ASBESTOSIS

Definition

Fiber Types and Occupational Exposure

• Chrysotile (most common in the US): wavy, long fibers that tend to split longitudinally, multiplying the asbestos effect even after exposure stops
• Amphiboles (crocidolite, amosite): needle-shaped, more biopersistent, associated with higher rates of asbestosis, mesothelioma, and lung cancer than chrysotile alone

Fishman's Pulmonary Diseases and Disorders, Ch. 85; Murray & Nadel, Ch. 101

Pathogenesis

1. Macrophage activation: Alveolar macrophages engulf fibers but fail to clear them completely. Long fibers (>5 µm) resist complete phagocytosis and induce apoptosis of macrophages.
2. Cytokine release: Activated macrophages release PDGF, IGF-1, TGF-β, IL-1β, TNF-α, IL-8, fibronectin, and oxygen free radicals. These stimulate fibroblast proliferation, chemotaxis, and collagen synthesis.
3. Asbestos bodies: Fibers coated by iron and hemosiderin form ferruginous (asbestos) bodies - beaded, clubbed structures identifiable on light microscopy. The vast majority of fibers remain uncoated in the lung.
4. Fibrosis: If the retained dust load is high and macrophage phagocytosis is incomplete, fibroblasts are recruited, proliferate, and lay down collagen, leading to irreversible chronic interstitial fibrosis.

Fishman's Ch. 85; Murray & Nadel Ch. 101

Pathology

• Early/mild: Foci of peribronchiolar fibrosis with chronic interstitial inflammation, air-space macrophage accumulation, and type 2 alveolar epithelial cell proliferation. Architecture of surrounding parenchyma is preserved.
• Progressive: Fibrosis spreads to involve 2nd and 3rd order bronchioles and alveolar ducts.
• Advanced: Lungs are small, with streaks of fibrosis outlining lobar and interlobar septa, thickened visceral pleura, and honeycombing subpleurally and in lower lobes.

Murray & Nadel, Fig. 101.7 - (A) Early asbestosis with asbestos body (arrow) and preserved parenchymal architecture. (B) Advanced diffuse interstitial fibrosis with honeycombing and asbestos bodies in air spaces (arrow). [Courtesy National Institute for Occupational Health, South Africa]

Murray & Nadel Ch. 101

Clinical Features

• Dyspnea on exertion - earliest and most consistently reported symptom
• Persistent cough (often spasmodic, with sputum production)
• Chest tightness; wheezing may occur
• Symptoms develop insidiously; may appear or worsen after exposure ceases

• Bilateral basal rales (crackles) - the most distinctive physical finding; late to pan-inspiratory, heard best at posterior lung bases, NOT cleared by coughing. They first appear in the mid-axillary line at the bases and spread posteriorly. In surveys, ~83% of patients with higher radiographic categories had bilateral rales.
• Digital clubbing - seen in ~52% of cases (22 of 42 patients in one clinical series)
• Signs of cor pulmonale in advanced disease

Fishman's Ch. 85

Pulmonary Function Tests

• Reduced FVC and total lung capacity (TLC)
• Reduced DLCO (diffusing capacity for CO) - may be abnormal even before spirometric changes
• FEV1/FVC ratio: generally well preserved (distinguishing from obstruction)
• Arterial hypoxemia at rest or on exercise
• In one longitudinal study of 77 workers, mean annual decline: FVC = 92 ± 28 mL/year, FEV1 = 66 ± 22 mL/year, TLC = 14 ± 53 mL/year

Fishman's Ch. 85

Chest Imaging

• Irregular opacities (s, t, u shapes) predominantly in lower lung zones (in contrast to silicosis with upper zone nodular opacities)
• ILO profusion categories 1/0 through 3/+ indicate progressive disease
• Pleural plaques (calcified or non-calcified) - parietal pleura, diaphragm, mediastinal pleura
• Diffuse pleural thickening - continuous pleural opacity, commonly blunts costophrenic angles

• More sensitive than plain radiography for early parenchymal fibrosis
• Findings: subpleural irregular lines, intralobular lines, honeycombing in lower lobes, traction bronchiectasis
• Demonstrates pleural plaques, diffuse pleural thickening, and their relationship to parenchymal disease
• Can identify rounded atelectasis (pseudotumor): characterized by a comet tail of vessels and bronchi sweeping into a wedge-shaped pleural-based mass, most often in the lower posterior lung - caused by folding over of fibrotic pleura trapping underlying lung

Fishman's Ch. 85; Murray & Nadel Ch. 101

Asbestos-Related Pleural Manifestations

Diagnosis

1. Adequate exposure history to asbestos (with appropriate latency)
2. Radiologic evidence of bilateral irregular lower zone opacities ± pleural disease
3. Restrictive physiology with reduced DLCO
4. Physical findings: bilateral basal crackles, digital clubbing
5. Exclusion of other causes of pulmonary fibrosis

Complications and Malignancy

1. Lung cancer: Risk is multiplicative (not merely additive) with cigarette smoking. The relative risk of lung cancer is substantially elevated in smokers with asbestosis. Low-dose CT surveillance is recommended for asbestos-exposed workers with smoking history >30 pack-years and age >55 years (annual basis).
2. Malignant pleural mesothelioma: 85% of affected men and 50% of women have an occupational asbestos history. Latency is typically >20-30 years. Histologic types: epithelial (50%), sarcomatous (20%), biphasic/mixed (30%). Differentiation from adenocarcinoma: mesotheliomas stain positive for hyaluronic acid (Alcian blue, colloidal iron stain), are negative for CEA and B72.3, and are TTF-1 negative.
3. Connective tissue disease: Asbestos exposure is associated with increased risk of autoimmune diseases.

Fishman's Ch. 85

Management and Prevention

• No established specific treatment alters the course of asbestosis. There is no proven disease-modifying therapy.
• Patients with end-stage disease may be considered for lung or heart-lung transplantation (though technical difficulties arise due to associated pleural disease).
• Smoking cessation is the single most important modifiable risk factor - smoking multiplies the lung cancer risk and may contribute to initiation and progression of parenchymal fibrosis.
• Medical surveillance: Annual low-dose CT for high-risk individuals (>30 pack-years, age >55).
• Further asbestos exposure should be avoided once the diagnosis is established - even without additional exposure, disease may progress because of the dust load already in the lungs.
• Occupational and employment advice should be given; notification of asbestos-related disease is mandatory in many jurisdictions.
• Workers with pleural fibrosis or plaques are at increased risk for parenchymal fibrosis, mesothelioma, and lung cancer and require ongoing surveillance.

Murray & Nadel Ch. 101; Fishman's Ch. 85

Key Differentiating Points (Exam-Relevant Summary)