Eosinophilic dermatosis as the initial presentation for Janus Kinase 2 positive chronic myelomonocytic leukaemia

Chronic myelomonocytic leukaemia (CMML) is a clonal haematopoietic stem cell disorder with overlapping myelodysplastic and myeloproliferative features and can be associated with extramedullary manifestations including cutaneous involvement. Eosinophilic dermatosis of haematological malignancy (EDHM) is a rare paraneoplastic entity and is infrequently reported in CMML. We describe an elderly man who presented with a diffuse, pruritic eosinophilic dermatosis preceding the diagnosis of JAK2 V617F-mutated CMML. Skin biopsy demonstrated a dermal hypersensitivity reaction with eosinophils, prompting haematological evaluation. Molecular testing revealed mutations in JAK2, RUNX1, SRSF2 and TET2, and bone marrow biopsy confirmed CMML. Initial treatment with hydroxyurea and corticosteroids provided only transient relief. Subsequent initiation and dose escalation of ruxolitinib resulted in rapid and sustained resolution of cutaneous symptoms. This case highlights EDHM as a rare presenting feature of CMML and suggests a potential therapeutic role for JAK inhibition in JAK2-mutated CMML-associated paraneoplastic dermatoses.