Extrinsic allergic alveolitis (EAA) or hypersensitivity pneumonitis (HP)

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What is Extrinsic allergic alveolitis?

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Hypersensitivity pneumonitis or extrinsic allergic alveolitis is a widespread disease of the lungs, involving inflammation in both the small airways of the lung and the alveoli (air sacs). It is not dissimilar to asthma, but the latter involves the bronchi and bronchioles. It is also known as extrinsic allergic alveolitis. Breaking up these terms, extrinsic means the disease is caused by an external agent not from the body with allergic component to it; alveolitis means inflammation to the alveoli (air sacs) in the lungs.

The lung is composed of large airways (beginning with the trachea and then two main bronchi), which branch out to form smaller airways (segmental bornchi and bronchioles) which finally lead to the alveoli - the small air sacs where gas exchange takes place.

Who gets Extrinsic allergic alveolitis?

This depends on exposure to the variety of predisposing factors identified for hypersensitivity pneumonitis. The disease is most common in poor, wet farming areas of the world, sometimes affecting 1 in 10 of the farmers. In the West, the cases appear to be declining as more mechanised farming procedures are introduced.

The cases of hypersensitivity pneumonitis vary widely in different countries, although it is thought to be low.

One of the commonest form of hypersensitivity pneumonitis is farmer's lung. The cases of farmer's lung in different countries is detailed below:

US: 420-3000 per 100,000 population

UK: 12-2300 per 100,000 population

France: 4370 per 100,000 population

Finland: 1400-1700 per 100,000 population

Predisposing Factors

A number of causes have been identified, involving exposure to substances that lead to allergic response in some people. The following occupations place the workers at risk of developing hypersensitivity pneumonitis.

Forking mouldy hay - farmers
Handling pigeons, cleaning lofts or budgie cages - bird fanciers
Turning germinating barley - malt-workers
Contaminated humidifying systems in air conditioners or humidifiers in factories - some factory workers
Turning mushroom compost - mushroom workers
Grain and flour processing and loading workers
Lumber milling, construction, wood stripping, paper and wallboard manufacturing
Plastic manufacture, painting, electronics industry, other chemicals
Textile workers

Interestingly, cigarette smoking is associated with a decreased risk of hypersensitivity pneumonitis. Although smoking seemed to be protective, once the disease is established, smoking does not reduce the disease severity.

Progression

This disease is similar to asthma in that it occurs due to a hypersensitivity response to an inhaled substance. However, the difference is that in this case, the allergic response occurs mainly in the alveoli and small vessels supplying them.

Lung symptoms of hypersensitivity pneumonitis can occur in 3 different stages:

acute: symptoms occur 4-6 hours after substance exposure

subacute: characterised by gradual cough development and other symptoms over 10-14 days

chronic: insidious onset where the patient may not complain of any symptoms until the disease reaches advanced stage.

If substance exposure continues on a long term basis, fibrosis begins to replace the inflammation, resulting in progressive, irreversible, fibrosis of the lungs. Fibrosis means that fibrous tissue (scar) replaces the inflammed tissue in the lungs.

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