Fibrosing mediastinitis, also known as sclerosing mediastinitis or mediastinal fibrosis, is a disorder characterized by an excessive fibrotic reaction in the mediastinum. It can result in compromise of airways, great vessels, and other mediastinal structures, with morbidity directly related to the location and extent of fibrosis. Fibrosing mediastinitis is usually a sequel of histoplasmosis [1,2].
The etiology, natural history, clinical presentation, diagnosis, treatment, and outcome of fibrosing mediastinitis are reviewed here. The presentation and treatment of pulmonary histoplasmosis are discussed separately. (See "Pathogenesis and clinical features of pulmonary histoplasmosis" and "Diagnosis and treatment of pulmonary histoplasmosis".)
Fibrosing mediastinitis usually results from an excessive host response to a prior infection that involved the mediastinal lymph nodes.
Histoplasmosis — The vast majority of cases of fibrosing mediastinitis are thought to be sequelae of infection with Histoplasma capsulatum, a dimorphic fungus that is found commonly in the southeastern, mid-Atlantic, and central United States. Infection with H. capsulatum is subclinical in the vast majority of patients. It begins as an asymptomatic pulmonary infection and disseminates to the mediastinal lymph nodes and other reticuloendothelial organs before specific cell-mediated immunity develops and contains the fungus. Depending upon the inoculum and the extent of host response, the involved mediastinal lymph nodes can enlarge and coalesce into an inflamed caseous mass referred to as a mediastinal granuloma, or can lead to a sclerosing process called fibrosing mediastinitis:
- Mediastinal granuloma – Mediastinal granuloma is more common (comprises 5 to 10 percent of mediastinal masses in most surgical series ) and more benign than fibrosing mediastinitis. It usually presents as a mediastinal mass that is discovered either incidentally or because it compresses compliant mediastinal structures such as the superior vena cava (SVC) or esophagus. The mediastinal granuloma consists of encapsulated caseous lymph nodes that are easily removed surgically.
- Fibrosing mediastinitis – The events that lead to the development and perpetuation of fibrosing mediastinitis are incompletely understood. It is believed that fibrosing mediastinitis results from the leakage of fungal antigens from lymph nodes into the mediastinal space, leading to a hypersensitivity reaction followed by an exuberant fibrotic response . Fibroblasts adjacent to evolving granulomata are stimulated and proliferate, leading to fibrosis, entrapment, and invasion of adjacent normal structures. The resulting tissue pathology consists of caseous central foci surrounded by mature, concentrically deposited acellular collagen . Fibrosing mediastinitis is not believed to reflect extranodal infection because the organism cannot typically be cultured from biopsy specimens [1,3]. (See "Diagnosis and treatment of pulmonary histoplasmosis".)
It is unknown why only a minority of patients infected with H. capsulatum develop fibrosing mediastinitis. However, genetic influences on the immune response may play a role. This was suggested by a case-control study that included 19 consecutive patients with fibrosing mediastinitis and 21,086 cadaveric kidney donors serving as controls . The study found that the relative risk of fibrosing mediastinitis was 3.3 among persons with the HLA-A2 antigen. (See "Human leukocyte antigens (HLA): A roadmap".)