Hypersensitivity Pneumonitis

Hypersensitivity pneumonitis (extrinsic allergic alveolitis, allergic interstitial pneumonitis, organic dust pneumoconiosis) is inflammation in and around the tiny air sacs (alveoli) and smallest airways (bronchioles) of the lung caused by an allergic reaction to inhaled organic dusts or, less commonly, chemicals.

Causes

Many types of dust can cause allergic reactions in the lungs. Organic dusts that contain microorganisms or proteins and chemicals, such as isocyanates, may cause hypersensitivity pneumonitis. Farmer's lung, which results from repeated inhalation of heat-loving (thermophilic) bacteria in moldy hay, is a well-known example of hypersensitivity pneumonitis. Air conditioner lung is another example; it occurs when contaminated humidifiers or air conditioners (especially large systems in office buildings) circulate antigens that are capable of causing a hypersensitivity reaction.

Only a small number of people who inhale these common dusts develop allergic reactions, and only a small percentage of those people who develop allergic reactions suffer irreversible damage to the lungs. Generally, a person must be exposed to large amounts of these antigens continuously or frequently over time before sensitivity and resultant disease develop.

Lung damage appears to result from a combination of immune complex reactions and cell-mediated allergic reactions. Initial exposures to the dusts sensitize lymphocytes. Some lymphocytes then help to produce antibodies that play a role in tissue damage. Other lymphocytes participate directly in inflammation after subsequent antigen exposure. Recurrent exposure to the antigen results in a chronic inflammatory response, which is manifested by a buildup of white blood cells in the walls of the alveoli and small airways. This buildup leads progressively to symptoms and disease.

Symptoms and Diagnosis

If a person has developed hypersensitivity to an organic dust, then fever, cough, chills, and shortness of breath typically appear 4 to 8 hours after reexposure to it. Wheezing is unusual. If the person has no further contact with the antigen, symptoms usually improve over a day or two, but complete recovery may take weeks.

In a slower form of hypersensitivity pneumonitis (subacute form), cough and shortness of breath may develop over days or weeks and sometimes may be so severe that the person needs to be hospitalized.

With chronic hypersensitivity pneumonitis, a person repeatedly comes in contact with an allergen over months to years, and lung scarring (fibrosis) may result. Shortness of breath during exercise, coughing up of sputum, fatigue, and weight loss may gradually progress over months or years. Eventually, the disease may lead to respiratory failure (see Section 4, Chapter 55).

The diagnosis of hypersensitivity pneumonitis depends on the clinical features, identification (if possible) of the dust or other substance causing the problem, and evidence of the person's exposure to the suspected agent, as determined by the presence of antibodies on a blood test.

Doctors may suspect the diagnosis based on finding something abnormal on a chest x-ray. Results of pulmonary function tests (see Section 4, Chapter 39)--which measure the lungs' capacity to hold air and their ability to move air in and out and to exchange oxygen and carbon dioxide--may help support a diagnosis of hypersensitivity pneumonitis. Blood tests for antibodies may show that the person has been exposed to the suspected antigen. When the antigen cannot be identified and the diagnosis is in doubt, reexposing the recovered person to the allergen and observing the person for symptoms or changes in lung function may occasionally be useful to confirm the diagnosis. Lung function can be determined using pulmonary function testing.

In cases that are not clear, especially when an infection is suspected, doctors may remove a small piece of lung tissue for examination under a microscope (lung biopsy). They may remove the tissue while examining the airways using a viewing tube (bronchoscopy (see Section 4, Chapter 39)). Sometimes, rather than (or in addition to) removing tissue using a sharp instrument, the person performing bronchoscopy may wash out the lung with fluid (bronchoalveolar lavage) to extract cells for examination. Rarely, an examination of the lung surface and pleural space using a viewing tube (thoracoscopy) or an operation in which the chest wall is opened (thoracotomy) may be needed (see Section 4, Chapter 39).

Prevention and Treatment

The best prevention is to avoid exposure to the antigen, but this may be impractical if the person cannot change jobs. Eliminating or reducing dust or wearing protective masks may help prevent a recurrence. Chemically treating hay or sugarcane waste and using good ventilation systems help to minimize exposure to the antigen, which may prevent workers from initially becoming sensitized to these materials.

People who have an acute episode of hypersensitivity pneumonitis usually recover if further contact with the substance is avoided. If the episode is severe, corticosteroids, such as prednisone, reduce symptoms and may help reduce severe inflammation. Prolonged or recurring episodes may lead to irreversible disease and progressive disability.

See the table What Causes Hypersensitivity Pneumonitis?