Objective Simple febrile delirium is usually reversible and benign in children; however, delirium associated with influenza recently attracted attention in Japan when several pediatric patients jumped from significant heights and were injured or died. The MR imaging finding of a reversible lesion with transiently reduced diffusion in the splenium of the corpus callosum (SCC) has been reported in patients with clinically mild encephalitis/encephalopathy, leading to a new clinical-radiological syndrome, clinically mild encephalitis/encephalopathy with a reversible splenial lesion (MERS). We have often experienced delirium in patients with MERS. The purpose of this study is to evaluate the frequency of delirium in a series of patients with MERS.

Methods We retrospectively reviewed clinical charts of 54 patients (45 patients less than 15 years; mean age, 9 years) with newly diagnosed MERS; who were collected from the members of the Japanese Society of Pediatric Neurology between September, 2006 and March, 2008 after institutional review board approval from Kameda Medical Center.

Results Delirium was the most frequent symptom (54%, 29/54), followed by seizures (35%, 19/54), and decreased consciousness level (33%, 18/54). Among the 10 patients associated with influenza, 9 presented with delirium.

Conclusion MERS should be considered as an underlying condition in children with delirium. This study suggested that influenza-associated MERS presents with delirium more frequently than MERS associated with other pathogens (90% (9/10) vs 45% (20/44)).