Pleural plaques are deposits of fibrous tissue that develop in the chest cavity as a result of asbestos exposure. These deposits usually are found on the parietal pleura – the membrane that lines the chest wall. Its opposite, the visceral pleura wraps around the lungs. They are also known to develop on the surface of the diaphragm and on the peritoneal mesothelium – the membrane that lines the abdominal cavity wall. These small growths are benign and do not undergo transformation into malignant growths.
They do commonly calcify however, which is the reason they are called plaques and the reason they can be seen on an X-ray. Pleural plaques that do not calcify won't appear on an X-ray and not all that do can be seen either – it depends on the density of the calcification. According to post mortem studies less than half of all cases of pleural plaque appear on an X-ray. The estimate is that somewhere between 10% and 40% of pleural plaque cases are discernible in a chest X-ray. When they do, they appear in the image as white strips on the chest wall.
Pleural Plaques and Asbestos
They are the most common symptom of asbestos exposure; if they are the only symptom they are indicative of a relatively low level of exposure and inhalation of asbestos fibers. They do not have any outwardly noticeable effect, although they may have minimal impact on lung capacity. One study reported in 1982 analyzed twenty cases of a post-mortem analysis of pleural plaques; sixteen of them had a history of asbestos exposure. Significantly, the majority of those cases showed higher levels of amosite and crocidolite types of asbestos fibers than comparative cases where there was asbestos exposure to the more common strain of asbestos, chrysotile, but no evidence of pleural plaque.
When pleural plaques are the only evidence of asbestos exposure there is very little impact on overall health. While the data is not sufficient for an accurate figure, it is estimated that after thirty years, one third to one half of individuals who had occupational exposure to asbestos will have developed calcified pleural plaques. If that is all they have developed, they are the fortunate victims of asbestos exposure; however it is common to find the development of pleural plaques in asbestosis victims. By definition, asbestosis is a form of fibrosis, but one that develops on the inner surface of the lung and destroys breathing capacity.
Patients with mesothelioma have a much higher instance of pleural plaques than people without the disease, and people with a history of asbestos exposure that remain disease free. Scientists believe that the reason some workers with asbestos exposure in their history develop pleural plaques and some do not has to do with difference in immunological systems. The cause appears to be the result of the body's effort to expel the asbestos fibers – but instead of inflaming and ultimately creating cancer cells, as with mesothelioma, the asbestos fibers create a fibrous collagen-based tissue which is benign and will ultimately calcify and pose no threat.
Health Indicators of Pleural Plaques
While pleural plaques pose no threat themselves, they are indicative of asbestos exposure and suggest that the patient should have regular checkups to ascertain that the impact of asbestos exposure has gone no further. Accumulated data shows that more than 80% of pleural plaque patients have some history of asbestos exposure. Some people exposed to asbestos will not develop pleural plaque but may have developed asbestosis or mesothelioma cancer. However medical studies have proven that individuals with pleural plaques are more likely to develop mesothelioma than those who have been exposed to asbestos and have not developed them.
While pleural plaques themselves are benign and do not inhibit physical function, a condition known as diffuse pleural fibrosis can inhibit respiratory capability. Individuals who have pleural plaques are at greater risk than others for developing diffuse pleural fibrosis. Cases have been sited where plaques have spread to the pericardial mesothelium, the ribs, and to other tissue layers around the parietal pleura. In all cases, if your physician has seen evidence of pleural calcification on your chest X-ray, you should be checked for the presence of malignant cells in the area as well.
- Numbers and Types of Asbestos Fibers in Subjects with Pleural Plaques, American Journal of Pathology, November 1982, Warnock et al.
- Asbestos Related Disease, eMedicine, March 2008, Khan, et al., http://emedicine.medscape.com/article/353015-overview
- Pleural Plaques and Risk for Bronchial Carcinoma and Mesothelioma, Chest, v.105, 1994, Hillerdal et al.