Silicosis and Silicatosis

Silicosis: Silicosis is a group of lung diseases that develop following the inhalation of crystalline silica dust (SiO2), usually quartz, but sometimes cristobalite or tridymite--other forms of crystalline silica [1-3]. Examples of occupations with exposure to silica include mining (e.g., coal [4], gold), tunneling, stonework, foundry work, sand blasting, and manufacture of ceramics. Chronic, accelerated, and acute forms occur, depending on dose and lag period since onset of exposure [3].

Definitions of Silicotic Diseases [3, 5]

Clinical features: Patients with chronic disease may be asymptomatic with an abnormal chest radiograph or have dyspnea. In some cases, the onset of dyspnea signifies a complication, such as progressive massive fibrosis (PMF), tuberculosis, or airway disease. Cough may accompany the disease or signify chronic bronchitis, tuberculosis, or lung cancer. In chronic silicosis, lung function may be normal, or there may be an obstructive, restrictive, or a mixed obstructive/restrictive pattern. Impairment of function is more rapid in accelerated disease. In acute disease, impairment of gas exchange is a prominent feature [3].

Radiographic changes: Classification of chest radiographs according to the International Labor Organization scheme for descriptive, epidemiologic purposes also aids in clinical evaluation of the patient by confirming work relatedness and establishing prognosis. Opacities are classified and graded by shape, size, and profusion. CT and HRCT scans are more sensitive for early detection of complicated disease [3]. In simple silicosis, nodules up to 1 cm in diameter are more prevalent in the upper than lower lobes. Coalescence to form nodules >1 cm in diameter signifies PMF, a complication of simple chronic or accelerated silicosis. Radiographs of acute silicosis show air space opacities.

Lymph node involvement ± calcification is prominent in silicosis. So-called "eggshell calcification" occurs in 3-6% of miners with silicosis. For diagnosis, peripheral solid or broken calcification up to 2 mm thick must be present in two or more nodes >1 cm in diameter. At least one ring shadow must be complete. "Eggshell calcification" has also been described in sarcoidosis, Hodgkin's disease after radiation, blastomycosis, histoplasmosis, scleroderma, and amyloidosis [6].

Gross features: Depending on the type and amount of pigmented dust deposited with the silica, the discrete nodules of simple silicosis may be pale (little dust), brownish (iron oxides), or black (coal dust). In complicated disease, nodules enlarge and become confluent, usually centrally in the upper lobes. Confluent lesions greater than 2 cm in diameter histologically or 1 cm in diameter radiographically are considered to represent PMF [5]. Extrathoracic silicotic nodules have been described in lymph nodes, liver, spleen, and bone marrow [7].

A. The photo shows a slice of lung from a 61-year-old ceramics worker. It shows diffuse pleural fibrosis (upper right), which involves interlobar fissures; and multiple, hard, black silicotic nodules (arrow marks one), which are confluent in the anterior upper lobe and upper portion of the lower lobe. This is an example of PMF.

Development of the silicotic nodule: Inhaled dust that is mostly less than 1 µm in diameter is deposited in alveoli. Dust that is not cleared is phagocytosed by epithelial cells or macrophages and transported to the interstitium or lymphatics. Silica in lymphatics is carried to hilar or mediastinal lymph nodes where the earliest nodules are formed. In more advanced disease, visceral pleura and parenchyma (upper > lower lobes) are studded with nodules similar to those in the lymph nodes. Early cellular lesions are composed of macrophages, lymphocytes, and plasma cells in the interstitium. With time, fibrosis develops centrally and the cellular periphery expands. The fibrous tissue forms concentric rings that eventually become homogeneous (hyalinized) centrally. Giant cells typical of a granuloma are absent [5].

B. Note the confluence of whorled, hyalinized, fibrous nodules. The actively proliferating edge is composed of pigmented macrophages. Most of the crystalline silica is located in these macrophages.

C. Necrosis, probably ischemic in origin, may occur centrally in nodules as they expand. Necrosis may, however, be the only histologic evidence of tuberculous infection, as typical granulomas with giant cells usually do not occur in the silicotic lung with active tuberculosis.

Silicatosis: Miners and workers handling silicates (minerals with metallic ions bound to silica), such as talc, mica, or kaolin, can develop a diffuse interstitial lung disease after prolonged exposure. Radiographically, it shows diffuse interstitial opacities, rather than nodules. Histologically, the interstitium is widened by pigmented and non-pigmented macrophages and chronic inflammatory cells, and there is disruption of some alveolar walls. Birefringent crystals are frequent. A van Gieson stain for collagen demonstrates little scarring (red stain), although progressive massive fibrosis and honeycombing can result [5].

D. This photo shows the cut surface of a lung with diffuse silicatosis. Air spaces are enlarged, and alveolar walls are stiffer than normal. Patients with this pattern have often been exposed to a mixture of dusts, which may include silica and asbestos, but the brightly birefringent silicate crystals predominate.

Diagnosis: Occupational history and chest radiographs are usually sufficient for diagnosis of uncomplicated silicosis. Biopsies may be performed when the diagnosis is unsuspected (as in mixed dust disease or silicatosis) or for a complication (cancer vs. a conglomerate lesion of PMF) [3, 5].

Course: Uncomplicated silicosis (or mixed dust/silicatosis pneumoconiosis) does not usually decrease life expectancy. Complications include PMF, infection (mycobacteria or fungi), cor pulmonale, pneumothorax, broncholithiasis, and tracheobronchial compression by lymph nodes. Cough, hemoptysis, fever, weight loss, or new radiographic changes suggest infection. Diagnosis should be pursued by cultures of sputum, BAL fluid, biopsies, or pleural fluid. Lung transplant has been performed in patients with accelerated disease [3].

References

1. Beckett W, Abraham J, Becklake M, Christiani D, Cowie R, Davis G, Jones R. et al. Adverse effects of crystalline silica exposure. Am J Respir Crit Care Med 1997; 155:761-765.

2. Becklake M. Pneumoconiosis. In: J Murray, J Nadel (eds): Textbook of Respiratory Medicine, 2nd ed. Philadelphia, WB Saunders 1994:1955-2001.

3. Mossman B, Churg A. Mechanisms in the pathogenesis of asbestosis and silicosis. Am J Respir Crit Care Med 1998; 157:1666-1680.

4. Green F, Althouse R, Weber K. Prevalence of silicosis at death in underground coal miners. Am J Indust Med 1989; 16:605-615.

5. Silicosis and Silicate Disease Committee. Diseases associated with exposure to silica and nonfibrous silicate minerals. Arch Pathol Lab Med 1988; 112:673-720.

6. Gross B, Schneider H, Proto A. Eggshell calcification of lymph nodes: an update. AJR 1980; 135:1265-1268.

7. Slavin R, Swedo J, Brandes D, Gonzalez-Vitale J, Osornio-Vargas A. Extrapulmonary silicosis: a clinical, morphologic, and ultrastructural study. Hum Pathol 1985; 16:393-412.

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