Silicosis Workup

Although various serologic abnormalities have been noted in patients with silicosis, they are not diagnostic of the disease, and tests to detect these abnormalities are not indicated routinely (see Other Tests).

Humoral immune system abnormalities observed in silicosis include increased incidence and titer of rheumatoid factor, antinuclear antibodies, and immune complexes. No consistent abnormality is noted in the cell-mediated immune system.

Chest radiograph

Radiographic studies of the chest are essential to the diagnosis of silicosis. While some chest radiographic findings of silicosis may be nonspecific, others are sufficiently characteristic of the disease.

Bilateral alveolar filling and a ground-glass appearance mimicking alveolar proteinosis are observed in silicoproteinosis.

Simple silicosis manifests as multiple small (< 10 mm) nodules that are scattered diffusely throughout the lungs but may be more prominent in the upper lung fields. These radiographic findings have been found to correlate well with pathologic findings. Chest radiographic findings in persons with simple silicosis have a high degree of sensitivity and specificity. Calcification of the hilar lymph nodes, particularly in the rim of the nodes (ie, eggshell calcification) is very characteristic of silicosis. However, eggshell calcifications are observed only in a minority of cases of silicosis. Other causes of eggshell calcification include sarcoidosis, histoplasmosis, and irradiation. Rarely, the pulmonary nodules also may show calcification.

Progressive massive fibrosis (PMF) manifests as bilateral upper lobe masses, which are formed by the coalescence of nodules. Cavitation may be seen. As these masses retract toward the hilum because of fibrosis, the lower lung fields may appear overinflated. Hilar calcifications may be present.

CT scan

A high-resolution CT scan has more sensitivity than chest radiography in discerning the nodules of simple silicosis but is unnecessary if the chest radiographic appearance shows characteristic nodules.

In progressive massive fibrosis, a CT scan provides more detail of emphysematous changes, pleural thickening, and the confluent masslike lesions (eg, the presence of cavitation) than a chest radiograph. Although cavitation in silicosis can occur without a mycobacterial infection, such a possibility should be considered when cavitation is observed.

Although pleural effusions are quite rare, pleural thickening is not an unusual finding.

Pulmonary function test (spirometry) results may be normal in simple silicosis. However, silicosis is associated with an excessive decline in lung function. In more advanced disease, airflow obstruction or a mixed pattern of obstruction and restriction is observed. In smokers with airflow obstruction, separating the effect of silicosis from the confounding factor of tobacco smoke is difficult. Progressive massive fibrosis causes severe restriction, decreased compliance, and hypoxemia.

A tuberculin skin test using purified protein derivative (PPD) is indicated in all persons with silicosis. If the test result is positive (ie, 10 mm or more of induration at the site), treatment for latent or active infection is indicated. In individuals who have a positive skin test result, sputum samples should be examined for acid-fast bacilli (AFB) by microscopy and cultured for AFB to identify active disease.

Bronchoscopy rarely is needed. The rare indication is in a cavitary or mass lesion when mycobacterial disease or lung cancer is suspected and examination of sputum samples is nondiagnostic.

The diagnosis of silicosis is based on history of exposure, chest radiographic appearance consistent with silicosis, and an absence of other diagnoses that simulate the radiographic abnormalities of silicosis. Clinical manifestations, symptoms, and physical examination findings provide support to the diagnosis. Thus, examination of lung tissue is seldom warranted.

The initial histopathologic changes of silicosis are pigmented macrophages and reticulin fibers in peribronchial, paraseptal, and perivascular areas.

The characteristic histopathologic finding is the silicotic nodule mostly located near the respiratory bronchiole. The nodule is composed of refractile particles of silica surrounded by whorled collagen in concentric layers, with macrophages, lymphocytes, and fibroblasts in the periphery. Emphysematous blebs surround the silicotic nodule, especially in the subpleural area. Birefringent crystals of silica in the center of a silicotic nodule may be identified by polarized light microscopy. For definitive identification, scanning electron microscopy combined with x-ray spectroscopy may be needed.

In PMF, the masslike areas may show cavitation caused by a necrotic process.

Acute silicosis silicotic nodules are seldom seen and the histology is similar to pulmonary alveolar proteinosis with alveolar filling with proteinaceous material that stains with periodic acid-Schiff stain.