Dyspnea and cough are always present. The severity of dyspnea is highly variable 3. Respiratory failure necessitating mechanical ventilation, however, is very rare 1415, in contrast with idiopathic acute eosinophilic pneumonia. Wheezing occurs in about half of the cases. Chest auscultation findings are non-specific, with possible inspiratory crackles or expiratory wheezes 3.

Asthenia, weight loss (sometimes marked), and nocturnal sweat or fever are frequent 237.

Extrathoracic features are absent in ICEP. When the symptoms and signs of extra-pulmonary involvement are present, a diagnosis of Churg-Strauss syndrome (CSS) or idiopathic hypereosinophilic syndrome (IHS) should be considered. A few patients with an initial diagnosis of ICEP, however, may develop minor extrathoracic manifestations without further fulfilling the diagnostic criteria for either CSS or IHS. It is noteworthy that a continuum between ICEP and CSS has been suggested 161718.

ICEP is characterized by peripheral parenchymal infiltrates on chest imaging. These infiltrates can be either unilateral or most often bilateral. The most distinctive chest X-ray images mimic the photographic negative of acute pulmonary edema 10. This typical pattern, however, is absent in the majority of cases of ICEP 2 and is not specific for the disorder, since it has also been described in cryptogenic organizing pneumonia, sarcoidosis, or drug-induced pneumonia. The parenchymal infiltrates are alveolar in nature, ranging from ground-glass opacities to consolidation with air bronchogram. They are sometimes migratory 37. Pleural effusion is uncommon.

Computing tomography (CT) may show discrete ground-glass opacities that are not discernible on X-rays. Bilateral parenchymal abnormalities are commonly seen with CT 3. Mediastinal lymph node enlargement has been described 319.

Although anecdotal cases without blood eosinophilia have been reported 2920, it is present in most cases, usually in excess of 1000/mm³ 3. In the absence of significant blood eosinophilia, a diagnosis of ICEP is supported by the demonstration of bronchoalveolar lavage eosinophilia (≥ 40%). Erythrocyte sedimentation rate and C-reactive protein level are usually increased. Total immunoglobulin E (IgE) levels are elevated in about half of the cases 3, reflecting the high proportion of patients with an atopic background.

Bronchoalveolar lavage in ICEP always reveals abnormally high levels of eosinophils, representing 12% to 95% (mean: 58%) of the total cell count 3. In contrast to cryptogenic organizing pneumonia with which it shares many common features, ICEP is typically associated with eosinophil counts higher than lymphocyte counts in BAL.

ICEP can be associated with either a restrictive or an obstructive pattern on pulmonary function tests. The latter pattern is more often present in patients with a history of asthma 216. However, spirometry remains within normal limits in up to one third of the cases 3. Diffusion tests often show a reduced carbon monoxide (CO) transfer factor (DLCO); the CO transfer coefficient (KCO) is reduced in up to one quarter of the patients with ICEP 3. Arterial blood gases usually demonstrate a mild to moderate hypoxemia 2916.

Pathological confirmation of ICEP is usually not necessary to establish the diagnosis. When performed, lung biopsy shows an interstitial and alveolar inflammation with a clear-cut predominance of eosinophils 29. Infiltration of pulmonary vessels with eosinophils may be observed but necrotising or granulomatous vasculitis are not present in ICEP 6. Foci of organising pneumonia are frequently observed 2.

Several drugs may cause eosinophilic pneumonias. The most frequently involved drugs are nonsteroidal anti-inflammatory agents, including salicylates used in inflammatory bowel diseases (e.g. sulfasalazine) and antibiotics, including minocycline and cotrimoxazole. A detailed history with respect to drugs consumption is needed in order to exclude any potential pharmacological cause of eosinophilic pneumonia. Updated information on causative drugs is available 2122.

Allergic broncho-pulmonary aspergillosis (ABPA) and other allergic broncho-pulmonary mycoses are characterized by type I and III immune reactions against Aspergillus fumigatus or other fungi. Characteristic features of ABPA are asthma, migratory pulmonary infiltrates associated with blood and alveolar eosinophilia, central bronchiectasis, high total IgE levels, positive immediate and late skin tests in response to the causative agent, as well as positive specific IgE and precipitins 23.

Some parasitic infections that are generally transient are associated with eosinophilia and pulmonary infiltrates. Causative agents are Ascaris lumbricoides, Toxocara canis,Taenia saginata, Trichinella spiralis and Fasciola hepatica. For patients living in tropical areas or with a history of travel in tropical areas, other parasites must be included in the search: Strongyloides stercoralis, Wuchereria bancrofti, Brugia malayi, Schistosoma mansoni and Schistosoma haematobium. Diagnosis relies on serologic tests and/or stool examination 24.

Other conditions such as malignancy (e.g. lung cancer) and other infections (e.g. tuberculosis) are exceptionally associated with eosinophilic lung disease.

Idiopathic acute eosinophilic pneumonia (IAEP) differs from ICEP by the more rapid onset and the severity of its clinical presentation. The time period between the onset of symptoms and the diagnosis is usually inferior to 14 days 2526. In contrast to ICEP, an atopic background is usually absent and there is no sex preponderance. Mean age at presentation is about 30. Patients present with cough and dyspnea, which is often severe and associated with tachypnea. Chest pain of pleuritic character and fever are frequently observed. Hypoxemic respiratory failure is frequent at presentation, often requiring mechanical ventilation 2526. On imaging, IAEP is characterized by diffuse bilateral air-space or interstitial opacities without peripheral predominance. Pleural effusion is frequent and sometimes bilateral 25. The white blood cell count rarely points towards the diagnosis at presentation, with the eosinophil count being lower than 500/mm³ in 2/3 of the patients 2526. The BAL, however, is the key to the diagnosis 252627, showing an average percentage of eosinophils of 40% in one series 25. IAEP dramatically improves with or without corticosteroid therapy but, in contrast to ICEP, relapses are not described after tapering or withdrawing treatment in IAEP.

Churg-Strauss syndrome (CSS) is a systemic eosinophilic vasculitis which usually develops in patients with a long history of atopic disease including asthma and, most often, allergic rhinitis. Dramatic blood eosinophilia and infiltration of a variety of tissues with eosinophils which ensue, are eventually followed by a systemic vasculitis. The latter affects many organs including the heart (heart failure, acute coronary syndrome, pericarditis), the gastro-intestinal tract (abdominal pain, gastrointestinal bleeding or perforation), the skin (purpura, urticaria, nodules), and the nervous system (mononeuritis multiplex, central nervous system involvement). Pulmonary infiltrates, similar to those described in ICEP, are observed in only half of the cases at the time of diagnosis 28. Antineutrophil cytoplasmic autoantibodies (ANCA) with a perinuclear pattern (p-ANCA) and an anti-myeloperoxydase specificity are found in about 50% of patients with CSS 29. As already mentioned, it is likely that a continuum exists between ICEP and CSS, at least in some patients.

Idiopathic hypereosinophilic syndrome (IHS), a rare and heterogeneous disorder, has been defined by blood eosinophilia greater than 1500/mm³ for more than 6 months, and signs or symptoms of organ damage related to eosinophilic infiltration 30. Cardiac involvement including mural thrombosis, endocardial fibrosis, which lead to restrictive cardiomyopathy, is the most important complication of IHS and the major cause of morbidity and mortality 31. Pulmonary involvement is observed in 40% of cases and is characterized by interstitial infiltrates on chest imaging; pleural effusion is seen in 50% of affected patients 32.

Cryptogenic organizing pneumonia shares many features with ICEP with respect to clinical presentation and imaging 33. However, it is not associated with blood eosinophilia and lymphocytes outnumber eosinophils in BAL.