Clinicopathological findings of four cases of pure influenza virus A pneumonia.

Objective The purpose of this study was to perform clinicopathological evaluations of patients with pure influenza A virus pneumonia. Methods We performed clinicopathological analyses of four cases of pure influenza A virus pneumonia. Patients Among the four cases, three were caused by the pandemic (H1N1) 2009 virus. Three patients were analyzed during autopsy, and one underwent transbronchial lung biopsy. Results We suggest that the interval between influenza virus A pneumonia onset and our analysis affected the pathological findings. Diffuse alveolar damage was observed during the acute phase. After ten days, organizing pneumonia and marked proliferation of premature type II alveolar epithelium were observed. Clinically, intra-alveolar hemorrhage was observed in two patients. Pathologically, hyaline membrane formation and intra-alveolar hemorrhage were observed in all cases. Conclusion Severe epithelial damage was determined as the main mechanism of respiratory failure caused by influenza A virus pneumonia.