Module 4: Asbestos And Disease - Note (Continued)

THE ASBESTOS RELATED DISEASES:

The diseases caused by asbestos range in severity from mild to lethal, but are incurable, progressive and may occur decades after exposure, so that the legacy of past exposure is still with us and will be continue to be so for the foreseeable future. The scientific mystery of the ability of asbestos to cause fibrosis and lung cancer, even decades after exposure has ended, has stimulated a huge amount of research, but many questions remain unanswered. The fibrous physical nature of asbestos plays an important role, for example, by its effect on macrophages (see picture on the left), but chemical factors may also play a role.

Fibrotic and inflammatory diseases:

1. Asbestosis:

This is a diffuse interstitial fibrosis of the lungs and, strictly, is the only condition to which the term asbestosis applies.

The main symptom in the early symptomatic stages is breathlessness. Clubbing may be seen in some cases and late inspiratory fine crackles heard best in the axillae are the characteristic lung finding. The chest x-ray is characterised by irregular opacities at the lung bases (and not diffuse nodules in the upper zones, which distinguishes it easily from silicosis). It is not infrequently found in conjunction with pleural plaques, which helps with the diagnosis. Lung function is restrictive with an abnormal gas transfer.

A high resolution CT scan of the lungs is useful to confirm the diagnosis in uncertain cases. Histology shows lung fibrosis in the region of respiratory bronchioles (see the pictures above - the normal lung on the right).

There is no treatment and asbestosis may be progressive, or even appear for the first time, after exposure has ceased (see the x-rays above) The risk of lung cancer is substantially increased where there is asbestosis, and cessation of smoking (as for anyone exposed to asbestos) is an important goal of medical care.

Circumcised pleural plaques (see picture on the right):

These are convex or ovoid opacities seen on the chest x-ray abutting the lateral lung wall, typically in the lower or mid zones or on the diaphragm. The may be seen over the lung fields away from the chest wall ("face on") and can be difficult to recognise if not calcified. Once plaques calcify, they may present an irregular edge, strange shape or show spicules within the plaque.

Diffuse pleural thickening:

This is pleural thickening starting in the lower zones, but sometimes extending the whole length of the lung, with a smooth concave edge which obscures the costophrenic angle. Diffuse thickening may occur after tuberculosis or any other inflammation of the pleura and is thus less specific for an asbestos effect.

Benign pleural effusion:

Recurrent benign effusions may occur. This is a diagnosis of exclusion of the typical causes of effusion such as TB and cancer (and perhaps cardiac failure). Malignant mesothelioma may present with an effusion without a mass.

In general, asbestos related pleural thickening is the most common asbestos related abnormality and is often found incidentally. It does not carry mesothelioma risk over and above that associated with the fact of past asbestos exposure. Plaques may progress slowly over time (or appear for the first time after exposure ceases) and calcify.

Most circumscribed pleural plaques typically do not interfere with lung function, but extensive plaques or diffuse thickening may do so (either on their own or because there is asbestosis not apparent on the chest x-ray). Only if there is lung function impairment is pleural disease on its own compensatable.

Pleuritic pain is a rare manifestation of pleural plaques and one should look for other causes of pleuritic pain.

2. Malignant disease:

Malignant mesothelioma of the pleura or peritoneum:

This may arise either in the chest or the abdomen. The mechanism of peritoneal mesothelioma is unknown. The background rate of this malignancy is very low so that most mesotheliomas can be attributed to asbestos exposure. (See reading by Rees et al.).

Pleural mesothelioma typically presents with a painless effusion or with pain. An irregular mass may be seen on the lateral chest wall (once the effusion is drained). There is no effective treatment and the prognosis is almost uniformly poor with median time to death 9 to 18 months after diagnosis.

Diagnosis may be difficult as pleural biopsies may not yield enough material to show the typical malignant cells. The main differentiation is from (in the early stages) benign pleural disease and from adenocarcinoma of the lung involving the pleura. Given enough material histochemical staining techniques can make a more specific diagnosis.

Bronchogenic carcinoma (see picture on the right):

There is nothing specific about the presentation of lung cancer due to asbestos, other than it may occur against a background of asbestosis. Any histological variant may occur. There has been debate in the scientific literature over whether asbestosis is necessary to make an attribution of asbestos associated lung cancer. However, under our compensation system, asbestosis is not required for a diagnosis of asbestos associated lung cancer.

Carcinoma of the larynx:

Tobacco is the main cause in the general population, but asbestos is a risk factor.

Carcinoma of the gastro-intestinal tract:

This includes cancer of the oesophagus, stomach, colon and rectum. This association has been controversial as not all studies show an excess risk among asbestos exposed workers. The mechanism is unknown. It has been suggested that excess risk seen in some epidemiologic studies may be due to misclassification of metastases from other sites as gastrointestinal primaries.

In South Africa, Botha et al. (1986) in a geographical mortality study found that there was an excess of stomach cancer in residents of crocidolite mining districts with residents of non-mining districts used as controls. However, Sluis-Cremer et al. in a cohort study of white amosite miners found no excess of gastrointestinal cancer.

Gastrointestinal cancer is not compensatable under COIDA.