Loss of the epigenetic mark, 5-Hydroxymethylcytosine, correlates with small cell/nevoid subpopulations and assists in microstaging of human melanoma.

BACKGROUND: Adult-onset Still's disease (AOSD) is a rare systemic autoinflammatory disorder and a diagnosis of exclusion. Kikuchi-Fujimoto disease (KFD) is a benign, usually self-limiting necrotizing histiocytic lymphadenitis. The clinical and histopathological overlap between the two can be diagnostically challenging.

CASE REPORT: A 24-year-old female presented with a 6-week history of quotidian fever, migratory polyarthralgia, a fleeting salmon-coloured rash, and cervical lymphadenopathy. Initial workup revealed pronounced leucocytosis, highly elevated C-reactive protein, and critically high serum ferritin (6,500 μg/l). An excisional lymph node biopsy, performed to exclude malignancy, demonstrated features consistent with KFD. Active hemophagocytic lymphohistiocytosis (HLH)/macrophage activation syndrome (MAS) was ruled out as the calculated H-score was low (5), indicating a low probability of HLH, and the patient did not have the cytopenias and organomegaly necessary for a diagnosis of HLH/MAS. The patient's presentation was ultimately confirmed as AOSD based on the Yamaguchi criteria.

CONCLUSION: This case illustrates a rare presentation where AOSD was masked by KFD-like lymphadenitis. Extreme hyperferritinaemia appears to serve as a vital, specific biomarker to establish the correct diagnosis of AOSD and ensure timely, life-saving immunomodulatory treatment. Also, prompt calculation of the H-score and the strict application of clinical and laboratory criteria are critical steps when managing patients with extreme hyperferritinaemia and systemic inflammation.

LEARNING POINTS: Adult-onset Still's disease (AOSD) may, in rare cases, present with the histopathological features of Kikuchi-Fujimoto disease, posing a significant diagnostic challenge and potentially leading to error.The persistence of systemic symptoms (spiking fever, severe polyarthralgia) and most critically, extreme hyperferritinaemia (often >5,000 μg/l) necessitate the immediate calculation of the H-score. A low H-score and the presence of neutrophilic leucocytosis are key factors in differentiating AOSD from macrophage activation syndrome (MAS) and hemophagocytic lymphohistiocytosis.This case highlights the necessity of rapid initiation of targeted immunosuppressive therapy (e.g., IL-1 inhibitors) to treat the underlying AOSD and prevent complications like MAS.