Asbestosis is an interstitial lung disease caused by the inhalation of asbestos fibers. These fibers are different mineral silicates (mainly hydrated magnesium silicates) and are classified into two main categories based on their shape, i.e., serpentine and amphibole. Serpentine fibers include chrysolites that are curly and flexible and are less pathogenic than amphibole fibers. Chrysolite being more flexible, curvy, and more soluble settles in the upper part of the respiratory tract. The mucociliary function is more prominent in the upper respiratory tract, so chrysolite fibers are easily removed. Amphiboles (crocidolite, amosite, tremolite, and anthophyllite) are straight, stiff, more brittle fibers. They are more toxic than serpentine fibers as they are less soluble and straight. They usually align with the airstream and reach deeper into the lungs and the interstitium by penetrating the epithelium. Asbestos fibers have been historically chosen for construction, shipping, mining, and aerospace engineering commercial use because of their high electrical and thermal resistance and low cost.
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There are three main forms of asbestos exposure. Direct work-related environmental exposure is common among workers at shipyard, mining, aerospace, etc. Bystander exposure is the second form of exposure commonly seen in certain professionals like electricians, masons, and painters. The third and most common form of asbestos exposure is the general community exposure, such as the use of asbestos for road surfaces, playground material, landfills, and chemical paints. The disease is dose-dependent; the dose of exposure is higher in the first group and, respectively, lower in the second and third forms of exposure. Also, the risk of the disease is more with amphibole variety compared to the serpentine asbestosis fibers.
The prevalence of the disease is more in those disciplines where the intensity of exposure is high. A study conducted in Okayama, Japan, among construction workers revealed that workers working in the installation of the asbestos board have the highest number of cases (39.1%), followed by sprayer of asbestosis (38.5%). And also longer the duration of exposure, the more the chances of asbestosis. In the UK, insulation workers have the highest risk of asbestosis, followed by asbestos stripping. It is estimated that asbestos causes around 55000 deaths per year in the world. More prevalent in construction workers.
The latent period for benign disease is shorter than other pulmonary complications. The incidence of pleural effusion is associated with the site of exposure and was found to be 9 cases per 1000 in heavily exposed groups. Although mesothelioma is less prevalent than lung cancer, over 2000 cases per yr in the twenty-first century were reported in the united states of America. The incidence of lung cancer is comparatively more than mesothelioma. The use of asbestos in developed countries has been reduced but use in developing countries like Nepal, Pakistan is considerably high.
Interstitial fibrosis is regarded as the principal pathogenic mechanism of asbestosis. It is believed that after deposition and transmigration of asbestos fibers in the lung, there is an accumulation of macrophages followed by fibroblasts that lay the foundation for fibrosis. Reactive oxygen species produced by immune cells and phagocytes in response to asbestos fibers result in oxidative injury. These reactive oxygen species and transepithelial migration of fibers injure the type 1 alveolar cells. Injured epithelial cells also produce fibroblast growth factor-beta, which induces fibrosis. In an attempt to phagocytose the foreign body, macrophages produce inflammatory mediators such as tissue necrosis factors, interleukins, and stimulation of the phospholipase C pathway. These mediators have a key role in stimulating the other cells like lymphocytes, myofibroblasts. This leads to the proliferation of fibroblasts and an increase in the number of cells in the matrix by about 2-fold. Macrophages also produce fibroblast growth factors, platelet-derived growth factors, and insulin-like growth factors that cause fibrosis. Plasminogen activator produced by macrophages degrades the matrix glycoproteins, which further damages the interstitium.
As this is a progressive disease, fibrosis increase over time. Asbestos fibers also activate serum complement C5a, a chemotactic factor for macrophages. In most cases, asbestos fibers coated by other toxins, including transition metals, induce the formation of reactive oxygen species. In one cell-free system, iron-coated on these fibers has a role in producing hydroxyl ions, causing free radicle injury.
A recent investigation revealed that the length of fibers has a direct relation to pathogenesis. Longer fibers are more potent than shorter fibers in stimulating the NF-kB pathway and gene promoter activity. Exposure to amphibole is linked to autoantibodies production. It is believed that patients with positive antinuclear antibodies test have more chances of developing interstitial and pleural abnormalities. In addition to cellular and fibrotic reactions, asbestos possibly functions as both a tumor initiator and promoter. Amphibole type has more tendency for carcinogenesis of pleura. The severity of asbestos-related pulmonary fibrosis is related to the total dose of exposure.
On gross examination, the visceral pleura is markedly thickened, especially on the lateral and diaphragmatic surface of the lung with localized fibrous plaques and pleural effusions. Diffuse fibrosis of the lower lobe of the lung is visible on the cut section.
On microscopic examination, asbestosis is marked by interstitial fibrosis with characteristic asbestos bodies and ferruginous bodies. The presence of both asbestos bodies and ferruginous bodies helps pathologists differentiate between asbestosis and interstitial lung fibrosis. Asbestos bodies are golden yellow, beaded rod-like structures with a translucent center. It is formed when asbestos fibers get coated with iron-containing proteinaceous material. A ferruginous body is an indication that the patient may have had prior exposure to asbestos and appear as golden-brown fusiform rods resembling dumbells. Fibrosis causes distortion of the lung parenchyma architecture, which causes the formation of enlarged air space surrounded by a thick fibrous wall and hyperplastic type 2 pneumocytes called honeycomb appearance.
History and Physical
The history of occupation is crucial for both diagnosis and management. History of exposure to asbestosis helps to exclude other chronic interstitial lung diseases. Usually, there is a history of 10 to 20 years of exposure to asbestos and progressively worsening dyspnea. Cough with sputum and wheezing are unusual, though, if present, are mainly associated with smoking. The patient may complain of chest discomfort due to heart failure following pulmonary hypertension. The severity of the disease depends on the duration and intensity of exposure and whether the patient has direct occupational contact with asbestos. History of smoking and dyspnea on exertion are important, and nonproductive cough is quite common. History of loss of appetite and weight, hemoptysis indicates the suspicion of lung tumors. Gradual onset of localized pain or breathlessness and then radiating to the shoulder may be evident in pleural involvement.
Physical examination reveals clubbing in 32% to 42%, asbestos warts, reduced chest expansion due to restrictive lung disease (38%). Bibasilar rales are best auscultated at lower lateral and basal areas. In advanced disease, the patient may show signs of cor pulmonale such as pedal edema, jugular venous distension, right ventricular heave, hepatojugular reflux, cyanosis. These signs are mainly due to right-sided heart failure from pulmonary vascular remodeling.
The diagnosis of asbestosis is largely clinical. First, the history of asbestos exposure is central to the diagnosis. The overall clinical picture is characterized by progressive restrictive pulmonary disease with interstitial fibrosis on radiography. The pulmonary function test reveals characteristic restrictive disease.
Pulmonary Function Tests
- Spirometry and Lung Volum: This includes forced vital capacity (FVC), forced expiratory volume in 1 sec (FEV1), total lung capacity (TLC), functional residual capacity, and residual volume. Like any other restrictive lung disease, all earlier mentioned lung volumes are reduced. FEV1/FVC ratio is either normal or increased.
- Diffusing Capacity: Total carbon monoxide diffusion capacity (DLCO) reduction is a common but nonspecific finding. Reduced total carbon monoxide diffusion capacity (DLCO) is manifested earlier. This is due to the mismatching of ventilation and perfusion (V/Q).
Arterial Blood Gas
Arterial oxygen tension (pao2) may be normal or may reveal hypoxia and respiratory alkalosis. Carbon dioxide retention is rare; if present indicates the end-stage disease, there may also be airflow obstruction due to small airway dysfunction. Nevertheless, it is important to note that asbestosis is a diagnosis of exclusion. It is clinically relevant to differentiate asbestosis from idiopathic pulmonary fibrosis (IPF) because both essentially have many similar presenting symptoms. Both asbestosis and IPF are characterized by progressive interstitial pulmonary fibrosis, with a restrictive lung disease picture in PFT. There are subtle differences between the two radiographically and histologically. It is important to differentiate between the two because distinctly targeted therapies are available, although not that effective.
Chest Radiograph and HRCT Chest
Chest radiograph shows diffuse reticulonodular infiltrates at the lung bases causing shaggy heart borders. High-resolution computed tomography (HRCT) is often diagnostic of asbestosis. HRCT shows ground-glass opacities, along with diffuse interstitial fibrosis in asbestosis, whereas, in idiopathic pulmonary fibrosis, there is evidence of patches of opacities. Pleural thickening and calcified pleural plaques in tomography are the hallmarks of the disease. Asbestosis mimics idiopathic pulmonary fibrosis radiographically, especially in CT. The important distinguishing point is that asbestosis begins more centrally and dissipates peripherally following a centrifugal pattern. In contrast, idiopathic pulmonary fibrosis begins peripherally, especially at the bases, and progresses centrally and upwards as the disease progresses. At least theoretically, IPF follows a centripetal pattern. Differentiating asbestosis and idiopathic pulmonary fibrosis (IPF) on CT can be challenging, and histopathology can be conclusive.
It shows interstitial fibrosis with characteristically peribronchial fibrosis. The distribution of fibrosis in asbestosis is from the center to the periphery, i.e., centrifugally. The microscopic view of asbestosis shows distinct asbestos bodies coated with iron-containing proteinaceous material and diffuse pulmonary interstitial fibrosis. Still, in contrast, idiopathic pulmonary fibrosis (IPF) displays patchy interstitial fibrosis. The profuse proliferation of fibroblasts, i.e., fibroblastic foci and subsistence of early and late proliferative lesions (temporal heterogeneity), is well defined in IPF and rare or absent in asbestosis.
A lung biopsy can be achieved by endobronchial, transbronchial, and video-assisted thoracoscopic (VAT) approaches. The endobronchial biopsy is done with ultrasound-guided bronchoscopy to visualize the airway and adjacent structures. This technique has been routinely used in many centers due to its high diagnostic value and low risk. Endoscopically-guided forceps do transbronchial lung biopsy. Conventional transbronchial biopsy has a higher risk of complications and bleeding. The modern technique is the minimally invasive video-assisted thoracoscopic (VAT) guided biopsy. VAT has fewer complications, is more sensitive, specific, and has more diagnostic yield than transbronchial biopsy. So, VAT is preferred over transbronchial lung biopsy, though VAT has some discrepancies like cost and expertise requirement.
Bronchoalveolar lavage has a limited role. Samples can be taken from suspected patients for cellular analysis of asbestos bodies count, inflammatory cells like macrophages, neutrophils, eosinophils, and dust particles. Asbestos body formation is more common in amphibole fibers, while chrysotile fibers have a shorter half-life and have fewer chances of asbestos body formation. So the absence of asbestos bodies in chrysotile asbestosis leads to a false-negative result.
Lab analysis shows elevated C-reactive protein, erythrocyte sedimentation rate, rheumatoid factor, and antinuclear antibodies.
Treatment / Management
Asbestosis has no specific treatment, so supportive care is the only available option. Therefore, prevention is the best management. Monitoring the occupational environment and minimizing asbestos exposure have a significant role in asbestosis control.
The goal of corticosteroid therapy is to suppress the acute and chronic inflammatory process, thereby reducing lung damage, but the success rate is low. Steroids suppress the release of reactive species and mediators in the ongoing inflammatory process. Because of the lack of placebo-controlled trials, there is no direct evidence of the benefits of steroids in improving the survival of life. The proper dose and length of therapy of steroids are not known. High-dose corticosteroids and other immunosuppressive drugs such as azathioprine have some roles in reducing exacerbation of acute asbestosis. The starting dose of prednisolone is usually considered 0.5 to 1 mg per kg in a once-daily oral dose for 4 to 12 weeks. The patient is monitored and reevaluated. The dose is tapered to 0.25 to 0.5 mg/kg for a further 4 to 12 weeks if the patient is stable and improving. But some studies show that the use of agents like cyclophosphamide accelerates the fibrotic process, so its use is limited. If the condition is not improving and the patient is unstable, then either other drugs are added or steroid is withdrawn. Colchicine is found to be a mild antifibrotic agent. Antibiotics therapy should be prompt for superimposed respiratory infections as well as immunization against pneumococcal pneumonia and influenza.
Patient with asbestosis sometimes has also a history of smoking and they develop chronic obstructive pulmonary disease (COPD). In such cases, inhalers containing long-acting beta 2 agonists, inhaled corticosteroids, or long-acting muscarinic antagonists are also prescribed. Smoking cessation is also advised.
Patients with hypoxemia (Pao2 less than 55 mmHg) at rest or on exertion should be supplemented with oxygen. Pulmonary rehabilitation and management of cor pulmonale have been shown to improve the quality of life as the disease progresses.
Decortication of subpleural fibrosis by thoracotomy improves atelectasis. Pleurectomy can be performed in pleural fibrosis. But if the collection of pleural fluid is rapid, then only palliative removal of fluids to relieve breathlessness should be done.
This is the ultimate treatment in severe asbestosis, where all other therapies have failed in the case of chronic and irreversible fibrosis.
Differentials of asbestosis include the diseases involving the interstitium of lungs such as idiopathic pulmonary fibrosis, interstitial pneumonitis, rheumatoid arthritis, hypersensitive pneumonitis, pleuropulmonary fibroelastosis, drug-induced pneumonitis, combined pulmonary fibrosis, and emphysema. Diagnosis of asbestosis requires the exclusion of these differentials.
Non-specific Interstitial Pneumonitis
This may be idiopathic or associated with other connective tissue disorders. Lower lobe involvement, ground-glass opacities, impairment of pulmonary functions mimic this condition with asbestosis, but female predominance and rare honeycombing help in differentiation. History of exposure and histopathology is important to rule out other similar conditions.
Chronic Hypersensitivity Pneumonitis
It is an allergic reaction based upon the antigen-antibody interaction following exposure to allergens. Clinical presentation of hypersensitive pneumonitis such as dyspnea, cough resembles that of asbestosis. So lung biopsy with histopathology evaluation is required. It involves interstitial inflammation with granulomas and giant cells, usually throughout the whole lung, as compared to more pronounced fibrosis in the lower lobes of the lung in asbestosis.
The rheumatic disease involves the interstitium of the lung in about 10% of cases. Unlike asbestosis, other systemic manifestations like arthritis, Raynaud phenomenon, muscle weakness, skin changes are present in rheumatic disease.
Drug-induced Interstitial Pneumonitis
Some drugs used for purposes other than pulmonary diseases have pulmonary toxicity involving interstitium as their side effects. Anagrelide, a phospholipase A2 inhibitor, lamotrigine, cyclophosphamide, bleomycin, nitrofurantoin, etc are the drugs that cause pulmonary toxicity. This interstitial lung toxicity may confuse with asbestosis, but the history of drug intake helps to distinguish the real cause of the disease.
Pleuropulmonary fibroelastosis is a rare benign condition. Though this disease simulates asbestosis, this condition involves pleura and subpleural parenchyma almost simultaneously. Fibrosis is due to elastic fiber proliferation. But in asbestosis, pleural involvement is not parallel to parenchymal fibrosis as pleural involvement is usually the late presentation. And also, pleuropulmonary fibroelastosis may involve the upper lobe of the lungs.
Silicosis has more tendency to undergo fibrosis than asbestosis, forming a whorled pattern of collagen fibers that can be appreciated in histology. Occupational history makes an easy way out to exclude silicosis. Fibrosis may also involve hilar lymph nodes and sometimes the calcification called egg-shell calcification.
Sarcoidosis is a chronic granulomatous disease of the lung which involves the multisystem. CD4+ to CD8+ cells ratio is increased as in asbestosis, but it has a more pronounced increment than in asbestosis. Histopathology reveals noncaseating granuloma, asteroid bodes, Schaumann bodies are characteristic of sarcoidosis not found in asbestosis.
Coal Worker Pneumoconiosis (CWP)
It is also called black lung disease. It is the coal dust-induced progressive massive fibrosis disease. Cut section and biopsy of the lung of workers working in coal mines show black patches mixed with fibrosis.
Combined Pulmonary Fibrosis and Emphysema
Usually, emphysema is highly associated with smoking and occurs without fibrosis. The co-existence of fibrosis and emphysema was first established in 1990. Upper lobe emphysema and lower lobe fibrosis are seen in some male smokers.
The severity of the disease depends upon the duration and intensity of exposure. More intense contact for a long period of time has a poor prognosis. Early intervention after the first symptom has a good outcome and prolongs survival. The condition worsens in acute exacerbations and superimposed infections. Radiologic screening in workers helps in picking the earliest abnormal changes. Steroids have only symptomatic treatment. So outcomes after steroid therapy do not always favor the alleviation of disease. Progression of the disease process can be stopped by removing the offenders. In later stages, the disease process becomes irreversible and may finally end with lung cancer. Patients with pleural involvement rarely survive more than a year after diagnosis has been made. But survival from the first symptom or earliest screening is a maximum of 4 years. Radical surgery or radiotherapy of mesothelioma may aid in the extension of tumors to the chest wall.
Complications increases with the duration and intensity of exposure. Following complications occur in patients having exposure to asbestos:
Asbestosis is a restrictive lung disease characterized by the restricted filling of the lung. Total lung capacity and forced vital capacity reduce significantly. Patients complain of progressive dyspnea on exertion and cough. Deterioration of diffusing capacity and oxygenation is common. Pleural fibrosis prevents the expansion of the lung. Carbon dioxide retention is another hazard that leads to respiratory acidosis. In many cases, benign pleural effusion occurs early, followed by pleural plaques formation. Effusion is generally bilateral and exudative and mostly remains asymptomatic. Fibrosis causes the derangement of the pulmonary vascular system, especially capillaries, which causes pulmonary hypertension. The decrease in diffusing capacity has a direct impact on hypoxemia.
Patients with asbestosis have a high risk of developing pleural malignancy. Patients who have low exposure to asbestos fibers have sequelae of asbestosis followed by malignancy, but those with intermediate or high-intensity exposure have high chances of lung cancer even in the absence of asbestosis. Lung cancer has more propensity to develop when the smoker is exposed to asbestos due to the additive effect of asbestos and cigarette smoke. Studies show that adenocarcinoma (primary bronchogenic carcinoma) is the major type of cancer that accounts for about 45%, followed by squamous cell carcinoma in about 42% of cases, and 13% is undifferentiated. Unlike fibrosis, the majority of tumors are present in the upper lobes (69%) and only 13% in the lower lobes. Mesothelioma of the pleura and peritoneum is common but is less prevalent than lung cancer. In contrast, to bronchogenic carcinoma, the risk of mesothelioma does not increase with smoking in patients with asbestosis. Epidemiologic studies have explored that greater than 80% of mesothelioma is due to asbestos exposure.
Heart and Other Organs
It was found that heart weight is increased due to hypertrophy of right ventricle pumping against high pulmonary vascular resistance. More commonly, left auricle hypertrophy has been appreciated. Pulmonary and tricuspid valves are generally normal, but the mitral valve is fibrosed and fused with thickening chordae tendineae. Advanced asbestosis causes right heart failure leading to cor pulmonale. The liver is congested in the centrilobular pattern due to the right-sided damming of blood.
Cancers of Other Organs
Cohort's study has unveiled that asbestosis is related to cancers of the gastrointestinal tract, ovaries, adrenals, larynx, and kidney. Gastrointestinal tract cancer is mainly due to exposure to asbestos through asbestos-lined cement water pipes.
Management of asbestosis requires a multidisciplinary team consisting of:
- Thoracic surgeon
Deterrence and Patient Education
Asbestosis is an occupational disease, so it is more prevalent in workers. Prevention is the utmost step in the management of asbestosis. Despite the usefulness of asbestos, it is hazardous for long-term use and exposure. In the modern world, concern over the harmful effects is rising continuously. Research has been conducted to evaluate the effects of asbestos on health. Because it takes a long time to develop the disease as it is a progressive process, cessation of exposure stops the disease process. Avoiding potential risks, health education and safety training, monitoring of the work environment, periodic examination, and providing treatment to the workers help to control the disease. Workers should be alarmed and have the right to know about the symptoms, outcomes of treatment, the toxicity of drugs, health problems of asbestosis, and they should consult the health team as soon as possible. Patients suffering from asbestosis should stop smoking, minimize the exposure by changing the workplace or occupation.
Enhancing Healthcare Team Outcomes
Competent healthcare systems in collaborations with other organizations and government bodies should work together to control occupational diseases. The healthcare team has more diagnostic, therapeutic, and supportive care. So prevention or minimization of exposure necessitates the government legislation to act accordingly, international labor organizations, World Health Organization, dust management teams. Intersectoral coordination has a huge impact on optimizing the working environment and working hours. The application of engineering techniques to maintain appropriate working sites for optimum workers' health is essential. Healthcare team consisting of general providers, radiologists, pulmonologists, pharmacists, surgeons, oncologists, nurses, and histopathologists assists in making a proper diagnosis and appropriate treatment. Proper diagnosis with the help of a multi-disciplinary healthcare team helps to devise the right way to the suitable treatment. Nursing care during the treatment has its importance. Complications of asbestosis require multimodality treatment options such as oncologists to treat lung cancer, surgeons for decortication and pleurectomy, etc.
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