Silicosis and Coal Workers’ Pneumoconiosis

Silicosis

The three main clinical presentations of silicosis are acute silicosis (silicoproteinosis), accelerated silicosis, and classic silicosis, and they are determined primarily by the intensity and the length of exposure to silica. Silicoproteinosis (acute silicosis) is an acute and progressive form of silicosis that often results in death from respiratory failure. This variant of silicosis can develop within a few weeks or months of exposure to very high concentrations of silica and is associated with occupations such as sandblasting, surface drilling, tunneling, silica flour milling, and quartzite milling. The lungs show a ground-glass appearance similar to that of pulmonary edema and alveolar proteinosis. The lesion may consolidate into appearances more characteristic of PMF over a short time. Workers usually present with rapidly progressive dyspnea, cough, and weight loss and develop cyanosis and respiratory failure. Death usually occurs within a short time despite intensive treatment. It has been proposed that acute silicosis occurs in workers exposed to freshly fractured silica dust and that surface Si* and SiO* radicals generated during fracturing play an important role in the rapid onset of this variant of silicosis.

With accelerated silicosis, the exposure time after which the disease becomes clinically evident is much shorter than classic silicosis, ranging from 5 to 10 years of exposure, with the rate of disease progression noticeably faster. The symptom of breathlessness occurs 1 year after exposure, with the patient's condition rapidly deteriorating to hypoxic respiratory failure and death within 5 years. High concentrations of dust in a relatively confined space are thought to predispose an individual to this form of silicosis, which is common in certain occupations, such as sandblasting, stone masonry, and other crushing operations. Except for this aggressive time course, the radiographic, clinical, and pathologic features of this entity are nearly indistinguishable from classic silicosis.

Long-term exposure to low concentrations of silica is associated with a slow progressive nodular infiltration in both lungs, predominantly in the upper lung zones. Classic silicosis is the most common presentation whereby patients remain asymptomatic until after 10 to 20 years of continuous silica exposure, by which time radiographic abnormalities are evident. In contrast to other inhalational occupational lung diseases, lung changes in silicosis often progress even after the individual has been removed from the causative environment. Although most patients are asymptomatic initially, dyspnea on exertion and then at rest is common. A relationship between severity of dyspnea and radiographic abnormalities has been documented. The lung lesions are usually progressive and may result in PMF. PMF is the result of the coalescence and agglomeration of several smaller nodules together with increased profusion and enlargement of nodules. According to the Silicosis and Silicate Disease Committee, a PMF lesion is defined as a lesion greater than 2 cm in diameter, in contrast to the 1 cm or larger radiographic size established by the ILO. Cavitation and extensive destruction of the lung parenchyma, including bronchioles and blood vessels, are common in PMF and may signify superimposed infection from anaerobic bacteria or tuberculosis.

With progressive lung damage, pulmonary hypertension and right heart failure eventually supervene. Emphysema is common in silicosis and has been considered the major cause of cor pulmonale and disability by some investigators. Additionally, risk of other cardiovascular diseases is increased, including stroke, peripheral vascular disease, and congestive heart failure. Pneumothorax can complicate silicosis and occurs more frequently among patients with accelerated or acute silicosis. Other conditions that may complicate silicosis include Caplan syndrome, tuberculosis, carcinoma, and connective tissue disease. The two most serious complications, lung cancer and tuberculosis, may affect the prognosis and natural history of the underlying disease. Chronic silica inhalation is associated with a threefold increased risk of mycobacterial infections (silicotuberculosis). The risk increases with more severe disease in terms of nodular profusion and PMF ; this predisposition depends on the prevalence of tuberculosis in the population from which the workers originate.

Rheumatoid pneumoconiosis (Caplan syndrome) was initially identified in CWP but is now known to occur in silicosis as well, although rarely. The incidence of this disease ranges from 0.48% to 0.74% and is characterized by rapidly developing large opacities (1–5 cm) located mostly in the periphery of the lungs, often with only mild silicosis.

The association between silicosis and lung cancer has been documented in several studies and recognized as an occupational carcinogen by the International Agency for Research on Cancer since 1996. The risk is greatest for workers with established silicosis.

The strength of the association between silicosis and connective tissue disease varies with the type of connective tissue disorder. The risk of developing systemic sclerosis particularly in workers with high exposure to silica dust is well established, although such causal associations between silicosis, rheumatoid arthritis, and systemic lupus erythematosus are less widely reported.

Coal Workers' Pneumoconiosis

Inhalation of coal mine dust can lead to the development of several disease entities, including CWP, bronchitis, emphysema, Caplan syndrome, and silicosis. The two most common patterns of disease found in coal miners are simple CWP and complicated CWP. With increasing duration of coal dust exposure, CWP may progress to complicated CWP, in which the nodules coalesce to form black, rubbery parenchymal masses usually in the upper posterior lungs, resulting in PMF. Progression from simple CWP to PMF has been related to radiographic severity of disease, coal mine dust–exposure level, and total dust burden, with the average transition from simple CWP to complicated CWP occurring over 12.2 years.

CWP is usually asymptomatic, and most chronic pulmonary symptoms in coal miners are attributed to other lung conditions, such as industrial bronchitis from coal dust or coincident emphysema from smoking. The main symptom even in nonsmokers is chronic cough, which persists even after patients leave the workplace. PMF causes progressive dyspnea with production of black sputum (melanoptysis) when the PMF lesion liquefies and ruptures into the airways. As with silicosis, PMF lesions often progress to pulmonary hypertension with right ventricular and respiratory failure.

An association between CWP and features of rheumatoid arthritis is well described. It is unclear whether CWP predisposes miners to developing rheumatoid arthritis, whether rheumatoid arthritis takes on a unique form in patients with CWP, or whether rheumatoid arthritis alters the response of miners to coal dust. Multiple rounded nodules in the lung appearing over a relatively short time (Caplan syndrome) represent an immunopathologic response related to rheumatoid diathesis. In contrast to lesions caused by PMF, which congregate in the upper lobes, these new lesions (known as Caplan lesions) tend to coalesce in the lung periphery. Histologically, the lesions resemble rheumatoid nodules, but they have a peripheral region of more acute inflammation.

As in silicosis, patients with CWP are at an increased risk of developing active tuberculous and nontuberculous mycobacterial infections. Weak associations have been reported between CWP and progressive systemic sclerosis and stomach cancer. Cumulative dust exposure has been shown to have a significant relationship with increased mortality from cancers of the digestive system. It has been suggested that nitrosation of ingested coal dust in the acidic gastric environment could result in the production of carcinogenic products, which may lead to the higher incidence of gastric cancer in coal miners.

Silicosis in coal miners is usually found in conjunction with simple CWP and rarely as an isolated form of pneumoconiosis. It is difficult to distinguish between silicosis and CWP on chest radiography. The prevalence of silicosis in coal miners can be reliably determined only in autopsy studies. In the National Coal Workers' Autopsy Study from 1972 to 1996, pathologic evaluations of 4115 autopsy cases found 23% of coal miners with pulmonary silicosis and 58% with lymph node silicosis.

Pathology

The pathogenesis of most pneumoconioses is a result of chronic inflammation that involves phagocytosis of the inhaled dust by alveolar and tissue macrophages and its deposition in the lung interstitium. Free particulate silica that is not ingested by macrophages enters the perivascular lymphatic channels to be translocated to the draining mediastinal lymph nodes as free particles or within macrophages. Inflammatory cytokines, such as tumor necrosis factor-α and interleukin-1, also are released by damaged epithelial cells and macrophages. These inflammatory mediators destroy the lung parenchyma by attracting other inflammatory cells (macrophages, neutrophils, and lymphocytes), resulting in alveolitis. In vivo and in vitro studies have shown that these silica-exposed macrophages release fibroblast growth factors that facilitate the accumulation of fibroblasts and fibroblast products, which induce inflammatory and fibrogenic reactions in the interstitium, alveoli, and lymph nodes. Various growth factors stimulate fibroblast and type II pneumocyte activity. Collagen and fibronectin production rapidly increase and eventually lead to fibrosis. Animal models have shown that even after the exposure to silica ceases, dust-laden macrophages continue to produce inflammatory mediators, such as interleukin-1β and tumor necrosis factor-α, propagating the inflammation-fibrosis cycle.