OVERVIEW OF ASBESTOS RELATED DISEASE
"There are several medical diseases that occur as a result of asbestos exposure. The ones of greatest concern and importance are pleural plaques and thickening; asbestosis; lung cancer; colon, laryngeal, pharyngeal cancer; and
mesothelioma. For many workers, these diseases are disabling or fatal. For each disease there is a standard set of tests, and generally accepted criteria, for diagnosis.
Pleural Plaques and Thickening
Pleural plaques are also called pleural fibrosis, pleural thickening, and pleural asbestosis. A majority of persons with heavy exposure to asbestos develop pleural abnormalities. The pleura is a thin lining that surrounds the lung.
Asbestos fibers that are breathed into the lung are transported to the outside of the lung and cause a scar to form in the pleural lining. When these scars reach a certain size they are visible on chest x ray as a plaque.
Most of these plaques alone do not cause disability, but they do tell us that significant exposure has occurred, and that other asbestos related diseases may be present. However, some types of plaques can cause loss of lung function. Scars that involve the costophrenic angle, the angle between the base of the lung and the diaphragm, can cause loss of lung function, as can extensive plaques on both sides of the lung.
Parenchymal Asbestosis (Pulmonary Asbestosis)
Parenchymal asbestosis is a scar formation in the substance of the lung itself. These scars can interfere with lung function, for they block the transport of oxygen from the air in the lungs into the blood vessels that travel through the lungs. Oxygen can only cross the membranes of the lung if they are thin; asbestosis causes them to thicken. As a general rule the greater the exposure the more the disease, i.e. there is a dose-response relationship between exposure and disease. However, some people seem to form scars more readily and so we see a variety of disease from the same level of exposure. These scars are visible on x ray in most cases but certainly not all cases. High resolution CT scan of the chest can find disease not seen on a plain chest x-ray, and is becoming an important component of the standard practice for the diagnosis of asbestosis. The International Labor Organization developed a way of grading chest x-rays for dust diseases of the lung. The most recent version is the1980 Classification of the Radiographic Appearance of Pneumoconioses (dust diseases of the lung). This system is accepted around the world. It provides a standard notation, so that if one reader calls a film a “1/1” another reader will know what the first reader is referring to. The classification uses a 12-point scale to define the degree, or severity, of increased lung markings. Classification of pleural changes (involvement of the membrane lining the chest wall and the lung) uses a separate scale, with specific notations made for side of the chest, whether or not the plaques contain calcium deposits, and the specific type, length, and width of the thickening of the pleura.
This 12-point scale runs from 0/- to 3/+; a “0” film is normal and a “3” film is the most severe scarring. Each reading on the scale is characterized by a number between 0 and 3, and a second number, separated by “/”. The first number, preceding the “/”, is the final number assigned to that film by that reader. The second number, following the “/”, is a qualifier. The numbers 0, 1, 2, and 3 are the main categories. An x-ray read as a category 1 film might be described as 1/0, 1/1, or 1/ 2. When the reader uses 1/1, he is rating the film as a 1, and only considered it as a 1 film. If he uses 1/0, he is saying is rating the film as a “1”, but considered calling it a “0” film before deciding it was category 1. Finally, when the reader uses 1/2, he is saying he is rating the film as a “1”, but did consider calling it a “2” film. In clinical practice, any category “1” film is abnormal; therefore a 1/0 film is consistent with asbestosis.
Even though the ILO system was designed to standardize reading x-rays for asbestosis, studies using the classification in asbestos exposed workers have found readers often disagree about classification of the same x-rays. Using the classification is somewhat of an art. Body size, weight, position of the person during the x-ray, and x-ray technique affect the amount of scarring that is visible on an x-ray. If an x-ray is less than perfect, one reader may think he can be sure scarring is present, while another cannot be sure and grades the film with a lower score for scarring.
The “best” readers agree 80% of the time with each other; 20% of the time they assign a different score to the same x-ray. If the scarring is extensive, a difference of one grade on the scale is not important. But if the x-ray shows less extensive scarring, a difference of one grade can be the difference between making diagnosis of asbestosis or deciding asbestosis is not present. For this reason experts agree that the x-ray alone should not be used to make a diagnosis of asbestosis; the examining physician should use the occupational and medical history, results of pulmonary function testing, and other medical data to reach a diagnosis. Experts also agree that asbestosis can be present in the lung even though the x-ray is normal using the ILO classification system.
High resolution computed tomography (HRCT) is now widely accepted as a diagnostic tool for asbestosis and asbestos-related pleural scarring. HRCT is an excellent technique for diagnosis of asbestosis and asbestos-related plaque. Recent studies show that readers using a scoring index were more accurate and reliable in the diagnosis of asbestosis that when using plain chest x-rays. This study concluded that “the examined HRCT scoring method proved to be a simple, reliable, and reproducible method for classifying lung fibrosis and diagnosing asbestosis also in large populations with occupational disease, and it would be possible to use it as a part of an international classification”. Expert consensus supports this conclusion.
Disease from asbestos is also detected on pulmonary function testing, and PFTs are used to quantity the level of lung impairment due to asbestosis. Asbestosis makes the lung stiffer and smaller, so the volume of air in the lungs is decreased. Oxygen transport as measured by the diffusion capacity is also decreased. Abnormalities are measured using spirometry, lung volumes, and gas exchange testing. Spirometry is reliable and reproducible when performed according to the specifications set by the American Thoracic Society (ATS) . Determination of lung volumes can be done by the gas dilution method or by body plethysmography; both are standard measures and also are reliable and reproducible. The ATS also sets standards for diffusion capacity , which ensure uniformity among laboratories and reproducibility.
Asbestosis can affect each of these tests without necessarily showing an abnormality in the other two. Spirometry and total lung capacity both measure lung volume, but one may be abnormal while the second remains normal. The diffusion capacity measures a decrease in oxygen exchange in the lung, and so is measuring a different function of the lung than lung volumes. Asbestosis can just as easily be manifest with a decreased lung volume or a decrease in gas exchange; neither is a better, more sensitive or more accurate test, and both types of tests must be used in any set of diagnostic criteria. The diffusion capacity has been shown to correlate with the severity of fibrosis found on pathologic examination of the lung, and a reduction in diffusion capacity can precede x-ray changes.
The changes in pulmonary function at times can be subtle, and test results should be interpreted by someone with experience in asbestos related diseases. Pulmonary exercise testing can be used to clarify subtle abnormalities, and any compensation system must allow the examining physician to submit a medical report and rationale based on accepted medical tests. Because the diagnosis of asbestosis or any other asbestos-related disease can be made with a range of medical tests, it is essential that any compensation system include a medical panel to review cases that do not meet the most common diagnostic criteria. As just one example of a study that supports the need for a medical panel, Kipen reported that 18% of insulators who had asbestosis found on pathological examination the lung had a normal chest x-ray . If we were to require a 1/0 film in all cases of asbestosis, these workers would be excluded. Pathological examination is not required in the absence of x-ray abnormalities; a combination of CT scan and exercise testing can reasonably approximate the specificity as tissue examination.
Once this scar formation takes place it is irreversible. It gets worse in some cases, even after exposure stops. Factors that are associated with worsening scarring include the severity of disease (the more the scarring, the more likely it is to get worse), and the amount and intensity of exposure to asbestos. Because of the damage to the lungs a person with asbestosis is at increased risk of lung infections and so should get regular medical care and influenza vaccines."
Testimony of Dr. Laura S. Welch - Part I
Testimony of Dr. Laura S. Welch - Part III
