Description
Pulmonary silicosis is a form of pneumoconiosis, resulting from inhalation of crystalline silica dust. It is considered a fibrotic and progressive lung disease primarily affecting the lung interstitium and hilar lymph nodes. Pulmonary silicosis is most common among workers in industries such as mining, quarrying, and stone cutting, where there is high exposure to silica dust.
Pathogenesis
Silicosis arises from the inhalation of small particles (<10 micrometres in diameter) of respirable crystalline silica. Once inhaled, these particles are engulfed by alveolar macrophages which, unable to process the silica, die and release their intracellular contents, triggering an inflammatory reaction. This inflammatory cascade results in fibroblast proliferation and collagen deposition, leading to fibrosis and the characteristic nodular changes seen in silicosis.
Subtypes
- Chronic silicosis: The most common form, often occurring after 15-20 years of moderate to low exposures.
- Accelerated silicosis: Occurs after exposure to high concentrations of silica dust for 5-15 years.
- Acute silicosis: A rare form, resulting from short-term exposure to very high concentrations of silica dust.
Epidemiology, Risk Factors & Associations
- Exposure to silica dust in industrial settings, such as mining and stone cutting (prevalence depends on occupational health regulations and adherence).
- Smokers are at increased risk of developing silicosis and have worse outcomes.
- Chronic silicosis is associated with increased risk of tuberculosis.
- Silicosis increases the risk of lung cancer (RR 2.2).
Clinical Features
Patients often present with chronic cough, dyspnoea, and fatigue. They may also present with chest pain and weight loss. In severe cases, cor pulmonale and respiratory failure can occur.
Complications
- Progressive massive fibrosis
- Increased susceptibility to tuberculosis
- Lung cancer
- Respiratory failure
- Cor pulmonale
Pathological Features
Histopathology
- Macroscopic: Lungs appear heavy and firm. Silicotic nodules may be seen throughout the lung parenchyma.
- Microscopic: Silicotic nodules show a dense fibrotic centre with concentrically arranged collagen and a cellular periphery. Birefringent silica particles can be identified under polarised light.
Radiological Features
General Features
- Characteristically demonstrates multiple small (2-5 mm) round opacities, predominantly in the upper lobes. Nodules follow a perilymphatic distribution.
- Hilar and mediastinal lymph node enlargement due to reactive lymph node hyperplasia is relatively common. Punctate calcifications in the lymph nodes are common.
- Characteristic eggshell calcifications of lymph nodes, due to calcium salt deposits in the sinus of the marginal lymph node, can be seen in up to 5% of workers with silicosis.
- As the disease progresses, these can coalesce to form progressive massive fibrosis with characteristic conglomerate masses.
CT
- Non-contrast: Shows small perilymphatic nodules, ground-glass opacities, and interstitial thickening. Large conglomerate masses may be seen in progressive disease. Enlarged hilar and mediastinal lymph nodes with or without eggshell calcifications can be identified.
- Contrast-enhanced: Not typically used in the assessment of silicosis.
MRI
- Not typically used in the assessment of silicosis.
Grading and Staging
Silicosis is not typically graded or staged.
Diagnosis
Diagnosis requires a history of silica exposure, radiological findings consistent with silicosis, and exclusion of other diseases. Lung biopsy is rarely required but can be used for confirmation.
Differential Diagnosis
- Coal worker’s pneumoconiosis: Silicosis and CWP are indistinguishable radiographically.
- Sarcoidosis: May also show hilar lymphadenopathy and upper lobe nodules, but sarcoid nodules are usually larger and show a ‘galaxy sign’ on imaging. Long-standing sarcoidosis may also exhibit eggshell calcifications.
- Metastatic disease: Usually demonstrates randomly distributed nodules, not upper lobe predominant.
- Treated lymphoma: Can also exhibit eggshell calcifications in lymph nodes.
Management
There is no specific treatment for silicosis. Management is focused on prevention through occupational safety measures, symptomatic relief, and management of complications. Pulmonary rehabilitation and immunisations are recommended. In severe cases, lung transplantation may be considered.