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Eosinophilic pulmonary syndromes are a diverse group of disorders typically characterized by peripheral blood eosinophilia (typically > 500 cells/mcL [0.5 × 109/L]), eosinophilic pulmonary infiltrates, dyspnea, and cough. Many patients have constitutional symptoms, including fever. Common causes include exposure to medications (nitrofurantoin, phenytoin, ampicillin, acetaminophen) or infection with helminths (eg, Ascaris, hookworms, Strongyloides) or filariae (eg, Wuchereria bancrofti, Brugia malayi, tropical pulmonary eosinophilia). Löffler syndrome refers to acute eosinophilic pulmonary infiltrates in response to transpulmonary passage of helminth larvae. Pulmonary eosinophilia can also be a feature of other illnesses, including ABPA, eosinophilic granulomatosis with polyangiitis, systemic hypereosinophilic syndromes, eosinophilic granuloma of the lung (properly referred to as pulmonary Langerhans cell histiocytosis), neoplasms, and numerous interstitial lung diseases (eFigure 9–16). If an extrinsic cause is identified, therapy consists of removal of the offending medication or treatment of the underlying parasitic infection.

eFigure 9–16.

A: Eosinophilic pneumonitis. This patient with parasite-induced eosinophilic pneumonitis has ill-defined nodular and fluffy, patchy densities scattered throughout both lungs. B: The lateral film from the same patient shows a better-defined infiltrate at the left lung base.

One-third of cases are idiopathic, and there are two common syndromes. Acute eosinophilic pneumonia is an acute, febrile illness characterized by cough and dyspnea, sometimes rapidly progressing to respiratory failure. The chest radiograph is abnormal but nonspecific. BAL fluid frequently shows eosinophilia, but peripheral blood eosinophilia is rare at the onset of symptoms. The response to corticosteroids is usually dramatic. Chronic eosinophilic pneumonia has a subacute-chronic presentation, characterized by fever, night sweats, weight loss, and dyspnea. Asthma or atopy is present in half of cases. Chest radiographs often show peripheral infiltrates (eFigure 9–17), the “photographic negative” of pulmonary edema. BAL typically has a marked eosinophilia, and peripheral blood eosinophilia is present in greater than 80%. Therapy with oral prednisone (1 mg/kg/day for 1–2 weeks, followed by a gradual taper over months) usually results in dramatic improvement; however, most patients require at least 10–15 mg of prednisone every other day for a year or more (sometimes indefinitely) to prevent relapses.

eFigure 9–17.

Eosinophilic pneumonia. The pulmonary infiltrate in this case is confined to the lingular portion of the left lung and extends into the upper lobe. There is also a small pleural effusion. This is one of the manifestations of eosinophilic pneumonia.

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Rosenberg  CE  et al. Approach to eosinophilia presenting with pulmonary symptoms. Chest. 2021;159:507.
[PubMed: 33002503]  
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Suzuki  Y  et al. Eosinophilic pneumonia: a review of the previous literature, causes, diagnosis, and management. Allergol Int. 2019;68:413.
[PubMed: 31253537]  

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