The following article is provided as a public service of Daniel Powers, M.D.:
B-Reader, Board Certified Diagnostic & Nuclear Radiologist, Certified by the American Board of Radiology
Imaging Findings in Silicosis/Coal Workers' Pneumoconiosis
Silicosis and Coal Workers' Pneumoconiosis (CWP) are two forms of occupational dust disease that give similar imaging appearances, but are caused by different pathologic changes within the lungs. Silicosis is the more fibrotic of the two entities and can be an isolated disease process or overlap with and be part of CWP. Imaging changes in silicosis and coal workers' pneumoconiosis most often begin with the simple type of the disease process in which interstitial changes appear as small rounded opacities (micronodules) in the posterior upper and superior aspects of the lower, greater than middle lung zones (probably due to lesser lymphatic clearance than in the lung bases). With increasing amounts of coal and/or free silica dust exposure, increased profusion of the nodules results. Although coal workers' pneumoconiosis looks like silicosis, the silicotic nodules are more likely to be larger and more sharply defined, with a greater likelihood to develop into progressive massive fibrosis.
The nodules are of two types - the centrilobular nodules and the subpleural nodules. Subpleural nodules can coalesce to form pseudoplaques, which may or may not calcify and can go on to develop rounded atelectasis (ra). Centrilobular nodules can rarely centrally calcify (cn) and in addition, they can coalesce (ax) and form masses of less than 1 cm in greatest dimension, usually with persistent well-defined nodular margins. This coalescence can eventually evolve into progressive massive fibrosis (PMF or large opacities), taking at least 5 years to develop, measuring 1 cm or more in greatest dimension, where the mass is more homogeneous, losing its nodular margination and eventually developing a fibrous capsule or rim. (PMF in silicosis tends to be matted aggregates of silicotic nodules whereas in CWP it is often more amorphous and homogeneous with a collagen capsule). Once progressive massive fibrosis develops, this is considered the complicated type of the disease process. These masses are commonly bilateral, usually posterior in location in the upper lung zones, have a lentiform or lens/ovoid shape with well-defined lateral margins that parallel the lateral chest walls (and can also be close to and parallel the major fissure). There is associated volume loss and upward distortion of the intrathoracic (di) contents associated with elevation of the hila and resultant lower lung zone compensatory hyperexpansion with the development of centrilobular emphysema (em) and bullae (bu), unrelated to smoking, which can become a significant etiology of symptoms and disability. Bilateral upper lung zone PMF clearly separated from the pleura by aerated lung has been compared to an "angel wings" appearance. PMF can develop micro-calcifications within their masses and rarely because they develop a fibrotic margination, can be rim calcified. Over a 10-year period or more, the areas of progressive massive fibrosis can migrate towards the hila. As they migrate, the scarring and contraction causes the airways to become obstructed and distorted, leading to paracicatricial emphysema along the lateral margins of the masses, associated with fibrous bands/scarring extending to the pleural surfaces. The centrilobular nodules become less apparent or disappear between the lateral margin of the progressive massive fibrosis and the lateral chest wall, which now has intervening emphysema. Silicosis is more likely to cause progressive massive fibrosis (PMF). Severe ventillatory failure and right-sided heart failure from secondary pulmonary hypertension - cor pulmonale (cp) and refractory spontaneous pneumothorax (px) is more common in silicosis than in CWP. In coal workers', it often takes working 20 or more years in the mines to develop progressive massive fibrosis (PMF). PMF can eventually cavitate (cv), especially if large, due to ischemic central necrosis, which can rarely perforate a bronchus to spill black contents, which is coughed up (melanoptysis). However, tuberculosis (tb)/tuberculomas, atypical mycobacterial infection, primary and metastatic carcinoma (ca), nodular sarcoidosis, and if there is concurrent rheumatoid arthritis, rheumatoid pneumoconiosis (rp) with Caplan's nodules must be considered in the differential of the large opacities. Sarcoidosis is the "great mimicker" and can cause micronodules, progressive massive fibrosis and all of the findings often associated with coal workers' pneumoconiosis and/or silicosis. Hilar and mediastinal adenopathy (hi) can develop associated in some cases with eggshell calcifications (eg). This can precede the development of micronodules. In rare instances massive lymph node enlargement can compress the superior vena cava and/or the esophagus. Tracheal deviation can occur if there is only a unilateral PMF lesion or if one PMF lesion predominates.
More on CWP:
Coal is rated according to its heat value and hardness. Anthracite is rated highest, followed by bituminous coal followed by sub-bituminous coal followed by lignite. Anthracite and bituminous coal cause CWP. Because the profusion of small rounded opacities present on the plain radiograph, in general, are proportional to the amount of coal dust inhaled, they provide an approximate basis for the judgment concerning the disposition of whether workers should remain in the mining environment. In contrast to silicosis, once the coal miner is removed from further dust exposure, the micronodules usually, but not always in the very heavily exposed) become stable, without progression. Other factors in addition to dust load influencing CWP development is the hardness of the particles (greatest with Anthracite) and the size of the inhaled particles (1 to 2 micron particles are more likely to cause problems). The profusion is not usually related to the silica content of the coal; although, the silica apparently plays a role in the development of progressive massive fibrosis. There are 2 basic types of appearances of coal workers' pneumoconiosis (CWP). The simple type of the disease, with nodules having a posterior upper greater than middle lung zone predominance, and the complicated type of the disease, which involves the evolution of progressive massive fibrosis (PMF) usually involving the upper lung zone(s). The lesion in coal workers' pneumoconiosis is a coal macule which develops when the clearance mechanism of the respiratory tree is overwhelmed. It consists of dust and fibroblasts resulting from the interactions of dust-laden macrophages primarily affecting the respiratory bronchioles (peribronchiolar).
There is an exception to the imaging appearance rule in some coal workers - 10 to 20% can develop an atypical variety of CWP with a basilar predominant small irregular opacities, decreased lung volumes and with time, traction bronchiectasis and honeycombing (ho), similar to the appearance of asbestosis. They may or may not have associated small rounded opacities elsewhere. These small irregular opacities tend to correlate better than the small rounded opacities with diminished pulmonary function. Coal miners that develop this form of lung fibrosis have an increased incidence of carcinoma compared to the simple small rounded opacity type.
In general, in coal workers' pneumoconiosis, increasing chest x-ray profusion of nodules corresponds to increased coal dust exposure but, does not correspond with greater pulmonary function impairment unless there is progressive massive fibrosis or the small irregular opacity (reticular) pattern subset. Even patients with large opacities/ progressive massive fibrosis measuring less than 5 cm, i.e., of the A-type, are often asymptomatic. The patient's pulmonary functional impairment, whether with the simple or complicated type of the disease process, often correlates better with the amount of chronic industrial bronchitis or associated emphysema. There is increased emphysema of both the paracicatricial and centrilobular types in those individuals having complicated coal workers' pneumoconiosis associated with progressive massive fibrosis, unrelated to smoking. If there is no progressive massive fibrosis and emphysema is present, then smoking plays a more significant role in the etiology of the emphysema and is often considered part of the etiology of the patient's disability.
Miners who never smoked with simple CWP can live a normal life expectancy, although some authors attribute an increased cancer risk of 1.3 to 6.9 times the normal non-exposed population, in part related to or increased with smoking or the radon gas exposure encountered in mining operations or associated with the atypical reticular basal subset of this disease. Complicated CWP usually leads to a premature death. There is also an increased risk of tuberculosis (far less than silicosis) and secondary lung infections.
More on Silicosis:
Silicosis is caused by the inhalation of high concentrations of free crystalline silica. These particles are less than 10 microns in diameter, and those reaching the alveolar spaces and deposited there are usually 1-3 microns in diameter. X-ray and CT pathology increases with the number of respirable free silica particles deposited in the lungs. Silicon Dioxide is responsible for the development of silicosis and can occur in nature in three different crystalline forms: a) quartz, b) cristobalite and c) tridymite. The industrial exposure can occur in such industries as mining, quarrying, tunneling; stonecutting, polishing, cleaning monumental masonry; sandblasting and glass manufacture; foundry work, pottery, porcelain, lining bricks, boiler scaling and vitreous enameling. Silicotic nodules result from a process in which the macrophages ingest the silica particle and are killed by intracellular liberation of enzymes. These materials attract other macrophages and fibroblasts to form a fibrous nodule with an acellular center and at the periphery of the nodule, a concentric laminated ring-like arrangement of collagen fibers, giving it a distinctive "onion skin" appearance on microscopy. These nodules are usually situated near the bronchiolar entrance to the alveoli. Silicosis has both the simple and complicated types of the disease, but these findings occur at a variable time after the exposure to free crystalline silica. There is an accelerated form that occurs within 4 to 10 years (some say 5 to 15 years) after exposure and has a more rapid progression of lung changes and a poorer prognosis and a chronic form that usually occurs greater than 10 and often 20 years (some say 20 to 40 years) after the initial sustained exposure to often lower levels of free crystalline silica and has a slower progression of changes compared with the accelerated type, often being asymptomatic or associated with chronic bronchitis, if of the simple type of the disease. Both can progress to the complicated type of disease, the accelerated form more frequently and sooner than the chronic form of the disease, at which time there are usually clinical symptoms. In addition, there is an acute form of silicosis, which is not found with coal workers' pneumoconiosis. This entity is also known as silicoproteinosis, looking similar to alveolar proteinosis, which can occur weeks or months after exposure and progress. This entity is associated with medial upper lung zone consolidated opacities, some with air bronchograms, can have symmetric hilar and mediastinal adenopathy, volume loss with progressive fibrosis, bullae and distortion of the mediastinal structures. Additionally, pneumothorax (px), recurrent pneumonia and centrilobular nodules may develop. It often occurs in those individuals who have experienced a massive inhalation, usually within an enclosed space such as with sandblasting (but some say can occur with very heavy exposure over a period of less than 5 years). Progression leading to death in 1 to 3 years can occur.
Free crystalline silica forming silicotic nodules is known to be cytotoxic and thus, decreases resistance to invading pathogens, increasing susceptibility to secondary infections. There is a 3x increased risk for tuberculosis in chronic silicosis. Other types of atypical mycobacterial infections are also increased. Infection risk is even greater with the acute more than the accelerated forms of silicosis and in those with progressive massive fibrosis. These patients, especially those with the accelerated form of the disease, also have increased risk for auto-immune connective tissue disorders including scleroderma, systemic lupus erythematosus, potentially rheumatoid arthritis and mixed connective tissue diseases. There is a 1% - 3% increased cancer risk from silicosis.
Silocotuberculosis may be difficult, if not impossible to differentiate radiographically from the changes of complicated silicosis.
Silicosis predisposes to tuberculosis and/or atypical mycobacterial infection. It is almost always present in those individuals with acute silicoproteinosis. In accelerated silicosis, mycobacterial infections are found in approximately one-quarter (25%) of the cases, but either typical or atypical organisms may be responsible for the disease. Atypical organisms often include Mycobacterium kansasii in the Gulf Coast region and Mycobacterium avium-intracellulare (battery bacillus) in the upper Midwest. Although cavitation may occur, these often heal when given anti-tuberculosis chemotherapy. Although tuberculosis is the most likely cause of a cavitating large opacity, it must be remembered that progressive massive fibrosis without tuberculosis or infection by atypical mycobacteria can also cavitate. In accelerated silicosis, the large opacities are often caused by tuberculosis (tuberculomas) and often found in atypical locations, predominantly with lower lobe involvement. PMF versus tuberculomas are difficult to distinguish by plain radiographs, although tuberculosis-caused large opacities can on occasion abut upon the pleura and incite pleural thickening unlike PMF. Often the tuberculomas resolve with tuberculous chemotherapy with only the silicotic progressive massive fibrosis and/or larger silicotuberculous lesions remaining.
Rheumatoid Pneumoconiosis - Caplan's Syndrome:
Coal miners with rheumatoid arthritis, who are exposed to free silica in addition to coal dust, can develop pulmonary nodules called Caplan's nodules more rapidly than and before the onset of progressive massive fibrosis. Small rounded opacities, if present, are often of low profusion. Caplan's nodules lesions resemble metastases on chest x-rays and can occur in various sizes (0.5 to 5 cm, averaging 1 to 2 cm in diameter) and in various locations throughout the lungs, often occurring at the junction of the middle and outer 1/3 of the lungs and frequently bilateral (20% are unilateral). Some cavitate or develop calcifications. Pleural effusions may develop. Recurrent nodules can occur with clinical flare-ups of extra-thoracic manifestations of rheumatoid disease such as general malaise and aggravation of joint symptoms, although clinical disease may occur before, during or after the pulmonary changes. The Caplan's nodules contain silica and other dust, if exposure is mixed and show characteristic rheumatoid reactions with palisading inflammatory cells surrounding a zone of central necrosis. The diagnosis of rheumatoid pneumoconiosis is usually based upon clinical findings and serological changes. (Patients with rheumatoid arthritis, but without CWP or a coal working/miner history can develop necrobiotic lung nodules, which look similar, but are histologically different from Caplan's nodules). In addition, miners exposed to silica appear to be more likely to develop progressive systemic sclerosis (systemic or diffuse scleroderma), a type of autoimmune connective tissue disease, which ordinarily is more common in women. Sandblasters with accelerated silicosis have shown an increased incidence of auto-immune connective tissue disorders which approaches 10%. In addition to rheumatoid arthritis, systemic lupus erythematosus and localized and diffuse scleroderma complicating silicosis have been identified as well. These complicated varieties of silicosis usually have an accelerated course, but in the case of generalized scleroderma, sometimes they have been suppressed or stabilized with steroids.
My Training and Acknowledgements:
The above diagrams and article are based upon my education and training, multiple peer review journal articles and textbooks, as well as lectures.
My initial training for chest disease was at L.A. County/USC Medical Center. In private practice, I self-studied and passed the Federal B-Reader Certification testing in 1984 and re-certified every four years thereafter. I also independently tutored with Dr. Gordon Gamsu at the University of California at San Francisco, when he was first publishing his articles on the CT and HRCT findings in pneumoconioses. Based upon his training and my understanding of his information, I began performing large-scale CT/HRCT imaging studies utilizing terminology learned through him and from various course lectures, journal articles and textbooks. Multiple other authors and educators have contributed to my understanding as have their lectures, articles and textbook chapters. Recent experiences have included the latest American College of Radiology Symposium on Radiology of the Pneumoconioses, presented between March 5-8, 2004, and re-certification as a Federal Government "B-Reader" during that same period. Also included in the review were the "Guidelines for the Use of the ILO International Classification of Radiographs of Pneumoconiosis", Revised Edition 2000, booklet #22 from the International Labor Office in Geneva. I also attended the Radiological Society of North America's 90th Scientific Assembly and Annual Meeting, November 28 - December 3, 2004 and Chest Imaging for the Clinician and Radiologist, 2005, presented by the American College of Chest Physicians, June 10 - 12, 2005, in which multiple lecturers contributed information including Theresa McCloud, M.D., David P. Naidich, M.D., and W. Richard Webb, M.D. There was a syllabus with that course. Other texts utilized included The CIBA Collection of Medical Illustrations, Volume 7, Respiratory System, illustrated by Frank Netter, M.D. (1979); Diseases of the Lung, Radiologic and Pathologic Correlations, by Muller, Fraser, Lee and Johkoh, published by Lippincott, Williams, and Wilkins, in 2003; Thoracic Imaging, by W. Richard Webb and Charles B. Wiggins, published by Lippincott, Williams and Wilkins, in 2005; and the Fourth Edition, Imaging of Diseases of the Chest, by Hansel, Armstrong, Lynch and McAdams, published by Elsevier Mosby, in 2005. Additional anatomic texts included, Clinically Oriented Anatomy, Fourth Edition by Moore and Dalley, published by Lippincott, Williams and Wilkins in 1999 and Chest Atlas, Radiographically Correlated Thin-Section Anatomy in Five Planes by Littleton and Durizch, published by Springer-Verlag in 1994. Special thanks Jeff Miller who helped put my ideas for the modified ILO-like forms into Microsoft Publisher®, to Hide Konishi who worked with me on making the visual diagrams come into reality on Adobe Illustrator® and Bill Malin who completed the web presentation.
My Thoughts and Wisdom:
Medicine is an "art" based upon "science." We as educators and clinicians are constantly learning and updating our teachings and knowledge base. Certainly, by my presentation, I have added addition "art" (the visual diagrams) to this science with the hope that it will help further clarify the learner's understanding of what Radiologists look for when observing emphysema.
Unfortunately, many authors use individualized coined terms, many of which overlap and some with the same name, that mean different things to different people. There is no easy fix. I have tried to standardize the language.
Regarding measurements and statistics - It must be remembered that common sense must prevail when using numbers and that there are false positive (overcalls) and false negatives (undercalls), plus there are always exceptions to the rules. There is no absolutely correct statistic, since involved study groups and technology change and individual patients have their own select responses to insults. The quoted statistics are for understanding general concepts only. The bottom line is, that imaging studies are part of the Sherlock Holmes investigation of a patient's disease - identifying if the anatomy is normal or abnormal and trying to direct one to a specific diagnosis when possible. Classifications of emphysema visually overlap both visually and in terms of originating insult and thus, the significance and type of emphysema observed is made based upon the patient's history including whether he or she smoked, the clinical examination, pulmonary function testing and imaging findings, taken all together.
If you detect any errors, have additional information to point me to, use other useful terms or have comments please do e-mail them to me at firstname.lastname@example.org.
Daniel Powers, M.D.
American Board of Radiology Certified
Federal Government Certified "B-Reader"