61-year-old man went to his physician because of cough and hemoptysis lasting for one week. A lung biopsy had been performed several years previously when the patient had a coronary bypass procedure. Radiographic studies are shown below (Figures 1–4).

DIAGNOSIS: Silicosis.

DISCUSSION

Silicosis falls under the classification of pneumoconiosis. Silicosis, a slowly progressive, chronic process is one of the many diseases produced by dust inhalation. Tissue fibrosis is caused by a reaction to dust that contains silicon dioxide, which is generated in mining, quarrying, or tunneling operations. Approximately 3% of heavy metals miners who have worked >=20 years are affected. At lesser risk for silicosis are stone masons, sandblasters, coal miners, potters, and foundry workers (1). Chronic silicosis is most frequently associated with hard-rock mining, whereas acute silicosis is most commonly caused by sandblasting (2). The patient in this case worked for many years as a painter and sandblaster.

Workers at risk for silicosis are often monitored at intervals with chest radiography, because clinical pulmonary function and chest radiograph findings do not always correlate. Extensive abnormalities may exist without much clinically decreased pulmonary function (3). The reverse also may be true on rare occasions. A silicosis diagnosis requires the combination of appropriate history of silica exposure and characteristic chest radiograph appearance (2). The diagnosis can be confirmed with bronchoalveolar lavage and transbronchial lung biopsy, combined with energy-dispersive x-ray analysis. Daily prednisolone may suppress alveolitis in some cases of chronic silicosis (4).

The pathogenesis of silicosis involves the lung's reaction to inhaled dust. Inhaled silica dust particles <10 ?m in diameter become trapped in alveoli and are engulfed by the phagocytic alveolar macrophages. Lysosomal enzymes escape, resulting in cell death. Released mediators stimulate collagen production, and hyalinization of collagen eventually occurs (1). Acellular nodules and fibrosis develop. The long latency period might be explained by the following hypothesis: adsorbed protein on the silica acts as an antigen and eventually results in an antibody reaction (3).

Four types of silicosis exist: simple, complicated, acute (or accelerated), and Caplan's syndrome. The severity of silicosis, like other pneumoconioses, is quantified in accordance with the 1980 International Labor Organization Classification System. This 12-point scale codifies radiograph changes in a reproducible manner.

Simple silicosis produces no clinical symptoms or changes in pulmonary function. Early findings of silicosis are similar to many other diseases; therefore, careful clinical history of dust exposure is required to suggest the diagnosis. Fibrotic nodules with the characteristic whorled appearance are located mostly in the mid and upper zones of the lungs—the posterior and apical upper lobes and the apical region of the lower lobes. The lung bases generally are spared (3). Radiographically, these nodules appear as multiple, small, rounded opacifications of 1 mm to 10 mm, although the initial 1-mm to 2-mm opacities may be very difficult to detect. Enlarged lymph nodes often precede the finding of nodular opacities. Peripheral calcification of hilar and mediastinal lymph nodes in an “eggshell” pattern classically is seen. (This is the observation referred to in Figure 4.) Cervical and intra-abdominal node calcification occasionally is seen and can help differentiate silicosis from the similarly appearing coal worker's pneumoconiosis (1).

Complicated silicosis is symptomatic, often producing cough and disabling dyspnea. Aggregation of silicosis nodules occurs, forming large conglomerate areas of progressive massive fibrosis. Confluent silicotic nodules >1 cm are the hallmark. Usually appearing in the mid lung zone or in the peripheral upper lobes, they are often vertically oriented and migrate to the hilum. Occasionally, cavitation of these larger nodules from ischemic necrosis or tuberculosis infection occurs. Superimposed tuberculosis or atypical mycobacterial infection can be difficult to detect. The large nodules of complicated silicosis can obliterate bronchi and blood vessels. Emphysematous lung is found in the bases. Overall, however, there is volume loss in the lungs due to conglomeration (1).

Acute silicosis, or silicoproteinosis, occurs over a short time, with a few weeks' exposure to silica dust. This is a rapidly progressive disease with high mortality from respiratory failure. Alveoli fill with a lipoproteinaceous fluid, causing proteinosis. Radiologic features include air-space consolidation in a perihilar distribution, with air bronchograms producing an identical appearance to alveolar proteinosis. Accelerated silicosis occurs with 5 years to 15 years of silica dust exposure (4).

In Caplan's syndrome, large necrotic rheumatoid nodules are superimposed on either silicosis or coal worker's pneumoconiosis, with the latter being more prevalent. The nodules measure 0.5 cm to 5.0 cm and often may calcify or cavitate (1).

High-resolution computed tomography has been used recently to diagnose silicosis by depicting the characteristic nodules as well as fine parenchymal detail. However, conventional 10-mm computed tomography should be used in conjunction with high-resolution computed tomography so as not to underestimate nodule prevalence or misinterpret small nodules as pulmonary vessels. Plain radiography is less sensitive than high-resolution computed tomography for detecting micronodules. Characteristic interstitial changes and lymphadenopathy are detected on the chest radiograph (2). However, bullae formation, emphysema, subpleural nodules, coalescent nodules, and obliteration of vessels in complicated silicosis are more readily seen on computed tomography images.