Hypersensitivity pneumonitis (HP) is an interstitial lung disease caused by repeated inhalation of certain fungal, bacterial, animal protein or reactive chemical particles, called antigens. While most people who breathe in these antigens don't develop problems, in some people, the body's immune reaction to these particles causes inflammation of the lung. In some cases, parts of the lungs may become scarred.
It's not known why a minority of people exposed to these antigens develop HP. Their genetics and environment may interact to make them more susceptible to the disease.
HP should not be confused with the more common types of allergies, which are caused by small amounts of proteins in the environment such as dust mites, cat dander, pollen and grass. Having seasonal or environmental allergies has nothing to do with having or developing HP.
Our approach to hypersensitivity pneumonitis
Hypersensitivity pneumonitis will often go away if the patient avoids the substance that's triggering the lung inflammation. Some cases of HP persist, however. For these patients, UCSF offers specialized care that includes medication management, treatment for other health problems that often affect people with HP, and a special exercise and education program designed for patients with chronic lung disease. Our specialists review each case as a team to ensure that every patient gets the right diagnosis and most effective care.
For patients who get worse despite treatment, lung transplantation may be an option. UCSF is home to a high-performing lung transplant program with the expertise to handle the most complex, challenging cases.
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Signs & symptoms
Hypersensitivity pneumonitis is subdivided into two forms: acute and chronic. Symptoms differ for each form.
Acute HP symptoms
The acute form of HP occurs after heavy, often short-term exposure to the antigen. Symptoms appear relatively suddenly and include:
- Chest tightness
If the person is removed from the antigen exposure, the symptoms usually resolve over 24 to 48 hours. Recovery is often complete.
Chronic HP symptoms
The chronic form of HP is thought to occur due to longer term, low-level exposure to the antigen, and it often causes more subtle symptoms. Patients with chronic HP often describe chronic symptoms, such as shortness of breath or cough, that have gotten worse. Symptoms may worsen at work, at home or wherever the patient is being exposed to the antigen, but most often, patients with chronic HP haven't had acute episodes. Most patients seen in our clinic have the chronic form of HP.
Diagnosing hypersensitivity pneumonitis can be difficult and requires input from pulmonologists, radiologists and, in many cases, pathologists experienced in evaluating patients with interstitial lung disease. A face-to-face discussion among these various specialists is often necessary to make an accurate diagnosis.
To determine if you have HP, your doctor will start by conducting a thorough medical history and physical examination. The medical history will include discussing other medical problems you have that could be related to lung scarring, such as connective tissue diseases, and reviewing any medications you're taking. A thorough review of potential occupational and environmental exposures to antigens is also essential, and we will take a detailed home and work history. This will include exposures to mold, birds and bird products, such as down. The physical exam will include listening to your chest with a stethoscope and carefully examining your skin and joints.
In addition to a thorough medical history and physical exam, your doctor may conduct the following tests:
- Pulmonary function test (PFT). This test involves a series of breathing exercises that measure the airflows, volume of air in your lungs and ability of your lungs to extract oxygen from the air. This allows your doctor to assess the function of your lungs.
- High resolution computed tomography (HRCT). This is a special type of CT scan that provides your doctor with high-resolution images of your lungs. Images are taken in several different ways, including lying on your back (supine), lying on your chest (prone) and while breathing air out of your chest (dynamic expiration). These images are extremely valuable in determining whether or not you have HP. Having an HRCT is very similar to having a regular CT scan; they both take only a few minutes.
- Blood tests. We may order blood tests, also called serologies, to test for the presence of various antibodies – proteins created by the immune system to deactivate or destroy antigens – to look for evidence of exposure to various antigens and for evidence of connective tissue diseases, such as rheumatoid arthritis or scleroderma. Blood testing isn't useful in diagnosing most cases of HP, but it may be used in certain situations to help with the diagnosis.
- Six-minute walk test. This test evaluates the distance you can walk within six minutes and the oxygen saturations, measured by finger or ear probe, you achieve while walking.
- Bronchoscopy. In this test, the doctor passes a flexible fiberoptic scope, about the diameter of a pencil, into the lungs to take fluid and tissue samples. This test doesn't require an overnight stay in the hospital. Bronchoscopy is used to aid in a diagnosis of HP and may be performed in certain circumstances.
- Surgical lung biopsy. This test is required for some patients to definitively diagnose HP. A cardiothoracic surgeon performs the surgical lung biopsy, usually with small tools and cameras through one-inch-long incisions. Patients will need to stay in the hospital for a few days.
- Home evaluation. Your doctor may recommend a professional inspection of your home by a certified industrial hygienist. Industrial hygienists are trained in evaluating homes for potential hazardous exposures, particularly molds. This can be quite expensive but may uncover important exposures that are not otherwise obvious. Whether or not to perform a home evaluation should be discussed with your doctor.
Treating hypersensitivity pneumonitis (HP) involves both identifying and removing the antigen that's causing the condition, and taking anti-inflammatory medication.
Removing the antigen
If the inhaled antigen can be recognized and removed, the lung inflammation in acute HP is often reversible. If you have chronic HP, however, the inflammation may persist even when the antigen is removed. If the antigen can't be identified, you may need to change your work or home environment, if possible.
If you don't improve or continue to worsen, we may recommend anti-inflammatory medications. Prednisone is the mainstay of medication therapy and is often very effective. If you require long-term medication or don't tolerate prednisone, you may need to take an alternative medication, such as mycophenolate or cyclophosphamide.
In addition to medications and removal of the antigen, pulmonary rehabilitation – a structured exercise and educational program designed for patients with chronic lung disease – is an important and effective treatment for patients with chronic HP.
We will also need to pay careful attention to common medical problems associated with HP, such as gastroesophageal reflux disease and pulmonary hypertension. Lastly, lung transplant may be an effective treatment option for some patients.
UCSF Health medical specialists have reviewed this information. It is for educational purposes only and is not intended to replace the advice of your doctor or other health care provider. We encourage you to discuss any questions or concerns you may have with your provider.