Leukocytoclastic Vasculitis in JAK2-Positive Essential Thrombocythemia: A Case of Cytokine-Driven Urticarial Rash Responsive to Ruxolitinib.

Essential thrombocythemia (ET) is a chronic myeloproliferative neoplasm characterised by sustained thrombocytosis and frequently associated with JAK2 V617F mutations. It is primarily known for its thrombotic and haemorrhagic complications, which dominate its clinical presentation. However, inflammatory and dermatologic manifestations are uncommon and less well recognised. Among these, cutaneous vasculitis - particularly leukocytoclastic vasculitis (LCV) - is a rare and underreported feature. We present the case of a 54-year-old woman with JAK2-positive ET who developed a persistent urticarial rash that coincided with worsening thrombocytosis. Bone marrow biopsy revealed megakaryocytic hyperplasia, confirming the diagnosis of ET. Initially, the patient responded to hydroxyurea, but her disease eventually became refractory, and anagrelide failed to control either her platelet count or symptoms. Her skin rash progressed in parallel with uncontrolled thrombocytosis, and biopsy confirmed LCV. Initiation of ruxolitinib, a JAK1/2 inhibitor, led to haematologic improvement and complete resolution of the vasculitis. This case highlights a rare but clinically significant association between ET and cutaneous vasculitis and suggests a potential role for JAK inhibition in managing such refractory inflammatory manifestations. LEARNING POINTS: Cutaneous vasculitis is a rare but significant manifestation of essential thrombocythemia.Leukocytoclastic vasculitis may present with urticarial lesions resistant to standard therapy.Janus kinase (JAK) inhibition with ruxolitinib can achieve both haematologic and dermatologic remission in refractory cases.