Eosinophilic Lung Disease

Approach to Diagnosis

Eosinophilic lung diseases can be challenging to diagnose, and a combined clinical, radiologic, and pathologic approach is essential. The clinical history and physical examination are important for determining the extent and duration of signs and symptoms. A diagnosis of asthma, travel history, occupational and environmental exposures, and previous and current medications are also important to identify.

Laboratory studies that aid in the diagnosis of eosinophilic lung disease include a white blood cell count with differential to look for peripheral blood eosinophilia, serum immunoglobulin E (IgE) level, and serum Aspergillus precipitins, and stool studies for cysts, ova, and parasites. Peripheral blood eosinophilia is defined as an eosinophil count >0.4 × 10 ⁹ /L. Peripheral blood eosinophilia is elevated in most eosinophilic lung diseases.

Diagnostic procedures that can help establish a diagnosis of eosinophilic lung disease include pulmonary function tests and bronchoalveolar lavage (BAL). Pulmonary function tests in patients with eosinophilic lung disease can show different patterns of disease. A restrictive defect usually occurs with acute eosinophilic pneumonia (AEP), chronic eosinophilic pneumonia (CEP), and tropical eosinophilic pneumonia. An obstructive defect usually occurs with allergic bronchopulmonary aspergillosis (ABPA) and Churg-Strauss syndrome (CSS). BAL is an extremely helpful test in diagnosing eosinophilic lung disease. Normal BAL fluid contains <1% eosinophils, whereas eosinophilic lung diseases are associated with BAL fluid with usually >10% eosinophils.

Sputum and tissue analysis can show Charcot-Leyden crystals, which are derived from a protein released by eosinophils. These crystals are present whenever there are large numbers of eosinophils present and thus are typically seen in eosinophilic lung diseases.

Both chest radiography and chest CT can show pulmonary opacities suggestive of eosinophilic lung disease. Transbronchial or surgical lung biopsy may be helpful to confirm the diagnosis in challenging cases.

A diagnosis of eosinophilic lung disease can be made if there is pulmonary opacity present on imaging studies and peripheral blood eosinophilia, biopsy confirmation of tissue eosinophilia, or increased eosinophils in BAL fluid. Additionally, Hodgkin lymphoma, sarcoidosis, infection, and hydatid disease must be excluded before a diagnosis of eosinophilic lung disease can be established.

Available Imaging Modalities to Evaluate Eosinophilic Lung Disease

Imaging procedures available to evaluate pulmonary eosinophilic disease include chest radiography and CT. Chest radiography is useful as a screening examination to detect pulmonary opacities, given its relatively low cost and low patient exposure to ionizing radiation. However, chest radiographic findings are nonspecific and may even be absent in patients with eosinophilic lung disease. In contrast, CT is more sensitive to the presence of lung opacities and sometimes shows a more specific pattern of disease, helping to narrow the differential diagnosis. Furthermore, CT can also be helpful in planning a transbronchial or surgical biopsy. However, compared to chest radiography, CT is more costly and results in higher levels of patient exposure to ionizing radiation.

Diseases Without a Known Cause

Simple Eosinophilic Pneumonia

SEP, also known as Löffler syndrome, is an idiopathic disease characterized by migrating pulmonary consolidation and peripheral blood eosinophilia. Symptoms are often mild and include cough, fever, and dyspnea.

Chest radiographs show migratory foci of consolidation that usually clear spontaneously within 1 month. Foci of consolidation are typically upper lung predominant and peripheral in location. CT shows peripheral ground-glass opacity, consolidation, or both in the mid and upper lungs ( Fig. 22.1 ). Nodules with surrounding ground-glass opacity may also be present. Pleural effusion, cavitation, and lymphadenopathy are absent. Histopathologically, eosinophils are seen infiltrating the alveolar septa and interstitium. The main differential diagnostic considerations include vasculitis and organizing pneumonia. Response to corticosteroids is excellent if treatment is necessary, and the prognosis is excellent.

Chronic Eosinophilic Pneumonia

CEP is an idiopathic eosinophilic lung disease with insidious onset. Signs and symptoms are usually present for more than 6 months before the diagnosis is established and include cough, fever, night sweats, dyspnea, and weight loss. Patients usually are middle-age women, and approximately 50% have a history of asthma or atopy.

Patients usually have leukocytosis, an elevated erythrocyte sedimentation rate, and a variable IgE level. Peripheral blood eosinophilia is present in approximately 90% of patients, but may be absent later in the disease process. Pulmonary function tests usually show a restrictive defect, and BAL usually shows >25% eosinophils. Characteristic chest radiographic and CT findings include peripheral areas of upper lung predominant ground-glass opacity and consolidation, which sometimes contain air bronchograms ( Fig. 22.2 ). However, other patterns of disease, including mixed peripheral and perihilar consolidation and unilateral disease, are also possible. Nodules, reticulation, and pleural effusion are less common findings and usually occur later in the disease course. On histopathology, eosinophils infiltrating into the alveoli and interstitium are present, sometimes forming eosinophilic abscesses.

Treatment of CEP is corticosteroids, often long term because up to 60% of patients relapse. Despite treatment, some patients subsequently develop CSS.