Silicosis and Coal Worker’s Pneumoconiosis

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Chapter 51 Silicosis and Coal Worker’s Pneumoconiosis

Pneumoconiosis has been defined as the non-neoplastic reaction of the lung to inhaled mineral or organic dust. Prolonged inhalation of coal mine dust may result in the development of pneumoconiosis, silicosis, and industrial chronic bronchitis and emphysema, either singly or in various combinations. Coal worker’s pneumoconiosis (CWP) is the term generally applied to interstitial disease of the lung resulting from chronic exposure to coal dust, its inhalation and deposition, and the tissue reaction of the host to its presence, whereas silicosis refers to lung disease due to inhalation of dust containing silica. Pneumoconioses differ in a number of ways from acute allergic and toxic interstitial diseases associated with exposure to organic dusts, principally because of their long latency periods (usually 10 to 20 years or more) between exposure onset and disease recognition.

Sources of Exposure

Coal is not a mineral of fixed composition. It is graded by rank, reflecting its carbon content and thus combustibility: Anthracite is the highest-ranked coal, with a carbon content of approximately 98%. Lower-ranked coals, bituminous and subbituminous, have carbon contents of approximately 90% to 95% carbon. The rank of coal has an influence on the risk of disease: Higher-rank coals entail higher risk than lower-rank coals. However, exposure to coal dust with a quartz concentration greater than 15% is associated with a high risk for development of a rapidly progressive form of pneumoconiosis that has the characteristics of silicosis. In open mines, dust levels rarely approach those of underground mines.

The most common form of crystalline silica is quartz. Quartz is almost pure silicone dioxide but often contains traces of other elements. Other crystalline forms of silica are cristobalite and tridymite. The importance of silica as a health hazard is due to its ubiquity (Table 51-1). Diatomite is a siliceous sedimentary rock used for filtration; for heat and sound insulation; as an adsorbent and filtering agent; as a filler material in plastics, paper, and insecticides; and in the manufacture of floor coverings.

Table 51-1 Major Industries Associated With Silica Exposure

Occupation	Exposure
Sand blaster	Ship building, oil rig maintenance, preparing steel for painting
Miner	Surface coal mining, roof bolting, shot firing, drilling, tunneling
Miller	Silica flour
Glass maker	Polishing with sand and enamel work
Potter cleaner	Crushing flint and fettling, foundry work, mold making and vitreous enameling, manufacture of cultured quartz crystal
Quarry and stone worker	Cutting of slate, sandstone, and granite
Abrasive worker	Inhalation of fine particles during grinding

It seems that development and progression of silicosis depend on the total amount of quartz to which workers are exposed, the time over which that exposure occurs, and the presence of other minerals that may interfere with the toxicity of the quartz.

Epidemiology

CWP was first recognized in Scottish miners in 1830. In recent decades, the incidence of CWP has been declining in industrial countries thanks to improved dust controls, although increased mechanization in the mid-1960s led to a temporary increase in dust levels. In parallel, in a report from the United Kingdom, for the period 1950 to 1980, the annual rate of CWP recognition for state compensation in current and retired miners decreased from approximately 7% to 1% to 2%. The overall prevalence of CWP, which reflects more distant exposure and earlier incidence, declined from approximately 13% to 5%, but regional differences in reported rates were substantial. Similar regional differences and similar declines have been noted in the United States and other countries.

Since the 2000s, however, new sources of silicosis have emerged, especially in developing countries—for example, in Turkey, where the denim industry has been responsible for more than 75 cases thus far. Denim sandblasting is used to give jeans a more “worn-out” appearance and requires highly pressurized sand projection, often performed by young persons without any respiratory protection. In a recent epidemiologic study, among 145 workers recruited from the outpatient clinic at Atatürk University, 77 (53%) presented with radiologic evidence of silicosis.

Other, more anecdotal sources of silica exposure have been described, such as heat-dried mud inhalation in workers engaged in the manufacture of tatami mats in China, and handling quartz-containing fillers by dental supply factory workers in the United States. All of these “new forms” of silicosis underscore the fact that it will always remain a concern for respiratory clinicians worldwide, despite the decline of the mining industry in Western countries.

Pathophysiology

Three groups of factors are known to influence the character and severity of lung tissue reaction to mineral dusts. The first category is the intensity and duration of exposure, followed by individual susceptibility, which explains why, among a group of workers exposed to the same dust, only a fraction will develop pneumoconiosis. Finally, the nature and properties of the dust are to be considered. For each mineral, geometric and aerodynamic properties, chemistry, and surface properties vary. Particles that can cause pneumoconiosis are aerodynamically and geometrically small enough to reach the respiratory bronchioles and be deposited there—this generally means spherical particles of 0.5 to 5 µm in diameter.

The pathogenesis of pneumoconiosis is similar to that of all interstitial lung diseases. The condition begins as a chronic inflammatory state (alveolitis) in which inflammatory cells are activated, with consequent damage to the pulmonary architecture. Inorganic particles are phagocytosed by alveolar macrophages, causing their activation and the release of inflammatory mediators such as cytokines and arachidonic acid metabolites. These mediators, in turn, induce the recruitment of other inflammatory cells within the alveolar wall and on the alveolar epithelial surface. Toxic oxygen derivatives and proteolytic enzymes are released by the inflammatory cells, which cause cellular damage and disruption of the extracellular matrix.

The inflammatory phase is followed by a reparative phase, in which growth factors stimulate the recruitment and proliferation of mesenchymal cells and regulate neovascularization and reepithelialization of injured tissues. During this phase, abnormal or possibly uncontrolled reparative mechanisms may result in the development of fibrosis. Fibrogenic particles activate proinflammatory cytokine production within the respiratory tract. Tumor necrosis factor (TNF)-α seems to play a key role in the recruitment of inflammatory cells induced by toxic dusts (Figure 51-1). In addition, neutrophils recruited in the area of inflammation may contribute to the alveolitis, and respiratory and endothelial cells may play a further role by releasing various chemokines such as interleukin (IL)-8. Finally, growth factors such as platelet-derived growth factor, insulin-like growth factor, fibroblast growth factor, and transforming growth factor-β are involved in the pathogenesis of lung fibrosis and in the proliferative response of type II epithelial cells, which occurs in progressive massive fibrosis (PMF).

Figure 51-1 Tumor necrosis factor-α messenger RNA (as demonstrated by in situ hybridization) is expressed in alveolar macrophages (arrows) in this lung section from a patient with coal worker’s pneumoconiosis.

Genetics

More recently, associations of polymorphisms in genes coding for inflammatory cytokines such as TNF-α, IL-6, IL-18, and their receptors with CWP incidence, prevalence, and progression have been reported. In silicosis, a significant association was found between disease severity and the TNF-α −238 variant. Irrespective of disease severity, the TNF-α −308 and IL-1 receptor type A (RA) +2018 variants conferred an increased risk for development of silicotic disease. The TNF-α polymorphisms in positions 238, 376, and 308 of the promoter region also were found to be associated with severe silicosis in South African miners. In French coal miners differentially exposed to coal dust and cigarette smoke, the TNF-α −308 single-nucleotide polymorphism (SNP) showed an interaction with erythrocyte glutathione peroxidase (GSH-Px) activity in workers with high occupational exposure, whereas the lymphotoxin-α (LTA) NcoI polymorphism was associated with CWP prevalence in miners with low blood catalase activity. Certain NFKB1 and FAS-1377 gene polymorphisms also have been described as potential risk factors in a population of Chinese silicotic patients, as compared with a similar group of disease-free silica-exposed miners.

An understanding of genetic variability and environmental factors is crucial to the identification of persons at high risk for development of CWP and to the prevention and treatment of this disease.

Histopathologic Changes

CWP lesions are focal. Simple CWP is associated with macular and nodular lesions, whereas complicated CWP is associated with PMF and lesions of rheumatoid pneumoconiosis (Caplan syndrome, discussed later on).

The pleural surfaces of a coal worker’s lung show an irregular pattern of bluish-black pigmentation that corresponds to the junction sites of septal-lymphatic vessels and the pleura. Peribronchial, hilar, and paratracheal lymph nodes are enlarged, black, and firm. The initial lesions in the lung are the coal dust macules, which correspond macroscopically to focal areas of black pigmentation. On microscopic examination, the macule is seen to be composed of coal dust–laden macrophages within the walls of the respiratory bronchioles and adjacent alveoli (Figure 51-2). Focal emphysema around the coal dust macule is common and is considered an integral part of the lesion of simple CWP.

Figure 51-2 Macular lesion of coal worker’s pneumoconiosis. This coal macule consists of collections of macrophages, laden with coal dust, seen extending into the connective tissue surrounding the respiratory bronchioles.

The histopathologic hallmark of simple CWP is the nodule. The nodules are rounded lesions with collagenous centers. On microscopic examination, the nodule can be divided into three zones: a central zone composed of whorls of dense, hyalinized fibrous tissue; a middle zone made up of concentrically arranged collagen fibers (onion-skinning); and a peripheral zone of more randomly oriented collagen fibers mixed with dust-laden macrophages and lymphoid cells (Figure 51-3). “Old” inactive nodules often are relatively acellular. Particles of silica may be demonstrated in the nodules as birefringent particles under polarized light. Nodules represent a form of mixed-dust fibrosis (i.e., coal dust plus silica exposure), usually are found in association with macules, and in some instances may develop from preexisting macules. They are not confined to the respiratory bronchioles but also are seen in the subpleural and peribronchial connective tissues. Nodules tend to cluster and eventually coalesce to produce PMF. Degenerative changes commonly are observed in the nodular lesions, including calcification, cholesterol clefts, and cavitation. In severe silicosis, structural alterations of the pulmonary vasculature may result from the accumulation of dust in the adventitia of large vessels, and involvement of the smaller blood vessels by silicotic nodules also may be seen.

Figure 51-3 Coal nodule, seen here as a rounded lesion with a collagenous center and a peripheral anthracotic pigmented area. The nodule shows a smooth, sharp border, with dust-laden macrophages and laminated collagen deposition within the interstitium of the lung. The small central area is pale and rich in collagen, and the periphery contains a variable amount of fibrogenic dust.

Progressive massive fibrosis is defined as an opacity or fibrotic pneumoconiotic lesion of 1 cm in diameter or greater. These lesions appear as black fibrotic masses that may be round, oval, or irregular in shape. The lung and bronchovascular rays become markedly distorted. Microscopically, the lesions are composed of bundles of haphazardly arranged hyalinized collagen fibers and/or reticulin fibers and coal dust. Dust particles near the periphery of the lesion are found mainly within macrophages, whereas in the center, the dust tends to lie free in clefts and cavities. The nodules are confluent, emphysematous bullae often surrounding the areas of massive fibrosis. Focal interstitial fibrosis can be observed in the lungs of workers exposed to dust containing a combination of silica and silicates, with sometimes sufficiently advanced lesions to result in honeycombing-type changes. Areas of liquefactive necrosis containing fragments of degenerating collagen as well as cholesterol crystals are frequently observed.

The histopathologic pattern in acute silicosis is quite different from that in the chronic form. Infiltration of the alveolar walls with plasma cells, lymphocytes, and fibroblasts, with some collagenation, is typical. The alveoli are filled with an eosinophilic coagulum (Figure 51-4). Electron microscopy shows widening of alveolar walls, with some collagen and clusters of type II cells; the alveolar spaces contain degenerating cells that probably are type II alveolar cells and macrophages. Silica particles may be demonstrated in the lungs and lymph nodes; silicotic nodules are few or absent.

Figure 51-4 Acute silicosis. Infiltration of the alveolar walls by plasma cells and lymphocytes is evident. Note the alveoli filled with an eosinophilic coagulum.

Clinical Features and Diagnosis

CWP and silicosis generally are first recognized from characteristic changes on the plain chest radiograph, which also is critical in evaluating disease progression. Requirements for the diagnosis include a history of significant exposure, radiographic features consistent with these illnesses, and the absence of any other disease that may mimic pneumoconioses (primarily infections with a predominantly miliary radiographic pattern, such as tuberculosis, fungal infections, or sarcoidosis). The radiographic appearances are most usefully described using the coding system devised for classification of findings on standard chest films in pneumoconiosis under the auspices of the International Labour Office (ILO) (Table 51-2). In clinical practice, simple CWP is characterized by small rounded opacities (nodules) rather than small irregular opacities, although the latter may be seen in much lesser profusion (as described in Table 51-2, profusion categories 0 to 3 refer to the relative abundance of small opacities apparent on the chest radiograph). For designation of disease characterized by large opacities, either PMF or complicated CWP is in common use.

Table 51-2 International Labour Organization (ILO) Radiographic Classification of Pneumoconioses

Small Opacities*	Regular	Irregular
<1.5 mm in diameter	p	s
>1.5 mm but <3 mm in diameter	q	t
>3 mm but <10 mm in diameter	r	u
*Categories*
Small Opacities
The four profusion categories, 0 to 3, refer to the concentration (density) of small opacities apparent on the radiograph: category 0, small opacities are absent or less profuse than in category 1; 1, opacities are few in number; 2, opacities are numerous; 3, opacities are very numerous and obscure the normal radiographic markings.
Large Opacities
Complicated pneumoconiosis or progressive massive fibrosis (PMF) is divided into categories A to C based on the size of the large opacities. To be classified as PMF, at least one nodule should be 1 cm or greater in diameter.
A—Sum of diameters of lesions is not more than 50 mm. B—Total area occupied by lesions is not greater than the area of the right upper zone. C—Total area occupied by lesions is greater than the area of the right upper zone.

* Small opacities are defined by their average size and profusion.

Clinical Features

Simple CWP and category A–complicated CWP are not associated with respiratory symptoms. As in most populations engaged in manual work, breathlessness and cough in coal miners usually are a consequence of cigarette smoking. However, coal mine dust may itself cause chronic bronchitis and chronic obstructive pulmonary disease, which together are known as industrial bronchitis. The evidence that coal dust exposure is associated with the development of significant respiratory impairment has led to its designation as a compensable disease despite the absence of CWP.

By contrast, patients with complicated pneumoconiosis (i.e., PMF) of categories B and C may present with undue breathlessness and productive cough. Melanoptysis is the result of necrosis within the conglomerate, coal-containing lesions that characterize PMF. Progressive, undue exertional dyspnea usually is the dominant symptom, but rarely, breathlessness may be present at rest.

No specific abnormal physical signs are found in CWP. Finger clubbing and fine inspiratory crackles are not features of the disease, so if these are present, another explanation should be sought. Only in a small proportion of severe cases of complicated disease does CWP evolve to produce chronic respiratory failure and cor pulmonale.

Other than PMF, CWP may be associated with a number of other disorders—most notably the autoimmune disorders of rheumatoid disease and progressive systemic sclerosis. The combination of rheumatoid disease and CWP is known as Caplan syndrome (Figure 51-5). This diagnosis is suggested by the association of coal dust exposure, rheumatoid arthritis, and multiple well-defined large, rounded opacities (nodules greater than 10 mm in diameter) on the chest radiograph. Spontaneous disappearance is common, with or without initial cavitation, and new nodules frequently emerge in different locations. The Caplan nodule also is more likely to cavitate, thereby producing a concentric ring pattern, so this lesion also is known as a necrobiotic nodule. Central necrosis is rare in nodules of CWP, although it may occur in conglomerate lesions.

Figure 51-5 Caplan syndrome. High-resolution computed tomography scan obtained at the level of the upper lobes shows bilateral parenchymal micronodules and coalescence in the right upper lobe. Note the cavitation of the nodule in the left upper lobe.

CWP has been linked with a number of specific infections, the most prominent of which has been tuberculosis. Silicosis is recognized to increase the risk for pulmonary tuberculosis, which can be difficult to diagnose in patients with preexisting abnormalities on plain chest radiographs. Diagnostic skin tests also may show lesser sensitivity in silicotic patients, owing in part to their age and general condition. Silicotic patients can suffer from malnutrition in later stages of the disease because of chronic respiratory failure and elevated metabolism. Protein deficiency may impair immunoglobulin production and thus lessen the skin tests’ sensitivity (as they rely on adaptive immune response). In these particular patients, interferon-gamma assays could be used to improve diagnostic performance. In contrast with silicosis, however, CWP does not increase significantly the risk for infection with Mycobacterium tuberculosis. The association observed with coal mining (and hence CWP) in some countries appears to have been a consequence only of close contact during long hours of work in the confined mine environment. Nontuberculous mycobacteria, on the other hand, may infect lungs damaged by CWP and other types of pneumoconiosis with greater-than-usual frequency, so CWP does seem to increase the risk for infection with opportunistic organisms. Mycobacterium avium probably is the most important of these and is poorly sensitive to antibiotic agents. Mycobacterium kansasii and Mycobacterium malmoense also may be pathogenic in this setting. It may similarly be difficult to attribute change in the radiographic appearances to advancing infection or worsening PMF. Experimental studies also suggest that mycobacterial infection is a factor that helps explain the progression from simple to complicated pneumoconiosis.

Other opportunistic infections reported in association with CWP have included nocardiosis, sporotrichosis, and cryptococcosis. Aspergillus spp. have been noted to colonize cavities in conglomerate lesions of complicated CWP.

A further association with complicated CWP, if manifested by bullous emphysema, is spontaneous pneumothorax. The advanced stages of complicated CWP also are associated with recurrent episodes of acute and subacute bronchitis. Persistent productive cough is common in coal miners in the absence of CWP. No evidence of a causal relationship between silicosis and carcinoma of the lung has been found, although association is consistent for silicotics and limited nonsilicotic workers. The available data leave open the issue of whether silica per se materially increases lung cancer risk in absence of silicosis. In 1997, the International Agency for Research on Cancer (IARC) classified crystalline silica dust exposure as a known human carcinogen, group 1. Currently, CWP and silicosis should be considered conditions that predispose workers to an increased risk of lung cancer.

Accelerated silicosis is rare and is clinically identical to the classic forms of silicosis, except that the time from initial exposure to the onset of disease is shorter and the rate of progression of disease is dramatically faster.

Acute silicosis is rare. Presenting signs and symptoms include cough, weight loss, and fatigue, with rapid progression to fulminant respiratory failure over several months in some cases. Chest auscultation reveals diffuse crackles, and in patients with this clinical picture, the rapid development of cor pulmonale eventuating in respiratory death is characteristic. Survival after the onset of symptoms often is less than 2 years. Diffuse alveolar filling, most apparent at the bases, is the most prominent finding on the chest radiograph (Figure 51-6). Although serial chest radiographs from workers with this illness have been infrequently reported, it seems that the bibasilar filling pattern progresses into large opacities located in the middle zones rather than the upper zones.

Figure 51-6 Chest radiograph showing alveolar filling in acute silicosis. The patient was a 28-year-old woman who had inhaled fine particles of silica from abrasive powder. The clinical disease that developed was rapidly progressive silicosis, which typically is fatal over the next several years.

Chest Radiology

The radiographic pattern in simple CWP typically is one of small rounded opacities that appear first in the upper zones (Figure 51-7). The middle and lower zones become involved as the number of opacities increases. The nodules increase in profusion with increasing dust exposure; a change in profusion after dust exposure has ceased is very unusual. Calcification of the nodules may occur (in 10% to 20% of cases).

Figure 51-7 Chest radiograph appearance in simple coal worker’s pneumoconiosis. The patient was a 55-year-old man who had worked for 18 years in an underground coal mine. He was a nonsmoker with a chronic cough and normal lung function. The radiograph shows a diffuse distribution of small rounded opacities, more prominent in the upper zones than in the lower zones.

Complicated pneumoconiosis is defined as presence of a lesion of 1 cm or greater in longest diameter. The large opacities usually are predominant in the upper lobes, may be unilateral or bilateral, and are symmetrically or asymmetrically distributed (Figure 51-8). The pattern of change in size is variable and unpredictable. In most but not all cases, PMF occurs on a background of simple pneumoconiosis, and it may appear after dust exposure has ceased. Cavitation can develop within a PMF lesion (Figure 51-9), and occasionally a dense peripheral arc or rim may be seen at its lower pole that represents calcification. Dense calcification with the lesion also is sometimes seen. PMF often is associated with bullous emphysema and fibrotic scarring, leading to distortion of the lung and shift of the trachea and mediastinum to the affected side. Irregular, mainly basal opacities also may be seen on standard radiographs. Eggshell calcification is uncommon in CWP but may occur in intrapulmonary, hilar, or mediastinal lymph nodes, possibly because of concomitant exposure to silica. Pleural effusion is uncommon in CWP. Its presence may be related to an associated infection or an interaction with a systemic collagen vascular disease.

Figure 51-8 Chest radiograph appearance in progressive massive fibrosis. The patient was a 66-year-old man who had worked for 30 years in an underground coal mine. The radiograph shows bilateral masses corresponding to the lesions of progressive massive fibrosis.

Figure 51-9 Cavitation within progressive massive fibrosis lesion. High-resolution computed tomography scan shows cavitation of the left lung masses. Note the diffuse micronodulation and associated emphysema.

In simple CWP, computed tomography (CT) shows parenchymal lesions that can be detected in miners with normal-appearing chest radiographs. CT thus shows greater sensitivity than plain radiographs for detection of simple CWP but less obvious benefit with complicated pneumoconiosis. A posterior and right-sided predominance of lesions in the upper zones is evident on these imaging studies.

Nodules usually are observed against a background of parenchymal micronodules and generally are associated with subpleural micronodules. Two categories of lesions can be observed in PMF: lesions with irregular borders that are associated with disruption of the pulmonary parenchyma and lead to typical scar emphysema (Figure 51-10) and lesions with regular borders that are unassociated with scar emphysema. When the lesions are greater than 4 cm in diameter, irregular areas of aseptic necrosis can be observed with or without cavitation (Figure 51-11).

Figure 51-10 Progressive massive fibrosis. High-resolution computed tomography scan shows bilateral masses consistent with progressive massive fibrosis. There is a background of nodules associated with bullous changes around progressive massive fibrosis lesions; this imaging pattern defines paracicatricial emphysema. Note the calcifications within the masses and the lymph nodes.

Figure 51-11 Aseptic necrosis. Computed tomography scan shows bilateral masses with necrosis of the right upper lobe mass and bilateral emphysema.

Two major forms of emphysema occurring in coal workers can be detected on the CT scan: bullous changes around PMF lesions, representing paracicatricial or scar emphysema, and nonbullous lesions, defined as irregular emphysema (Figure 51-12). Lesions of diffuse pulmonary fibrosis can be detected on high-resolution CT as honeycombing or areas of ground glass attenuation. Two specific etiopathogenic mechanisms for fibrosis in coal miners should be considered: (1) a direct effect of deposited coal or silica particles and (2) an indirect effect resulting from an association with scleroderma. In addition, determination of the extent of air trapping on expiratory thin-section CT scans may assess obstructive abnormalities.