National Organization for Rare Disorders, Inc.
It is possible that the main title of the report Fibrosing Mediastinitis is not the name you expected.
Mediastinal fibrosis is the least common, but the most severe, late complication of histoplasmosis. Many physicians believe mediastinal fibrosis 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 mediastinal fibrosis, which are termed idiopathic mediastinal fibrosis. 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 mediastinal fibrosis is characterized by invasive, calcified fibrosis centered on 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 the scar tissue can be slow growing in some cases and in others the scar tissue may grow at a rapid rate.
Histoplasmosis, the most common endemic parasitic fungus in the United States, is caused by Histoplasma capsulatum. In the endemic area, along the Mississippi and Ohio River valleys, most 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 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. For example, H. capsulatum has been traced to chicken feathers in pillows. 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 invaded buildings through air ducts.
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, human 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, in which patients have excessive immune response to the organism. The spores are merely contained in place, and may actually be dead, but are not really destroyed, and that may be part of the problem. They persist for years, maybe indefinitely, and can release antigen to stimulate ongoing immune response.] Calcification of the infected lymph nodes is a component of healing of a chronic granulomatous process. When calcium stones of substantial size are extruded into the airways, they may cause bronchial obstruction.
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This is an abstract of a report from the National Organization for Rare Disorders, Inc. ® (NORD). A copy of the complete report can be obtained for a small fee by visiting the NORD website. The complete report contains additional information including symptoms, causes, affected population, related disorders, standard and investigational treatments (if available), and references from medical literature. For a full-text version of this topic, see http://www.rarediseases.org/search/rdblist.html