Idiopathic acute eosinophilic pneumonia (ORPHA:724)
Idiopathic acute eosinophilic pneumonia (IAEP) is an eosinophilic pneumonia of undetermined etiology that is characterized by acute febrile hypoxic respiratory failure associated with diffuse radiographic infiltrates and pulmonary eosinophilia, but without concurring allergy or infection.
Epidemiology
The prevalence of IAEP is still unknown but <100 cases have been reported to date. A recent study has described only 3 pediatric cases in a 5 year-time frame in France. Men are affected approximately twice as frequently as women.
Clinical description
IAEP can occur at any age, even in previously healthy children, although most patients are between 20 and 40 years of age. IAEP presents itself as an acute pneumonia in previously healthy subjects and the clinical features include acute respiratory failure with fever, dry cough, progressive dyspnea, tachypnea and severe hypoxemia. Crackles on chest auscultation and marked chest wall retractions can be observed. Myalgia and abdominal pain may also be present.IAEP can be precipitated or exacerbated by smoking tobacco and cannabis.
Etiology
The pathophysiology of IAEP is still not well understood. It appears to result from damage caused by eosinophils infiltrating the lung parenchyma and releasing toxic substances such as basic proteins, lipid mediators, and cytokines.
Diagnostic methods
Diagnostic criteria for IAEP are as follows: 1) acute onset of febrile illness (usually less than 7 days), 2) diffuse bilateral pulmonary infiltrates, 3) severe hypoxemic respiratory failure (with blood oxygen saturation <90% by pulse oximetry, or PaO2<60mmHg on room air or PaO2/FiO ≤300 mmHg), 4) lung eosinophilia (bronchoalveolar lavage [BAL] fluid eosinophilia >25% or predominance of eosinophils in lung biopsy), and 5) no known causes of acute eosinophilic lung (recent onset of tobacco smoking or exposure to inhaled dust). Chest radiographs show diffuse bilateral infiltrates, air space opacities, interstitial reticulo-nodular densities and/or pleural effusion. These interstitial infiltrates usually progress to extensive alveolar and interstitial infiltrates involving all lobes in several hours. Chest computed tomography (CT) scans show diffuse areas of ground-glass attenuation, alveolar infiltrates, poorly defined nodules and interlobular septal thickening.
Differential diagnosis
Differential diagnosis includes acute interstitial pneumonia (AIP), acute lung injury (ALI), acute respiratory distress syndrome (ARDS) (see these terms), severe community-acquired pneumonia, aspiration pneumonia, acute hypersensitivity pneumonitis and fungal or parasitic infections.
Management and treatment
Complete clinical and radiological recovery without relapse may be achieved rapidly with corticosteroid therapy (between 2 and 12 weeks intravenous dosage followed by a switch to oral administration).In patients not properly treated, respiratory failure is observed, requiring mechanical ventilation or extracorporeal membrane oxygenation (ECMO).
Prognosis
Prognosis of IAEP is excellent as long as corticosteroid therapy is instituted promptly. Without the appropriate diagnosis and treatment, IAEP may be fatal.

                            Last update: 
                            November 2013


                            - Expert reviewer(s): 
                            Pr Jean-Christophe DUBUS