Asbestosis

Definition

There are several diseases associated with asbestos exposure which can affect the lungs or pleura (lining membrane of the lung):

• Benign disease of the pleura

               - Pleural plaques (small smooth localised areas of thickening, often

                  calcified)

               - Pleural effusion ( collections of fluid around the lungs)

               - Diffuse pleural thickening (more extensive thickening surrounding the

                  lungs)

               - Rounded atelectasis (localised areas of thickening which cause

                  in-folding of the adjacent lung)

• Malignant disease of the pleura (mesothelioma)
• Asbestosis
• Lung cancer

Asbestosis is a diffuse fibrotic (scarring) disease of lung tissue, caused by long-standing and heavy inhalation of asbestos fibres. It is a slowly progressive disease for which there is no cure. It is associated with the development of the other asbestos-related diseases mentioned above. The pleural diseases can occur alone with rather mild asbestos exposure (including growing up in asbestos mining area or from fibres brought home on workers’ clothes).

Cause and risk factors

Asbestos fibre inhalation
The cause is exposure and inhalation of asbestos fibres, a generic term for naturally occurring fibrous mineral silicates. Asbestos fibres were extensively used in industry, so that occupational exposure was high in many job categories, like construction work, boilermakers, welders, insulation workers, electricians, shipbuilders, dockyard workers, railway workers, car mechanics (especially clutch and brake work) and yarn mill workers. It is particularly important in certain asbestos mining areas of South Africa (Northern Cape, Limpopo & Mpumalanga).

Environmental contamination

Exposure and intensity
Over 25 million people have been exposed to asbestos over last 40 years. There is usually a long latent period (10-30 years) between exposure and the development of lung or pleural disease, but the greater the degree and intensity of asbestos exposure, the sooner symptoms arise.

Cigarette smoking causes earlier onset of symptoms, and significantly increases the risk of malignancy developing.

Pathology

There are two main types of asbestos fibre:

• Long straight fibres, like crocidolite (mined in the Northern Cape, “Cape Blue asbestos”) – highly toxic, and implicated in the development of malignancy 
• Curly fibres, like chrysotile – found in most commercially used asbestos, and considered less toxic

When inhaled, the irritant fibres set up a severe inflammatory response as the lungs try to expel them. The extent of this response ultimately causes fibrosis throughout the lungs, making them stiff and unable to function properly in providing the body with enough oxygen. Crocidolite is difficult to remove from the lungs, so the tissues lay down a coating of iron and protein in an attempt to seal off the fibres from the lung, thereby reducing the irritation. The fibre and coating are visible in lung tissue under the microscope as “asbestos bodies”, and the number present correlates with the severity of the disease

Symptoms and signs

The condition typically takes a long time to develop, so there are no specific symptoms at the time of exposure. In milder exposure, the latent period (time before symptoms are apparent) may be as long as 30 years.

When established, the main symptoms of asbestosis are progressive shortness of breath with even mild exertion, due to stiff lungs (restrictive lung disease), and a dry cough. Wheezing and sputum production are not characteristic, but may occur if the patient smokes or has other unrelated respiratory problems. Weight loss may occur in advanced stages, or if there is associated malignancy.

The signs include clubbing of the fingers (enlargement of the fingertips with drumstick deformity if advanced) in about 30% of cases, an increased breathing rate and crackles (bubbling sounds in the lower and mid zones of the lungs) in 30-60%.

Complications

In advanced cases the outcome is respiratory failure, and/or heart failure, secondary to the restrictive lung disease, causing bluish discolouration of the fingers, lips and tongue (due to low oxygen levels in the blood); severe shortness of breath and swollen feet and abdomen..

Every year, 6 percent of those with asbestosis develop malignant mesothelioma, a uniformly fatal cancer of the pleura. The average survival rate from time of diagnosis is 6 to 18 months. It can occasionally also affect the lining membranes of the heart (pericardium) and abdomen (peritoneum).

Other malignancies (cancers) occurring more frequently in asbestos-exposed persons are:

• Lung (especially if the asbestosis is severe)

                - Larynx and pharynx (throat)

                 - Kidney

                 - Oesophagus (swallowing tube)

• Liver & bile ducts

Lung cancer is much more likely to occur if there is asbestosis present. Occurrence is also significantly increased in those who also smoke ( the incidence of lung cancer is 11 times increased in smokers, 5 times in asbestosis and over 50 times in smokers with asbestosis).

Diagnosis

The diagnosis suspected on clinical grounds, based on the above symptoms and signs with a history of asbestos exposure with a latent period. This exposure is often not immediately apparent, because the exposure may have been up to 20 years before and forgotten or not obvious to the patient. Most cases will also require the following tests: 

Chest X-ray
This might be within normal limits in early and mild cases. It may show pleural plaques (often calcified) or fluid, suggesting previous asbestos exposure. Asbestosis is suggested by an irregular linear shadowing of the lower zones of both lungs. The heart shadow might have a “shaggy” appearance due to adjacent lung scarring. In more severe cases, the linear shadowing can become coarser and spread to involve all zones, giving a “honeycomb” appearance.

High-resolution CT Scan (HRCT)

This is the definitive investigation and the characteristic features tissues show lower zone coarse fibrous bands both parallel and perpendicular to the pleural lining. Again in advanced disease, a honeycomb pattern is seen. In 85% there is evidence of associated pleural disease (plaques, fluid, diffuse thickening or mesothelioma). An associated lung cancer may be seen much less commonly.

HRCT showing Asbestosis

Lung function tests
These typically show a pattern of restricted volume and function, without airways obstruction.

Biopsy

This is not usually necessary, but may be done if there is doubt about the test results. Finding asbestos bodies and fibrosis is diagnostic. Malignancy may also be excluded or confirmed.

Treatment

There is no known treatment for asbestosis. However, all efforts should be made to treat symptoms, for instance with supplemental oxygen use, and to prevent or treat respiratory and heart failure. Preventing further known asbestos exposure and stopping smoking are important, as is prompt treatment of any added lung infection. Patients and their families should also be assisted to get Workman’s Compensation or compensation from asbestos companies in those with environmental exposure.

Dr AG Hall, Health24, January 2008, Reviewed by Prof Gillian Ainslie, UCT Lung Institut