Coal workers’ pneumoconiosis (CWP a.k.a. black lung disease) and silicosis 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 can overlap with, and be part of CWP. Imaging changes in coal workers’ pneumoconiosis and silicosis begins with a simple type of disease process in which the interstitial changes appear as small rounded opacities (micronodules) occurring in the upper and 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 nodules results. Although coal workers’ pneumoconiosis looks like silicosis, the silicotic nodules are more likely to be larger and more sharply defined, with the greater likelihood to develop into progressive massive fibrosis (PMF).
Both silicosis and coal workers’ pneumoconiosis cause micronodules, which are of two types – the centrilobular nodules and the subpleural nodules. Subpleural nodules can coalesce and form pseudo-plaques, which may or may not calcify and 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.
The coalescence of micronodules (ax) can eventually evolve into progressive massive fibrosis (PMF) or “Large Opacities”, taking at least five years to develop, measuring 1 cm or more in greatest dimension, where the mass is more homogeneous, losing its nodular margination and even 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 chest walls (and can also be close to and parallel the major fissure). There is associated volume loss and upward distortion of the intrathoracic contents (di), associated with elevation of the hila and resulting 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 having an “angel wings” appearance. PMF can develop micro-calcifications within their masses and rarely when they develop a fibrotic margination, can become 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 progressive massive fibrosis, 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. Severe ventilatory failure, right-sided heart failure, cor pulmonale (cp), from secondary pulmonary arterial hypertension (PAH) and refractory spontaneous pneumothorax (px) are more common in silicosis than in CWP. In coal workers, it often takes 20 or more years in the mines to develop progressive massive fibrosis. PMF can eventually cavitate (cv), especially if it is large, due to ischemic central necrosis, which can rarely perforate a bronchus and spill its black coal related contents into the bronchus, which is coughed up (melanoptysis). However, tuberculosis (tb)/tuberculomas, atypical mycobacterial infection, primary and metastatic carcinoma (ca), nodular sarcoidosis and if there is current rheumatoid arthritis, rheumatoid pneumoconiosis (rp) with Caplan’s nodules must be considered in the differential diagnosis of PMF/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.
More on Coal Worker’s Pneumoconiosis:
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 coal workers’ pneumoconiosis (CWP). Because the profusion of small rounded opacities present on the plain radiographs, in general, are proportional to the amount of coal dust inhaled, they provide an approximate basis for the judgment concerning the disposition of whether worker should remain in the mining environment. In contrast to silicosis, once the miner is removed from further dust exposure, the micronodules usually, but not always in the heavily exposed individual, 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 silica apparently plays a role in the development of progressive massive fibrosis (PMF)/Large Opacities.
There are two basic types of appearances of coal workers’ pneumoconiosis (CWP). The simple type of the disease with small rounded opacities (micronodules) having a posterior upper greater than middle lung zone predominance, and the underlying 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 clearance mechanisms of the respiratory tree are 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 where 10% to 20% can develop an atypical variety of CWP with basilar predominant small irregular opacities, decreased lung volumes and with time, traction bronchiectasis and honeycombing (ho), similar to the appearance of asbestosis. There may or may not be 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 rounded opacity type.
ILO Classification of Coal Workers’ Pneumoconiosis (CWP):
Interstitial (or Parenchymal) Fibrosis/Coal Workers’ Pneumoconiosis: Diffuse, micronodular scarring of the lung tissue from coal dust exposure in the correct pattern and distribution is called Coal Workers’ Pneumoconiosis (CWP) also known as black lung disease. On chest x-rays, this occurs as small rounded opacities (micronodules) involving initially the upper and mid lung zones – classified on the ILO (International Labor Organization) in terms of size and shape, location (upper, middle or lower lung zones) plus profusion (the visual amount of fibrosis per visual unit area). The shape is rounded as opposed to linear/irregular. The size and shape is classified as either “p”, “q” or “r”, depending on the size of the small rounded micronodules. The system of grading the size and shape of the small rounded opacities uses a numerator and a denominator, with the numerator (top number of the fraction) representing the main impression of the reading physician and the denominator (the bottom number of the fraction) representing the second choice (the in addition to) of the reading physician; i.e., p/q means that the physician interprets that the primary size and shape of the small rounded opacities as “p”, but there are in addition, but of a lesser amount, “q” size and shape small rounded opacities. Location is noted – most frequently in the upper and middle lung zones. When recording affected zones, all zones with interstitial changes are recorded, regardless of profusion. The profusion or number of small rounded opacities noted per visual unit area is determined by comparison to a standard reference set of plain radiographs and is rated as either mild, moderate or severe disease by a numbering system put out by the International Labor Organization (ILO) for international epidemiological purposes – 0 for no disease, 1 for mild disease (previously stated to be associated with normal lung markings, that are still visible), 2 for moderate disease (previously stated to be associated with normal lung markings, that are partially obscured) and 3 for severe disease (previously stated to be associated with normal lung markings, that are usually obscured). The system for grading the profusion also uses a numerator and a denominator, with the numerator (top number of the fraction) representing the main impression of the reading physician and the denominator (the bottom number of the fraction) representing the second choice (the tending towards) of the reading physician, i.e., 1/2 means that the physician interprets the primary diagnosis is mild disease, but on second thought it is tending towards moderate disease. The calculation of the overall profusion on chest x-ray involves reviewing all six lung zones for evidence of interstitial fibrosis. It requires a mental averaging of all of the affected lung zones with one caveat – that one removes from the calculation, those zones having 3 sub-categories or more lesser profusion compared to the zone of greatest/highest profusion (or put another way, those zones that have a profusion in which there are two sub-categories or more between them and the zone of greatest/highest profusion are removed from the calculation). As an example, if a patient had small irregular opacities in both mid and lower lung zones of profusion values 1/1, 1/2, 2/1 and 2/2, then the zone having a profusion of 1/1 is removed from the calculation because it is three sub-categories lower than the 2/2 zone of greatest/higher profusion or because there are two sub-categories between 1/1 and 2/2. The overall average profusion would be 2/1. Another example would be 1/1, 1/2, 2/3 and 3/3. The 1/1 and 1/2 zones would be removed and the overall average would be 3/2. The classification of mild, moderate and severe disease is a visual classification unrelated to the patient’s clinical symptoms or physiologic changes.
Progressive massive fibrosis is classified by the International Labor Organization (ILO) as large opacities. Remember that coalescence of nodules in either silicosis or coal workers’ pneumoconiosis totaling 1 cm or less, is not considered progressive massive fibrosis. Progressive massive fibrosis occurs when the large opacities become more ill-defined in terms of their nodular components. The ILO defines coalescence of small opacities (ax) as small opacities with the “margins of the small opacities remaining visible, whereas a large opacity demonstrates a homogeneous opaque appearance.” Coalescence of small opacities may be recorded in the presence or absence of large opacities (progressive massive fibrosis). To identify the complicated type of coal workers’ pneumoconiosis or silicosis, one needs progressive massive fibrosis/large opacities. The ILO defines the large opacity of progressive massive fibrosis as “an opacity having the longest dimension exceeding 1 cm.”
Three categories of large opacities are identified and include:
Category A: A large opacity having its longest dimension greater than 1 cm, but less than 5 cm or several large opacities, each one of which is greater than 1 cm in longest dimension, but the sum of which, does not exceed 5 cm.
Category B: One large opacity having its longest dimension exceeding 5 cm, but not exceeding the equivalent area of the right upper lung zone or several large opacities, each of which exceeds 1 cm in longest dimension, with the sum of their longest dimensions exceeding 5 cm, but not exceeding the equivalent area of the right upper lung zone.
Category C: One large opacity whose longest dimension exceeds the equivalent area of the right upper lung zone or several large opacities, each of, which exceeds 1 cm in longest dimension, whose combined longest dimensions exceed the equivalent area of the right upper lung zone.
In general, in coal workers’ pneumoconiosis, increasing chest x-ray profusion of nodules corresponds to increased 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 occurs. 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 function 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 centrilobular and paracicatricial 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 emphysema and is often considered part of the etiology of the patient’s disability.
Miners who never smoked with simple coal workers’ pneumoconiosis 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 with silicosis) and secondary lung infections.
My Training and Acknowledgments:
The above diagrams and articles are based upon my education, training and experience, as well as from multiple peer-reviewed journal articles, textbooks and lectures.
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 additional “art” vis-a-vis the visual diagrams (no pun intended) to the “science” with the hope that it will help further clarify the learner’s understanding of what Radiologists look for when observing coal workers’ pneumoconiosis (CWP or black lung disease).
Unfortunately, many authors use individualized coined terms and some terms with the same name, mean different things to different authors. There is no easy fix. I have tried to standardize the language. 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.
This article is provided as public service by Daniel Powers, M.D.: B-Reader, Board-Certified Diagnostic Radiologist, Certified by the American Board of Radiology.
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.