Hypersensitivity Pneumonitis

Signs and Symptoms

Hypersensitivity pneumonitis (HP) is a group of rare inflammatory interstitial lung diseases that result from repeated inhalation of certain fungal, bacterial, animal protein, or reactive chemical particles, called antigens. The body's immune reaction to these particles causes inflammation of the very small airways in the lungs, called alveoli.  If exposure to the antigen is continuous, parts of the lungs may become scarred or fibrous.

Most people breathe in the antigens that cause HP every day. However, evidence suggests that genetics and environment may play a role in the development of HP, making some people more susceptible to the disease. Changes in certain genes may predispose a person toward developing HP. In addition, in many cases, environmental factors are the leading cause of HP. For example, some bird handlers may develop HP because of the species of bird raised and the handling practices used.

HP should not be confused with the more common types of aero-allergies, which are caused by small amounts of proteins in the environment, such as dust mites, cat dander, pollen and grass. The body's response to these proteins is very different than to the proteins that cause HP. Thus, having seasonal or environmental allergies has nothing to do with having or developing HP.

HP is a very complex condition, which has been subdivided into three forms: acute, sub-acute and chronic. Symptoms differ for each form.

• The acute form of HP occurs following intermittent, heavy exposure to the antigen. Symptoms include fever, chills, fatigue, breathlessness, chest tightness and cough. They occur rather quickly after inhalation exposure to the antigen, typically within four-eight hours. If the person is then removed from the antigen and is no longer breathing it in, the syndrome usually resolves over 24 to 48 hours.
• The sub-acute form of HP is thought to occur due to chronic, low-level exposure to a causative antigen, such as in a persons home or workplace. The symptoms are much more subtle than those of the acute form, including progressive breathlessness, cough and fatigue developing over months. Patients with sub-acute HP can have episodic attacks of symptoms that resemble acute HP, such as fevers and sudden worsening of pulmonary symptoms. These attacks may correspond to intermittent heavy antigen exposure.
• The chronic form of HP, like the sub-acute form, is also thought to occur due to chronic low-level exposure with more subtle symptoms. However, patients with chronic HP often have no history of acute episodes and in some cases may develop scarring of the lungs.

Diagnosis

There is no single test available to diagnosis hypersensitivity pneumonitis (HP). Therefore, your doctor will perform a series of tests to make a definite diagnosis. A careful and detailed medical and antigen exposure history, including environmental and occupational exposure, will also be recorded. In confirming a diagnosis of HP, your doctor will rely heavily on your medical and exposure history and the results from the tests listed below.

• Pulmonary Function Testing (PFT) -- This test involves a series of breathing maneuvers that measure the airflow and volume of air in your lungs. This allows your doctor to objectively 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. These images are extremely valuable in determining which type of ILD you have. Having a HRCT is no different than having a regular CT scan; they both are performed on an open-air table and take only a few minutes.
• Six-Minute Walk Testing -- This test is a formal evaluation of the distance you can walk and the oxygen saturations (measured by finger or ear probe) you achieve while walking. This is an important marker of functional status used by your doctor in managing your condition.
• Bronchoscopy -- This test involves the passage of a flexible fiberoptic scope about the diameter of a pencil into the lungs to obtain fluid and sometimes tissue samples to aid in diagnosis. This test is an outpatient procedure, which means you do not have to stay overnight in the hospital, performed by your doctor.
• In some cases in which test results suggest more than one diagnosis, a surgical lung biopsy may be recommended. This test is required for some patients to definitively diagnose the cause of their lung disease. Surgical lung biopsy is usually performed with small tools and cameras through three-four inch long incisions. The procedure is performed by a cardiothoracic surgeon and requires a few days stay in the hospital.

Treatment

It is extremely important to understand that the identification and avoidance of the antigen that is causing symptoms of hypersensitivity pneumonitis (HP), called the causative agent, is the most important step in treating HP. Medications are also available to help reduce the effects of HP.

Medication Therapy

Corticosteroids are often prescribed for patients with HP because they reduce the inflammation of the air sacs in the lungs. The type of corticosteroid used is typically prednisone. If your condition does not respond quickly to prednisone, your doctor may recommend adding another immune-suppressant to your treatment, such as azathioprine (imuran). You doctor will discuss with you the dosage amount, maintenance and tapering process.

Identification of the Causitive Antigen

There are a wide range of environmental exposures, occupations and avocations, which result in contact with causative antigens that can increase the risk of developing HP. Identification of the antigen responsible for HP is of critical importance for successful treatment. If the inhaled antigen can be recognized and removed, the lung inflammation in the acute and sub-acute forms of HP is often reversible. In these cases, patients can have a complete recovery. However, finding the antigen is often more difficult than it may seem. If the antigen cannot be identified, the patient may need to consider moving away from their potential exposure, either work or home environments, if possible.

In many cases, patients with chronic HP who have had the disease for many years, have lung scarring and therefore, a full recovery is not possible. However, the hope is to halt the progression of the disease by identifying the antigen and removing it. Often times, immunosuppressive therapy is also recommended. For patients whose condition continues to worsen despite all attempts of treatment, lung transplantation may be an option. Your doctor will discuss this with you if he/she thinks that it is an option.

Experts at our Interstitial Lung Disease Center of Excellence can help facilitate detailed evaluations to identify the causative agent of HP. For more information, please see Identifying the Causative Antigen of Hypersensitivity Pneumonitis (HP).