Answer: Pulmonary bleb with reactive eosinophilic pleuritis

Histology: This section shows a pleural bleb that has recently ruptured and is presumably the immediate cause of the patient’s spontaneous pneumothorax. The fibrous tissue wall of the bleb shows local accumulation of mesothelial cells admixed with eosinophils and some macrophages. This lesion is Reactive Eosinophilic Pleuritis. The associated pulmonary parenchyma shows no specific abnormality. There is a focal interstitial perivascular eosinophilic infiltrate.

Discussion: This wedge biopsy shows characteristic features seen in patients with spontaneous pneumothorax, although the finding of an almost intact bleb is a relatively rare occurrence. Blebs represent dissection of air from the lung into the fibrous layers of the pleura with formation of an air-filled space. Subsequent rupture of this air-filled space leads to a bronchopleural fistula with the production of pneumothorax. Bulla, on the other hand, represents large emphysematous spaces in the lung and are not related to interstitial dissection of air. Reactive eosinophilic pleuritis (REP) represents a reaction to irritation of the pleural surface by air. The cells involved are mesothelial cells, macrophages and eosinophils. It is important to recognize the existence of this reactive phenomenon in order to avoid making an erroneous diagnosis of pulmonary eosinophilic granuloma (Langerhans’ cells histiocytosis). As seen in this case, interstitial perivascular eosinophilic infiltrates may also be seen in patients with REP. The lung segments obtained in this situation usually represent apparent blebs resected by the surgeon during a procedure to obliterate the pleural space. Other reported findings in the lung of patients with spontaneous pneumothorax have been non-specific interstitial fibrosis, perhaps representing the sequella of previous clinical or subclinical pneumothoraces. Separation of REP from pulmonary eosinophilic granuloma is made by review of the chest radiograph, which should show no interstitial lung disease, by the lack of interstitial lesions in the lung. and by the clinical history. In cases where the diagnosis of eosinophilic granuloma is of serious clinical concern, immunostaining with antibodies to S100 protein and to CD1A may be of help. Remember, that patients with pulmonary eosinophilic granuloma may also have spontaneous pneumothorax and so it is possible to have a patient with PEG and REP. In the pediatric population, metastatic sarcoma is a serious consideration in patients with pneumothorax, since sarcoma may metastasize to the lung and present with pneumothorax in the absence of a clinically recognizable mass in the lung parenchyma.