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Silicosis

by Lee S. Newman, MD, MA

Silicosis is caused by inhalation of unbound (free) crystalline silica dust and is characterized by nodular pulmonary fibrosis. Chronic silicosis initially causes no symptoms or only mild dyspnea but over years can advance to involve most of the lung and cause dyspnea, hypoxemia, pulmonary hypertension, and respiratory impairment. Diagnosis is based on history and chest x-ray findings. No effective treatment exists except supportive care and, for severe cases, lung transplantation.

Etiology

Silicosis, the oldest known occupational pulmonary disease, is caused by inhalation of tiny particles of silicon dioxide in the form of unbound (free) crystalline silica (usually quartz) or, less commonly, by inhalation of silicates, minerals containing silicon dioxide bound to other elements, such as talc. Workers at greatest risk are those who move or blast rock and sand (miners, quarry workers, stonecutters) or who use silica-containing rock or sand abrasives (sand blasters; glass makers; foundry, gemstone, and ceramic workers; potters). Coal miners are at risk of mixed silicosis and coal workers’ pneumoconiosis (see Coal Workers’ Pneumoconiosis).

Factors that influence the likelihood of development of silicosis include

  • Duration and intensity of exposure

  • Form of silicon (exposure to the crystalline form poses greater risk than the bound form)

  • Surface characteristics (exposure to the uncoated form poses greater risk than the coated form)

  • Rapidity of inhalation after the dust is fractured and becomes airborne (exposure immediately after fracturing poses greater risk than delayed exposure)

Pathophysiology

Alveolar macrophages engulf inhaled free silica particles and enter lymphatics and interstitial tissue. The macrophages cause release of cytokines (tumor necrosis factor-α, IL-1), growth factors (tumor growth factor-β), and oxidants, stimulating parenchymal inflammation, collagen synthesis, and, ultimately, fibrosis.

When the macrophages die, they release the silica into interstitial tissue around the small bronchioles, causing formation of the pathognomonic silicotic nodule. These nodules initially contain macrophages, lymphocytes, mast cells, fibroblasts with disorganized patches of collagen, and scattered birefringent particles that are best seen with polarized light microscopy. As they mature, the centers of the nodules become dense balls of fibrotic scar with a classic onion-skin appearance and are surrounded by an outer layer of inflammatory cells. In low-intensity or short-term exposures, these nodules remain discrete and do not compromise lung function (simple chronic silicosis). But with higher-intensity or more prolonged exposures (complicated chronic silicosis), these nodules coalesce and cause progressive fibrosis and reduction of lung volumes (total lung capacity, ventilatory capacity) on pulmonary function tests, or they coalesce, sometimes forming large conglomerate masses (called progressive massive fibrosis).

Chronic silicosis is the most common form of the disorder and generally develops only after exposure over decades.

Acute silicosis and the rarer accelerated silicosis are caused by intense silica dust exposure over short periods (several months or years). Mononuclear cells infiltrate alveolar septa, and alveolar spaces fill with a proteinaceous material that stains periodic acid-Schiff (PAS) positive and is similar to that found in pulmonary alveolar proteinosis (silicoproteinosis—see Pulmonary Alveolar Proteinosis). The occupational history of acute exposure is needed to distinguish silicoproteinosis from the idiopathic variety.

Conglomerate (complicated) silicosis is the advanced form of chronic or accelerated silicosis and is characterized by widespread masses of fibrosis, typically in the upper lung zones.

Complications

Patients with silicosis are at risk of other disorders:

  • TB

  • Nocardiosis

  • Lung cancer

  • Progressive systemic sclerosis (scleroderma)

  • Possibly RA

All patients with silicosis are at about a 30-fold increased risk of pulmonary TB or nontubercular mycobacterial disease and are more likely to develop both pulmonary and extrapulmonary manifestations. Increased risk may result from impaired macrophage function and an increased risk of activation of latent infection. People exposed to silica but without silicosis have 3 times the risk of developing TB compared with the nonexposed general population.

Other complications include spontaneous pneumothorax, broncholithiasis, and tracheobronchial obstruction. Emphysema is a common finding in areas immediately peripheral to conglomerate nodules and in areas of progressive massive fibrosis.


Symptoms and Signs

Chronic silicosis is often asymptomatic, but many patients eventually develop dyspnea during exertion that progresses to dyspnea at rest. Productive cough, when present, may be due to silicosis, coexisting chronic occupational (industrial) bronchitis, or smoking. Breath sounds diminish as the disorder progresses, and pulmonary consolidation, pulmonary hypertension, and respiratory failure with or without right ventricular failure may develop in advanced disease.

Patients with accelerated silicosis experience the same symptoms as those with chronic silicosis, but symptoms develop over a shorter period.

Acute silicosis patients experience rapid progression of dyspnea, weight loss, and fatigue with diffuse bilateral crackles. Respiratory failure often develops within 2 yr.

Conglomerate silicosis causes severe, chronic respiratory symptoms.

Diagnosis

  • Occupational history of silica exposure

  • Chest CT or chest x-ray

  • Sometimes tissue biopsy for confirmation

  • Adjunctive tests for distinguishing silicosis from other disorders

Imaging

Silicosis is usually recognized on the basis of chest x-ray or CT appearance in patients with a history of exposure. CT is more sensitive than x-ray, especially when helical CT and high-resolution (thin-section) techniques are used. In most cases, chest CT is preferable because it is more sensitive for detecting silicosis as well as the transition from simple to conglomerate silicosis. Chest CT can also better distinguish asbestosis from silicosis, although this differentiation can usually be made on the basis of chest x-ray and exposure history. In patients who develop RA, 3- to 5-mm pulmonary rheumatoid nodules are visible on chest x-ray or CT.

Chronic silicosis produces multiple 1- to 3-mm rounded opacities or nodules recognized on chest x-ray or CT, usually in upper lung fields. Severity is graded on a standardized scale developed by the International Labor Organization ( International Classification of Radiographs of Pneumoconioses ), in which specially trained readers examine the chest x-ray for size and shape of opacities, concentration of opacities (profusion), and pleural changes. An equivalent scale does not exist for CT appearance. Calcified hilar and mediastinal lymph nodes are common and occasionally resemble eggshells. Pleural thickening is uncommon unless a severe parenchymal disease abuts the pleura. Rarely, calcified pleural thickening occurs in patients with little parenchymal involvement. Bullae commonly form around the conglomerate masses. Tracheal deviation may occur when the masses become large and cause volume loss. True cavities may indicate TB.

Numerous disorders resemble chronic silicosis on x-ray; they include welders’ siderosis, hemosiderosis, sarcoidosis, chronic beryllium disease, hypersensitivity pneumonitis, coal workers’ pneumoconiosis, miliary TB, fungal pulmonary diseases, and metastatic cancer. Eggshell calcifications in hilar and mediastinal lymph nodes may help distinguish silicosis from other pulmonary disorders but are not a pathognomonic finding and are not commonly present.

Accelerated silicosis resembles chronic silicosis on x-ray but develops more rapidly.

Acute silicosis is recognized by rapid progression of symptoms. X-ray findings include diffuse alveolar bibasilar opacities representing fluid-filled alveoli. On CT, areas of ground-glass density consisting of reticular infiltration and areas of patchy increased attenuation and inhomogeneity occur. These areas are best observed on high-resolution (thin-section), spiral CT views. The multiple rounded opacities of chronic and accelerated silicosis are not characteristic of acute silicosis.

Conglomerate silicosis is recognizable by confluent opacities > 10 mm in diameter against a background of chronic silicosis findings.


Adjunctive tests

Sputum culture and cytology, PET, and bronchoscopy all may assist in distinguishing silicosis from disseminated TB or cancer.

Pulmonary function tests and measures of gas exchange (diffusing capacity for carbon monoxide [DL co ], ABGs) are not diagnostic but help monitor progression. Early chronic silicosis may manifest with reduced lung volumes that are at the lower end of the predicted range and with normal functional residual capacity and residual volume. In conglomerate silicosis, pulmonary function tests reveal decreased lung volumes, decreased DL co , and airway obstruction. ABGs show hypoxemia usually without CO 2 retention. Measurement of gas exchange during exercise, using pulse oximetry or preferably an indwelling arterial catheter (see Procedure), is one of the most sensitive measures of pulmonary impairment.

Antinuclear antibodies and elevated rheumatoid factor are detectable in some patients and are suggestive but not diagnostic of a coexisting connective tissue disorder (eg, systemic sclerosis, RA).


Treatment

  • Sometimes whole lung lavage

  • Sometimes oral corticosteroids

  • Rarely lung transplantation

  • Empiric use of bronchodilators and inhaled corticosteroids for obstruction

  • Removal from further exposure

Whole lung lavage may be useful in some cases of acute silicosis. Whole lung lavage can reduce the total mineral dust load in the lungs of patients with chronic silicosis. Some studies have shown short-term reduction in symptoms after lavage, but controlled trials have not been done. Anecdotal evidence supports the use of oral corticosteroids in acute and accelerated silicosis. Lung transplantation is a last resort.

Patients with airway obstruction may be treated empirically with bronchodilators and inhaled corticosteroids. Patients should be monitored and treated for hypoxemia to forestall pulmonary hypertension. Pulmonary rehabilitation may help patients carry out activities of daily living. Workers who develop silicosis should be removed from further exposure.

Management of TB is the same as for other patients with TB except that longer courses are usually recommended because relapse is more common in patients with silicotuberculosis.

Prevention

The most effective preventive interventions occur at an industrial rather than clinical level and include dust suppression, process isolation, ventilation, and use of non–silica-containing abrasives. Respiratory masks provide imperfect protection and, although helpful, are not an adequate solution. Surveillance of exposed workers with respiratory questionnaires, spirometry, and chest x-rays is recommended. Frequency of surveillance depends to some degree on the expected intensity of the exposure. (See also guidelines from the American College of Occupational and Environmental Medicine: Medical Surveillance of Workers Exposed to Crystalline Silica.) Other preventive measures include smoking cessation and pneumococcal and influenza vaccination.

Physicians must be alert to the risk of TB and nontuberculous mycobacterial infections in silica-exposed patients, especially miners. People exposed to silica should have annual tuberculin testing. Those with a positive skin test should have sputum culture for TB. In some cases, CT and bronchoscopy may be needed to confirm TB. Patients with a positive tuberculin test and negative TB cultures should be given isoniazid chemoprophylaxis in keeping with standard guidelines for tuberculin reactors.

Key Points

  • Silicosis is usually chronic, but acute, accelerated, and conglomerate forms are possible.

  • Patients who have silicosis are at risk for pulmonary complications and other disorders (eg, TB, nocardiosis, lung cancer, progressive systemic sclerosis).

  • Base the diagnosis on chest imaging (eg, multiple 1- to 3-mm rounded opacities) in patients with a history of exposure.

  • Consider whole lung lavage and treat supportively.

  • Monitor silica-exposed patients (eg, miners) for TB and nontuberculous mycobacterial infections.

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