Silicosis
Marisa Dolhnikoff, M.D., Ph.D.
Fabio R. Tavora, M.D., Ph.D.
Definition
Silicosis is a fibrotic pulmonary disease caused by the inhalation and deposition of crystalline silica particles.1
Incidental silicoanthracotic nodules are common in lymph nodes and occasionally lung parenchyma resected for lung cancer and are of no clinical significance. However, the macrophage reaction may result in increased uptake on positron emission tomography scans and cause false-positive results.2
Epidemiology and Exposure Risks
Silicosis is the most prevalent occupational disease worldwide and an occupational health concern, especially in developing countries.
As sources of silica are common, disease potential is high. Decreases in incidence largely depend on preventive and control measures. From 1975 to 2007, the proportions of White and Black South African gold mine workers with silicosis increased from 18% to 22% and from 3% to 32%, respectively3; in 2013, there were an
estimated 11.5 million workers exposed to silica dust in India.4 A prevalence as high as 54% is reported in Brazilian stone carvers,5 and high prevalence has also been reported in Chinese pottery workers and tin and tungsten miners.6 Although the incidence of silicosis has decreased dramatically in the last decades in countries like the United States and in Europe, where appropriate measures have been taken, more than 100,000 US workers are still in highrisk jobs.7
estimated 11.5 million workers exposed to silica dust in India.4 A prevalence as high as 54% is reported in Brazilian stone carvers,5 and high prevalence has also been reported in Chinese pottery workers and tin and tungsten miners.6 Although the incidence of silicosis has decreased dramatically in the last decades in countries like the United States and in Europe, where appropriate measures have been taken, more than 100,000 US workers are still in highrisk jobs.7
The disease is associated to a number of occupational activities, the greatest risk being associated to mining and processing of stone, well drilling, sandblasting, ceramic and glass production, and constructionrelated activities such as drilling of rock, abrasive blasting of concrete, sawing, hammering, drilling, grinding, and chipping of concrete or masonry.1
Silica is a major component of sand, rock, and mineral ores and is the second most common mineral in the earth’s crust, next to feldspar. The general term silica refers to silicon dioxide (SiO2), which occurs naturally in crystalline, amorphous, and glassy states.8 Crystalline silica, the form of silica that poses the occupational respiratory hazard, is a basic component of soil, sand, granite, and many other minerals. Quartz is the most common form of crystalline silica; cristobalite and tridymite are two other. All three forms may become inhalable size particles when workers chip, cut, drill, or grind objects that contain crystalline silica.
The risk of developing silicosis is related to the concentration of dust-containing silica in workplace air, the percentage of respirable silica in the total dust, the particle size, the nature of the silica (crystalline or noncrystalline), and the duration of the exposure.9 Silicosis is a progressive disease and may appear many years after exposure to silica ceases.3
Exposure to crystalline silica has been also associated with other health effects, including increased susceptibility to tuberculosis, lung cancer, chronic obstructive pulmonary disease, autoimmune diseases, and chronic renal disease.10
Pathophysiology
Pulmonary lesions in silicosis are modulated and triggered by the immune system, involving both innate and adaptive immunities. The activation of receptors expressed on the surface of macrophages such as the macrophage receptor with collagenous structure (MARCO) seems to have a crucial role in silica-induced lung injury and apoptosis, as well as in the pulmonary expression of proinflammatory cytokines such as TNF-alpha.11 Phagocytosis of crystalline silica causes lysosomal damage, activating the NALP3 inflammasome that triggers the inflammatory cascade with subsequent fibrosis.12 Silica also stimulates the release of chemotactic agents, such as platelet-activating factor (PAF), macrophage inflammatory protein 2 (MIP-2), and cytokine-induced neutrophil chemoattractant (CINC) from alveolar macrophages, inducing a pronounced recruitment of inflammatory cells both in the alveolar walls and on the alveolar epithelial surface.11,13
The proinflammatory cytokines released by macrophages and also by neutrophils, mast cells, and B lymphocytes participate in the exaggerated deposition of matrix protein by producing significant levels of profibrotic mediators that stimulate the recruitment and the proliferation of mesenchymal cells as well as regulate neovascularization and reepithelialization of injured tissues. These mediators mainly include fibrogenic cytokines (TNF-alpha, IL-1, and TGF-beta), growth factors such as platelet-derived growth factor (PDGF), the alveolar macrophage form of insulin-like growth factor (IGF), and fibronectin.11 Silica particles also induce the generation of reactive oxygen species (ROS) directly or by stimulation of cells.13 The generation of oxidants by silica particles and silica-activated immune cells results in macrophage apoptosis, lung damage, inflammation, and cell transformation. Silica-induced acute inflammation is also accompanied by thrombosis, suggesting that inhaled silica particles may also induce extrapulmonary lesions.11
Clinical Forms, Pathology, and Radiologic Presentation
There are three main forms of silicosis, depending on the airborne concentration of respirable crystalline silica: (1) acute silicosis (also referred to as silicoproteinosis), (2) accelerated silicosis, and (3) chronic (or classic) silicosis, the most common form of the disease.1
Incidental silicoanthracotic nodules do not form symptomatic lung disease and are not considered a form of silicosis.
Acute Silicosis (Silicoproteinosis)
Acute and accelerated silicosis describe rapidly progressive forms, usually associated with intense silica exposure.14 Acute silicosis is a rare presentation of silicosis and occurs following a heavy inhalational exposure to silica dusts for a few weeks to 5 years. It most commonly affects sandblasters.1 Clinically, it may resemble pulmonary alveolar proteinosis (PAP)15; besides dyspnea and dry cough, constitutional symptoms can be present, such as fever, fatigue, and weight loss. The clinical course is usually progressive, and respiratory failure and death often occur within a few months.1
The pathologic features of acute silicosis differ substantially from the chronic form and resemble those of primary PAP, that is, alveolitis and filling of the airspaces with proteinaceous material that are PAS positive (Fig. 57.1). Minimal collagen deposition and fibrosis can be present.1,16
Despite the pathologic similarities, PAP and silicoproteinosis have distinct imaging features. Silicoproteinosis presents with bilateral dependent consolidation often with areas of calcification and/or ground-glass opacities. Bilateral crazy-paving pattern with areas of geographic sparing, characteristic of PAP, is not usually seen in silicoproteinosis.16,17
Accelerated Silicosis
Accelerated silicosis shares similar clinical and radiographic findings with chronic silicosis but tends to progress rapidly.1,18 It appears after more intense exposures to silica dust and is characterized by an earlier onset (usually 5 to 10 years after exposure) than the chronic form (10 to 15 years or more).19 It can produce symptoms as early as within a year of first exposure, and deaths may occur within 5 years.20 Due to the improvement in working conditions, accelerated silicosis has become uncommon in developed countries. However, serious health threat still exists, especially in small-scale mining in developing countries.19 Patients with accelerated silicosis are at a higher risk of developing
progressive massive fibrosis (PMF) as well as other complications.21 The pathologic picture is similar to that of classic silicosis or may show a combination of acute and chronic features, with areas of alveolar proteinosis associated with the presence of silicotic nodules that develop sooner and are more cellular than in chronic silicosis.18,21 Sequential biopsies may show interstitial inflammation and alveolar proteinosis with minimal fibrosis progressing to diffuse fibrosis within 6 months.21
progressive massive fibrosis (PMF) as well as other complications.21 The pathologic picture is similar to that of classic silicosis or may show a combination of acute and chronic features, with areas of alveolar proteinosis associated with the presence of silicotic nodules that develop sooner and are more cellular than in chronic silicosis.18,21 Sequential biopsies may show interstitial inflammation and alveolar proteinosis with minimal fibrosis progressing to diffuse fibrosis within 6 months.21
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