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Asbestosis

Definition
  • Disease onset occurs ≥10 years following the initial exposure to asbestos.
  • Patients may be asymptomatic or have progressive shortness of breath.
  • CXR is the preferred initial test.
  • There is no definitive treatment.
  • Cigarette smoking cessation is important to reduce risk of lung cancer.
  • Prognosis is related to extent of fibrosis noted at diagnosis and past cumulative exposure to asbestos.
Risk Factors
  • Asbestos inhalation
    • Incl. family of asbestos workers
Differential diagnosis
Epidemiology
  • The number of deaths from asbestosis is approximately 1500 per year and there are 10,000 to 20,000 hospitalisations per year.
  • Both have been increasing over the last 30 years. [3] 
  • There is a latency period of around 20 years from time of first exposure to asbestos to development of radiographical changes
  • Exposure was common among:
    • workers in the shipyard, construction, and building maintenance industries
    • production workers of brakes, floor tiles, gaskets, asbestos cement products, fireproof textiles and insulation materials. [4] 
  • Family members of these workers were also exposed because of asbestos contamination of work clothes. [5] 
  • Currently in the US and Europe, the risk of exposure is greatest among construction and/or maintenance workers who work with or around previously installed asbestos. [1]
  • Asbestos use in the US and Europe has markedly decreased (its use is banned in many European countries), in contrast to developing countries where the amount of asbestos being used is increasing
  • Exposures to workers in Africa, Asia and South America are significantly greater than those currently occurring in the US. [6]
Aetiology
  • Asbestos is a fibrous silicate, which exists as a naturally occurring mineral.
    • Chrysotile is the primary asbestos mined.
    • The other 5 types of asbestos that are mined commercially are known as amphiboles and include actinolite, amosite, anthophyllite, crocidolite, and tremolite. [7] 
      • Tremolite is also typically a contaminant of chrysotile.
  • Airborne asbestos particles <10 microns can be inhaled.
    • The more asbestos inhaled the greater the risk of developing asbestosis.
  • The minimum amount required to cause asbestosis is unknown, but a level of 10 fibre/mL-years is a reasonable estimate. [8] 
  • The first legal standard in the US in 1971 was 5 fibres/mL; voluntary standards prior to that allowed up to 30 fibres/mL.
  • Asbestos-related pleural changes can occur at appreciably lower levels than those needed to cause asbestosis.
  • There is no difference in the risk of asbestosis or pleural-related changes by mineral type of asbestos. [9]
Clinical features
  • Dyspnoea on exertion
    • Typically the first sign of asbestosis and increases with progression of disease.
    • Can be absent in patients with early asbestosis and is usually absent in patients with pleural changes alone
  • Cough
    • Dry, non-productive cough; frequency increases with progression.
    • It can be absent in patients with early asbestosis and is usually absent in patients with pleural changes alone.
    • A productive cough may be seen if the patient has also developed COPD. [1] 
  • Crackles 
    • Initially heard at bases and increases with progression of disease.
    • It can be absent in patients with early asbestosis and is absent in patients with pleural changes alone.
    • Not specific to asbestosis. [1] 
    • indirect exposure (uncommon)
  • Chest pain (uncommon)
    • Not typically seen in patients with asbestosis or pleural changes.
    • Symptoms of chest tightness from shortness of breath may be confused with chest pain.
    • Severe unremitting chest raises concern about cancer, particularly mesothelioma.
  • Clubbing (uncommon)
    • Only found in advanced asbestosis, not found with pleural disease alone.
    • Not specific to asbestosis
Pathophysiology
  • When asbestos fibres are inhaled, they deposit at alveolar duct bifurcations and cause an alveolar macrophage alveolitis.
  • These activated macrophages release cytokines, such as tumour necrosis factor and interleukin-1beta and oxidant species, which initiate a process of fibrosis. [4]
  • The initial interstitial fibrosis typically occurs in the lower lobes and may progress to extensive fibrosis and honeycombing.
  • Peri-bronchial fibrosis with a cellular infiltrate may narrow the airway and cause reduced air flow. [4]
  • The presence of asbestos bodies, coating of asbestos fibres that are typically amphiboles not chrysotile with iron and haemosiderin, are useful diagnostically but are rare (<15% with asbestosis). [10]
  • The presence of asbestos fibres is associated with radiographical changes.
  • Since chrysotile is more easily broken down and cleared from the lung than amphibole fibres, the correlation is with the amphibole fibres, such as tremolite, that contaminate the chrysotile. [7]
  • Part of the mechanism of the clearance of asbestos fibres is through lymphatic drainage and the pleural cavities.
  • Mechanical irritation and inflammation is the presumed initiating mechanism for the pleural scarring.
  • Typically few fibres are found in pleural plaques.
  • When present, the fibres are preferentially chrysotile. [11] 
  • The plaques usually occur on the parietal pleura and are acellular collagen that may calcify.
  • When the plaques are found on the visceral pleura, they may extend into an interlobar fissure.
  • Individuals who develop a benign pleural effusion are likely to go on to have diffuse pleural thickening, which involves the visceral pleura. [1] 
  • Diffuse pleural thickening may also occur from confluence of smaller parietal pleural plaques or extension of subpleural interstitial fibrosis.
  • Rounded atelectasis from both parietal and visceral pleural scarring and partial collapse of adjoining lung tissue can mistakenly appear as a tumour on a CXR. [7]
Investigations
  • CXR - posterior-anterior (PA) and lateral
    • It is less sensitive than a CT scan and more specific then pulmonary function testing.
    • Left and right oblique views have been used to increase sensitivity in comparison to PA view to identify pleural changes.
    • The presence of interstitial fibrosis in the lower zones and bilateral pleural thickening is highly specific
    • => Lower zone linear interstitial fibrosis; progressively involves the entire lung; pleural thickening
  • Pulmonary function tests
    • Non-specific.
    • Restrictive: reduced FVC, normal FEV1/FVC ratio, reduced slow vital capacity (SVC), reduced TLC, reduced lung diffusion capacity testing (DLCO).
    • Obstructive changes: reduced FEV1, reduced FEV1/FVC ratio, increased RV/TLC ratio, reduced DLCO.
    • May have mixed restrictive and obstructive changes.
    • Reduced DLCO is the most sensitive pulmonary function change. [1]
  • High-resolution CT chest
    • High-resolution CT chest scan is more sensitive than CXR
    • => Lower zone linear interstitial fibrosis; progressively involves the entire lung; pleural thickening
  • Lung biopsy
    • Open lung biopsy is rarely needed for diagnosis.
    • Its use should be limited to when cancer is suspected or in the absence of a known history of exposure to asbestos.
    • Bronchoscopic biopsy generally provides insufficient tissue to rule out asbestosis and is limited to evaluation for cancer, or if other clinical conditions are suspected.
    • Quantification of the mineral content is the most sensitive procedure.
    • => Interstitial fibrosis; pleural changes with asbestos bodies; increased parenchymal asbestos mineral fibres
  • Bronchial lavage
    • Asbestos bodies maybe found in bronchial lavage fluid and their presence is diagnostic of significant exposure and a high likelihood of asbestos-related disease and or radiographical changes. [17]
    • However, asbestos bodies are unusual and their absence cannot be used to rule out asbestosis
Management

a) conservative
  • Advice on importance of not smoking
b) medical
  • Supportive care
  • Pulmonary rehabilitation ± oxygen therapy
c) surgical
  • Pleural decortication or lung transplant
Prognosis
  • Many patients with asbestosis will not progress and will go on to die from other conditions.
  • A patient with only pleural changes is unlikely to develop asbestosis, as pleural changes occur after a long latency from first exposure and if asbestosis were to occur it is likely to have done so already
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