Distinguishing Acute Encephalopathy with Biphasic Seizures and Late Reduced Diffusion from Prolonged Febrile Seizures by Acute Phase EEG Spectrum Analysis

The term acute encephalopathy (AE) encompasses various etiologies with acute insult to the brain and clinical manifestations of seizures, impaired consciousness and other neurological symptoms. This includes bacterial meningitis, viral encephalitis, hypoxic-ischemic encephalopathy (HIE), head injury, cerebrovascular disorders, and encephalopathies secondary to hepatic or renal failure.1, 2 Among others, AE as a complication of common viral (rarely bacterial) infections often affects young children and causes death or severe neurological sequelae. This subgroup of AE is a main cause of acute cerebral injury in Japan.3, 4 Acute encephalopathy with biphasic seizures and late reduced diffusion (AESD) is the most frequent subtype,3, 5 characterized by no abnormalities on magnetic resonance imaging (MRI) at the disease onset, and evolution of second phase of encephalopathy following 4–7 days of latent period, accompanied by reduced diffusion due to cytotoxic edema.6–8 It is often difficult to identify which patient represents the first phase of AESD, and which just manifests with a prolonged febrile seizure (FS) with favorable outcome. Given that a sustained increase in excitatory transmitters play a role in the provocation of second phase in AESD,8 early initiation of treatment may lead to a better outcome. Therefore, early diagnosis of AESD is desirable for decision of treatment strategy that possibly prevents or ameliorates the second phase of AESD to improve the prognosis of this burden for previously healthy children. Electroencephalogram (EEG) during the acute phase of AE shows diffuse slowing and attenuation/ flattening in encephalopathy due to various etiologies,1, 9 including AESD.10, 11 However, it is not easy to distinguish AE from prolonged FS or infection-induced epileptic status (ES), because diffuse delta activities are also often seen in these conditions for a few days after seizure onset.10, 11 The aim of this study was to examine whether qualitative analysis of EEGs during acute phase of AEs was able to predict the prognosis, and specifically to find any findings that distinguish EEGs of AESD from those of other disorders including FS and ES, through quantitative EEG analysis by using fast Fourier transform (FFT).