Abstracts

Rationale: Magnetic resonance imaging (MRI) studies have shown increased T2 signal intensity in the hippocampi of some infants immediately following febrile status epilepticus (FSE). It is assumed that the increased T2 signal represents increased water content in the affected hippocampi. Measurements of the apparent diffusion coefficient of water (ADC) in the affected hippocampi have yielded mixed results with some revealing increased ADC, compatible with vasogenic edema or increased extracellular space, and others finding reduced ADC, compatible with cytotoxic edema or reduced extracellular space. We have measured the hippocampal ADCs in infants within 72 hours following FSE as part of a prospective study of the consequences of prolonged febrile seizures in children. Our results indicate that the hippocampal ADCs are often reduced in the infants with increased hippocampal T2 signal compared to the infants without increased T2 signal immediately following FSE.Methods: During our multicenter study of the consequences of prolonged febrile seizures, diffusion weighted imaging (DWI) and coronal oblique T2 weighted images of the hippocampi have been performed on a large group of infants within 72 hours of an episode of FSE. In the DWI images, hippocampal ADCs are sampled by placing regions of interest entirely within each slice of each hippocampus in coronal ADC maps. The average of the ADCs for all slices of each hippocampus was designated the mean ADC for that hippocampus. The T2 weighted images of each infants hippocampi were reviewed for T2 signal abnormality as well. Hippocampi were classified for this study into two groups, those with normal T2 signal and those with definitely increased T2 signal throughout the hippocampus. We then compared the ADCs in the hippocampi with definitely abnormal T2 signal to those with normal signal intensity. Results: Eleven hippocampi, one hippocampus in each of 11 infants, had definitely increased T2 signal intensity. Satisfactory ADC maps were available on 5 of these 11 infants. The mean ADC of the 5 hippocampi with definite increased T2 signal was 0.0010 compared to the mean hippocampal ADC of the 106 subjects with normal hippocampi of 0.0012(P = 0.04). These preliminary data suggest that, for DWI done within 72 hours of FSE, a definite increase in hippocampal T2 signal intensity is associated with reduced ADC. Conclusions: We tentatively interpret the reduction of ADC in hippocampi with increased T2 signal intensity as indicative of cytotoxic edema in the acute phase of hippocampal injury following FSE. Supported by Grant NS 43209 from NINDS.