Abstracts

Prolonged febrile convulsion (PFC) is the most common form of status epilepticus in childhood and may have a causative relationship with mesial temporal sclerosis (MTS), the most common structural lesion identified in patients requiring surgery in the treatment of epilepsy. Our recently published data on children investigated within 5 days of a PFC is suggestive of hippocampal oedema (large hippocampal volume (HCV) and prolonged T2 relaxation time (T2)) that is resolving over a 5 day period, suggesting that PFC can injure the hippocampus. However, those data do not rule out the possibility of a pre-existing lesion such as hippocampal dysgenesis. Thus, the aims of the current follow-up study are; to determine whether HCV and T2 have changed 4-8 months after a PFC, to determine whether follow-up HCV and T2 are different in patients and control subjects, and to determine whether HCV or T2 side-to-side asymmetry has increased.
Patients were requested to have follow-up magnetic resonance (MR) investigations 4-8 months after their initial scan. They were assessed for evidence of seizures, developmental delay or behavioural abnormalities. Subsequently, MR investigations including a FLASH 3-D dataset for HCV measurement, and T2 mapping were obtained. Paired sample t-tests and general linear modelling, after adjustment for age and intracranial volume, were used for statistical analysis. Hippocampal asymmetry was characterised by an asymmetry index (AI). HCV, T2 or AI that falls outside the 95% prediction limits for control subjects is considered abnormal.
14 patients (14-31 months) have had follow-up investigations. The second scan was carried out a mean of 5.5 months after the acute scan. 14 age matched controls had HCV and 15 age matched controls had T2. Of the 14 patients, 4 have had further seizures. 2 had short febrile convulsions, one had PFC and one had non-febrile seizures. There was a significant reduction in HCV (p=0.002) and T2 (p=0.018) between the first and second investigations. There is now no difference in HCV (p=0.91) or T2 (p=0.99) in patients when compared to controls. There is an increase in HCV asymmetry in patients at follow-up (p=0.044). 5/14 patients had an abnormal AI for HCV. Three had one hippocampus outside the lower 95% prediction limit for control subjects.
A bilateral reduction in HCV and T2 between the first and second scans confirms that the original findings are temporary and are more consistent with hippocampal oedema than with a pre-existing abnormality. The change in hippocampal symmetry in the patient group is consistent with injury and neuronal loss at the time of PFC, especially in the 3 individuals who now have a single small hippocampus. As there is no T2 abnormality, the hippocampi do not meet the criteria for MTS. There may be a lag period of several years between a PFC and the onset of epilepsy, and therefore some of these patients may be in the process of developing MTS.
[Supported by: The Wellcome Trust]