Silicosis in the 1990s, Revisited

Abstract

Silicosis is a fibrotic lung disease that results from exposure to crystalline silica. Rooted in antiquity, silicosis had its highest human toll in the Industrial Age. It remains, however, a tragedy of our time that this preventable disease has not disappeared. In the previous issue of CHEST, Rosenman et al1Rosenman KD Reilly MJ Kalinowsky DJ et al.Silicosis in the 1990s.Chest. 1997; 111: 779-786Abstract Full Text Full Text PDF PubMed Scopus (45) Google Scholar reported that silicosis indeed is not a disease of bygone days. Using a silicosis surveillance system that was part of the Sentinel Event Notification System for Occupational Risks (SENSOR), these authors identified 577 workers between 1985 and 1995 in the state of Michigan. Most cases were identified from hospital discharge records and physicians' reports, while a lesser number were reported through Workmen's Compensation claims. Most were men in their 70s, and 93% were in jobs with silica exposure for at least 20 years. About a third had progressive massive fibrosis (PMF), and another third had advanced (categories 2 and 3) simple silicosis. Most had abnormalities in spirometry, suggesting restrictive or obstructive impairment. The distribution of patients with more severe categories of pneumoconiosis (categories 2, 3, and PMF), as well as those with more severe decrements in pulmonary function deserves comment. Since silicotics with more severe disease or more impaired lung function are likely to be hospitalized more frequently than those with lesser degrees of this illness, it is not surprising that these reports originating from hospitals recognize most cases and those with the greatest impairment. Although the authors observed that never-smokers seemed to have more severe decrements in FVC than smokers and ex-smokers, solid conclusions cannot be made because of possible inaccuracies in self-reported smoking status, as well as the probable variable quality of pulmonary function tests. Perhaps the most important lesson from this paper is the reminder that silicosis can occur in workers exposed for 10 years or less. Forty of these cases were identified, with some workers' exposures beginning as recently as the 1970s and 1980s. These workers were more likely to have done sandblasting, a job apparently associated with the most significant exposures, and a likelihood of accelerated or even acute silicosis. Education remains the key tool in preventing this disease. This, with enforcement of regulatory standards of dust levels in the workplace, has led to a tremendous reduction in pneumoconiosis cases, yet cases continue to occur in industrialized and developing countries. For example, excessive silica exposures occurred in over 40% of the facilities where sampling was done. Few of the workplaces provided medical surveillance. Corrections of these deficits would help protect these workers. How can we help workers with silicosis? Since silicosis is well recognized to progress despite ceasing exposure, the lack of any way to alter the natural history frustrates both the clinician and patient. Over the past 20 years, investigators have tried to develop strategies to interrupt the inflammatory pathways pharmacologically or physically to reduce the dust and/or mediator burden in the lungs of humans and in animal models.2Banks DE Cheng YH Weber S et al.Strategies for the treatment of pneumoconiosis.Occup Med: State of the Art Reviews. 1993; 8: 205-232PubMed Google Scholar Examples of pharmacological interventions include corticosteroids, inhaled aluminum powder, Poly-2[or 4]-vinylpyridine-N-oxide (PVNO), and more recently, tetrandrine, an alkaloid isolated from the root of Stephania tetrandra, which has been used extensively in Chinese medicine. Corticosteroid use has been shown in a small case series to lead to improvements in radiographic and pulmonary function severity for the duration of treatment, yet a long-term effect was not shown.3Sharma SK Pande JN Verma K. Effect of prednisolone treatment in chronic silicosis.Am Rev Respir Dis. 1991; 143: 814-821Crossref PubMed Google Scholar PVNO shows therapeutic effects in animal and human studies4Chen SY Lu XR. Clinical studies on the therapeutic effect of Kexiping on silicosis.in: Proceedings of the therapeutic effect of Kexiping on silicosis. Institute of Occupational Medicine, CAPM Press, Beijing, China1970: 162-165Google Scholar, 5Li YR Yao PP. Studies on the absorption, distribution, and excretion of PVNO-C14. Institute of Occupational Medicine, CAPM Press, Beijing, China1970: 143-156Google Scholar (particularly when administered directly into the lung parenchyma via a bronchoscope),6Zhou Z Wang FY Hu YB et al.Clinical investigations on therapeutic effects of combined use of polyvinylpyridine-N-oxide and tetrandrine.J Labour Med. 1996; 13: 1-6Google Scholar but there remains concerns regarding toxicity.7Hasirci VN Holt PF. Poly-2-pyridine-l-oxide in silicosis therapy: Effect of molecular weight.Int Arch Occup Environ Health. 1977; 38: 177-188Crossref PubMed Scopus (6) Google Scholar, 8Schamdl D. Prufung von polyvinylpyridine-N-oxide auf die carcinogene wirkung bei ratten und mausen.Arzneimittel-Forsch. 1969; 19: 1313-1314PubMed Google Scholar Recent US work has shown that tetrandrine has clinically important antifibrotic activity. Animal studies have shown histologic evidence of significantly less silicosis in those animals given tetrandrine compared to untreated animals receiving silica exposure,9De-Hwa Chao Ma JYC Malanga CJ et al.Multiple emulsion-mediated enhancement of the therapeutic effect of tetrandrine against silicosis.Appl Occup Environ Hygiene. 1996; 11: 1008-1018Crossref Scopus (7) Google Scholar yet again, concerns remain regarding toxicity.10Lu XR Li QR. Clinical studies on the therapeutic effect of tetrandrine on silicosis.Chin J Ind Hyg Occup Med. 1983; 2: 106-110Google Scholar Finally, whole lung lavage has been used as an attempt to reduce the dust and inflammatory cell and mediator burden in the lungs. In data first reported by Mason,11Mason GR Abraham JL Hoffman L et al.Treatment of mixed dust pneumoconiosis with whole lung lavage.Am Rev Respir Dis. 1982; 126: 1102-1107PubMed Google Scholar and more recently in China by Liang,12Liang YP Sun Y Chen CY et al.Clinical evaluation of whole lung lavage for the treatment of coal workers' pneumoconiosis.He Bei Liao Yang (J Hebei Convalescence). 1992; 1: 1-9Google Scholar and in the United States by Wilt,13Wilt JL Banks DE Weissman DN et al.Reduction of lung dust burden in pneumoconiosis by whole-lung lavage.J Occup Environ Med. 1996; 38: 619-624Crossref PubMed Scopus (35) Google Scholar it appears that this procedure can be done safely, and that significant amounts of dust can be removed. Whether this procedure, alone or in combination with pharmacologic intervention, halts or ameliorates the fibrotic process can only be answered by controlled, long-term studies. Cases of silicosis continue to occur. The optimum approach to workers with silica exposure must be two-pronged: an emphasis on education of employers and workers with vigilance by regulatory agencies and an aggressive, collaborative effort by basic scientists and clinical researchers to find ways to halt progression, or even better, to reverse the onslaught of aggressive disease in those unfortunate workers who develop this process despite public health measures.