Acute eosinophilic pneumonia

Acute eosinophilic pneumonia is an eosinophilic lung disease which can mimic community-acquired pneumonia or present with acute lung injury ¹⁰:

Epidemiology

The true incidence is unknown due to under-diagnosis and under-reporting. Studies of military personnel indicate approximately 10 per 100,000 person-years. In this population, the disease is predominantly smoking-related ¹⁰.

Diagnosis

A history of exposure may be elicited ¹⁰:

• cigarette smoking:

  • first-time smokers

  • resumption of smoking

  • increase in number of cigarettes smoked

• other inhaled substances:

  • cocaine

  • crystal amphetamine

  • heroin

  • marijuana

  • inorganic or organic dusts or smoke

• drugs:

  • antimicrobials

  • anti-inflammatories

  • chemotherapy

  • antidepressants

  • miscellaneous, including amiodarone

• infectious agents:

  • parasites

  • fungi

  • viruses

Some cases are branded idiopathic because no inciting cause is identified.

Clinical Presentation

Symptoms and signs are non-specific and can lead to misdiagnosis as community-acquired pneumonia or viral infection. Disease may be mild and self-limiting or severe, presenting as acute lung injury which may be fatal.

Dry cough and dyspnea may be accompanied by myalgia, chills, pleurisy, inspiratory crackles and fever ¹⁰.

The criteria for diagnosis of AEP have been revised ¹⁰:

• acute respiratory illness of up to 1 month duration

• imaging features (see below)

• pulmonary eosinophilia (>25% eosinophils in BAL fluid or eosinophilic pneumonia on lung biopsy)

• absence of other eosinophilic lung disease (eosinophilic granulomatosis with polyangiitis, hypereosinophilic syndrome, allergic bronchopulmonary aspergillosis)

Previously suggested diagnostic criteria are not necessary for diagnosis:

• hypoxemia and respiratory failure (variable)

• response to corticosteroids (AEP may also resolve spontaneously)

• asthma and atopy as exclusion criteria (they may predispose to AEP) ¹⁰

Pathology

It is likely that an environmental trigger acts on a susceptible individual to induce an immune cascade with cytokine generation, eosinophil recruitment and degranulation. Epithelial and endothelial cell injury elevates levels of IL-33. Eosinophils infiltrate the interstitium and alveoli and there is a protein and fibrin exudate and an increase in surfactant-type proteins with hyperplasia of type II pneumocytes. Inflammation affects airways and bronchovascular bundles and there may be eosinophilic abscesses and diffuse alveolar damage. Pleural effusions are eosinophilic exudates. Peripheral eosinophilia may be absent initially but may increase over time ¹⁰.

Radiographic features

Imaging features should be interpreted in the correct clinical context.

CT 

Described features include ^4,6,9:

• patchy bilateral ground-glass opacity or air-space consolidation

• interlobular septal thickening

• pleural effusions: can be present in ~80% (range 60-100%) of cases

• thickening of bronchovascular bundles: present in around two-thirds of cases

• ill-defined centrilobular nodules (possibly more common in smokers)

Distribution tends to be random.

Treatment and prognosis

Cases may be mild and regress spontaneously or, if required, will respond to glucocorticoids within 48 hours. Chest radiographs may take 1 month to clear.

Severe acute lung injury requires ventilatory support and oxygen. Fatalities are uncommon.

Relapse is rare and is typically due to resumption of smoking ¹⁰.