Pneumoconiosis

Pneumoconioses represent a group of lung diseases caused by the inhalation of occupational inorganic dusts leading to progressive parenchymal lung disease.

1. Silicosis is a fibrotic lung disease attributable to the inhalation of crystalline silica, usually in the form of quartz. Silica dust penetrates to the pulmonary parenchyma, undergoes phagocytosis by macrophages, and causes lysis of the macrophages and the release of substances responsible for pulmonary fibrosis.

Three clinicopathologic types of silicosis have been described: chronic silicosis, accelerated silicosis, and silicoproteinosis. Chronic silicosis is the most common form, which usually follows decades of sustained exposure. Accelerated disease follows shorter, heavier exposures after 3 to 10 years. The pathogenic hallmarks are silicotic nodules, which can become confluent and lead to the development of progressive massive fibrosis. Silicoproteinosis follows intense exposure to fine dust of high silica content and shows pathologic features of alveolar proteinosis. Patients exposed to silica dust are also at increased risk of developing chronic obstructive pulmonary disease (COPD), lung cancer, mycobacterial infection, and immune-mediated diseases, such as scleroderma and rheumatoid arthritis. Occupations at risk of exposure include employment in tunnel and shaft construction, stone pits, metallurgical industry, pottery and porcelain manufacture, and production of heat-resistant and abrasive materials.

Chronic silicosis usually develops after >10 years of exposure and remains asymptomatic for a prolonged period. As fibrosis progresses, patients develop dyspnea and cough, sometimes accompanied by cor pulmonale and respiratory failure. It is important to note that the appearance of breathlessness or worsening bronchitic symptoms may be reflective of a separate disease associated with silica dust exposure, rather than silicosis itself, including COPD, tuberculosis, or lung cancer. The changes of chronic silicosis are irreversible and have a tendency to continued progression despite the discontinuation of the exposure. In acute silicosis, dyspnea may be disabling within months of exposure, followed by impaired gas exchange and respiratory failure.

Additional tests:

1) Chest radiography in chronic silicosis is characterized by fine, round nodular opacities that are usually dense and well demarcated, sometimes with calcifications. Lesions are usually bilateral with upper-lobe predominance. Enlargement of hilar lymph nodes with eggshell calcifications is strongly suggestive of the diagnosis.

2) High resolution computed tomography (HRCT) is useful in identifying early conglomeration or the infectious or malignant complications of silica dust exposure.

3) Pulmonary function tests have value in longitudinal assessment of disease progression. Patients with chronic silicosis often demonstrate a mixed ventilatory impairment with restriction and obstruction related to interstitial lung diseases (ILDs) and comorbid COPD, respectively. Functional impairment is often more closely associated with airflow obstruction than restriction.

Diagnosis is based on significant occupational exposure and radiologic changes.

Differential diagnosis:

1) Other diseases causing upper-lobe nodularity: Sarcoidosis, miliary tuberculosis, lymphangitic carcinomatosis, pulmonary Langerhans cell histiocytosis, respiratory bronchiolitis–associated ILD, or chronic hypersensitivity pneumonitis.

2) Lung tumors: Cancer, tuberculoma.

Treatment: Treatment is focused on stopping the exposure (personal protective equipment) and symptomatic management. Smoking cessation is critical. Lung transplant should be considered in patients with accelerated and acute disease.

2. Coal workers’ pneumoconiosis (CWP) involves focal fibrosis of the pulmonary parenchyma caused by inhalation of coal dust. Coal miners may also develop silicotic nodules when the coal is mined from hard rocks. Coal mining, even in the absence of CWP, is associated with chronic bronchitis and airflow obstruction. Progressive symptoms can develop with progressive fibrosis. Chest radiographs reveal nodules that are less sharply demarcated and less dense than those observed in silicosis; consolidations >3 mm and calcifications within the nodules occur less frequently. Caplan syndrome includes the rapid development of multiple pulmonary nodules 0.5 to 5 cm in diameter visible on chest radiographs accompanied by features of rheumatoid arthritis and positive rheumatoid factor in blood.

3. Asbestosis is a diffuse interstitial pulmonary fibrosis, frequently associated with pleural lesions, which is caused by the inhalation of asbestos fibers. Exposure is most often occupational and may occur in mining, milling, manufacturing, pipefitting, remote work with automobile brake pads, and applying or removing asbestos fiber products. Retention and clearance of fibers is inhomogeneous. Fibers <3 microm are cleared by alveolar macrophages, subsequently translocated to lymphatic channels, and eventually drained into the pleural space. Longer fibers are incompletely phagocytosed by macrophages and are retained in the lungs as asbestos bodies. The fibers induce inflammation that leads to fibrosis.

Clinical manifestations: Patients are normally asymptomatic for at least 20 to 30 years after the initial exposure. Determinants of disease include burden of exposure, duration of exposure, pattern of exposure, age of exposure (young age is associated with a longer fiber residence time), and type of fiber inhaled. Symptoms are similar to other types of interstitial fibrosis (dyspnea on exertion worsening with the progression of lesions; crackles at lung bases may be audible). Changes are irreversible and have a tendency to progress despite the discontinuation of exposure. Pleural changes related to exposure to asbestos may coexist with fibrosis or occur independently. They include nonmalignant lesions: limited pleural thickening (plaques that are usually located on the parietal pleura and have a tendency to develop calcifications) and diffuse pleural thickening (usually affecting the visceral pleura; depending on the extent, it may impair pulmonary function)­, as well as malignant lesions.

Additional tests:

1) Chest radiographs are crucial for diagnosis and reveal irregular basilar-predominant linear shadows. Subpleural parenchymal changes may be obscured by overlying asbestos-related pleural thickening or pleural calcification. Early fibrotic changes are better visualized by HRCT. Mediastinal lymphadenopathy and progressive massive fibrosis are not typically observed. In advanced disease the usual interstitial pneumonia (UIP) pattern (see Idiopathic Pulmonary Fibrosis) may be observed.

2) Pulmonary function tests reveal a restrictive pattern with decreased carbon monoxide diffusing capacity of the lungs (DLCO) and static compliance.

3) Bronchoalveolar lavage using special staining may confirm the presence of asbestos bodies; however, this finding proves only the exposure to asbestos.

Diagnosis is based on radiologic features and the patient’s history of occupational exposure. Differential diagnosis includes idiopathic pulmonary fibrosis, connective tissue disease–related ILD, hypersensitivity pneumonitis, or lymphangitic tumor spread.

Treatment is symptomatic only.

Complications: Asbestos exposure has been also linked to pleural plaques, pleural effusion, malignant mesothelioma, and lung cancer.