Silicosis/Coal Workers’ Pneumoconiosis

What every physician needs to know:

Coal workers’ pneumoconiosis (CWP) and silicosis are preventable occupational lung diseases that are typically associated with significant dust exposure. Although these diseases are two separate entities, they may occur concomitantly, especially in coal miners. Despite mandated dust controls, silicosis and coal workers’ pneumoconiosis remain significant public health care burdens. No specific therapy for either of these occupational lung diseases is available. Prevention strategies and therapeutics offering symptomatic relief remain the mainstay of management.

CWP is classically a slowly progressive parenchymal and airway lung disease that results from inhalation and deposition of coal dust in the lungs. Early in the disease, patients are typically asymptomatic and may come to attention only secondary to medical screening programs and/or incidental findings on imaging. A subset of miners has been described that develop a rapid decline in lung function. Patients may have significant radiographic abnormalities and be relatively asymptomatic. Patients may also develop chronic respiratory failure. Recent evidence demonstrates certain regions having a significant increase in miners developing an advanced form of the disease termed progressive massive fibrosis. The diagnosis of CWP is usually straightforward in the appropriate clinical context with compatible imaging. Lung biopsy is typically not required to make the diagnosis. Therapy is aimed at prevention and management of the expiratory airflow obstruction and other complications of CWP.

Disease associated with the inhalation of a respirable form of crystalline silica, most commonly quartz, has been recognized in numerous occupations. Classically, workers involved with mining, quarrying, drilling, tunneling, and sandblasting have been at high risk for development of disease. However, this list does not nearly encompass all the jobs with potential for silica exposure and development of disease. Even exposure to silica dust through environmental phenomena with non-industrial exposure has been linked to the development of silicosis. The pathological expression of silicosis takes numerous forms depending on the exposure history, latency periods, natural history, and host response to various silica particles.

Classification:

Classification of CWP is based on radiographic findings. Simple CWP is defined in a patient with an appropriate clinical history who has classic findings of small, rounded opacities (less than 1 cm in size) with an upper lobe distribution, although other zonal distributions may occur. As the disease advances, these nodules conglomerate, leading to opacities greater than 1 cm in size. The radiographic and pathologic presence of these larger opacities characterizes complicated CWP, also known as progressive massive fibrosis.

A recent update to the classification scheme of occupational dust exposure more accurately describes the spectrum of lung disease associated with coal mine dust exposure beyond the classically recognized CWP. This scheme classifies the manifestations under the umbrella of coal mine dust lung disease (CMDLD) which includes classic CWP, silicosis, mixed dust pneumoconiosis and dust related diffuse fibrosis (clinically indistinguishable from idiopathic pulmonary fibrosis absent the exposure history or pathologic evidence).

Atypical features of CMDLD have been observed based on recent evidence. The radiographic opacities in CWP may occur in lower lung zones and often appear irregular in shape, rather than the classic rounded appearance. Progression to advance pneumoconiosis may be considerably more rapid than the previously considered norm of 15-25 years. This accelerated decline in FEV₁ (>60 ml/year) was associated with work in roof bolting, lack of use of respiratory protection, exposure to explosive blasting fumes, use of stored mine water for dust suppression sprays and elevated silica exposures from work in smaller mines, mining the thin coal seams or coals with high carbon content.

The current evidence suggests the etiological role of coal dust inhalation for COPD and emphysema, and the rate of associated function decline is similar for dust exposure and tobacco smoke. Small airways disease from coal dust exposures has been increasingly implicated as one of the underlying pathogenic mechanisms for development of clinically significant respiratory symptoms, spirometry deficits and accelerated decline in lung function. Lastly, clinical expression in individual miners is often a result of multiple effects of coal mine dust, such as chronic bronchitis, emphysema and interstitial lung disease.

Similarly, silica exposure can lead to a multitude of disease processes with highly variable presentations that depend on exposure history/burden, host factors, and the underlying pathology.

Chronic Silicosis (or classic silicosis) is asymptomatic in its early form. The pathologic hallmark of this form of silica lung disease is the silicotic nodule. These nodules manifest radiographically as multiple, well-defined nodular opacities 1-10 mm in diameter with occasional calcification, and an upper lobe predilection. As with CWP, these nodules conglomerate into lesions greater than 1 cm in size, leading to progressive massive fibrosis or complicated silicosis. This nodule coalescence leads to clinically important destruction of the lung parenchyma, fibrotic disease, and cavitation.

Accelerated Silicosis is pathologically similar to chronic silicosis in terms of the formation of the classic silica nodules and the ability to progress to progressive massive fibrosis. The difference is in the time course from exposure to the development of disease. Patients with accelerated silicosis typically have a history of intense exposure with onset of symptoms that may occur within two years of exposure, compared to ten years for chronic silicosis. Patients with this form of the disease typically develop rapid deterioration in lung function and are likely to be at higher risk for complicating autoimmune and/or mycobacterial diseases compared to those with chronic silicosis.

Acute silicosis (silicoproteinosis) may develop in weeks to years following massive inhalation of freshly fractured or broken silica particles, which may be seen in jobs like sandblasting, surface drilling, and mining. Patients with this form of silica-associated disease typically have severe dyspnea, weakness, weight loss, and cough at presentation. Radiographically, acute silicosis differs from the more chronic forms of silicate disease, as this manifestation of silicosis has a diffuse alveolar filling pattern that typically affects the middle and lower lobes. Histological evaluation does not show the classic silicotic nodules. Instead, a severe alveolitis and alveolar filling that stains positive with periodic acid-Schiff, as seen in alveolar lipoproteinosis, is present. This form of silicosis is usually fatal.

Are you sure your patient has silicosis/coal workers' pneumoconiosis? What should you expect to find?

CWPcan be diagnosed with confidence in the appropriate clinical setting, coal dust exposure history of at least five years, and compatible radiographic findings. The clinical presentation of patients with CWP can vary widely. Many miners with simple pneumoconiosis have no symptoms or signs of disease and come to attention secondary to radiographic screening or incidental findings on imaging. Advanced disease can lead to early disability and mortality.

Chronic cough, sputum production, and dyspnea in a patient with significant coal dust exposure history should raise the possibility of CWP. Less common manifestations that typically present in advanced disease include cor pulmonale, edema, and melanoptysis (expectoration of black sputum). Melanoptysis is worrisome, but it is a harmless and rare finding that results from excavation or liquefaction of a PMF lesion.

As described above, chest radiographs in simple CWP typically reveal small, rounded opacities in the upper lung zones. In some instances, the chest radiography may be normal in early disease, but high-resolution CT scanning may reveal nodularity and emphysematous changes. Nodules typically have a perilymphatic distribution, but they may also have a centrilobular predominance.

Silica-related lung disease can have a variety of presentations, depending on numerous factors, which include chronic silicosis, accelerated silicosis, and acute silicosis (silicoproteinosis) as described above. As with CWP, the diagnosis of silicosis is typically straightforward in patients with appropriate exposure history and radiographic findings. Significant silica exposure can occur in many occupations. Known high-risk occupations include mining, quarrying, drilling, tunneling, and sandblasting. Although these occupations specifically place patients at high risk for silicosis, disease development may occur in a large number of settings. The type of silica exposure also plays an important role in disease development. Settings in which freshly fractured silica is produced, such as sandblasting and drilling settings, carry an increased risk of silicosis.

Early in disease, patients are typically asymptomatic. Advanced forms may lead to respiratory failure and cor pulmonale. This progressive disease may develop 10-30 years following significant silica exposure. Accelerated silicosis typically requires heavier exposures, with a latency of 5-10 years. Acute silicosis may develop in a few months or up to five years following intense silica exposure. The simple silicotic nodules described above typically manifest as small, rounded opacities with an upper lobe distribution on chest radiograph, although other radiographic distribution patterns have been described.

Beware: there are other diseases that can mimic silicosis/coal workers' pneumoconiosis:

Patients with respiratory symptoms typically undergo radiologic evaluation as part of their diagnostic work-up. In patients with CWP and silicosis, chest radiography and CT typically reveals multiple pulmonary nodules. The differential for multiple pulmonary nodules includes infection (abscess, septic emboli, mycobacterial, fungal), auto-immune disorders (granulomatous polyangiitis [GPA], sarcoid, rheumatoid arthritis), malignancy (primary lung or metastatic disease), and less commonly, arteriovenous malformations. The clinical context and radiographic distribution of these nodules will, in most cases, enable the healthcare provider to make a clinical diagnosis of CWP or silicosis without extensive testing.

How and/or why did the patient develop silicosis/coal workers' pneumoconiosis?

The most important factor in diagnosing a patient with CWP or silicosis is obtaining a good occupational history. Obviously, workers cannot develop CWP or silicosis if they are not exposed to these respirable dusts. Clinicians should be aware that different occupations and different duties that are part of high-risk occupations may place workers at particularly high risk for the development of these two occupational lung diseases.

Coal remains an important source of energy for both developed and developing countries. Approximately 30% of world’s recoverable coal is in the US and China is estimated to have over 6 million coal miners. Coal is obtained through strip and/or surface mining or through underground mining. Underground miners typically have increased respirable dust exposure compared to their surface-mining counterparts. Coal is removed in underground mines from intact seams via various methods. These seams are the coal-cutting face of the mine and are where the coal dust exposure is the highest.

Since 2000, the surveillance study of working US coal miners has shown an unexpected increase in the proportion of radiographic evidence of pneumoconiosis. In 2005, the National Institute for Occupational Safety and Health (NIOSH) reported that certain miners were showing an accelerated form of pneumoconiosis, often PMF, which appeared to occur in certain geographic regions. Disease risk has been shown to increase with the rank of coal and with the level of bioavailable iron in coal.

Silica is the most abundant element in the Earth’s crust, and any occupation that disrupts the Earth’s crust and exposes respirable-size silica particles (0.5-5 microns) may lead to disease. Disease associated with the inhalation of a respirable form or crystalline silica, most commonly quartz, has been recognized in numerous occupations. Although silicosis is the classic occupational lung disease, silica dust exposure through environmental phenomena without industrial exposure has also been linked to the development of silicosis.

Although the basic mechanisms of the disease are not fully understood, ongoing dust deposition in the airways and alveoli leads to a host response that includes important interactions between pulmonary macrophages and inflammatory mediators. These interactions lead to pathological injuries and the clinical manifestations. Akin to COPD, significant debilitating disease will be developed by certain individuals exposed to coal and silica dust, whereas others will have no evidence of disease and no clinical burden.

Which individuals are at greatest risk of developing silicosis/coal workers' pneumoconiosis?

Silicosis is a worldwide phenomenon that is associated with significant morbidity and mortality. The risk of silicosis increases with the amount of respirable dust exposure and the type of silica exposure. Patients with exposure to freshly fractured silica are more likely to develop disease, and intense exposure to silica in the short-term places patients at risk for acute silicosis. The precise quantitative relationship between exposure and disease remains controversial, though most nations regulate silica workplace dust levels at about 0.1 mg per cubic meter of air. High-risk occupations, including mining, quarrying, blasting, and sandblasting, involve grinding or crushing of silicates. Emerging settings of silica exposures include the construction industry, natural gas extraction by hydraulic fracturing, agriculture activities in dry sandy soil, denim sandblaster textile industry, manufacturer of “Tatami” mats in China, dental laboratories, and supply factories.

As described above, coal dust exposure can vary significantly depending on a person’s duties at a mining site. Workers whose duties take them in proximity to the coal-cutting face of the mine have increased respiratory exposure. These jobs include operation of mining machines, loading of coal onto train cars, roof bolting, drilling operations, and performing as motormen. These miners are also at risk for exposure to crystalline silica. Other workers in the industry have duties at preparation plants, where crushing, sizing, washing, and blending of coal is completed. These workers also have higher dust exposures compared to others in the industry. Most nations regulate coal mine dust exposure levels between 1 and 2 mg per cubic meter of workplace air.

What laboratory studies should you order to help make the diagnosis, and how should you interpret the results?

No specific routine laboratory studies are diagnostic for either silicosis or CWP. Laboratory studies are useful in the work-up of these patients if the diagnosis remains unclear. As in the evaluation of other interstitial lung diseases, anti-neutrophil cytoplasmic antibodies (ANCA), anti-nuclear antibody (ANA), angiotensin-converting enzyme (ACE), and erythrocyte sedimentation rate (ESR) testing may be helpful in eliminating autoimmune disease if it is suspected. Urine histoplasma antigen and serum quantiferon testing may be indicated if an infectious etiology is suspected. Again, laboratory evaluation will help if the diagnosis is unclear, but no specific diagnostic test exists for either CWP or silicosis.

What imaging studies will be helpful in making or excluding the diagnosis of silicosis/coal workers' pneumoconiosis?

Basic chest radiography is essential to the proper diagnosis of both CWP and silicosis. These patients often come to attention during routine radiography as part of medical screening for high-risk occupations. Chest radiographs typically reveal small, rounded opacities that can be up to 1 cm early in the disease.

Many approaches have been used to classify the abnormalities in occupational lung disease. The most widely used system today is the International Labour Organization (ILO) classification scheme. Based on this system, simple pneumoconiosis is classified on a four-point major scale based on the concentration (profusion) of small opacities in the lung parenchyma. A category “0” represents no abnormalities, and category “3” represents the most severe radiographic disease.

These four major categories are further sub-divided into subcategories, thus giving a final range of a twelve-point scale with which to classify pneumoconiosis. An interpretation of profusion category 1/0 indicates the presence of opacities consistent with simple CWP or silicosis. Progressive massive fibrosis is defined by opacities greater than 1 cm, which are classified according to the size of the longest dimension of the opacities. These large opacities are categorized as A, B, or C lesions, with category A describing the smaller opacities and category C representing the larger opacities. No official ILO endorsed classification system exists for CT scans in patients with occupational lung disease. A widely used CT classification system is in use in Germany for surveillance and compensation and has been used for research purposes in the U.S. and many other nations studying occupational lung disease.

Chest radiography may be unremarkable early in disease in both silicosis and CWP. However, high-resolution CT scanning may reveal abnormalities not seen on the chest radiograph in the earliest stages in these diseases. CWP typically manifests as nodules in a perilymphatic distribution, but they may have a centrilobular predominance. These nodules may have calcifications and be associated with areas of emphysematous changes. Approximately 30% of patients have hilar or mediastinal lymph node enlargement.

Silicosis manifests on CT scanning as nodules in a centrilobular, paraseptal, and/or subpleural distribution. Hilar and mediastinal lymphadenopathy may be present, with an “eggshell” calcification pattern being highly suggestive of silicosis. Intra-alveolar accumulation of proteinaceous material, as seen in acute silicosis, manifests with multifocal areas of centrilobular ground-glass opacities and consolidation.

What non-invasive pulmonary diagnostic studies will be helpful in making or excluding the diagnosis of silicosis/coal workers' pneumoconiosis?

Patients with significant dust exposure who are suspected or diagnosed with silicosis and/or CWP should undergo pulmonary function testing. Pulmonary function testing is not specific in these disorders, and patients may manifest with normal, restrictive, or mixed physiologic defects. However, testing is helpful in the clinical evaluation, as physiologic deficits may manifest prior to any radiographic abnormality. Pulmonary function testing commonly reveals air flow obstruction that may not improve with bronchodilator therapy.

Important subsets of miners with rapid declines in lung function (defined as loss of 60 ml/yr or greater in FEV₁) have been evaluated. Results suggest that these miners develop more respiratory symptoms and chest illnesses and experience a higher risk of death from cardiovascular and nonmalignant respiratory diseases compared to matched miners with more stable lung function. A role of small airways disease has been implicated in the cohort of rapid decliners which showed a constellation of findings including increased airway resistance, decreased mid-expiratory flow rates, and air trapping which improved following inhaled bronchodilators. Therefore, lung function may provide clinicians with an opportunity to intervene early in the course of the disease and mitigate some of the potential serious consequences.

Patients with early chronic forms of disease may have normal spirometry or minor ventilatory abnormalities with normal diffusion capacity. Advanced or end-stage disease (progressive massive fibrosis) is heralded by significant ventilatory and gas exchange abnormalities, leading to marked hypoxemia and chronic respiratory failure.

What diagnostic procedures will be helpful in making or excluding the diagnosis of silicosis/coal workers' pneumoconiosis?

Silicosis and CWP are typically diagnosed in patients with sufficient exposure history and characteristic radiographs, with diagnosis made after alternative diagnoses have been excluded. Rarely, atypical radiographic findings, complications of disease, alternative diagnoses (TB, malignancy), and/or uncertain exposure history necessitate more invasive means to establish a diagnosis. Successful use of bronchoalveolar lavage and transbronchial biopsy in diagnosis has been reported. However, when tissue diagnosis is needed, a thoracoscopic or open-lung biopsy is generally required to obtain adequate tissue for analysis. Tissue diagnoses is made based on the findings described below.

What pathology/cytology/genetic studies will be helpful in making or excluding the diagnosis of silicosis/coal workers' pneumoconiosis?

In the face of sufficient exposure history and a characteristic radiograph, CWP and silicosis can be diagnosed with confidence. Alternate diagnoses or unclear exposure history occasionally necessitates tissue confirmation of disease.

The pathologic expression of silicosis takes numerous forms, depending on the exposure history, the latency period, the natural history, and the host’s response to various silica particles. The initial insult occurs when silica dust particles of appropriate size are deposited into the small airways. The majority of dust particles are likely cleared by the mucocillary elevator and/or phagocytosed with clearance through lymphatics. These particles have complex interactions with macrophages, lymphocytes, and other cells of the interstitium, which leads to the clinical manifestations. A large amount of evidence supports the notion that the generation of reactive oxygen species and interaction with alveolar macrophages are of central importance in silica toxicity.

Classically, these complex interactions lead to the histologic hallmark of chronic silicosis, the silica nodule. These nodules contain an acellular zone with silica particles surrounded by whorls of collagen and fibroblasts, and an active peripheral zone composed of macrophages, fibroblasts, plasma cells, and additional free silica. These lesions are unique, and they may be present in simple or complicated silicosis.

Silicoproteinosis, as seen in acute silicosis, has a different histologic manifestation. Acute silicosis specimens reveal severe alveolitis and alveolar filling that stain positive with acid-Schiff, as seen in alveolar lipoproteinosis. The complex pathways that lead to alveolar filling with eosinophillic-granular, lipid rich pulmonary edema are not completely understood.

The fundamental pathologic lesion in CWF is the coal macule. Anthracosis is the initial asymptomatic accumulation of coal pigment without consequent cellular reaction. This coal dust is engulfed by macrophages and removed via the mucocillary elevator through mucus or lymphatics. Current understanding suggests that, with prolonged exposure, this system becomes overwhelmed, and these dust-laden macrophages accumulate in the alveoli, triggering an inflammatory response. The coal macule is a collection of these coal-dust-laden macrophages around respiratory bronchioles and tapering off toward the alveolar duct. Focal centriacinar emphysema typically develops adjacent to these regions, with increased severity as dust exposure accumulates.

If you decide the patient has silicosis/coal workers' pneumoconiosis, how should the patient be managed?

No specific interventions for CWP or silicosis are available. Associated expiratory airflow obstruction should be managed as in COPD, with bronchodilators and, at times, inhaled corticosteroids. For the successful control of CWP and silicosis around the world, industries and governments will need to implement prevention strategies, medical surveillance programs, research, and education. The mainstay of management is implementation of preventative strategies and, when prevention fails, therapeutics that offer symptomatic relief from the complications associated with these diseases.

At-risk workers with significant dust exposure should receive regular medical evaluations, although these evaluations should not substitute for aggressive preventive measures. Health evaluations typically involve spirometry and chest radiographs. New onset of respiratory symptoms should lead to prompt evaluation, and newly diagnosed workers at a particular site should provoke evaluation of other workers with similar exposure patterns. Patients with evidence of disease should consider transfer to alternate jobs and/or duties to minimize dust exposure. The risks associated with ongoing dust exposure should be discussed with these patients.

Among the potential complications of silica and coal dust exposure are chronic bronchitis and obstructive lung disease. Bronchodilator therapy may be implemented in patients with symptomatic obstructive disease. As with other pulmonary disorders, supplemental oxygen should be provided in patients who meet criteria. The criteria for supplemental oxygen in chronic lung disease include an arterial PO2 (PaO2) of less then or equal to 55 mm Hg or an arterial oxygen saturation (SaO2) less then or equal to 88%. Supplemental oxygen is also indicated if PaO2 is less than or equal to 59 mmHg, or if the SaO2 is less than or equal to 89 percent in the setting of cor pulmonale, right heart failure, or erythrocytosis (hematocrit >55 percent).

Serious complications may arise from chronic hypoxemia, including pulmonary hypertension, cor pulmonale, polycythemia, and cerebral dysfunction. As with other advanced lung diseases, complicated pneumoconiosis may lead to chronic respiratory failure, potentially requiring mechanical ventilation.

Counseling on smoking cessation should be provided to all workers with significant tobacco dependence. Cessation efforts can be supplemented by support groups, nicotine replacement therapy, pharmacologic aides (varenicline, bupropion, etc.), and/or behavior modification techniques as needed.

Mycobacterial infection deserves special mention in this population of occupational lung disease. Silicosis is a well established risk factor for tuberculosis, and clinicians must be vigilant in recognizing this complication. Infection should also be considered in coal workers’ lung disease, as these patients often have concomitant silica exposure. Workers who develop cavitation of a PMF lesion or worsening respiratory symptoms, hemoptysis, or constitutional symptoms require immediate evaluation for tuberculosis. In nations where PPD testing is of value, a reaction of more than 10 mm with PPD administration is considered positive in this population.

Microbiological evaluation is of utmost importance in patients with silicotuberculosis, as the silica burden may mask ongoing infection. Sputum evaluation is not always reliable, and specimens obtained through bronchoscopy may aid in establishing the diagnosis. Although the standard multidrug regimens are used in the treatment of active TB in patients with silicosis, these patients may require extended therapy. A multiplicative increase in the risk of mycobacterial disease occurs in patients with both silicosis and HIV.

Concern often arises in these patients as their PMF lesions coalesce and enlarge, mimicking malignancy. The expected increase in the size of PMF lesions leads to the diagnostic dilemma for clinicians in differentiating malignancy from unusual presentation of CWP or silicosis. Serial radiologic evaluations of these lesions, which are often available from medical screening programs, can aid clinicians in assessing for stability over time. Fluorodeoxyglucose positron emission (FDG-PET) scanning is often not helpful in the evaluation of PMF lesions as these lesions are typically PET positive, leading to further diagnostic dilemma. Miners with radiographically ambiguous lesions may require surgical excision/biopsy to determine the true nature of the lesion.

Acute silicosis may rapidly progress to respiratory failure. Glucocorticoid therapy has been used in an attempt to arrest ongoing inflammation in the disease process; however, it is of unproven benefit. As previously described, this disease may resemble pulmonary alveolar proteinosis. In the face of severe hypoxemia, investigators have used whole-lung lavage in an attempt to improve gas exchange and remove alveolar debris. The efficacy of this approach has yet to be established.

A number of autoimmune diseases have been linked to silica exposure. The most persuasive evidence suggests a likely association between occupational silica exposure and scleroderma and rheumatoid arthritis. Evidence regarding silica exposure and other connective tissue disorders, such as systemic lupus erythematosus, is less clear. Clinicians should be aware of these potentially complicated illnesses in order to provide appropriate therapeutic interventions.

What is the prognosis for patients managed in the recommended ways?

The prognosis for early-stage CWP and silicosis is generally good, with most patients dying with but not of their occupational lung disease. However, in advanced cases, especially PMF cases, the disease may lead to respiratory failure, infectious complications, and premature disability and death.

The radiographic abnormalities may progress in the absence of further workplace exposure.

What other considerations exist for patients with silicosis/coal workers' pneumoconiosis?

As with other advanced lung diseases, complicated CWP and silicosis (progressive massive fibrosis) may lead to chronic respiratory failure that requires mechanical ventilator support. These support measures may be warranted if a potentially reversible complication is present, but patients’ wishes with respect to long-term supportive measures should be addressed frankly by physicians and outlined in advanced directives.

Young patients with extensive disease or with rapidly advancing disease may be candidates for lung transplant. Early referral and evaluation should be considered in appropriate candidates.

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