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NORD is very grateful to James Loyd, MD, Pulmonary and Critical Care at Vanderbilt University Medical Center, for assistance in the preparation of this report.
Synonyms of Fibrosing Mediastinitis
- mediastinal fibrosis, sclerosing mediastinitis
- fibrosing mediastinitis as a late complication of histoplasmosis
- fibrosing mediastinitis, Idiopathic
Fibrosing mediastinitis is the least common, but the most severe, late complication of histoplasmosis. Many physicians believe fibrosing mediastinitis to be an abnormal immunologic response to antigens released by the soil-based fungus histoplasma capsulatum. It should be differentiated from the many other less-severe mediastinal complications of histoplasmosis, and from other causes of fibrosing mediastinitis, which are termed idiopathic fibrosing mediastinitis. Idiopathic fibrosing mediastinitis is even less common, but may have multiple causes, none of which are related to histoplasmosis. Accordingly, there are two types of fibrosing mediastinitis; histoplasmosis-related fibrosing mediastinitis, and idiopathic fibrosing mediastinitis which may have multiple causes unrelated to histoplasmosis.
Both types are rare disorders caused by proliferations of collagen, fibrosis tissue and associated inflammatory cells within the mediastinum (the space between the lungs).
Post histoplasmosis fibrosing mediastinitis is characterized by invasive, calcified fibrosis centered at locations of lymph nodes, which, by definition, occludes major vessels or airways.
Often symptoms of fibrosing mediastinitis do not develop until the disease has progressed to a level at which there is damage to some vessel or organ. The build-up of scar tissue occurs very slowly (1 mm/year) in most cases, but in others the scar tissue may grow at a rapid rate. Symptoms may appear suddenly, even though the scar progressed slowly for months or years, because the scar growth itself causes no symptoms. When scar has developed in a major vessel to the extent that it restricts blood flow to or from a lung, the decreased blood flow may cause infarction, with pain of pleurisy.
Histoplasmosis is due to the most common endemic parasitic fungus in the United States, Histoplasma capsulatum. In the endemic area, along the Mississippi and Ohio River valleys, nearly all persons are infected in childhood. Histoplasmosis also occurs in isolated spots around the world, but is most common in North and Central America, with isolated cases reported from Southeast Asia and Africa. Pulmonary infection is typically asymptomatic or only mildly symptomatic in the infected person. Some infected persons may suffer flu-like symptoms. H. capsulatum appears to have precise growth requirements related to humidity, acidity, temperature and nitrogen content. It flourishes in soil fertilized by bird droppings, and is carried in bat guano, although birds themselves are not infected with H. capsulatum, bats’ intestinal systems may be colonized with the organism. However, chickens are known to harbor the organism in their feathers. Chicken houses and bat guano under bridges and their environs are notorious sources of H. capsulatum infection.
Histoplasmosis has also been found in urban settings and is occasionally referred to as an urban disease as well. In urban settings where the soil is disturbed, the fungus spores become air borne. The University of Texas Southwest Medical Center reported between 600 and 700 cases of Histoplasma infection during a 20-year period when buildings were under construction. Although a bird sanctuary existed in the area, most cases occurred in employees who had no direct contact with the sanctuary. The spores were drawn into buildings through air conditioning systems.
An outbreak of histoplasmosis occurred in 384 students in a junior high school in Ohio. On Earth Day, a courtyard was raked and swept, and the entire school building was contaminated with air containing Histoplasma spores. The epidemic was short-lived and influenza-like. In 1975, bird droppings swept from the roof of a courthouse in Arkansas were distributed through the building by window air-conditioners. Overall, histoplasmosis is considered usually to be an asymptomatic and clinically insignificant infection. In the vast majority of the many millions of infected persons, infection and recurrent infection follow a generally benign course.
The number of persons with the more severe complication, fibrosing mediastinitis, is a small fraction, estimated to total only a few hundred in the US, of the millions of individuals infected by histoplasmosis. It is not known why some individuals are predisposed to excessive immune response to the organism, which leads to excessive scarring and obstruction of major vessels or airways that characterizes FM. [Patients with deficient immune systems who are exposed, develop disseminated histoplasmosis, which is at the opposite end of the spectrum from FM, .] The yeasts are immobilized in lymph nodes around critical mediastinal structures, and may actually be dead organisms, but are not destroyed nor removed, and that may be part of the problem. They persist for years, maybe indefinitely, and may release antigen to stimulate an ongoing immune response. Calcification of the infected lymph nodes is typical, but may require years to develop. When calcium stones (broncholith) of substantial size work their way into the airways, they may cause bronchial obstruction.
Fibrosing Mediastinitis Resources
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