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Retroperitoneal fibrosis. Retroperitoneal fibrosis is a rare immune-mediated disease characterised by a periaortoiliac fibro-inflammatory tissue that often encases neighbouring structures (eg, ureters). Idiopathic retroperitoneal fibrosis can be isolated or part of IgG4-related disease, whereas secondary forms recognise different aetiologies, such as histiocytosis, malignancies, and infections. Idiopathic retroperitoneal fibrosis has a multifactorial origin, with genetic, environmental, and lifestyle factors being main contributors. The immunopathogenesis of the disease involves B-lymphocyte and T-lymphocyte crosstalk, macrophage and fibrocyte chemotaxis, and recruitment of eosinophils and mast cells. Idiopathic retroperitoneal fibrosis can cause severe complications, such as acute or chronic kidney injury, caval occlusion, and deep vein thrombosis. Although interventional or surgical procedures can be necessary to manage these complications, medical therapy remains the cornerstone of treatment. Glucocorticoids are effective, and B-cell-targeting therapies are increasingly used. However, relapses are frequent after treatment discontinuation. In this Seminar, we provide a contemporary overview of retroperitoneal fibrosis focusing on pathophysiology, differential diagnosis, and management.