Abstracts

There is mounting evidence that prolonged febrile convulsion (PFC) causes acute hippocampal oedema (prolonged T2 relaxation time and large hippocampal volume). The hippocampal diffusion characteristics associated with PFC may be useful in the understanding of the mechanisms of the oedema. In addition, the age dependency of apparent diffusion coefficient (ADC) in the hippocampus is not well characterised. Therefore the aims of the study are (1) to characterise ADC abnormalities within 5 days of a PFC, and to determine whether any such abnormalities had evolved 4-8 months later and (2) to characterise ADC age dependency in the hippocampus. Diffusion weighted imaging, with calculation of ADC maps, was acquired in 23 patients (median age 18 months, range 7-21 months) within 5 days of a PFC, and in 14 of these children a mean of 5.5 months later. 20 control subjects (median age 12 months, range 6-39 months) were also enrolled. A region of interest was placed in the hippocampus on the ADC map and the mean ADC for the region was recorded. The PFC was generalised at the time it was first observed in all patients. 3 children had a further PFC between the magnetic resonance investigations. One patient has developed non-febrile seizures, with no associated structural brain abnormality. In control subjects there is a strong dependence of ADC on age (p = 0.001) but no age dependence was identified in patients at the initial timepoint (p=0.35) or at follow-up (p=0.65). On a paired analysis there is a reduction in ADC between the acute and follow-up investigations in patients investigated within 2 days of a PFC (p=0.048), but not in those children investigated 3-5 days after the PFC (p=0.9). The 2 major findings in the current study are (1) the reduction in ADC over time in patients investigated within 2 days of a PFC but not in those investigated 3-5 days after the event, and (2)the difference in the age trajectory between patients and controls. The reduction in ADC over time provides additional evidence for hippocampal oedema, that is vasogenic in nature, within 2 days of PFC. The oedema has largely recovered within 5 days of the PFC. The difference in the dependence of ADC upon age in the children with a PFC, compared to control subjects, at either the initial or follow-up timepoints, supports the view that hippocampal development is abnormal in children who are prone to PFC. Understanding the cellular mechanisms for such developmental differences may provide insight into mechanisms of a predisposition to PFC. (Supported by The Wellcome Trust)