Severe form of encephalopathy associated with 2009 pandemic influenza A (H1N1) in Japan.

Background: Every year, an estimated 200-500 children in Japan develop influenza-associated encephalopathy (IAE), and 10-30% of these children die.

Objective: To clarify the clinical features of a severe form of acute encephalopathy seen with 2009 pandemic influenza A (H1N1).

Study Design: This retrospective survey examined 20 children with acute encephalopathy associated with the 2009 pandemic influenza A (H1N1) who died or were in a prolonged deep coma with a flat electroencephalogram tracing and loss of spontaneous respiration. We obtained demographic, clinical, laboratory, and neuroimaging data through interviews with the attending physicians and chart reviews.

Results: Subjects were 13 boys and seven girls. Their median age was 45 (range 11-200) months. Five patients had one or more pre-existing conditions. Acute encephalopathy developed within 2 days after influenza onset in 16 patients. As the initial neurological symptom, delirious behavior was seen in six children, and brief seizures in six. Eighteen patients were comatose within 6h of the onset of encephalopathy. Marked brain edema on computed tomography (CT) was seen in all but one patient. Brainstem lesions on CT were recognized in 12 patients. Sixteen patients died 0-45 (median 2.5) days after the onset of acute encephalopathy, and the others remained in deep comas without spontaneous respiration.

Conclusions: The clinical course of the patients was characterized by an onset with mild neurological symptoms and rapid deterioration of consciousness into coma. Head CT revealed marked cerebral edema, often associated with brainstem lesions.