Hypersensitivity Pneumonitis

Hypersensitivity pneumonitis (HP) is an inflammation of the lung (usually of the very small airways) caused by the body's immune reaction to small air-borne particles. 

Acute hypersensitivity pneumonitis tends to occur 4-12 hours after exposure (usually heavy exposure) to the particles.

Symptoms of acute hypersensitivity pneumonitis include:

• Fever
• Chills
• Coughing
• Shortness of breath
• Body aches, malaise

Chest X-ray may show diffuse small nodules in the lungs. Typically, the symptoms will subside hours to days after exposure (provided there are no repeated exposures). The abnormalities on chest X-ray abnormalities will also disappear. The patient's condition can deteriorate after exposure; therefore it is imperative that a medical evaluation be performed to determine the best course of treatment.

Chronic (long-term) hypersensitivity pneumonitis causes lung scarring (fibrosis).

Symptoms of chronic hypersensitivity pneumonitis include:

• Shortness of breath
• Cough

Chronic disease is believed to occur after prolonged low grade exposure to the offending particles. It is sometimes quite surprising that individuals with a passion for their hobbies or occupation will continue to allow exposure to lung damage (if the offending particles are related to the hobby or job) despite the knowledge that it is harmful.

Examples of hypersensitivity pneumonitis include:

• Farmer's lung disease from exposure to mold spores in hay
• Pigeon breeder's disease from exposure to protein particles in pigeon droppings
• Sauna takers' disease from exposure to mold growing in wet containers
• Mushroom workers' disease from exposure to moldy compost
• Bagassosis from exposure to moldy sugar cane
• Winemaker's lung from exposure to a fungus on grapes called Botrytis cinerea
• An unusual case was published involving a case of hypersensitivity to Canadian goose droppings. The individual was a physician who was exposed to both indoor and outdoor antigens while living in a suburban Illinois community. One can only imagine the tremendous detective work necessary to make this diagnosis.

A more detailed analysis is listed in the table, which includes the types of compounds, bacteria, and molds known to cause hypersensitivity pneumonitis.

Many people with episodes of hypersensitivity pneumonitis are probably unrecognized and undiagnosed. Some cases believed to be viral pneumonias may actually be hypersensitivity pneumonitis. The patient's history of repeated episodes of typical symptoms, hours after exposure to certain environments are important in establishing the diagnosis.

Most patients with this disorder have symptoms of shortness of breath and/or cough. Quite often a chest X-ray may show a variety of abnormalities, but primarily increased lung markings. A specialist in lung disease will primarily depend on a history of exposure either by occupation or hobby. With the abnormal chest X-ray, a high resolution cat scan of the chest is often reviewed. This study shows a detailed image of the appearance of the lung tissue. Lung function tests that examine the lung volumes and the ability for gases to move through the lungs (diffusing capacity) are then performed. Not only are the CT scan and pulmonary function tests useful in diagnosis but they are also useful in following response to therapy.

Blood antibody tests and skin tests against certain offending molds, bacteria, or particles are available, but their results are usually inconclusive. Other more recent tests such as the serum KL-6 (Krebs von den Lungen-6; this test may act as a marker for activity seen in diffuse lung disease) may be used. Infectious causes as well as collagen vascular diseases and cancer must be excluded as a cause of the symptoms and signs, and this may involve a biopsy. Unfortunately the small biopsies obtained with a bronchoscope or a needle are usually inadequate. It is often necessary to perform Video Assisted Thoracoscopic Surgery (a.k.a. VATS), which is an open lung biopsy technique performed by thoracic surgeons under general anesthesia. This procedure allows for a larger sample size and more accurate diagnosis.

The interpretation of these tests is complicated, and is best performed by a doctor experienced in hypersensitivity pneumonitis, often with the help of a pathologist experienced in lung diseases. The most accurate determination of the cause of hypersensitivity pneumonitis is almost always a detailed exposure history. It is not uncommon, however, to be unable to find the causative agent.

The most important treatment of hypersensitivity pneumonitis is avoidance of repeated exposures to the offending particles. With early diagnosis and prevention, the prognosis is good.