Acute Interstitial Pneumonia: Pathophysiology, Clinical Features, Diagnosis and Treatment

What Is Acute Interstitial Pneumonia?

Acute interstitial pneumonia (AIP) is an idiopathic lung disease (of unknown cause) denoted by rapidly evolved dyspnoea (difficulty in breathing). It is defined as a rapidly progressive respiratory failure disorder that occurs to be associated with involvement in the lung. There is a rapid onset of respiratory failure in a patient already suffering from lung disease.

What Is the Other Name for Acute Interstitial Pneumonia?

Acute interstitial pneumonia is also known as Hamman-Rich syndrome.

What Is the Pathophysiology of Acute Interstitial Pneumonia?

The disturbance activates acute interstitial pneumonia (AIP) in the alveolar epithelium. This further leads to the initiation of fibroblastic activity.

What Is the Histopathology of Acute Interstitial Pneumonia?

1. Acute interstitial pneumonia (AIP) shows a different histologic pattern from acute respiratory distress syndrome. The condition represents a histological pattern of diffuse alveolar damage (DAD) depending on the timing of the biopsy. As endothelial and epithelial injury limits the virtue of the alveolar-capillary membrane.

2. If the biopsy is done in the early phase, that is, within a week of the initial tissue injury, edema occurs in the interstitium and alveoli. In the late stage, also known as the organizing phase, there is fibroblastic proliferation and type 2 cell hyperplasia (an increase in the number of type II cells).

3. An imbalance between proinflammatory and anti-inflammatory mediators. Proinflammatory causes increased vascular permeability, altered vascular morphogenic responses, leukocyte adhesion, and transmigration, increased procoagulant activities, and increased platelet adhesion and aggregation. Anti-inflammatory mediators, such as lipoxins (LXs) and cyclopentenone prostaglandins (cyPGs), attenuate cell migration and promote the apoptosis and clearance of leukocytes from the inflammatory site.

4. Increased neutrophils in alveoli and interstitium release metabolites, leading to lung injury. Alveolar epithelial cells show epithelial-mesenchymal transition and become myofibroblasts. Therefore, resulting in interstitial organization and fibrosis.

What Are the Clinical Features of Acute Interstitial Pneumonia?

The following are the clinical features of acute interstitial pneumonia:

1. Influenza-like elements such as cold, cough, sneezes, watering of eyes, and running nose.

2. Shortness of breath (dyspnea).

3. Respiratory failure is seen in severe acute respiratory distress syndrome.

4. Oxygen saturation: PaO2/FIO2 ≤ 100 mm Hg.

5. Multiple organ failure.

What Are the Microscopic Features of Acute Interstitial Pneumonia?

Acute interstitial pneumonia is similar to acute respiratory distress syndrome( ARDS). The following phases are seen in these conditions:

1. Exudative Phase:

• There are hyaline membranes present in alveolar ducts or sacs.
• Interstitial and intra-alveolar edema is seen.
• Crumpled alveoli.
• Denudation and necrosis of type I pneumocytes.
• Mild hemorrhage.

2. Proliferative or Organizing Phase:

• There may or may not be remnants of the hyaline membrane.
• The proliferation of fibroblasts or myofibroblasts.
• Lymphocytic infiltration.
• The proliferation of type II pneumocytes with cellular abnormalities (atypia).
• Endothelial injury and fibrinous thromboembolism in arterioles/arteries.

3. Fibrosis Phase:

• Diffuse collagenous fibrosis.
• Microscopic honeycomb-like change.
• Traction bronchiectasis.
• Squamous metaplasia.
• Organized thrombus.
• Thickening of pleura with dilatation of lymphatic or blood vessels.

How To Diagnose Acute Interstitial Pneumonia?

Diagnosis requires excluding other reasons for respiratory failures, such as acute respiratory distress syndrome, acute exacerbation of interstitial lung disease, and interstitial lung disease.

1. Chest X-ray: It is done to rule out acute interstitial pneumonia from other diseases such as acute respiratory distress syndrome (ARDS). The chest X-ray shows bilateral air-space diffuse opacities.

2. Echocardiography: It is done to check for underlying cardiomyopathy or valve dysfunction.

3. High-Resolution Computed Tomography (HRCT): In the case of acute interstitial pneumonia, it appears abnormal in the first 12 hours. There are ground-glass opacities and air space unification. These consolidations are either diffuse or patchy.

4. CT (Computed Tomography): The scan shows bronchiectasis (a condition in which airways of the lungs become damaged).

5. The following methods make a histological diagnosis of diffuse alveolar damage:

   1. Open Lung Biopsy: It is done by removing a small piece of tissue from the lung. It is helpful to define the site and extent of the disease.

   2. Transbronchial Lung Biopsy: It is done to discover hyaline membranes of diffuse alveolar damage.

What Is the Treatment and Management of Acute Interstitial Pneumonia?

Treatment includes the following:

1. Management is mainly based on supportive care. Adequate oxygenation is required along with mechanical ventilation. In the case of acute respiratory distress syndrome (ARDS), arterial blood gases specify the severity of respiratory distress based on the Berlin Criteria (PaO2/FiO2), that is, one of the criteria to check for acute respiratory distress syndrome (ARDS).

2. Ventilator management depends upon low tidal volume ventilation. Tidal volume is the amount of air that moves in or out of the lungs with each respiratory cycle. It measures around 500 mL in an average healthy adult male and approximately 400 mL in a healthy female. It is a vital clinical parameter that allows for proper ventilation.

3. Broad-spectrum antibiotics, such as

   • Doxycycline.

   • Minocycline.

   • Aminoglycosides (except for Streptomycin).

   • Ampicillin.

   • Amoxicillin and Clavulanic acid (Augmentin).

   • Azithromycin.

   • Carbapenems (Imipenem).

   • Piperacillin or Tazobactam.

What is the differential diagnosis of acute interstitial pneumonia?

Differential diagnosis of acute interstitial pneumonia is as follows:

1. Cryptogenic organizing pneumonia (COP) is a form of interstitial lung disease where the bronchioles and alveoli become inflamed and thus leads to difficulty in breathing and flu-like illness (cold, cough, and watering eyes).

2. Acute eosinophilic pneumonia is a severe and rapidly progressive lung disease that causes fatal respiratory failure.

3. Hypersensitivity pneumonitis (HP) is a lung disease causing inflammation of the lungs. This leads to dyspnea and irreversible scarring of the lungs.

4. Acute exacerbation of underlying interstitial lung disease or pulmonary exacerbation of connective tissue disease.

5. Acute heart failure.

6. Infections.

7. Diffuse alveolar hemorrhage.

8. Drug-induced lung injury.

9. Radiation-induced lung injury.

What Is the Prognosis of Acute Interstitial Pneumonia?

The mortality rate (a measure of the number of deaths in a particular population) is greater than 50 percent, both in the early stages of presentation and within six months after commencement.

Conclusion:

Acute interstitial pneumonia is a disease with unknown etiology. Thus it becomes difficult to prevent or forecast its outset. Early symptoms are not related to the disease because many other respiratory diseases also show the same signs and symptoms. Lung biopsy is the most reliable method to diagnose acute interstitial pneumonia. It is managed mainly by supportive care.