Asbestosis

 

INTRODUCTION

Background: Pulmonary fibrosis caused by asbestos inhalation is called asbestosis. The word asbestos is derived from Greek and means inextinguishable, and asbestos is a group of naturally occurring, heat-resistant fibrous silicates. Pneumoconiosis is the general term for lung disease caused by inhalation and deposition of mineral dust.

Asbestos fibers are long and thin (length-to-diameter ratio >3) and may be either curved or straight. The curved fibers are called chrysotile, and the straight fibers are amphiboles. Researchers recognize 5 different amphiboles: (1) amosite, (2) anthophyllite, (3) tremolite, (4) actinolite, and (5) crocidolite. Chrysotile is by far the most common type of asbestos fiber produced in the world.

Production and use of asbestos increased greatly between 1877 and 1967. In the 1930s and 1940s, scientists recognized a causal link between asbestos exposure and asbestosis. In the 1950s and 1960s, researchers established asbestos as a predisposing factor for bronchogenic carcinoma and malignant mesothelioma.

 

Pathophysiology: The cumulative dose of fibers inhaled over a period of time and the type, durability, and dimensions of the fiber influence carcinogenicity and fibrogenicity. The incidence of asbestosis varies with the cumulative dose of inhaled fibers; the greater the cumulative dose, the higher the incidence of asbestosis. All types of asbestos fibers are fibrogenic to the lungs. Amphiboles, particularly crocidolite fibers, are markedly more carcinogenic to the pleura. Fibers with diameters smaller than 3 micrometers are fibrogenic because they penetrate cell membranes. Long fibers (ie, >5 micrometers) are incompletely phagocytosed and stay in the lungs, setting up cycles of cellular events and the release of cytokines.

The initial inflammation occurs in the alveolar bifurcations and is characterized by the influx of alveolar macrophages. Asbestos-activated macrophages produce a variety of growth factors, including fibronectin, platelet-derived growth factor, insulinlike growth factor, and fibroblast growth factor, which interact to induce fibroblast proliferation. Oxygen free radicals (eg, superoxide anion, hydrogen peroxide, hydroxy radicals) that are released by the macrophages damage proteins and lipid membranes and sustain the inflammatory process. A plasminogen activator, which also is released by macrophages, further damages the interstitium of the lung by degrading matrix glycoproteins.

Individuals probably differ in their susceptibility to asbestosis based on respiratory clearance and other unidentified host factors.

 

Frequency:

  • In the US: No reliable information exists regarding the number of people presently at risk in the United States and in other countries. Since the early 1940s, as many as 10 million workers in the United States may have been exposed to asbestos. In 1972, reports estimated that 250,000 persons were at risk. By the 1980s, the number of active asbestos miners and millers had fallen to a few hundred. Strict regulation (eg, prohibition of asbestos sprays in buildings, controls in the level of asbestos fibers in the air) has drastically reduced the risk of developing asbestosis.
  • Internationally: Trends in usage of asbestos and observational studies suggest that asbestosis and other asbestos-related diseases are likely to be continuing problems in developing countries.

Mortality/Morbidity: 

  • In 1992, nearly 6 of every 1 million deaths were attributed to asbestosis in the United States.
  • Smokers are likely to develop chronic bronchitis and obstructive airway disease and are prone to respiratory infections. Smokers are at high risk for development of bronchogenic carcinoma because asbestos and tobacco smoke are synergistic in carcinogenicity. Individuals who both smoke and are exposed to asbestos are 90 times more susceptible to developing lung carcinoma than individuals who either smoke or are exposed to asbestos only.
  • Asbestosis may coexist with other asbestos-related diseases, including calcified and noncalcified pleural plaques, pleural thickening, pleural effusion, rounded atelectasis, and malignant mesothelioma of the pleura.

CLINICAL

History:

  • Because the development of asbestosis is dose dependent, symptoms appear only after a latent period of 20 years or longer. This latent period may be shorter after intense exposure.
  • Dyspnea upon exertion is the most common symptom and worsens as the disease progresses.
  • Patients may have a dry (ie, nonproductive) cough. A productive cough suggests concomitant bronchitis or a respiratory infection.
  • Patients may complain of nonspecific chest discomfort, especially in advanced cases.

Physical:

  • Rales (ie, end-inspiratory crackles) are the most important finding during examination. The rales are persistent and dry and are described as fine cellophane rales or coarse Velcro rales. The rales are best auscultated at the bases of the lungs posteriorly and in the lower lateral areas. Initially, physicians hear the rales in the end-inspiratory phase. However, in advanced disease, rales may be heard during the entire inspiratory phase. Occasionally, the presence of rales precedes radiographic abnormalities and pulmonary function abnormalities.
  • Finger clubbing is observed in an estimated 25-50% of cases. This finding is not necessarily related to the severity of disease.
  • Reduced chest expansion in advanced disease correlates with restrictive ventilatory impairment and reduced vital capacity.
  • In advanced disease, patients may be cyanotic.

Causes:

  • See Pathophysiology for a discussion of various factors that cause asbestosis. Among them, the level of asbestos fiber exposure is of prime importance. Experts estimate a 1% risk of developing asbestosis after a cumulative dose of 10 fiber-year/m3.
  • In modern times, the risk to persons in the United States occurs mainly through the processing, manufacturing, and end-use of asbestos.
  • Manufacturers commonly use asbestos in the following products:
    • Products containing asbestos cement - Pipes, shingles, clapboards, sheets
    • Vinyl-asbestos floor tiles
    • Asbestos paper in filtering and insulating products
    • Material in brake linings and clutch facings
    • Textile products - Yarn, felt, tape, cord, rope
    • Spray products used for acoustical, thermal, and fireproofing purposes

DIFFERENTIALS

Coal Worker's Pneumoconiosis
Dermatomyositis
Hypersensitivity Pneumonitis
Pulmonary Fibrosis, Idiopathic
Sarcoidosis
Silicosis


Other Problems to be Considered:

Collagen vascular diseases
Other interstitial pulmonary diseases
See Complications.

Be aware of the predisposition for bronchogenic carcinoma and the variety of asbestos-related diseases that may coexist with asbestosis.

 

WORKUP

Lab Studies:

  • The diagnosis is based on the following:
    • A reliable and significant (ie, dose x time) history of asbestos exposure and an appropriate latency period between exposure and detection of disease
    • Characteristic changes of pulmonary fibrosis on imaging studies
    • Absence of other fibrotic diseases that mimic asbestosis
    • Dyspnea upon exertion
    • Bilateral basilar inspiratory crackles
    • Restrictive pattern on pulmonary function studies

Imaging Studies:

  • Radiography
    • Chest radiographs (ie, posteroanterior and lateral views) are basic and required diagnostic imaging studies.
    • 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.
    • 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 affected by tuberculosis and the lower third damaged by empyema, trauma, or past pleurodesis therapy.
    • A calcified pleural plaque located in the diaphragmatic pleura is a reliable indicator of asbestos exposure but is not a required element for diagnosis of asbestosis.
    • 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.
    • The International Labor Office standardized classification of radiographic abnormalities is useful in grading the extent of disease in asbestosis and in other pneumoconioses.
    • An oblique-view radiograph may be helpful in recognizing pleura-based abnormalities.
  • Computed tomography scan
    • CT scan of the chest is not required in all cases. CT scan is useful in delineation of pleural or pleura-based abnormalities (eg, effusion, thickening, plaque, malignant mesothelioma, rounded atelectasis) and in delineation of a parenchymal density that is suggestive of bronchogenic carcinoma.
    • A high-resolution computed tomography (HRCT) scan allows better definition of interstitial infiltrates and may be helpful in diagnosing early stages of asbestosis.

Other Tests:

  • Pulmonary function tests
    • Diffusing capacity reduction may precede lung volume changes, but findings from a diffusing capacity measurement are not specific.
    • Total lung capacity is reduced in asbestosis and in other restrictive disorders.
    • Using spirometry, vital capacity 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 are nonspecific for a diagnosis of small airway obstructive disease.
  • Oximetry
    • Evaluation of oxygenation is important because uncorrected hypoxemia causes pulmonary hypertension and may lead to cor pulmonale.
    • Physicians can use a noninvasive test of pulse oximetry as a screening 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.
  • A lung scan with gallium citrate Ga 67 is a nonspecific test that detects areas of inflammation in the lungs. This test is no longer recommended because an HRCT scan provides information that is more detailed.

Procedures:

  • 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, which simulate asbestosis, and BAL may aid in the diagnosis of a coexisting bronchogenic carcinoma.
    • In workers who are exposed to asbestos, BAL can provide quantitative information by asbestos fiber counts. More than 1 asbestos body (ie, coated asbestos fiber) per milliliter of lavage effluent suggests significant exposure.
  • Bronchoscopy
    • Fiberoptic bronchoscopy is performed to facilitate BAL.
    • In addition, bronchoscopy is needed for airway examination in cases suggestive of bronchogenic carcinoma.
    • Transbronchoscopic lung biopsy is not recommended. This procedure yields inadequate tissue, and alterations to the tissue caused by the procedure make it of little value in the diagnosis of asbestosis.
  • Open-lung biopsy is not indicated in most cases. However, this procedure provides sufficient tissue for the pathologist to make a definitive diagnosis.
Histologic Findings: Most often, physicians diagnose asbestosis without histopathological 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' scheme for assessing the severity of asbestosis grades fibrosis in 4 categories. Grade 1 is fibrosis in the wall of a respiratory bronchiole without extension to distant alveoli. Grades 2 and 3 define more extensive disease, and Grade 4 is 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 occasionally examiners find asbestos bodies in people without known exposure.

 

TREATMENT

Medical Care:

  • Control of asbestos in the workplace is the most effective method for preventing asbestosis. The current US standard is 0.1 fiber per milliliter of air. Cessation of further exposure to asbestos once the diagnosis of asbestosis is made is imperative because further exposure increases the rate of progression. A small minority (10-20%) of people has progressive disease after cessation of exposure.
  • Advise smokers to quit smoking, and provide referral to a smoking cessation clinic.
  • Assessment of disease severity and functional impairment are important in tailoring a treatment and follow-up plan (ie, frequency of clinic visits, chest radiographs, pulmonary function testing).
  • Treatment requires prompt attention to respiratory infections and immunizations against influenza and pneumococcal pneumonia.
  • Assess oxygenation status at rest and with exercise. If testing detects hypoxemia at rest or with exercise, prescribe supplemental oxygen.
  • Remain aware of the complications of asbestosis to expedite detection and treatment.

     

  • Provide palliative care for the relief of distressing symptoms in severe advanced disease.

Consultations:

  • Consult a pulmonologist to assess the need for long-term oxygen therapy and for the management of complications (see Mortality/Morbidity).
  • If patients smoke, refer them to a smoking cessation clinic.
  • Because of the likelihood of bronchogenic carcinoma, consult a thoracic surgeon if a solitary pulmonary nodule develops in a patient with asbestosis.
  • Provide hospice referral (preferably at home) when disease reaches the terminal phase.

MEDICATION

Drugs are not effective in the treatment of asbestosis. Corticosteroids and immunosuppressive drugs do not alter the course of the disease.

 

FOLLOW-UP

Deterrence/Prevention:

  • Control of asbestos in the workplace is the most effective method for preventing asbestosis. The current US standard is 0.1 fiber per milliliter of air.

Complications:

  • Pulmonary hypertension
  • Cor pulmonale
  • Right heart failure
  • Progressive respiratory insufficiency

MISCELLANEOUS

Medical/Legal Pitfalls:

  • Diagnosis, causation, and impairment are the major issues with regard to medical/legal pitfalls.
  • Physicians often make the diagnosis 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 between patient exposure and the manifestation of symptoms and signs of asbestosis. When lung tissue is available for histopathologic examination, confirmation of diagnosis requires both fibrosis and accumulation of asbestos bodies or fibers.
  • Determining the cause depends on assessment of the levels and duration of exposure and on knowledge of occupational epidemiologic studies.
  • Assessment of impairment, which is a key ingredient in determining disability, is based mainly on pulmonary function studies.
  • No evidence exists to confirm that small airway disease, which is detected by flow volume curves, progresses to asbestosis.
  • Pleural plaques may coexist with asbestosis, but pleural plaques alone usually are not associated with impaired pulmonary function.