Acute interstitial pneumonitis

Overview

Acute interstitial pneumonitis (AIP), also known as Hamman–Rich syndrome, is a rare and rapidly progressive lung disease characterized by sudden inflammation and widespread damage to the alveoli and interstitial tissue of the lungs. It is classified as an idiopathic interstitial pneumonia, meaning it arises without a known cause. AIP typically occurs in previously healthy individuals and presents similarly to acute respiratory distress syndrome (ARDS), requiring emergency care. Despite aggressive treatment, AIP carries a high mortality rate.

Causes

The exact cause of acute interstitial pneumonitis is unknown (idiopathic), but it is thought to result from an abnormal immune or inflammatory response in the lungs. Unlike other interstitial lung diseases, AIP develops spontaneously and without any identifiable environmental, infectious, or occupational triggers.

While not directly caused by infection, AIP may be preceded by a flu-like illness or upper respiratory tract symptoms. There is no known genetic or familial link, and it can occur in both men and women, typically in middle age.

Symptoms

Symptoms of AIP develop rapidly over days to a few weeks and resemble those of severe pneumonia or ARDS. Common clinical features include:

• Shortness of breath (dyspnea): Rapid onset and worsening

• Dry cough

• Fever and chills

• Fatigue or malaise

• Rapid breathing (tachypnea)

• Cyanosis: Bluish tint to the lips or fingers due to low oxygen levels

Most patients quickly progress to respiratory failure and require hospitalization and mechanical ventilation.

Diagnosis

Diagnosing acute interstitial pneumonitis is challenging and involves ruling out other causes of acute respiratory failure. Diagnostic steps include:

• Chest X-ray: Shows diffuse, bilateral infiltrates similar to ARDS

• High-resolution CT scan: Reveals ground-glass opacities and interstitial thickening

• Arterial blood gas (ABG): Shows hypoxemia (low oxygen in the blood)

• Pulmonary function tests: Not typically feasible in acute setting but would show restrictive patterns

• Bronchoscopy with lavage: To rule out infection or malignancy

• Surgical lung biopsy: Often needed to confirm the diagnosis and shows diffuse alveolar damage (DAD)

Other causes such as infections, drug toxicity, connective tissue diseases, and environmental exposures must be excluded.

Treatment

Treatment for AIP is supportive and aimed at managing respiratory failure and reducing inflammation. Common strategies include:

• Mechanical ventilation: Often required for severe respiratory distress

• High-dose corticosteroids: May be used to reduce lung inflammation, though their benefit is unproven and controversial

• Broad-spectrum antibiotics: Given initially until infection is ruled out

• Immunosuppressive agents: Occasionally tried in steroid-resistant cases

• Extracorporeal membrane oxygenation (ECMO): Considered in refractory respiratory failure in select patients

There is no specific cure, and treatment focuses on stabilizing the patient while the lungs recover, if recovery is possible.

Prognosis

The prognosis for acute interstitial pneumonitis is poor, with a high mortality rate, estimated between 50–70% within the first few weeks. Those who survive the initial phase may experience complete or partial lung recovery, though some may develop chronic interstitial lung disease or require long-term oxygen therapy.

Early recognition and intensive supportive care in a critical care setting offer the best chance for survival. Survivors often require long-term follow-up and pulmonary rehabilitation.