From Head to Toe: Granulomatosis with Polyangiitis.

Granulomatosis with polyangiitis (GPA) is an antineutrophil cytoplasmic antibody-associated vasculitis. It is an uncommon multisystem disease involving predominantly small vessels and is characterized by granulomatous inflammation, pauci-immune necrotizing glomerulonephritis, and vasculitis. GPA can involve virtually any organ. Clinical manifestations are heterogeneous and can be classified as granulomatous (eg, ear, nose, and throat disease; lung nodules or masses; retro-orbital tumors; pachymeningitis) or vasculitic (eg, glomerulonephritis, alveolar hemorrhage, mononeuritis multiplex, scleritis). The diagnosis of GPA relies on a combination of clinical findings, imaging study results, laboratory test results, serologic markers, and histopathologic results. Radiology has a crucial role in the diagnosis and follow-up of patients with GPA. CT and MRI are the primary imaging modalities used to evaluate GPA manifestations, allowing the differentiation of GPA from other diseases that could simulate GPA. The authors review the main clinical, histopathologic, and imaging features of GPA to address the differential diagnosis in the affected organs and provide a panoramic picture of the protean manifestations of this infrequent disease. The heterogeneous manifestations of GPA pose a significant challenge in the diagnosis of this rare condition. By recognizing the common and unusual imaging findings, radiologists play an important role in the diagnosis and follow-up of patients with GPA and aid clinicians in the differentiation of disease activity versus disease-induced damage, which ultimately affects therapeutic decisions. RSNA, 2021.