[A case of eosinophilic pneumonia with histologically proven mediastinal lymph node involvement]

Intrathoracic lymphadenopathy in eosinophilic pneumonia is very rare and few cases have been confirmed histologically. This is the first case of eosinophilic pneumonia with mediastinoscopic lymph node biopsy reported in Japan. The case is a 42-year-old man who was admitted to our hospital complaining of cough, general fatigue and dyspnea of one month's duration. Chest X-ray demonstrated bilateral hilar and mediastinal lymph node enlargement with interlobar pleural thickening and infiltrative shadows in right lower lung field. Computed tomography revealed infiltrative shadows at right S4, S5, S8 and S9 and S10 segments and #2, #3, #5, #6 lymph node enlargement. Biopsy of the lymph node via mediastinoscopy demonstrated that the architecture was preserved, sinusoids were filled with histiocytes and eosinophils, and lymphoid follicles were compressed. Immunohistochemical technique using monoclonal antibody EG2 which reacts with the secreted form of Eosinophilic Cationic Protein (ECP), demonstrated that eosinophils infiltrating the mediastinal lymph nodes were activated. The pathogenesis of mediastinal lymphadenopathy in eosinophilic pneumonia remains to be determined, but our case suggests that mediastinal lymph nodes may play a role in eosinophilic pneumonia.