized by high spiking fevers, evanescent rash, arthritis, and systemic inflammation. Its diagnosis is challenging due to the lack of specific biomarkers and clinical overlap with infections, malignancies, and autoimmune diseases. Case Report: We present the case of a 20-year-old female with a two-year history of recurrent fever, polyarthritis, and erythematous rash, accompanied by generalized lymphadenopathy and splenomegaly. Initial workup revealed severe anemia, leukocytosis, markedly elevated serum ferritin, and abnormal liver function tests. Infectious and malignant etiologies were excluded. Lymph node biopsies initially suggested dermatopathic lymphadenitis, but subsequent histology showed paracortical hyperplasia with features mimicking peripheral T-cell lymphoma. Immunohistochemistry revealed a polyclonal T-cell population and hemophagocytosis, leading to a revised diagnosis of reactive lymphadenopathy associated with AOSD. The patient fulfilled criteria for secondary hemophagocytic lymphohistiocytosis (HLH), a life-threatening hyperinflammatory syndrome. High-dose corticosteroid therapy resulted in significant clinical and biochemical improvement. Conclusion: This case highlights the diagnostic complexities of AOSD with secondary HLH, particularly when lymph node histology mimics malignancy. Awareness of the varied lymph node patterns in AOSD and integration of clinical, laboratory, and immunohistochemical data are crucial for accurate diagnosis and timely management, preventing unnecessary interventions and improving outcomes.