Silicosis

Silicosis is caused by inhalation of uncrystallized silica dust and is characterized by nodular pulmonary fibrosis. Chronic silicosis initially causes no symptoms or only mild dyspnea, but over years may progress to involve large lung volumes and lead to dyspnea, hypoxemia, pulmonary hypertension, and respiratory failure.

Diagnosis is based on history and chest X-ray. There is no effective treatment for silicosis other than supportive care and, in severe cases, lung transplantation.

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What causes silicosis?

Silicosis, the oldest known occupational lung disease, is caused by inhalation of tiny particles of silicone in the form of clear "free" quartz (common quartz) or, less commonly, by inhalation of silicates - minerals containing silicone dioxide mixed with other elements (e.g., talc). Those most at risk are those who work with rock or sand (miners, quarry workers, stone cutters) or who use quartz-containing tools or sand grinding wheels (sand miners; glassblowers; foundry, jewelry, and ceramic workers; potters). Miners are at risk of a mixed disease, silicosis and coal workers' pneumoconiosis.

Chronic silicosis is the most common form and usually develops only after exposure over decades. Accelerated silicosis (rare) and acute silicosis may occur after more intense exposures over several years or months. Quartz is also a cause of lung cancer.

Factors that influence the likelihood of developing silicosis include the duration and intensity of exposure, the form of silicone (exposure to the clear form carries a greater risk than the bound form), surface characteristics (exposure to uncoated forms carries a greater risk than coated forms), and the rate of inhalation after the dust is ground and becomes inhalable (exposure immediately after grinding carries a greater risk than delayed exposure). The current limit value for free silica in industrial atmospheres is 100 µg/m3, a value calculated from an average eight-hour exposure and the percentage of silica in the dust.

Pathophysiology of silicosis

Alveolar macrophages ingest inhaled free silica particles and enter the lymphatic and interstitial tissue. Macrophages induce the release of cytokines (tumor necrosis factor TNF-alpha, IL-1), growth factors (tumor growth factor FGF-beta), and oxidants, stimulating parenchymal inflammation, collagen synthesis, and ultimately fibrosis.

When macrophages die, they release silica into the interstitial tissue around the small bronchioles, causing the pathognomonic silicotic nodule. These nodules initially contain macrophages, lymphocytes, mast cells, fibroblasts with disorganized clumps of collagen and scattered biconvex particles that are best seen with polarized light microscopy. As they mature, the centers of the nodule become dense globules of fibrous tissue with the classic onion-skin appearance, surrounded by an outer layer of inflammatory cells.

At low-intensity or short-term exposures, these nodules remain discrete and cause no change in lung function (simple chronic silicosis). But at higher intensity or longer-term exposures (complicated chronic silicosis), these nodules coalesce and cause progressive fibrosis and reduction in lung volumes (VLC, VC) on pulmonary function tests, or they coalesce, sometimes forming large grouped masses (also called progressive massive fibrosis).

In acute silicosis, which is caused by intense exposure to silica dust over a short period, the alveolar spaces are filled with PAS-positive proteinaceous substrates similar to those found in pulmonary alveolar proteinosis (silicoproteinosis). Mononuclear cells infiltrate the alveolar septa. An occupational history of short-term exposure is necessary to differentiate silicoproteinosis from idiopathic changes.

Symptoms of Silicosis

Chronic patients with silicosis are often asymptomatic, but many eventually develop dyspnea on exertion, which progresses to dyspnea at rest. Productive cough, when present, may be due to silicosis, concomitant chronic occupational bronchitis, or smoking. Breath sounds become weaker as the disease progresses, and pulmonary consolidation, pulmonary hypertension, and respiratory failure with or without right ventricular failure in advanced cases may develop.

Patients with rapidly progressive silicosis experience the same symptoms as those with chronic silicosis, but over a shorter period. Similar pathological changes and radiographic features often develop over months or years.

Patients with acute silicosis experience rapidly progressive dyspnea, weight loss, fatigue, and diffuse bilateral wheezing. Respiratory failure often develops within 2 years.

Silicosis conglomerate (complicated) is a severe form of chronic or progressive disease characterized by widespread fibrotic masses typically localized in the upper zones of the lungs. It causes severe chronic respiratory symptoms of silicosis.

All patients with silicosis have an increased risk of pulmonary tuberculosis or nongranulomatous mycobacterial disease, possibly because of decreased macrophage function and increased risk of activation of latent infection. Other complications include spontaneous pneumothorax, broncholithiasis, and tracheobronchial obstruction. Emphysema is often found in areas immediately adjacent to grouped nodules and in areas of progressive massive fibrosis. Quartz exposure and silicosis are risk factors for lung cancer.

Diagnosis of silicosis

The diagnosis of silicosis is based on radiographic data in combination with the anamnesis. Biopsy plays a confirmatory role when radiographic data are unclear. Additional studies are performed to differentiate silicosis from other diseases.

Chronic silicosis is recognized by multiple, round, 1- to 3-mm infiltrates or nodules on chest radiography or CT, usually in the upper lung fields. CT is more sensitive than plain radiography, particularly when helical or high-resolution CT is used. Severity is graded using a standardized scale developed by the International Labor Organization, in which trained examiners evaluate chest radiographs for infiltrate size and shape, infiltrate concentration (number), and pleural changes. No equivalent scale has been developed for CT. Calcified hilar and mediastinal lymph nodes are common and sometimes have an eggshell appearance. Pleural thickening is uncommon unless severe parenchymal disease is not adjacent to the pleura. Rarely, calcified pleural deposits occur in patients with a small volume of parenchymal disease. Bullae usually form around conglomerates. Tracheal deviation may occur if the conglomerates become large and cause volume loss. True cavities may indicate a tuberculous process. Numerous disorders may mimic chronic silicosis on radiography, including welders' siderosis, hemosiderosis, sarcoidosis, chronic beryllium disease, hypersensitivity pneumonitis, coal workers' pneumoconiosis, miliary tuberculosis, fungal pulmonary lesions, and metastatic neoplasms. Eggshell calcification of the hilar and mediastinal lymph nodes may help differentiate silicosis from other pulmonary disorders but is not pathognomonic and is usually not present.

Rapidly progressive silicosis looks like chronic silicosis on x-ray but develops more rapidly.

Acute silicosis is recognized by rapid progression of symptoms and diffuse alveolar infiltrates in the basal regions of the lungs on chest radiographs due to alveoli filling with fluid. On CT, areas of ground-glass density consisting of reticular infiltration and areas of focal consolidation and heterogeneity appear. The multiple round opacities seen in chronic and progressive silicosis are not characteristic of acute silicosis.

Silicosis conglomerate is recognized by confluent darkening more than 10 mm in diameter against the background of chronic silicosis.

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Additional studies for silicosis

Chest CT can be used to differentiate between asbestosis and silicosis, although this is usually done based on the history of exposure and chest radiography. CT is more useful in detecting the transition from simple silicosis to silicosis conglomerate.

Tuberculin skin test, sputum examination and cytology, CT, PET, and bronchoscopy can help differentiate silicosis from disseminated tuberculosis or malignancy.

Pulmonary function tests (PFT) and gas exchange (carbon monoxide diffusing capacity (DL), arterial blood gas analysis) are not diagnostic but help monitor the progression of the disease. Early chronic silicosis may present with reduced lung volumes that are at the lower limit of normal, with normal functional residual volume and capacity. PFT in silicotic conglomerates reveals reduced lung volumes, DL, and airway obstruction. Arterial blood gas analysis shows hypoxemia, usually without CO ₂ retention. Exertion gas exchange studies using pulse oximetry or, preferably, an arterial catheter are among the most sensitive criteria for deterioration in pulmonary function.

Antinuclear antibodies and increased rheumatoid factor are occasionally detected in some patients and are suggestive but not diagnostic of an underlying connective tissue disorder. There is an additional risk of progression to systemic sclerosis (scleroderma) in patients with silicosis, and some patients with silicosis develop rheumatoid arthritis with pulmonary rheumatoid nodules of 3–5 mm detected by chest x-ray or CT.

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Treatment of silicosis

Total lung lavage may be effective in some cases of acute silicosis. Total lung lavage may reduce total mineral load in the lungs of patients with chronic silicosis. Short-term improvement in silicosis symptoms after lavage has been reported in some cases, but controlled studies have not been conducted. Some investigators support the use of oral glucocorticoids in acute and rapidly progressive silicosis. Lung transplantation is a last resort therapy.

Patients with obstruction may be treated empirically with bronchodilators and inhaled glucocorticoids. Patients should be monitored and treated for hypoxemia to prevent pulmonary hypertension. Pulmonary rehabilitation may help patients tolerate daily physical activities. Workers who develop silicosis should be isolated from further exposure. Other preventive measures include smoking cessation and pneumococcal and influenza vaccinations.

How to prevent silicosis?

The most effective preventive measures are those taken at the occupational rather than clinical level; they include dust control, isolation procedures, ventilation, and the use of non-silica containing abrasives. Respiratory masks, although useful, do not provide adequate protection. Monitoring of exposed workers with questionnaires, spirometry, and chest radiography is recommended. The frequency of monitoring depends to some extent on the expected intensity of exposure. Physicians should be alert to the high risk of tuberculosis and non-tuberculous mycobacterial infections in quartz-exposed patients, especially miners. Persons exposed to quartz but without silicosis have a 3-fold increased risk of developing tuberculosis compared with the general population. Miners with silicosis have a greater than 20-fold increased risk of tuberculosis and non-tuberculous mycobacterial infections compared with the general population and are more likely to have pulmonary and extrapulmonary manifestations. Patients exposed to quartz and with a positive tuberculin skin test and negative sputum cultures for tuberculosis should receive standard isoniazid chemoprophylaxis. Treatment recommendations are the same as for other patients with tuberculosis. Silicosis recurs more frequently in patients with silicotuberculosis, sometimes requiring longer courses than usually recommended.