Apparent diffusion coefficient (ADC) measurements were used to characterize hippocampal edema within 5 days of a prolonged febrile seizure (PFS) in a study at Great Ormond Street Hospital, London, UK. A reduction in ADC between acute and 4-8 month follow-up measurements was indicative of early vasogenic edema, followed by recovery in children investigated within 2 days, but not in those measured between 3 and 5 days of a PFS. An expected age dependence decrease in ADC observed in control subjects was not present in children following a PFS. The findings are consistent with resolution of an early onset acute vasogenic edema that follows a PFS. The authors propose that the ADC data reflect a preexisting developmental hippocampal abnormality that predisposes to a PFS. The data also suggest that the vasogenic edema developing within 2 days after a PFS resolves in 3-5 days. 
COMMENT. These ADC findings corroborate the longitudinal MRI data reported by these authors previously . Within 48 h of a PFC hippocampal volumes were enlarged and T2 relaxation times prolonged, whereas MRI studies delayed >48 h but within 5 days of PFC revealed large hippocampal volumes and normal T2 relaxation time. These findings were suggestive of hippocampal edema that is resolving within 5 days of a PFC. Repeat MRI at 4-8 month follow-up showed reduction in hippocampal volume with asymmetry and reduced T2 relaxation time compared to the first exam. It is postulated that mesial temporal sclerosis may develop after a lag period, or the hippocampal asymmetry represents a return to a preexisting hippocampal developmental abnormalty that antedates the PFC. (Ped Neur Briefs Nov 2003;17:83).
Hippocampal asymmetry in 6-year follow-up MRI study of complicated convulsion at University of Sheffield, UK. . Significant hippocampal asymmetry unrelated to edema was initially reported within 2 weeks of a first complicated early childhood convulsion in >50% of 17 subjects tested. At 6-year follow-up, 3 of 8 retested showed significantly greater asymmetry and 2, a modest increase in asymmetry; 2 showed no change and 1 a resolution of asymmetry. The asymmetry was detected only by volumetric analysis of MRI and relaxation times, and was consistent with a progressive pathology and developing hippocampal sclerosis in 2 patients.
Frontal subcortial white matter lesions following PFS with encephalopathy are described in a diffusion-weighted MRI study between 3 and 9 days after the seizure. Diffusion abnormalities disappeared between days 9 and 25, leaving cerebral atrophy after 2 weeks (Ped Neur Briefs May 2006;20:33).