Microscopic Polyangiitis With Temporal Artery Involvement Mimicking Giant Cell Arteritis.

A 63-year-old woman presented with a four-month history of bilateral hearing loss and three months of progressive, painful peripheral neuropathy. Her clinical course was complicated by deep vein thrombosis with pulmonary emboli and an incomplete response to initial empiric glucocorticoid therapy. Following glucocorticoid taper, she developed temporal headaches, jaw claudication, and scalp tenderness, raising concern for giant cell arteritis (GCA). Laboratory evaluation revealed markedly elevated inflammatory markers (erythrocyte sedimentation rate and C-reactive protein), microcytic anemia, mild hematuria, and strongly positive myeloperoxidase-antineutrophil cytoplasmic antibody (MPO-ANCA) by antigen-specific immunoassay with negative proteinase-3 (PR3)-ANCA. Temporal artery biopsy demonstrated transmural inflammation affecting all vessel layers without giant cells or granulomata. The combination of multisystem small-vessel involvement, MPO-ANCA seropositivity with negative PR3-ANCA, and characteristic histopathological findings established the diagnosis of microscopic polyangiitis (MPA) with temporal arteritis rather than granulomatosis with polyangiitis (GPA) or GCA. Treatment with high-dose glucocorticoids (prednisone 80 mg daily), avacopan (30 mg twice daily), and rituximab (375 mg/m² weekly for four weeks) resulted in rapid improvement of constitutional and cranial symptoms. This case highlights the importance of integrating MPO-ANCA serology and histopathological findings when temporal arteritis is the presenting feature of systemic vasculitis. It illustrates the utility of avacopan-based, glucocorticoid-sparing combination immunosuppression in this context.