An unusual case of eosinophilic lung disease with multiple cyst formation.

We report here an unusual case of eosinophilic necrotizing inflammation of the lung that mimicked chronic eosinophilic pneumonia. A 71-year-old man who lived in an unsanitary environment and was referred to our hospital with suspected pneumonia. Peripheral blood eosinophilia was observed, and computed tomography indicated extensive consolidation with multiple cystic lesions, mainly in the left lung. A histological analysis using video-assisted thoracic surgery revealed diffuse infiltration of inflammatory cells into the alveolar wall and massive accumulation of macrophages and eosinophils in the airspace. Many tiny eosinophilic abscesses were scattered through the tissue. These findings were more severe than those associated with chronic eosinophilic pneumonia. Immunostaining revealed the deposition of eosinophil granular protein and the presence of extracellular traps and Charcot-Leyden crystals, which suggested excessive eosinophil activation. Interestingly, the patient's symptoms and clinical findings gradually improved without treatment after admission. He was discharged to a clean residence and did not have a recurrence for 19 months. The observations suggest a hypersensitivity reaction to an environmental allergen and consequent multiple cyst formation in association with eosinophilic necrotizing inflammation, although further studies are warranted.