Management Dilemma: Myeloperoxidase (MPO)-Positive Vasculitis in a Patient With Metastatic Malignant Melanoma.

A man in his 80s with a history of eosinophilic granulomatosis with polyangiitis (EGPA) presented with progressive bilateral leg weakness, sensory loss, and pain, on a background of metastatic malignant melanoma awaiting pembrolizumab therapy. Investigations revealed a relapse of anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitis, manifesting as vasculitic mononeuropathy. This created a therapeutic dilemma: corticosteroids were required to control vasculitis but risked blunting the effect of imminent immune checkpoint inhibition, highlighting the need for multidisciplinary management. The patient was diagnosed with EGPA in 2008 following presentation with adult-onset asthma, peripheral eosinophilia, and neuropathic symptoms. He achieved sustained remission after induction therapy with intravenous cyclophosphamide followed by a prolonged tapering course of oral corticosteroids and had remained clinically quiescent for over a decade. In 2023, he was diagnosed with metastatic malignant melanoma with liver and suspected pulmonary metastases and was referred for immune checkpoint inhibitor therapy (pembrolizumab), which had not yet been initiated at the time of this admission. Approximately 15 years after his initial EGPA diagnosis - and prior to receiving any immunotherapy - he developed progressive lower limb weakness and sensory symptoms, prompting the current admission and revealing a relapse of myeloperoxidase (MPO)-positive ANCA-associated vasculitis.