Eosinophilic pneumonia is a clinical syndrome characterized by eosinophil infiltration of the lungs with or without eosinophilia in the peripheral blood, and can be divided into acute eosinophilic pneumonia (AEP) and chronic eosinophilic pneumonia (CEP).
Etiology
The etiology of acute eosinophilic pneumonia has not yet been determined. It is generally believed the disease is associated with inhaled allergic substances in the environment, and may be hypersensitivity reaction caused by unknown allergens.
The etiology of chronic eosinophilic pneumonia may resemble that of simple eosinophilia, or it may be an autoimmune disease caused by the synergistic effect of type III and IV hypersensitivity reactions, or type II hypersensitivity reaction. Although the exact immune pathogenesis is still unclear, there is a lot of evidence that eosinophils play an important initial role in lung tissue damage. Before the onset of clinical symptoms, the count of eosinophils in peripheral blood and bone marrow has increased. In BALF, there are also mainly eosinophils. Eosinophil-derived granule protein (EDGP) can be seen in the lung parenchyma and microvessels. Compared with the control group, EDGP in BALF of CEP patients is also increased. BALF-derived eosinophil activation markers, including type II histocompatibility antigens, are also increased. The increase in the expression of type II histocompatibility antigens and other activation markers in BALF-derived eosinophils rather than eosinophils in blood indicates that the immune inflammatory response is confined to the lungs. In vitro, immunoglobulins can amplify eosinophil chemotaxis and degranulation. Increased immune circulating complexes and IgE titers are associated with the clinical onset of the disease. So far, the relationship between eosinophil activation and immunoglobulins is unclear.
Pathogenesis
The main pathological changes of acute eosinophilic pneumonia are acute diffuse alveolar damage. Obvious eosinophil infiltration can be seen in the alveolar cavities, interstitium, and bronchial wall. In most patients, hyaline membranes may form and type II alveolar epithelial cells may proliferate. In the late stage, interstitial edema, massive infiltration of inflammatory cells, and fibrosis can be seen. There is no vasculitis or damage to extrapulmonary organs.
The lung damage of chronic eosinophilic pneumonia is manifested by infiltration of mainly eosinophils, but also macrophages, lymphocytes, and occasionally plasma cells in the alveoli and interstitium. Alveolar wall structure destruction, localized edema of capillary endothelium, focal type II epithelial cell hyperplasia, alveolar protein exudation, and multinucleated tissue cell infiltration are present. 1/3 of patients have manifestations of proliferative obstructive bronchiolitis. Mild non-necrotizing microvasculitis can also be seen, and venules are mainly involved. In few patients (less than 20%), obvious necrosis, eosinophilic microabscesses, or non-caseous granulomas can be seen. Lymphatic hyperplasia and eosinophil infiltration are seen in mediastinal lymph node biopsy specimens.
Clinical manifestations
Acute eosinophilic pneumonia is mainly characterized by acute onset, dyspnea, fever (mainly moderate fever), cough, thoracodynia, myalgia, and abdominal pain; and moist crackles in the lungs can be heard in most patients.
Chrome eosinophilic pneumonia is mainly manifested by cough, expectoration, tachypnea, dyspnea, and fever (mainly low-grade fever), accompanied by diaphoresis and emaciation. The disease can be easily misdiagnosed as pulmonary tuberculosis or lung tumor. Extrapulmonary manifestations are often atypical, and arthralgia, neuropathy, skin urticaria, and gastrointestinal lesions have also been reported.
Auxiliary examination
The typical manifestation of chest x-ray is dense alveolar exudation in the peripheral lung, with a clear central zone, termed photographic negative of pulmonary edema, and the exudative lesions are mostly in the upper lobe. 80% of patients have increased eosinophils in the peripheral blood. Increased serum IgE is also common.
Diagnosis
If patients have clinical manifestations and imaging features, and eosinophils in BALF are greater than 40%, eosinophilic pneumonia is highly suggested.
Treatment
Acute eosinophilic pneumonia
Systemic glucocorticoids are preferred. Symptoms can be relieved within several hours after initial treatment, and pulmonary infiltration can completely subside within 1 - 2 weeks. The treatment regimen is methylprednisolone intravenously 60 - 125mg once every 6 hours in the early stage, followed by prednisone orally 40 - 60mg once a day for 2 - 4 weeks after the symptoms are controlled, and then the dose is reduced gradually until discontinuation.
Chronic eosinophilic pneumonia
Prednisone is preferred for CEP, and the initial dose is 40mg/d. After treatment, fever subsides within 6 hours; dyspnea, cough, and, eosinophil infiltration are alleviated within 24 - 48 hours; and hypoxemia is relieved within 2 - 3 days. Symptoms are completely relieved within 2 - 3 weeks, and x-ray return to normal within 2 months. The dose is gradually reduced (about in 10 - 14 days) after the symptoms improve, and the treatment course is 4 - 6 months.
Prognosis
AEP can spontaneously relieve in some patients. After recovery, the disease will not relapse. The prognosis of acute eosinophilic pneumonia is generally good.
The prognosis of CEP is generally good, but CEP can occasionally cause death. Untreated patients rarely get relief, and the mortality of patients treated with glucocorticoids is significantly reduced. However, if prednisone is reduced or discontinued too quickly, the disease relapses in 58% - 80% of patients. Relapse often occurs at the original anatomical site, requiring 1 - 2 years of treatment, and 25% of patients require long-term maintenance doses (2.5 - 10 mg/d) until healing. Although there is no indicator that patients will relapse or require long-term maintenance treatment, short-term (1 - 3 months) treatment often causes relapse, and multiple relapses may occur, but prednisone is still effective.