Acute eosinophilic pneumonia

Acute eosinophilic pneumonia (AEP) is an uncommon, acute-onset form of eosinophilic lung disease which varies in severity. Though poorly understood, the pathogenesis of AEP likely varies depending on the underlying cause which may include smoking, inhalation exposure, medication, and infection. In most patients, AEP is idiopathic, or has no known cause.

AEP is characterized by airway injury, vascular injury, and the release of IL-33, a pro-inflammatory cytokine. This results in the recruitment of eosinophils, a type of white blood cell, to the lungs with subsequent inflammation and symptoms. Symptoms are nonspecific and include cough, shortness of breath, malaise, myalgia, night sweats, and pleuritic chest pain.

Pathogenesis
While not completely understood, it is thought that AEP may be triggered by offending agents such as smoking, parasites, or fungi that trigger a hypersensitivity reaction.

Diagnosis
Eosinophilic pneumonia is diagnosed by characteristic imaging and the presence of eosinophils on alveolar lavage, or washing of the lower respiratory system. Eosinophilia, or elevated eosinophils in the blood may be significantly elevated or absent. Lung biopsy is usually not necessary for diagnosis.

While there is no formal diagnostic criteria, the modified Philit criteria has been used and includes: acute illness with fever for less than 21 month, hypoxemia, diffuse infiltrates in both lungs on imaging, greater than 25% eosinophils in bronchoalveolar lavage fluid (or eosinophils on lung biopsy), and no other known causes of eosinophilic lung disease present.

Treatment
Treatment generally involves removing exposure of the causal agent, if identifiable, and systemic steroid therapy. Patients often respond quickly to systemic steroids. Relapse of disease is rare.

History
AEP was first described in 1989 by Allen, Pacht, Gadek, and Davis, with eight proposed diagnostic criteria. These criteria included: acute febrile illness, hypoxemia, pulmonary infiltrates on imaging, alveolar eosinophils, ruled out infection, ruled out asthma, quick response to steroids, and followed by resolution of disease with no long-term effects. Currently there is no agreement on diagnostic criteria for AEP.