Elsevier

Neurobiology of Disease

Volume 32, Issue 2, November 2008, Pages 293-301
Neurobiology of Disease

Short- and long-term limbic abnormalities after experimental febrile seizures

https://doi.org/10.1016/j.nbd.2008.07.010Get rights and content

Abstract

Experimental febrile seizures (FS) are known to promote hyperexcitability of the limbic system and increase the risk for eventual temporal lobe epilepsy (TLE). Early markers of accompanying microstructural and metabolic changes may be provided by in vivo serial MRI.

FS were induced in 9-day old rats by hyperthermia. Quantitative multimodal MRI was applied 24 h and  8 weeks later, in rats with FS and age-matched controls, and comprised hippocampal volumetry and proton spectroscopy, and cerebral T2 relaxometry and diffusion tensor imaging (DTI). At 9 weeks histology was performed.

Hippocampal T2 relaxation time elevations appeared to be transient. DTI abnormalities detected in the amygdala persisted up to 8 weeks. Hippocampal volumes were not affected. Histology showed increased fiber density and anisotropy in the hippocampus, and reduced neuronal surface area in the amygdala.

Quantitative serial MRI is able to detect transient, and most importantly, long-term FS-induced changes that reflect microstructural alterations.

Introduction

The impact of febrile seizures (FS) on neurodevelopment is still largely unknown. Though they are generally considered benign, case reports, epidemiological, and retrospective studies suggest that a subpopulation of patients is at risk for developing mesial temporal sclerosis-associated temporal lobe epilepsy (MTS-TLE) (Cendes et al., 1993, Offringa et al., 1991, Ojemann, 2001). Diagnostic tools that can identify this subpopulation before they clinically manifest epilepsy are necessary to develop a rational anticipatory treatment. Because MTS-TLE patients are characterized by structural and metabolic abnormalities of the limbic system, it may be possible to visualize the development of these abnormalities by serial magnetic resonance imaging (MRI). MRI has several important beneficial features. First, quantitative MRI can image and quantify disease related changes over time (Tofts, 2003). Second, its versatile capabilities allow investigation of distinct tissue characteristics, including microstructural (T2 relaxation time and diffusion tensor imaging (DTI)) and metabolic (proton spectroscopy) changes.

Longitudinal MRI studies addressing the potential causative association between FS and MTS-TLE are scarce and have primarily utilized hippocampal T2 and volumetric measurements. These studies presented cases with a unilateral increase in hippocampal T2 signal and volume within days after prolonged FS. A follow-up MRI 2–10 months later demonstrated normal or increased hippocampal T2 relaxation time and reduced volume (Scott et al., 2003b, Sokol et al., 2003, VanLandingham et al., 1998). It was therefore suggested that FS occasionally cause acute hippocampal edema that evolves into sclerosis. However, hippocampal sclerosis (HS, i.e. elevated T2 relaxation time and reduced volume) was not observed in all patients that showed post-FS edema. This raises the questions whether the follow-up MRI had been acquired too early and whether a possible developing MTS would have been noticed with other MR modalities. A severe difficulty in prospective studies is the long and variable latent period that may exist between FS and eventual HS allowing the occurrence of other factors (e.g. trauma, seizures, or infection) that potentially bias the outcome.

Animal models may prove to be valuable in prospective MRI as they allow a more standardized experimental design and conditions (Grohn and Pitkanen, 2007). Moreover, animal models open the possibility for comparing in vivo imaging data with postmortem histology. A recent T2 weighted MRI study has demonstrated early effects, i.e. 87% of rats with prolonged FS at postnatal day (PN) 11 had increased T2 signal in the limbic region 8 days after the seizure (Dube et al., 2004). Interestingly, the T2 abnormalities were not accompanied by histological evidence of neurodegeneration. A subsequent study showed that 35% of the FS rats had spontaneous electro-clinical seizures and 88% displayed interictal epileptiform discharges at PN180 (Dube et al., 2006). The present study examined this model using quantitative multimodal MRI and serially (24 h and 8 weeks after FS) assessed the brains of FS-induced rats. Histological analysis was performed 9 weeks after FS.

The aim of this study was to determine early and late microstructural and metabolic changes induced by experimental FS, which are possibly related to pathologic cellular processes linked to epileptogenesis. Revealing cellular processes underlying these changes may provide insights in how early-life FS, tissue abnormalities, and (late) epilepsy are related.

Section snippets

Hyperthermia treatment

Sprague–Dawley rats (Harlan, The Netherlands) were born and housed under standard conditions. Experiments were approved by the local Animal Experiments Committee.

Hyperthermia (HT) was induced as described previously (Baram et al., 1997, Lemmens et al., 2005). In brief, on PN9, rat pups (HT+) were placed in a cylinder, their core temperature raised with an adjustable stream of heated air to 41–42.5 °C for 30 min. Core temperatures were measured before and every 2.5 min during the HT treatment.

Animal model

Sixty-seven percent of the rats showed FS behavior after HT (HT+ rats) with a seizure duration of 9.1 ± 2.0 min. Mortality during the follow-up MR examinations at PN66 was 18% (2/11) and 0% (0/9) for the HT+ group and normothermia group, respectively, and was probably due to respiratory failure during anesthesia. During the MR experiments, all control and experimental animals were stable, displaying normal body temperature and respiratory signal.

Combined regional analysis

The ordinary least squares test revealed

Current findings

This study applied quantitative multimodal MRI, comprising hippocampal volumetry and proton spectroscopy, T2 relaxometry, and diffusion weighted imaging, to detect and monitor cerebral FS-induced abnormalities. Experimental FS were shown to induce short-term T2 relaxometric and diffusion changes, particularly in the limbic system. Most importantly, FS provoked long-term diffusion changes in the hippocampus and amygdala, and histological analysis revealed microstructural alterations.

Short-term effects

The present

Acknowledgments

We gratefully acknowledge W. Jennekens and J. Habets for their valuable assistance.

References (49)

  • CavassilaS. et al.

    Cramer-Rao bounds: an evaluation tool for quantitation

    NMR. Biomed.

    (2001)
  • CendesF. et al.

    Early childhood prolonged febrile convulsions, atrophy and sclerosis of mesial structures, and temporal lobe epilepsy: an MRI volumetric study

    Neurology

    (1993)
  • ConnellyA. et al.

    Magnetic resonance spectroscopy in temporal lobe epilepsy

    Neurology

    (1994)
  • de GraafR.A. et al.

    High magnetic field water and metabolite proton T1 and T2 relaxation in rat brain in vivo

    Magn. Reson. Med.

    (2006)
  • DubeC. et al.

    Prolonged febrile seizures in the immature rat model enhance hippocampal excitability long term

    Ann. Neurol.

    (2000)
  • DubeC. et al.

    Serial MRI after experimental febrile seizures: altered T2 signal without neuronal death

    Ann. Neurol.

    (2004)
  • DubeC. et al.

    Temporal lobe epilepsy after experimental prolonged febrile seizures: prospective analysis

    Brain

    (2006)
  • FarinaL. et al.

    Acute diffusion abnormalities in the hippocampus of children with new-onset seizures: the development of mesial temporal sclerosis

    Neuroradiology

    (2004)
  • GrohnO. et al.

    Magnetic resonance imaging in animal models of epilepsy-noninvasive detection of structural alterations

    Epilepsia

    (2007)
  • GundersenH.J. et al.

    The new stereological tools: dissector, fractionator, nucleator and point sampled intercepts and their use in pathological research and diagnosis

    Apmis.

    (1988)
  • HochbergY.

    A sharper Bonferroni procedure for multiple tests of significance

    Biometrika

    (1988)
  • JensenF.E. et al.

    Developmental seizures induced by common early-life insults: short- and long-term effects on seizure susceptibility

    Ment. Retard. Dev. Disabil. Res. Rev.

    (2000)
  • JuppB. et al.

    Hippocampal T2 signal change during amygdala kindling epileptogenesis

    Epilepsia

    (2006)
  • LäuterJ.

    Exact t and F tests for analyzing studies with multiple endpoints

    Biometrics

    (1996)
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