Approach Considerations
The diagnosis of asbestosis is based on the following [35] :
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A reliable and significant (ie, dose × time) history of asbestos exposure and an appropriate latency period between exposure and detection of disease
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Characteristic changes of pulmonary fibrosis on imaging studies
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Absence of other fibrotic diseases that mimic asbestosis
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Dyspnea upon exertion
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Bilateral basilar inspiratory crackles
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Restrictive pattern on pulmonary function studies associated with impaired gas exchange
In general, laboratory studies are nonspecific and rarely useful. Blood tests for antinuclear antibodies (ANAs), rheumatoid factor, and erythrocyte sedimentation rate (ESR) lack diagnostic sensitivity or specificity and are not useful in diagnosis, disease severity, or activity assessment.
A lung scan with gallium citrate (67Ga) is a nonspecific test that may detect areas of inflammation in the lungs. However, the results do not always correlate with other measurements of inflammation. This test is no longer recommended.
Physicians often make the diagnosis of asbestosis without histopathologic confirmation. Errors may occur because other, more common, interstitial diseases (eg, idiopathic pulmonary fibrosis) mimic the clinical, radiologic, and pulmonary functional features of asbestosis. Bear in mind the long latency period that exists between patient exposure and the manifestation of symptoms and signs of asbestosis.
When lung tissue is available for histopathologic examination, confirmation of the diagnosis requires both fibrosis and accumulation of fibers or asbestos bodies (ie, ferruginous bodies; these are asbestos fibers that develop a ferritin-protein coat and have a characteristic long-beaded appearance). Asbestos bodies alone are not diagnostic for disease, because examiners occasionally find asbestos bodies in people without known exposure.
Pleural plaques may coexist with asbestosis, but these plaques alone are usually not associated with impaired pulmonary function. Nonetheless, pleural plaques are a reliable indicator of asbestos exposure.
Radiography
Chest radiographs (ie, posteroanterior and lateral views) are basic and required diagnostic imaging studies. However, the diagnosis of asbestosis requires multiple elements. A chest radiograph alone has only a modest positive predictive value of less than 50% for the condition, but when it is combined with abnormal signs (rales) and pulmonary function test results, the positive predictive value is markedly increased. [36]
Typical findings include diffuse reticulonodular infiltrates, which are observed predominantly at the lung bases. The diffuse lung infiltrates cause the appearance of shaggy heart borders.
In early disease, an increase in interstitial markings, mostly linear, is seen. Honeycombing, with cystic spaces surrounded by coarse interstitial infiltrates and small lung fields, characterizes advanced disease. Honeycombing may also be seen with idiopathic pulmonary fibrosis, making it an important diagnostic consideration.
Bilateral pleural thickening may be observed. Asbestos-related pleural thickening more often involves the middle third of the pleura as opposed to the upper third, which is affected by tuberculosis, or the lower third, which can be damaged by empyema, trauma, or past pleurodesis therapy. (An oblique-view radiograph may be helpful in recognizing pleura-based abnormalities.)
A calcified pleural plaque located in the diaphragmatic pleura is a reliable indicator of asbestos exposure but is not a required element for the diagnosis of asbestosis. Besides the diaphragmatic pleura, other common sites for plaque formation in the parietal pleura are along the sixth through the ninth ribs. Noncalcified plaques may not be detected on chest radiographs. (See the image below.)
Asbestosis. Asbestos pleural plaques.
Rarely, pleural adhesions may cause peripheral atelectasis with a rounded border (rounded atelectasis) that may simulate a lung tumor. [37]
The International Labor Office standardized classification of radiographic abnormalities is useful in grading the extent of disease in asbestosis and in other pneumoconioses. [2, 38]
CT Scanning
Computed tomography (CT) scanning is useful in the delineation of pleural or pleura-based abnormalities (eg, effusion, thickening, plaque, malignant mesothelioma, rounded atelectasis) and in the delineation of a parenchymal density that is suggestive of bronchogenic carcinoma. [39]
A review of chest CT images for 35 individuals exposed to asbestos found a high incidence of pleural plaque (94%) and pulmonary fibrosis (77%y). The findings of lung parenchymal lesions were as follows: centrilobular opacities (94%), subpleural dot-like or branching opacities (80%), interlobular septal thickening (57%), intralobular interstitial thickening (46%), parenchymal bands (43%), and subpleural curvilinear line (29%). [40]
A high-resolution CT (HRCT) scan allows better definition of interstitial infiltrates and may be helpful in diagnosing asbestosis in the early stages. Typical HRCT findings in asbestosis include subpleural linear opacities seen parallel to the pleura; basilar lung fibrosis and peribronchiolar, intralobular, and interlobular septal fibrosis; honeycombing; and pleural plaques. In a minority of cases, HRCT abnormalities may be seen in individuals with normal chest radiographic findings.
Radiation delivered during repeated screening of asbestos-exposed workers is a major concern. Low dose or ultra-low dose CT scans can reduce radiation doses up to 87%, but image noise increases and may diminish image quality. Tekath et al compared ultra-low dose CT with standard CT scanning for detecting asbestos-related diseases and found that ultra-low dose CT scanning compares favorably with standard CT in detecting pleural plaques, diffuse pleural thickening, and pulmonary modules. However sensitivity for interstitial pulmonary abnormalities was poor in ultra-low dose CT. [41] Because early findings of asbestosis on CT scans are very subtle, it may be unsuitable to scan the patient with low dose or ultra-low dose CT scan.
Pulmonary Function Testing
In asbestosis, a reduction in diffusing capacity precedes lung volume changes, but findings from a diffusing capacity measurement are not specific. [2] Besides diffusing capacity reduction, the earliest physiologic abnormality is exertional hypoxemia. Total lung capacity is reduced in asbestosis as in other restrictive disorders.
Using spirometry, vital capacity on a pulmonary function test typically appears reduced, without a reduction in the ratio of forced expiratory volume in 1 second to forced vital capacity (FEV1 to FVC).
Small-airway flow rates (eg, midexpiratory forced expiratory flow [FEF25-75]) are reduced, but they are nonspecific for a diagnosis of small-airway obstructive disease.
Oximetry
The evaluation of oxygenation is important, because uncorrected hypoxemia causes pulmonary hypertension and may lead to cor pulmonale.
Clinicians can use a noninvasive test of pulse oximetry as a screening test, especially if oximetry is performed during rest and during exercise (eg, 6-minute walk test).
Obtain accurate information through measurement of arterial blood gases, which requires an arterial puncture. In selected cases, an exercise study may demonstrate desaturation during exercise.
Bronchoalveolar Lavage and Bronchoscopy
Bronchoalveolar lavage
Bronchoalveolar lavage (BAL) has only limited application in the diagnosis and management of asbestosis. BAL is helpful in diagnosing infections that may present with diffuse infiltrates and simulate asbestosis, and the procedure may aid in the diagnosis of a coexisting bronchogenic carcinoma. In workers who are exposed to asbestos, BAL can provide quantitative information through asbestos fiber counts. More than 1 asbestos body (ie, coated asbestos fiber) per milliliter (mL) of lavage effluent suggests significant exposure. [18]
Bronchoscopy
Fiberoptic bronchoscopy is performed to facilitate BAL. In addition, bronchoscopy is indicated for airway examination when findings from radiologic studies are suggestive of bronchogenic carcinoma.
Transbronchoscopic lung biopsy is not recommended for diagnosis of asbestosis. This procedure yields inadequate tissue and may cause crush alterations to the tissue.
Histologic Findings
In most cases, asbestosis is diagnosed without a histopathologic examination of lung tissue. A pathologic diagnosis of asbestosis requires visualization of both fibrosis and asbestos bodies through light microscopy, or a significant quantity of asbestos fibers observed through electron microscopy.
The American College of Pathologists uses a scheme for assessing the severity of asbestosis by grading fibrosis in the following four categories [42] :
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Grade 1: Fibrosis in the wall of a respiratory bronchiole without extension to distant alveoli
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Grade 2: Fibrosis extends to alveolar ducts and/or at least two tiers of alveoli adjacent to the respiratory bronchiole, with sparing of at least some alveoli between adjacent bronchioles
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Grade 3: Fibrotic thickening of the walls of all alveoli between two or more adjacent respiratory bronchioles
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Grade 4: Alveolar and septal fibrosis with spaces larger than alveoli, ranging up to 1 cm (ie, honeycombing)
Asbestos bodies (ie, ferruginous bodies) are asbestos fibers that develop a ferritin-protein coat and have a characteristic long-beaded appearance. Asbestos bodies alone are not diagnostic for disease, because examinersoccasionally find asbestos bodies in people without known exposure.
Lung biopsy
Open lung biopsy is not indicated in most cases of asbestosis. However, this procedure provides sufficient tissue for the pathologist to make a definitive diagnosis.
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Asbestosis. Asbestos pleural plaques.
