Pneumonitis refers to a condition in which the lung tissue is inflamed. It is caused when any foreign substance or irritant (allergen) inflames the alveoli (small air sacs in the lungs), which results in a decrease in lung function. An allergen is a substance that can cause an allergic reaction. Pneumonitis can be classified into different types based on the underlying cause. Different types of pneumonitis include 

• Acute interstitial pneumonitis: Allergens include bacteria, mold, or chemicals.

• Chemical pneumonitis: Allergens include chemical fumes.

• Hypersensitivity pneumonitis: Allergens include bird feathers or droppings, household mold, and animal dander.

• Radiation pneumonitis or Radiation-induced pneumonitis.

Any individual experiencing abnormal lung sounds should visit a doctor; diagnostic tests such as lung X-rays and blood tests can be used to identify this condition.

What is Hypersensitivity Pneumonitis (HP)?

Hypersensitivity pneumonitis is a condition in which an individual inhales an allergen that causes an allergic response in their body. 

Hypersensitivity pneumonitis triggers a different immune response in the body than the usual pet or pollen allergies. Unlike common allergies that cause hay fever or asthma, persistent exposure to HP-causing allergens may result in inflammation that can cause permanent lung damage.

Causes of Hypersensitivity Pneumonitis

Hypersensitivity pneumonitis can be caused by over 300 different substances. Some of the most common conditions are named after the allergen that cause them, such as:

Farmer's lung: This condition is most commonly seen in cattle breeders and farmers which is caused by inhaling mold that develops on hay, straw, and grain.

Bird fancier's lung or pigeon breeder's disease: The condition is caused by the inhalation of particles (proteins) from the feathers or excrement of a variety of birds.

Humidifier lung: This condition is caused by inhaling mold that grows on humidifiers, ACs and heaters, especially if they are poorly maintained.

Hot tub lung: This condition may be caused by inhaling bacteria found in indoor hot tub water vapors.

Hypersensitivity pneumonitis may be more likely to occur in individuals who are farmers, poultry handlers and bird breeders, veterinarians, animal handlers, wood workers, etc. Thus, it is most commonly considered an occupational disease. Additionally, it is possible that genetic predisposition may play a role in the development of hypersensitive pneumonitis, as it is commonly seen in families.

Symptoms of HP

The symptoms of HP can be either acute or chronic. Acute symptoms can appear within hours of exposure to the allergen and last for a period of a few hours or days, while chronic symptoms may develop gradually and worsen over time.

Acute hypersensitivity pneumonitis symptoms: 

• Shortness of breath 

• Dry cough

• Tightness in the chest

• Chills

• Fatigue

• Fever

• Muscle aches

Chronic hypersensitivity pneumonitis symptoms: 

• Breathing difficulties, especially during strenuous activity

• Cough

• Fatigue

• Weight loss

• Clubbed fingers or toes

Diagnosis of HP

There are a number of diagnostic tests that a physician may order if they suspect that a patient may have HP. The diagnosis is based on the patient's symptoms and the results of these tests. The diagnostic tests include: 

1. Allergy panel blood test:

• Total IgE blood test –The levels of total IgE antibodies are measured in the blood sample. Elevated levels of IgE antibodies may occur if the patient has any type of allergy. 

• Specific IgE blood test –The levels of specific IgE antibodies against specific allergens are measured in the blood sample. Elevated levels of specific IgE antibodies against particular allergens such as bird droppings, mold, etc. may be indicative of HP. 

2. Pulmonary Function Test (PFT): This test includes a series of breath tests that measure air flow, air volume, and lung capacity to draw oxygen from the air. This helps the healthcare provider evaluate lung function. 

3. High Resolution Computed Tomography (HRCT): This is a medical imaging technique that provides a high-resolution image of the lungs to the healthcare provider. These images are taken from different angles and play an essential role in the diagnosis of HP. 

4. Bronchoscopy: In this test, the healthcare provider inserts a flexible optical scope, approximately the size of a pencil into the lung to collect lung fluid and tissue specimens for further testing. 

5. Six-minute walk test: This test evaluates the walking distance an individual is able to complete in six minutes, as well as the level of oxygen saturation achieved while walking, which is determined by either finger or ear probes.

6. Surgical Lung Biopsy: This test may be necessary for a definitive diagnosis of HP. The surgical pulmonary biopsy is performed by a cardiothoracic surgeon using small instruments and cameras through one-inch long incisions. 

Treatment of HP

Once HP is diagnosed, the most important thing to do is to avoid exposure to the allergen(s) that are causing the reaction. Individuals are recommended to consult their healthcare providers to determine the steps to prevent or eliminate the allergens from their residence or workplace. If the disease is caught early, avoiding exposure to the source can help reverse the damage caused and within a short period of time, the lungs will return to their normal function.

In more serious cases, avoidance may not be sufficient. Subsequently, treatment may involve the administration of corticosteroids, or immunosuppressive medications to reduce inflammation and inhibit the immune system's response to the allergens inhaled. These medications may be prescribed for a period of up to three months or longer, depending on the severity of the condition. Additionally, supportive therapies may be necessary, including bronchodilators which relax the airways to facilitate breathing and oxygen therapy, thereby, increasing the oxygen levels in the blood.

The treatment will be evaluated through a combination of radiologic scans, pulmonary function tests, and blood tests. Healthcare professionals will assist patients in determining the duration of treatment, providing information on adverse drug reactions, and scheduling follow-up appointments.

If symptoms persist, it may be due to advanced scarring and healthcare providers may suggest a lung transplant. However, this is not a cure for HP and patients will still require additional precautions to prevent exposure and reduce the risk of persistent inflammation and damage to the new lung.

Diagnostic tests such as allergy panel blood tests can be used to detect HP in individuals, and early detection allows prompt treatment. Consequently, it is recommended to undergo an allergy panel test to prevent various complications associated with HP.