Influenza virus‐induced encephalopathy: Clinicopathologic study of an autopsied case

Background: Rapid progressive encephalopathy with a high fever, consciousness loss and recurrent convulsions has been occasionally reported in children during influenza pandemics in Japan since 1995. We examined a 2-year old girl with hemorrhagic shock and encephalopathy syndrome associated with acute influenza A virus infection (A/Nagasaki/76/98; H3N2), to answer several questions for which no histologic or virologic data exist. Methods: A clinicopathologic study using immunohistochemical staining and viral genome detection by reverse transcriptase polymerase chain reaction (RT-PCR) was performed with this autopsied case. Results: The virus antigen was positive in CD8+ T lymphocytes from the lung and spleen. The virus infected a very limited part of the brain, especially Purkinje cells in the cerebellum and many neurons in the pons, without inducing an overt immunologic reaction from the host. The RT-PCR used for detecting the hemagglutinin gene demonstrated positive bands in all frozen tissues and cerebrospinal fluid taken at autopsy and not in samples obtained on admission. Conclusions: The pathologic change induced by the direct viral invasion cannot be responsible for all of the symptoms, especially for the rapid and severe clinical course of the disease within 24–48 h after the initial respiratory symptoms. Together with the rapid production of several inflammatory cytokines, the breakdown of the blood–brain barrier may induce severe brain edema and can be a major pathologic change for the disease. Any therapeutic strategy to control this multistep progression of the disease could be effective.