B: Advise smoking cessation and avoidance of silica exposure.
Image A - Coronal and axial chest CT angiography images demonstrate
no pulmonary embolism but presence of innumerable perilymphatic and
centrilobular pulmonary nodules predominantly in a mid and upper lung
zone distribution.
Image B - Hematoxylin & Eosin stain from surgical lung biopsy
illustrating a centrally fibrotic and acellular pulmonary nodule
composed of whorled collagen, bordered by lymphocytes and dust-laden
macrophages. Under polarized light, weakly birefringent polarizable
particles (white arrow) and strongly birefringent particles (black
arrow) are visualized. Magnification 20X
Discussion
The correct diagnosis is suggested by the combination of clinical
history of prolonged silica exposure, CT findings, and VATS findings of
weakly birefringent silica crystals. The timing of exposure to silica is
particularly important as the cumulative dose of silica is the most
significant factor leading to silicosis.1 The majority of
times a silicosis diagnosis can be made by the history of silica
exposure and imaging features but other competing diagnoses must also be
excluded (e.g. miliary tuberculosis, endemic fungal infection,
metastatic carcinomas, sarcoidosis, and other fibrotic interstitial lung
diseases). Chest CT is more sensitive than chest radiograph in
detecting conventional features of silicosis such as bilateral air-space
disease with consolidation in the posterior portions of the lungs and
centrilobular nodules.1
Given the need to rule out other processes based on the broad
differential diagnosis generated from this patient’s history and
imaging, a bronchoscopy was conducted. In this case, the bronchoalveolar
lavage culture was negative and the transbronchial biopsy was
non-diagnostic. Therefore, a surgical lung biopsy was performed which
revealed classic, sharply circumscribed nodules with weakly birefringent
silica particles and strongly biregringent silicate particles
visualized under polarized light consistent with a diagnosis of
silicosis. Given the pattern of small innumerable nodules on chest
imaging and normal lung function, the patient was diagnosed with simple
silicosis. The biopsy samples were negative for mycobacterium, fungi,
and malignancy. This is important to note as silicosis is associated
with an increased risk of lung cancer and infections such as
tuberculosis. 1
Silicosis has been considered a risk factor for the development of
rheumatologic disease as silica particles are identified as danger
signals by the inflammasome and lead to activation of T and B
lymphocytes resulting in increases in antinuclear antibodies, rheumatoid
factor, and immunoglobulins. 2 Our patient’s autoimmune
workup during his initial presentation was concerning for existing
autoimmunity and in a follow-up encounter he presented with symptoms of
dry mouth and dry eyes in addition to positive Anti-Ro/SSA, Anti-La/SSB
and was diagnosed with Sjogren’s syndrome.
Currently, there is no treatment for silicosis and patients are
advised to avoid silica exposure and to wear respiratory protective
equipment to minimize the degree of exposure. There is no evidence that
corticosteroid therapy alters the course of the disease without removal
of the offending exposure. Bronchodilators may be considered for
symptomatic patients with airflow obstruction on spirometry testing.
There is no conclusive evidence that whole lung alveolar lavage is
beneficial in these patients.3
There are roughly 2.2 million workers exposed to silica in the US.
There are different types of silicosis: chronic silicosis which develops
after greater than 10 years of exposure and is associated with
progressive massive fibrosis, accelerated silicosis which develops after
5 to 10 years of exposure, and acute silicosis which develops within
weeks to 5 years of exposure and is associated with silicoproteinosis. 1
Given these different degrees of exposure leading to silicosis, it has
been key to reduce occupational exposure limits. In 2014, the US
Occupational Safety and Health Administration (OSHA) lowered the
occupational exposure limit from 0.1 mg/m3 (0.25 mg/m3 for the construction industry) to 0.05 mg/m3 with the hope of limiting silicosis and silicosis-associated illness such as lung cancer.4
Among smokers exposed to silica, the hazard ratio of lung cancer death
is 5.07 (3.41–7.52) versus 1.60 (1.01–2.55) among non-smokers exposed
to the same dose of silica (>1.12 mg/m3).5 Hence, this patient should be advised to avoid further silica exposure and quit smoking to help reduce his lung cancer risk.
References
- Leung CC, Yu IT, Chen W. Silicosis. Lancet 2012; 379(9830): 2008-18.
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Lee S, Hayashi H, Mastuzaki H, et. al. Silicosis and autoimmunity. Curr Opin Allergy Clin Immunol 2017; 17(2): 78-84.
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Greenberg MI, Waksman J, Curtis J. Silicosis: a review. Dis Mon 2007; 53(8): 394-416.
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Deslauriers JR, Redlich CA. Silica exposure, silicosis, and the new
Occupational Safety and Health Administration Silica Standard. What
pulmonologists need to know. Ann Am Thorac Soc 2018; 15(12): 1391-2.
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Liu Y, Steenland K, Rong Y, et al. Exposure-response analysis and
risk assessment for lung cancer in relationship to silica exposure: a
44-year cohort study of 34,018 workers. Am J Epidemiol 2013; 178(9): 1424-33.