Elsevier

Epilepsy Research

Volume 88, Issues 2–3, February 2010, Pages 221-230
Epilepsy Research

Quantitative MRI predicts status epilepticus-induced hippocampal injury in the lithium–pilocarpine rat model

https://doi.org/10.1016/j.eplepsyres.2009.11.013Get rights and content

Summary

Convulsive status epilepticus (SE) is a common medical neurological emergency and is associated with hippocampal injury and the subsequent development of epilepsy. However, pathophysiological mechanisms that underlie injury remain unclear, and a clinically useful prognostic biomarker of at-risk patients remains elusive. We hypothesised that non-invasive quantitative multi-parametric MRI characterisation of the early time course in the lithium–pilocarpine rat model would provide insight into pathophysiological processes, and may help to develop a non-invasive prognostic marker of hippocampal injury.

T1, T2, apparent diffusion coefficient (ADC), and cerebral blood flow (CBF) were measured before and after SE on days 0, 1, 2, 3, 7, 14 and 21. Hippocampal volume measurements were used to assess final structural outcome. MRI changes were found in the parietal cortex, hippocampus, piriform cortex, and thalamus. Each of the regions displayed time-dependent changes, and returned to baseline levels by Day 7. Hippocampal measurements peaked on Day 2, and further analysis revealed that the magnitude of these peak changes was predictive of the hippocampal volumes on Day 21. This time course is consistent with cell death and an inflammatory process. The maximal changes provide a potential clinically useful prognostic marker of final hippocampal volume.

Introduction

Convulsive status epilepticus (SE) is a common medical neurological emergency that has been associated with brain injury, subsequent epilepsy and cognitive dysfunction (Mathern et al., 2002, Raspall-Chaure et al., 2006). Hippocampal CA1 and CA3 subfields have been shown to be particularly vulnerable to injury associated with SE (Fujikawa, 1996), although other brain areas may also be injured. Furthermore, hippocampal injury may mature into mesial temporal sclerosis (MTS), the most common abnormality seen in patients with epilepsy who undergo surgical treatment (Engel, 1996, Sloviter, 2005). These data therefore support the long-standing hypothesis that there is a progression from SE to acute hippocampal injury and later MTS associated with epilepsy (Cavanagh and Meyer, 1956). Nevertheless, there is currently no clinically approved therapy to modulate this process. Therefore an understanding of how the pathophysiological mechanisms evolve following SE may help to identify windows of opportunity for possible novel interventions aimed at these processes. Such therapies could then be used to minimise the risk of developing MTS associated with epilepsy.

Epidemiological studies indicate that only a small proportion of people may develop MTS after an episode of SE (most commonly febrile status epilepticus in children) (for a review see Raspall-Chaure et al., 2006), suggesting that interventions would be most appropriately targeted to an at-risk population. Currently there remains no clinically approved method for identifying these patients. MRI is widely used to diagnose patients with MTS, characterised by reductions in hippocampal volumes and increased T2, and MRI signal hyperintensities in the hippocampus on T2-weighted imaging have been observed during the initial 72 h following SE (VanLandingham et al., 1998, Scott et al., 2002, Provenzale et al., 2008). In addition, at least subjectively, there is evidence of the predictive potential of MRI for progression on to MTS (Provenzale et al., 2008). A quantitative approach using various imaging techniques may therefore improve the predictive capacity of MRI for MTS. As ethical and practical considerations prevent a full longitudinal characterisation of the early MRI changes in patients for identification of a surrogate marker of injury, animal models can be useful for investigating and characterising these early events.

Animal models typically have SE induced either by chemical or electrical means, with the rat being the most commonly used species. Following cessation of SE, there is a period of apparent quiescence before progression to spontaneous recurrent seizures (White, 2002). Thus these models follow the hypothesised route of SE-induced hippocampal injury that evolves to MTS-associated temporal lobe epilepsy (TLE). MRI has previously been used to investigate these processes in animal models in which SE has been induced using electrical stimulation, kainic acid, pilocarpine (with or without lithium pre-treatment) or hyperthermia (Nairismagi et al., 2004, Roch et al., 2002, Dube et al., 2004, Bhagat et al., 2001, Nakasu et al., 1995, Greene et al., 2007). In all of these studies, hippocampal MRI changes have been detected during the initial few days following SE, which parallel those observed in humans (Scott et al., 2006). However, the previous studies have only sampled the early time course infrequently, a quantitative approach has not been universal, and a time course of cerebral blood flow changes has not been investigated.

We therefore used quantitative multi-parametric MRI to characterise the time course of brain injury with more frequent early sampling than in previous studies, and to determine whether early multi-parametric MRI findings in the hippocampus can be used as a prognostic marker for permanent hippocampal damage.

Section snippets

Experimental design

All animal care and procedures were carried out in accordance with the UK Animals (Scientific Procedures) 1986 Act. A baseline scan was performed 1 day prior to SE induction following which rats underwent MRI over a period of 21 days. Twenty-one days was chosen as the final timepoint as previous reports have observed substantial cell loss by this time, spontaneous field bursts from hippocampal neurons have been detected, and spontaneous seizures have begun (Roch et al., 2002, Klitgaard et al.,

Pre-existing abnormalities

Pre-existing brain abnormalities were identified during the pre-SE time point in three animals. In all three of these animals, the T2-weighted images revealed unilateral hyperintense regions which indicated an enlarged ventricle. These rats were removed from the study due to these abnormalities, and no further imaging was conducted. In total, 26 rats that did not have abnormalities on baseline scans were included in the present study.

Multi-parametric MRI mapping time courses

Overall, significant differences were found in the time

Discussion

There were three novel findings from this multi-parametric quantitative MRI study. Firstly, transient MRI changes in the hippocampus and piriform cortex after SE reached a peak about 2 days following the lithium–pilocarpine induced seizure. Secondly, the degree of the peak change measured in the hippocampus at Day 2 predicted the severity of later hippocampal damage, and finally, recurrent isoflurane exposure provided a partial neuroprotective effect on the hippocampus.

Pilocarpine induced

Conclusion

In conclusion, using the lithium–pilocarpine rat model, we have characterised the evolution of brain injury following SE using non-invasive, quantitative multi-parametric MRI with a focus on the early period post-SE. Furthermore, we have identified an early prognostic marker of hippocampal injury that predicts subsequent degree of atrophy. These data suggest that MRI might be used to identify patients that will progress on to a poor outcome following SE, and might in turn present an opportunity

Acknowledgements

This work was supported by Epilepsy Research UK, Wellcome Trust, and the British Heart Foundation. The authors thank Dr. Martin King for his advice on statistics, Prof. Matthew Walker for his guidance on the lithium-pilocarpine model, and Prof. Gennadij Raivich, Prof. Sebastian Brandner, Dr. Daniel Cuthill, and Dr. Maria Hristova for their help on histology.

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