Silicosis is currently the most prevalent chronic occupational disease in the world. It is caused by inhalation of crystalline silica, mostly in occupational settings. Workers involved in sandblasting and hard-rock mining are at particularly high risk. Silica occurs in both crystalline and amorphous forms, but crystalline forms (including quartz, cristobalite, and tridymite) are by far the most toxic and fibrogenic. Of these, quartz is most commonly implicated in silicosis. After inhalation, the particles interact with epithelial cells and macrophages. Ingested silica particles cause activation of the inflammasome and the subsequent release of inflammatory mediators by pulmonary macrophages, including IL-1, TNF, fibronectin, lipid mediators, oxygen-derived free radicals, and fibrogenic cytokines. When mixed with other minerals, the fibrogenic effect of quartz is reduced. This fortuitous situation is commonplace, as quartz in the workplace is rarely pure. Thus, for example, miners of the iron-containing ore hematite may have abundant quartz in their lungs yet have relatively mild lung disease because of the protective effect of hematite.
Fig. 13.17 Advancedsilicosis,seeninatransectedlung.Scarringhascontractedtheupperlobeintoasmalldarkmass(arrow). Notethedensepleuralthickening.(Courtesy of Dr. John Godleski,Brigham and Women’s Hospital,Boston, Massachusetts.)
MORPHOLOGYSilicotic nodules intheirearlystagesaretiny,barelypalpable,discrete,pale-to-black(ifcoaldustispresent)nodulesintheupperzonesofthelungs(
Fig.13.17 ).Microscopically,thesilicoticnoduledemonstratesconcentrically arranged hyalinized collagen fibers surroundinganamorphouscenter.The“whorled”appearanceofthecollagenfibersisquitedistinctiveforsilicosis(
Fig.13.18 ).Examinationofthenodulesbypolarized microscopy reveals weakly birefringent silica particles,primarilyinthecenterofthenodules.Asthediseaseprogresses,individualnodulesmaycoalesceintohard,collagenousscars,witheventualprogressiontoPMF.Theinterveninglungparenchymamaybecompressedoroverexpanded,andahoneycombpatternmaydevelop.Fibroticlesionsalsomayoccurinhilarlymphnodesandthepleura.
Fig. 13.18 Coalescentcollagenoussilicoticnodules.(Courtesy of Dr. John Godleski, Brigham and Women’s Hospital, Boston, Massachusetts.) http://ebooksmedicine.net
Silicosis usually is detected in asymptomatic workers on routine chest radiographs, which typically show a fine nodularity in the upper zones of the lung. Most patients do not develop shortness of breath until late in the course, after PMF is present. Many patients with PMF develop pulmonary hypertension and cor pulmonale as a result of chronic hypoxia–induced vasoconstriction and parenchymal destruction. The disease is slowly progressive, often impairing pulmonary function to such a degree that physical activity is severely limited. Silicosis is associated with an increased susceptibility to tuberculosis. It is postulated that silicosis depresses cell-mediated immunity, and crystalline silica may inhibit the ability of pulmonary macrophages to kill phagocytosed mycobacteria. Nodules of silicotuberculosis often contain a central zone of caseation. The relationship between silica and lung cancer is unsettled, but most studies suggest that silica exposure is associated with some increase in risk.