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ii. Asbestosis (Pneumonconiosis due to asbestos exposure)

Asbestosis is a chronic progressive disease that requires high exposures to asbestos over prolonged periods of time and is characterised pathologically by interstitial fibrosis and asbestos bodies.  Changes in industrial hygiene and other asbestos related work  practices has meant that the latency period for the development of asbestosis has lengthened.  Optimal diagnosis depends on a combination of histo-pathological and mineralogical information, although in the absence of  pathology the diagnoses is based on the following factors (Begin, 1998):

1. proven asbestos exposure

2. diffuse lung fibrosis on chest radiograph or CT

3. the absence of other medical illnesses which have similar symptoms and signs

4. an appropriate latency period i.e. time between exposure and disease development

5. changes in lung function - restrictive pattern

6. dry end inspiratory crackles

7. clubbing of the fingers and toes 

The first three points are essential in diagnosing asbestosis (Begin, 1998), whilst the remaining points are confirmatory.

During the initial stages of asbestosis, inhaled asbestos fibre's pierce the epithelium of the alveolar ducts resulting in an almost immediate low grade inflammatory response.  Initially, exposure results in a localised peribronchiolar fibrosing alveolitis but as the fibres migrate into the interstitium between the alveoli and towards the pleura, further inflammation follows causing more widespread interstitial fibrosis.   Disease progression requires chronic exposure to asbestos leading to further inflammation and fibrosis.  An inability to remove the fibre causes continued cellular responses by alveolar macrophages and neutrophils causing increased cytokine and growth factor production, and fibroblast proliferation, all of which elicits scar tissue production and causes irreversible damage to the lung structure.

To assess the severity of asbestosis histologically, pathologists use a grading scheme to determine the extent of fibrosis and the degree of respiratory bronchiole involvement (Craighead et al.,1982). The scheme is as follows:

Grade 0: No fibrosis is associated with bronchioles.
Grade 1: fibrosis involves wall of at least one respiratory bronchiole with or without extension into the septa of the immediately adjacent layer of alveoli; there must still be a zone of nonfibrotic alveolar septa between adjacent bronchioles.
Grade 2: fibrosis as in grade 1; and involvement of alveolar ducts or two or more layers of adjacent alveoli; there must still be a zone of non-fibrotic alveolar septa between adjacent bronchioles.
Grade 3: fibrosis as in grade 2; but with coalescence of fibrotic change such that all alveoli between at least two adjacent bronchioles have thickened fibrotic septa; some alveoli may be obliterated completely
Grade 4: fibrosis as in grade 3; but with formation of new spaces of a size larger than alveoli, ranging up to as much as 1 cm; this lesion has been termed honeycombing.  Spaces may or may not be lined by epithelium.

Conjoined with this grading scheme are three additional grades which include the level of involvement of the respiratory bronchioles and are as follows:

Grade A: Only occasional bronchioles are involved; most show no lesion
Grade B: More than occasional involvement is seen, but less than half of all bronchioles are involved.
Grade C: More than half of all bronchioles are involved.  

Clinically, patients present with dyspnea, dry cough and non-specific chest discomfort.  Dry-end inspiratory crackles are an important physical finding and as the disease worsens patients experience increasing breathlessness.  Lung function shows a restrictive ventilatory defect, with a greater loss of  (FVC) than FEV1.  The fibrosis causes a loss of compliance and a fall in gas transfer.

References:

1. Begin R. 1998. Asbestos-related lung diseases in Occupational Lung Disease: An international perspective Eds D E Banks & J E Parker. Chapman and Hall Medical. UK, pp 234-235

2. Craighead JE, JL Abraham, A Churg et al. 1982. The pathology of asbestos-associated diseases of the lungs and pleural cavities: diagnostic criteria and proposed grading schema. Report of the Pneumoconiosis Committee of the College of American Pathologists and the National Institute for Occupational Safety and Health. Archives of Pathology and Laboratory Medicine106(11):544-596
   

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