Quantitative diffusion MRI of hippocampus as a surrogate marker for post-traumatic epileptogenesis
1Epilepsy Research Group, 2Biomedical NMR Research Group, Department of Neurobiology, A.I. Virtanen Institute for Molecular Sciences and 3Department of Neuroscience and Neurology, University of Kuopio, FIN-70211 Kuopio, Finland
Correspondence to: Dr Asla Pitkänen, A.I. Virtanen Institute for Molecular Sciences, University of Kuopio, PO Box 1627, FIN-70211, Kuopio, Finland. E-mail: asla.pitkanen{at}uku.fi
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Summary |
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The need to use animal models to develop imaging markers that could be linked to electrophysiological abnormalities in epilepsy and able to predict epileptogenicity in human studies is widely acknowledged. This study aimed to investigate the value of early magnetic resonance imaging (MRI) in predicting the long-term increased seizure susceptibility in the clinically relevant model of post-traumatic epilepsy (PTE).
Moderate traumatic brain injury (TBI) was induced by lateral fluid-percussion in two groups of adult rats (34 injured, 16 controls). In Experiment 1, MRI follow-up was performed using a 4.7 T magnet at 3 h, 3 days, 9 days, 23 days, 2 months, 3 months and 6 months after TBI. T2 and 1/3 of the trace of the diffusion tensor (Dav) were quantified from a single slice using a fast spin-echo sequence. In Experiment 2, MRI was performed at 7 and 11 months post-injury. In both groups, seizure susceptibility was tested by injecting a single dose of pentylenetetrazol at 12 months post-injury. Electrographic and behavioural responses were monitored for 1 h. Total number of spikes, total number of epileptiform discharges (EDs) and latency to first spike were measured. Finally, the severity of mossy fibre sprouting was evaluated. In both experiments, EEG parameters such as total number of spikes or EDs proved to be reliable indicators of increased seizure susceptibility in injured animals when compared to controls (P < 0.05). In the hippocampus ipsilateral to TBI, Dav correlated with these EEG parameters at both early (3 h), and chronic (23 days, 2, 3, 6, 7 and 11 months) time points after TBI, as well as with the density of mossy fibre sprouting. These results for the first time demonstrate that quantitative diffusion MRI can serve as a tool to facilitate prediction of increased seizure susceptibility in a clinically relevant model of human PTE.
Key Words: traumatic brain injury; magnetic resonance imaging; surrogate marker; post-traumatic epilepsy; video-EEG monitoring
Abbreviations: MRI, magnetic resonance imaging; PTE, post-traumatic epilepsy; TBI, traumatic brain injury; FPBI, fluid-percussion brain injury
Received July 3, 2007. Revised September 24, 2007. Accepted October 16, 2007.
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Introduction |
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Epilepsy is a major source of morbidity after TBI, which can complicate up to 50% of cases of severe head injury and 4.2–6.7% of cases of mild to moderate injury (Salazar et al., 1985
; Asikainen et al., 1999; Frey, 2003). Post-traumatic epilepsy (PTE) is the most common cause of new-onset epilepsy in young people (Annegers, 1996) and the cause of epilepsy in 5% of patients referred to specialized epilepsy centres (Annegers et al., 1980; Hauser et al., 1991; Semah et al., 1998). PTE is often refractory to medical treatment (Herman, 2002). Attempts have been made to develop prophylactic antiepileptogenic therapy, but none have been successful so far (Temkin et al., 1990; Schierhout and Roberts, 1998; Haltiner et al., 1999; Temkin et al., 1999). It is now generally acknowledged that in the process of continuing the search for treatments that could prevent epileptogenesis after TBI, it is important to identify and characterize potential surrogate markers of epileptogenesis and epilepsy. That is, indices that can be objectively measured and evaluated as indicators of evolving pathogenic processes (Curing Epilepsy 2000, "Benchmarks" for Epilepsy Research, http://www.ninds.nih.gov/funding/research/epilepsyweb/benchmarks.htm).
Magnetic resonance imaging (MRI) has become the imaging method of choice for identifying the structural basis of seizure disorders (Duncan, 1997, 2002). Quantitative MRI procedures have been used successfully to characterize the structural abnormalities associated with head injury (Assaf et al., 1997; Wieshmann et al., 1999b; Rugg-Gunn et al., 2001a) and post-traumatic epilepsy (Wieshmann et al., 1999a; Gupta et al., 2005). Nevertheless, currently there is little information regarding potential correlations of quantitative MRI changes with histological and neurological outcomes after TBI.
The purpose of this study was to investigate the utility of quantitative diffusion MRI as a potential marker of secondary cerebral damage leading to seizures in PTE. We used longitudinal/serial MRI measurements during evolving epileptogenesis in a well-established model of closed head injury, in which 50% of animals develop epilepsy with structural and functional changes closely resembling those of human PTE (Kharatishvili et al., 2006). We studied the association between quantitative diffusion MRI measures obtained at various time points after the lateral fluid-percussion brain injury (FPBI) and the EEG indicators of long-lasting susceptibility to seizures after FPBI. We also studied the association of MRI and EEG changes with mossy fibre synaptic reorganization that is a common histological finding in surgically treated patients with TLE, and thus, a marker for hyperexcitable hippocampus.
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Material and Methods |
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Study design
The study design is summarized in Fig. 1 and consisted of two independent experiments. In Experiment 1, 14 animals subjected to lateral FPBI and 6 controls underwent MRI investigation at 3 h, 3 days, 9 days, 23 days, 2 months, 3 months and 6 months after the brain injury. In Experiment 2, 20 animals with lateral FPBI and 10 controls were imaged at 7 and 11 months after the brain injury to rule out the influence of repetitive imaging under anaesthesia on the natural course of post-traumatic epileptogenesis. Eleven months post-injury in both the experiments, animals were implanted with cortical electrodes and underwent continuous 3-week video-EEG monitoring. Twelve months post-injury animals were subjected to the pentylenetetrazol (PTZ) test and afterwards sacrificed for histology.
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Animals
Adult male Harlan Sprague-Dawley rats (50 injured, 16 controls, 305–390 g; Harlan Netherlands B.V., Horst, the Netherlands) were used for the study. The animals were housed in individual cages and kept under controlled laboratory conditions (light regime of 12 h light/12 h dark, light on at 07:00 a.m.; temperature, 22 ± 1°C; air humidity, 50–60%, ad libitum access to food and water). All animal procedures were approved by the Committee for the Welfare of Laboratory Animals of the University of Kuopio and the Provincial Government of Kuopio, and conducted in accordance with the guidelines set by the European Community Council Directives 86/609/EEC.
Lateral fluid-percussion brain injury
TBI was induced by lateral FPBI as originally described (McIntosh et al., 1989; Kharatishvili et al., 2006). Briefly, following anaesthesia, the head of the animal was mounted in a Kopf stereotactic frame (David Kopf Instruments, Tujunga, CA). A midline scalp incision was made and the underlying periosteum dissected. The scalp and left temporal muscle were reflected, exposing the skull and a 5 mm circular craniectomy was performed with a trephine over the left parietal lobe midway between lambda and bregma, with the lateral edge of craniectomy adjacent to the left lateral ridge. A modified Luer–Lock cap was cemented over the craniectomy and filled with saline. At 90 min after administration of anaesthesia, animals were connected to the fluid-percussion device (AmScien Instruments, Richmond, VA, USA) through the male Luer–Lock fitting and brain injury of 2.3–3.2 atm severity was induced as originally described by McIntosh et al. (1989). Control animals received anaesthesia and all surgical procedures without FPBI injury.
MRI
MRI data were acquired using a 4.7 T Magnex magnet interfaced to a Varian UNITYINOVA console (Varian Inc., Palo Alto, CA, USA). A quadrature half-volume radio frequency coil with two 1.8 cm loops (HF Imaging LLC, Minneapolis, MN, USA) was used as transmitter and receiver. Rats were anaesthetized with 1% halothane in N2O/O2 (70%/30%).
Animals were positioned in the MRI compatible stereotactic holder and the head was securely fixed using ear bars and a nose bar to prevent any movement artefacts. Warm water (37 ± 0.5°C) was circulated in a heating element to control the temperature of the animals.
MRI was performed 3 h, 3 days, 9 days, 23 days, 2 months, 3 months and 6 months after FPBI in Experiment 1, and at 7 months and 11 months in Experiment 2. Volumetric changes were detected using a T2-weighted spin-echo multi-slice sequence with adiabatic refocusing RF pulses to minimize the influence of a moderately inhomogeneous B1 field (echo time 70 ms, repetition time 3 s, field of view of 30 x 30 mm2 covered with 128 x 256 data points, slice thickness 0.75 mm, 19 consecutive slices covering rat cerebrum). T2 and 1/3 of the trace of diffusion tensor (Dav) were quantified from a single slice using a magnetization-prepared fast spin-echo sequence with adiabatic BIR-4 refocusing pulses (repetition time 3.0 s, 16 echoes/excitation, centre-out k-space filling, echo spacing 10 ms, field of view of 30 x 30 mm2 covered with 128 x 256 data points, slice thickness 1.5 mm). T2 relaxation time was measured using a spin-echo preparation block consisting of an adiabatic half passage (AHP), two hyperbolic secant (HS) adiabatic full passages, reverse AHP and crusher gradient in front of the fast spin-echo sequence (echo times 20, 38, 52, 76 ms). One-third of the trace of the diffusion tensor was quantified using the same pulse sequence with a diffusion sensitizing gradient pair positioned around each refocusing RF pulse in the magnetization preparation block. Three images with different degrees of diffusion weighting (b = 90, 496, 1014 s/mm2, diffusion time = 29 ms) were obtained in three different orthogonal orientations.
Quantitative relaxation and diffusion maps were calculated by fitting the data to standard single exponential formulae in Matlab. Seven regions of interest (ROI) were manually outlined in T2-weighted images and transferred to T2 maps and diffusion maps as follows: the parietal cortex (excluding the lesion area) bilaterally, the hippocampus bilaterally, the lesion area and the lateral ventricles (Fig. 2).
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Electrode implantation and video-EEG monitoring
Eleven months after the TBI, animals in Experiments 1 and 2 were anaesthetized with a single intraperitoneal (i.p.) injection (6 ml/kg) mixture containing sodium pentobarbital (58 mg/kg), chloral hydrate (60 mg/kg), magnesium sulphate (127.2 mg/kg), propylene glycol (42.8%) and absolute ethanol (11.6%). Animals were then mounted in a stereotactic frame (lambda and bregma on the same horizontal level). Two cortical screw electrodes (E363/20 Plastics One Inc., Roanoke, VA, USA) were placed over the parietal cortex, one rostral to the craniectomy, and the other contralateral to the centre of the craniectomy. The electrode implantation paradigm in the present study was based on the results from our previous experiments (Kharatishvili et al., 2006
). According to the data from long-term (12 months) video-EEG monitoring in rats with chronic spontaneous seizures, epileptiform activity detected by the depth electrode located in the ventral hippocampus ipsilateral to injury invariably rapidly propagated to the cortex, both ipsi- and contralateral to the lesion. We concluded consequently, that if any spontaneous epileptiform activity arises in the hippocampus it will be picked up by the cortical electrode as well. Therefore, the depth hippocampal electrode had not been implanted to avoid additional mortality in the brain-injured animals of this age due to breathing problems caused by prolonged anaesthesia. Two cortical screw electrodes were placed over the left and right frontal cortex and were used as supplementary electrodes if there was a problem in obtaining a good-quality signal from the parietal electrodes. Two stainless steel screw electrodes were inserted into the skull bilaterally over the cerebellum and served as indifferent and ground electrodes. All electrode pins were inserted into the plastic pedestal, and the entire assembly was cemented to the calvarium with dental acrylic.
Details of the methodology of long-term video-EEG monitoring were described previously (Nissinen et al., 2000). Briefly, electrical brain activity was recorded with the Nervus EEG Recording System connected with a Nervus magnus 32/8 amplifier (Taugagreining, Iceland), and filtered (High-pass filter 0.3 Hz cutoff, low-pass 100 Hz). The behaviour of the animals was monitored using the WV-BP330/GE video camera (Panasonic) that was positioned above the cages and connected to a SVT-N72P time lapse VCR (Sony) and a PVM-145E video monitor (Sony). A wide-angle lens allowed simultaneous videotaping of eight animals. Rats were first continuously video-EEG monitored for 3 weeks to detect the occurrence of epileptiform activity: spontaneous seizures or spiking. The spike was defined as paroxysmal potential (i.e. arising suddenly from the background), very sharp in contour, with the duration of 20–70 ms, usually followed by a low-voltage slow potential (about 200 ms duration) before re-establishing the baseline. Thereafter, animals were subjected to PTZ test and monitored for 60 min following the injection.
Pentylenetetrazol test
To detect any enhanced seizure susceptibility in post-TBI rats, we used a single dose of PTZ to challenge rats 12 months after TBI and compared these with age-matched controls that received the same dose of PTZ. To determine whether seizure threshold was reduced after TBI, we selected a normally subconvulsant dose of PTZ based on earlier works showing that 20–30 mg/kg of PTZ (intraperitoneal) is subconvulsant for adult male Sprague-Dawley rats (Golarai et al., 2001; Velisek, 2006). All tests were performed between 9.00 a.m. and 2.00 p.m. Pentylenetetrazol (1,5-pentamethylenetetrazole, 98%, Sigma-Aldrich YA-Kemia Oy, Finland) was dissolved in sterile 0.9% saline (12.5 mg/ml solution) and injected i.p. at a dose of 30 mg/kg of body weight in Experiment 1 and 25 mg/kg in Experiment 2. Each rat received a single injection of PTZ.
Following the PTZ injection, rats were placed separately into transparent plexiglas cages (47 x 29 x 50 cm3) where they could move freely, and video-EEG was recorded from parietal electrodes for 60 min after PTZ administration. An electrographic seizure was defined as a >5 s duration high-amplitude rhythmic discharge with a clear onset, temporal evolution in wave morphology and amplitude, and offset. Electrographic interictal epileptiform discharge (ED) was defined as a high-amplitude rhythmic discharge containing a burst of slow waves, spike-wave and/or polyspike-wave components and lasting <5 s. A spike was defined as a high-amplitude (twice a baseline) sharply contoured waveform with a duration of 20–70 ms. Latency to the first spike, total number of spikes and total number of epileptiform discharges were calculated during 60 min after PTZ administration. These parameters were chosen according to the unpublished results from our previous experiment (Kharatishvili et al., 2006). Spike counting did not include the electrographic seizure events. In addition to continuous video recording, behaviour was monitored by an observer. The time of the occurrence of epileptiform behavioural events was recorded and scored according to a modified Racine type scale (1 = twitching, freezing, 2 = myoclonic jerks of one forelimb; 3 = bilateral forelimb clonus; 4 = forelimb clonus with rearing; 5 = tonic–clonic convulsion).
Histology
Fixation and processing of tissue
The rats were perfused transcardially for histological analysis immediately after finishing the PTZ test. The animals were deeply anaesthetized and perfused according to the following fixation protocol: 0.37% sulphide solution (30 ml/min) for 10 min followed by 4% paraformaldehyde in 0.1 M phosphate buffer, pH 7.4 (30 ml/min), 4°C, for 10 min. The brains were removed from the skull and post-fixed in buffered 4% paraformaldehyde for 4 h and then cryoprotected in a solution containing 20% glycerol in 0.02 M potassium–phosphate-buffered saline (KPBS) for 24 h. The brains were then blocked, frozen in dry ice and stored at –70°C until cut. The brains were sectioned in the coronal plane (30 µm, 1-in-5 series) with a sliding microtome. The first series of sections were stored in formalin for thionin staining, for cytoarchitectonic characterization of various brain areas and localization of the lesion. The remaining series were stored in a cryoprotectant tissue-collecting solution (TCS, 30% ethylene glycol, 25% glycerol in 0.05 M sodium phosphate buffer) at –20°C until processed.
Timm histochemistry
Timm staining was used to visualize mossy fibre reorganization in the inner molecular layer of the dentate gyrus that accompanies epileptogenesis (Sutula et al., 1989; Cavazos et al., 1991). Mossy fibre sprouting was analysed from sections stained with the Timm sulphide/silver method (Sloviter, 1982; Nissinen et al., 2000). For staining, all coronal sections (30 µm, 1-in-5 series) where the hippocampus was present were mounted on gelatin-coated slides and dried at 37°C. Staining was performed in the dark according to the following procedure: working solution that contained gum arabic (300 g/l), sodium citrate buffer (25.5 g/l citric acid monohydrate and 23.4 g/l sodium citrate), hydroquinone (16.9 g/l) and silver nitrate (84.5 mg/l) were poured into the staining dish that contained the slides. The sections were developed until an appropriate staining intensity was attained (60–75 min). The slides were then rinsed in tap water for 30 min and placed in 5% solution of sodium thiosulfate for 12 min. Finally, sections were dehydrated through an ascending series of ethanol, cleared in xylene, and coverslipped with DePeX mounting medium (BDH, Laboratory Supplies, England).
Mossy fibre sprouting was analysed along the septotemporal axis of the hippocampus. The septal end included the coronal sections between AP levels 2.3 and 6.0 mm posterior from bregma. The dorsal mid-portion and ventral mid-portion of the dentate gyrus included dorsal and ventral parts of the hippocampus where the granule cell layer of the septal and temporal ends becomes fused and forms an easily identifiable and standardized ‘oval-shaped’ layer (AP level 6.1–6.7 mm) posterior to bregma. The density of mossy fibre sprouting was scored according to Cavazos et al. (1991): Score 0 = no granules, Score 1 = sparse granules in the supragranular region and in the inner molecular layer, Score 2 = granules evenly distributed throughout the supragranular region and the inner molecular layer, Score 3 = almost a continuous band of granules in the supragranular region and inner molecular layer, Score 4 = continuous band of granules in the supragranular region and in the inner molecular layer, Score 5 = confluent and dense laminar band of granules that covers most of the inner molecular layer, in addition to the supragranular region. The scoring was performed by an observer that was blinded to the identity of the rat.
Statistics
All data were analysed using SPSS for Windows (version 14.0). The Mann–Whitney U-test was used to assess differences between the two groups. The Spearman rank correlation coefficient was used as a non-parametric test to assess correlations between MRI data, EEG data and histological results. A P-value of <0.05 was considered to be significant. Data are presented as mean ± SD.
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Results |
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Injury severity and acute (<72 h) mortality
In Experiment 1, the total animal number was 20. One animal died during the anaesthesia before induction of FPBI. From the remaining 19 animals, subjected to FPBI, 5 died within 72 h of the injury, so the final number of animals included in the analysis was 14. In Experiment 2, the total number of animals was 30. One animal died immediately after anaesthesia, one during the operation before induction of FPBI. From the remaining 28 animals, 6 died within 72 h after trauma. One animal died 7 days after the injury (intracranial bleeding), one was euthanized 26 days after the injury because of the severe weight-loss and weakness. The final number of animals included in the study was 20.
Mortality during the first 72 h post-injury was used as a marker of injury severity (Thompson et al., 2005). In Experiment 1, the force of the applied pressure pulse in FPBI varied from 2.3 to 3.2 atm (mean 2.64 atm). The mortality rate was 25% (three animals died on the day of injury, two died within 72 h after FPBI). In Experiment 2, the force of the applied pressure pulse in FPBI was 3.0 to 3.1 atm (mean 2.86 atm). The mortality rate was 21% (four animals died on the day of injury, and two animals within 72 h after FPBI). Thus, in both experiments the severity of injury corresponded to ‘moderate’ injury (McIntosh et al., 1989; Thompson et al., 2005; Dubreuil et al., 2006). Mortality values as well as MRI data on lesion size and progression proved that the injury was similar in both experiments.
Mortality during the time course (>72 h post-injury) of the experiments
In Experiment 1, three animals died during the imaging sessions at 3 days, 9 days and 6 months time points, likely reason of death—halothane anaesthesia/overdose. One animal had to be euthanized on the third week after the injury because of the severe eye infection (probable cause the tattoo colour/rat number on the ear); this rat has had several spontaneous seizures within 2 weeks post-injury but was not included in the analysis. In Experiment 2, one animal was operated 10 months post-injury due to a large tumour in the jaw area, and died soon after the operation. After participating in the second MRI session, two animals with FPBI had to be euthanized due to the paresis of the intestine and severe weight-loss, two more animals were euthanized (did not recover from the anaesthesia during the electrode implantation) 11 months after the injury because of the tumours on the neck/ear and abdominal area, respectively. One control animal died after anaesthesia during the electrode implantation, another one lost the headset and had to be euthanized because of the strong bleeding from the wound.
Video-EEG monitoring
Eleven months post-injury animals were implanted with cortical electrodes and underwent continuous 3-week video-EEG monitoring. In both Experiments 1 and 2, video-EEG monitoring did not reveal spontaneous seizure activity in any of the experimental animals. This may relate to a low-seizure frequency in this model and the single video-EEG monitoring session at the end of the study which was probably not long enough for seizure detection (compare with Kharatishvili et al., 2006). Also, animals had milder TBI than in the previous study, thus lower expectancy of epilepsy. Epileptiform activity was, however, recorded in 80% of rats with FPBI in the form of isolated spikes (similar results in Experiments 1 and 2). In injured animals, spiking activity was registered by the cortical electrode placed over the parietal cortex on the side of the injury; in some cases bilateral spike discharges were registered over the parietal cortex with a spike of smaller amplitude on the contralateral side (propagation). No spiking activity was observed in any of the control animals during the 3 weeks of video-EEG monitoring.
Analysis of post-traumatic seizure susceptibility using PTZ test
The behavioural signs of seizure activity that typically occurred sequentially after the PTZ administration were as follows: (i) myoclonic twitch—sudden, involuntary jerking of the whole body, (ii) clonic movements of forelimbs with preservation of the righting reflex and (iii) generalized clonic seizure with loss of righting reflex that corresponds to the observations previously published by Löscher et al. (1991). In the EEG, a single dose of PTZ induced electrographic epileptiform discharges (brief bursts of high amplitude slow waves, spikes and/or spike-and-wave episodes) in all animals of both injured and control groups. Brief (
1 s) spike-wave and high-amplitude delta bursts corresponded to myoclonic twitches, while ictal epileptiform discharges of >5 s duration corresponded to clonic seizures. The general evolution of EEG record after the PTZ injection was as follows: (i) brief bursts of high-amplitude slow waves, (ii) isolated spike-waves or burst of several spike-waves lasting 1 s, (iii) electrographic seizure.
In Experiment 1, 6 MR-imaged controls and 10 out of 14 MR-imaged rats with FPBI survived 12 months and participated in the PTZ test. After a 30 mg/kg dose of PTZ injection, all animals survived for at least 60 min following the injection. One animal with FPBI lost a headset during a seizure induced by PTZ and was excluded from the EEG analysis. All rats with FPBI and four out of six controls experienced clonic convulsions (behavioural score 2–5). As summarized in Fig. 3, the total number of spikes during the 60-min follow-up was higher in injured animals (n = 9) as compared to controls (n = 6) (mean 140 ± 109 versus 40 ± 32, P < 0.05). Also, the total number of epileptiform discharges was higher in injured animals as compared to controls (mean 86 ± 73 versus 32 ± 26, P < 0.05). The latency to the first spike was shorter in injured animals as compared to controls (mean 102 ± 73 s versus 331 ± 240 s, P < 0.05) (Fig. 3). There was a negative correlation between latency to the first spike and the total number of spikes (r = –0.41, P < 0.05, n = 15). Similar to our previous experiment [animals included in the study by Kharatishvili et al. (2006), data not shown], no difference was found between the groups in the latency to the first behavioural symptom, latency to the (first) electrographic/behavioural seizure or in electrographic/behavioural seizure duration and severity.
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In Experiment 2, 8 of 10 MR-imaged controls and 15 out of 20 MR-imaged rats with FPBI survived 12 months. All control animals and 14 injured (one rat with FPBI was excluded because of the extracranial haematoma attached to the skull and suspicion of epidural haematoma in MR images) participated in the PTZ test. In this experiment, we reduced the dose of PTZ from 30 to 25 mg/kg to avoid a very rapid and robust response to PTZ which was seen in a majority of animals with TBI in Experiment 1. The wider range of convulsive response in injured animals also allowed us to better correlate the various outcome parameters of lowered seizure threshold with histological and MRI findings. After injecting 25 mg/kg of PTZ, all animals survived for at least 60 min. In both the FPBI and control groups, 50% of rats exhibited clonic convulsions (behavioural score 2–5). Two out of 14 animals with TBI developed status epilepticus and had a large amount of artefacts in EEG. In these animals, EEG analysis of the number of spikes and the number of EDs could not have been performed. As in Experiment 1, the total number of spikes during the 60-min follow up was higher in injured animals (n = 12) as compared to controls (n = 8) (mean 240 ± 111 versus 34 ± 60, P < 0.05). Also, the total number of epileptiform discharges was higher in injured animals as compared to controls (mean 144 ± 54 versus 64 ± 55, P < 0.05). The latency to the first spike was shorter in injured animals as compared to controls (mean 474 ± 372 s versus 1065 ± 396 s, P < 0.05) (Fig. 3). There was no difference between the groups in latency to the first behavioural symptom, latency to the electrographic/behavioural seizure and electrographic/behavioural seizure duration and severity.
The PTZ test results showed that a substantial proportion of the rats had developed a lowered seizure threshold and, hence, were likely undergoing epileptogenesis at the moment of PTZ test (Figs 3 and 4).
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MR imaging
The aim of the present study was to investigate the association between the severity of MRI changes during the course of epileptogenesis with seizure susceptibility that develops later on. Unlike expected, the cortical damage did not associate with the lowered seizure threshold. However, in the hippocampus ipsilateral to the injury, from two parameters measured (Dav, T2), the abnormality of Dav was consistently associated with the lowered seizure threshold, and therefore, the most of the description of imaging part of the study is focused on Dav in the hippocampus as a predictor of increased epileptogenicity. Data are summarized in Fig. 5.
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Severity and progression of the cortical lesion
In the cortex ipsilateral to injury, there was a progressive increase in T2 values in FPBI animals compared to controls at all time points (P < 0.05). However, we observed a substantial variation in injured animals in T2 values from 9 days time point onwards which can be explained by inter-animal variability in lesion progression, but also, by the problems we had experienced in drawing the ROI accurately (identification of boundary between the lesioned cortex and enlarged ventricle). We did not find any association between the T2 values and indicators of lowered seizure threshold at any of the time points. T2 values from the contralateral parietal cortex remained stable within 3 h–6 months time window and no difference was found between the control and TBI groups.
In the ipsilateral cortex, there was a trend towards Dav increase in FPBI animals when compared to controls starting from day 9. This increase was significant only at 2 months time point when there was also an association between the Dav and the number of epileptiform discharges at 12 months (r = 0.55; P < 0.05). In the contralateral cortex, Dav showed a non-significant trend towards slight increase during ageing both in injured animals and controls. No difference was found between the groups at any time point.
Severity and progression of the hippocampal lesion
A significant elevation in the hippocampal T2 ipsilateral to injury was found only at chronic stage (7 and 11 months in Experiment 2; Fig. 5A).
In Experiment 1, Dav measures of the hippocampus ipsilateral to the lesion were significantly different from those of controls at most of the time points (Fig. 5). At the acute phase (3 h post-injury), 4 out of 14 injured animals showed a severe magnetic susceptibility artefact in MRI caused by a large extracranial blood clot, and were excluded from the analysis at that time point. Temporal evolution of Dav changes showed the diffusion drop by 
0.05 x 10–3 mm2 s–1 at 3 h after the injury, followed by a brief temporary increase in some of the animals, and a secondary drop within the control levels during 3–9 days post-injury. Elevation in Dav was observed after day 9 and the increase became significant at 2 months after the FPBI (Fig. 5). Dav in the ipsilateral hippocampus at 7 months in Experiment 2 was similar to that at 6 months in Experiment 1 (0.876 ± 0.04 x 10–3 mm2 s–1 versus 0.850 ± 0.07 x 10–3 mm2 s–1, respectively), suggesting the results were reproducible (Fig. 5). Dav at 11 months (0.856 ± 0.04 x 10–3 mm2 s–1) did not differ from that at 6 months in Experiment 1 either.
Contralaterally, T2 values remained stable within 3 h–6 months time window and no difference was found between the control and TBI groups. Like in cortex, Dav showed a non-significant trend towards increase during ageing both in injured animals and controls. No difference was found between the groups at any time point.
Changes in ipsilateral hippocampal Dav associate with increased seizure susceptibility
Data are summarized in Fig. 6. In Experiment 1, the Dav drop at 3 h correlated with the increase in the total number of spikes at 12 months (r = –0.61, P < 0.05, n = 14). At later time points, Dav increase at 23 days (r = 0.73, P < 0.01, n = 14), 2 months (r = 0.72, P < 0.01, n = 14), 3 months (r = 0.80, P < 0.001, n = 14) and at 6 months (r = 0.76, P < 0.01, n = 14) post-injury correlated with the increase in the total number of epileptiform discharges at 12 months.
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Similar associations between the Dav increase and the hyperexcitability were obtained at more chronic time points in Experiment 2, in which Dav increase at 7 months correlated with all three EEG parameters of the increased seizure susceptibility, including shortening in the latency to the first spike (r = –0.50, P < 0.05, n = 16), increase in the total number of spikes (r = 0.62, P < 0.05, n = 16) and increase in the total number of epileptiform discharges (r = 0.55, P < 0.05, n = 16). At 11 months post-injury, Dav correlated with the shortening in the latency to the first spike (r = –0.60, P < 0.05, n = 14) and increase in the total number of spikes (r = 0.72, P < 0.01, n = 14).
Next we subdivided all FPBI animals into a ‘severe’ injury group (animals with Dav values <2 SD from the control mean at 3 h) and a ‘moderate’ injury group (the rest of the FPBI animals). We noticed that the Dav in the ‘severe’ group differed from controls at 23 days (P < 0.05), 2 months (P < 0.01), 3 months (P < 0.01) and 6 months (P < 0.01) post-injury. The ‘severe’ group also had a higher number of spikes (P < 0.05, Fig. 6) and epileptiform discharges (P < 0.05) in the PTZ test at 12 months post-injury as compared to controls. Interestingly, no difference was found between ‘moderate’ group and controls in any of the EEG indicators of the increased seizure susceptibility.
Changes in ipsilateral hippocampal Dav and increased seizure susceptibility associate with increased mossy fibre sprouting
Injured animals had mossy fibre sprouting into the inner molecular layer of the dentate gyrus ipsilateral to the lesion as compared to controls in both Experiment 1 [mean sprouting score 1.1 ± 0.7 (range 0.2–2.2, median 1) versus controls (mean 0.2 ± 0.2, range 0–0.5, median 0.15), (P < 0.01)] and Experiment 2 [2.2 ± 0.8 (range 1–3.5, median 2.2) versus 0.1 ± 0.1 (range 0–0.4, median 0.09) (P < 0.001)].
As summarized in Fig. 7, the severity of mossy fibre sprouting correlated positively with the EEG indicators of increased seizure susceptibility in both the experiments. Analysis of PTZ data from rats included in our previous experiment (Kharatishvili et al., 2006) indicated similar association between mossy fibre sprouting and seizure susceptibility in PTZ test in a population of rats, in which some of the animals had behaviourally and electrographically verified spontaneous seizures (data shown in Fig. 7C).
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Mossy fibre sprouting scores correlated with Dav values at 3 h (r = –0.74, P < 0.01, n = 14), 2 months (r = 0.75, P < 0.01, n = 14), 3 months (r = 0.73, P < 0.01, n = 14), 6 months (r = 0.64, P < 0.05, n = 14), 7 months (r = 0.77, P < 0.001, n = 16) and 11 months (r = 0.49, P < 0.05, n = 16) after lateral FPBI.
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Discussion |
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The present study describes the long-term MRI alterations in hippocampal T2 and Dav in a model of human PTE. We hypothesized that acute and subchronic alterations in these parameters reflecting the severity of initial hippocampal impact and consequent brain pathology can serve as surrogate markers for the development of long-term hyperexcitability in post-traumatic brain. There were three major findings. First, from the two common clinical MRI parameters analysed, hippocampal Dav showed more consistent differences between the control and injured brain than T2. Second, an early hippocampal diffusion drop at 3 h as well as a later diffusion increase observed first at 23 days post-injury were associated with the hyperexcitability at 12 months post-injury. Third, the changes in hippocampal Dav associate with the severity of mossy fibre sprouting that is a common pathological finding in patients operated on due to drug-refractory PTE. The implications of the present finding for using hippocampal Dav as a surrogate marker for post-traumatic epileptogenesis are discussed.
Moderate TBI results in hyperexcitability at 11 months post-injury
In the two independent experiments, we observed injury-induced mortality in 21–25% of animals, consistent with previous reports inducing FPI of moderate severity (McIntosh et al., 1989; Saatman et al., 1997; Dubreuil et al., 2006). In humans, moderate head injury is usually defined by the following criteria: (i) Glasgow Coma Score (GCS) 9–12, (ii) post-traumatic amnesia up to 24 h, (iii) loss of consciousness from a few minutes to few hours, (iv) operative intracranial lesion, (v) abnormal imaging findings. The risk of developing epilepsy after moderate TBI is 0.7% within 1 year and 1.6% to 4.2% in 5 years (Annegers et al., 1980, 1998). Thus, the present experimental data are in line with clinical observations that risk of epilepsy after moderate injury is substantially lower than after severe injury that results in epilepsy in 50% of rats and up to 53% in humans (Salazar et al., 1985; Frey, 2003; Kharatishvili et al., 2006).
In animals with FPBI, long-term video-EEG monitoring and quantification of chronic spontaneous seizures (i.e. severity of epilepsy) was not feasible in this study because of the interference of EEG with MRI measurements. In a single 3-week video-EEG recording at 11 months post-injury we did not observe any spontaneous seizures, but 80% of FPBI animals in both Experiments 1 and 2 had spiking, suggesting hyperexcitability. To further assess the presence of hyperexcitability as an index of increased epileptogenicity, we performed PTZ test under video-EEG control at 12 months after FPBI. Assessing the susceptibility to seizures induced by PTZ is a standard and widely used experimental model of clinical generalized seizures with both face and construct validity (Loscher et al., 1991). Systemic administration of the chemoconvulsant PTZ for induction of generalized clonic seizures in rodents is widely employed to identify potential anticonvulsants (Swinyard et al., 1989). PTZ test has been used to show lowered seizure threshold in many epileptogenic aetiologies, including genetically modified animals, and the PTZ seizure threshold has been shown to be lower in epileptic animals as compared to non-epileptic (Velisek, 2006). Although there was variability within each group that was comparable with the previously published work on PTZ in normal rats (Mason and Cooper, 1972; Golarai et al., 1992), we were able to observe lowered seizure threshold in injured animals as compared to controls in both Experiments 1 and 2. EEG parameters such as increase in total number of spikes and EDs as well as the shortened latency to the first spike in the injured rats proved to be reliable indicators of increased seizure susceptibility. The data obtained from the PTZ test confirmed our previous observations of consistently enhanced susceptibility to PTZ-induced convulsions at 12 months in animals with PTE after FPBI (Kharatishvili et al., 2006). Taken together, rats with lateral FPBI-induced moderate TBI develop lowered seizure threshold in chronic follow-up, which is in record with the data previously reported in a weight-drop animal model of impact head injury by Golarai and colleagues (Golarai et al., 2001). However, it is important to keep in mind that currently there are no long-term prospective studies that had directly addressed the question whether the lowered threshold for induced seizures triggered by PTZ reliably predicts the later development of spontaneous seizures, that is, the end product of the epileptogenic process.
Finally, the pressure of fluid pulse within the range of 2.3–3.2 atm applied to the brain did not correlate with the decrease in seizure threshold in the injured animals. Similarly, according to our previous data (Kharatishvili et al., 2006) no correlation was found between the development of spontaneous epileptic activity and pressure (2.6–3.3 atm) of the fluid pulse applied. Therefore, it is the brain injury triggered by the impact rather than the impact force itself that is critical for alteration in seizure threshold and imaging parameters.
Post-traumatic abnormalities in hippocampal water diffusion are progressive
Diffusion MRI is a non-invasive technique allowing quantification of water diffusion due to thermal Brownian motion. We measured apparent average diffusion constant (Dav) = 1/3 of the trace of diffusion tensor which is an orientation-independent measure of water diffusion in the tissue. Previous studies on diffusion after TBI in humans have reported that diffusion MRI identifies the largest number of overall lesions as well as the largest volume of trauma-related signal abnormalities in diffuse axonal injury (DAI) compared with conventional MRI sequences that include T2-weighted fast spin-echo, fluid attenuated inversion recovery (FLAIR) and T2*-weighted gradient echo sequences (Huisman et al., 2003). The total volume of diffusion-weighted imaging (DWI) signal abnormalities encountered in DAI correlate better than other imaging variables with the acute GCS and the subacute Rankin scale score (Schaefer et al., 2004). Two animal studies showed a decreased diffusion within 1 h of injury of moderate severity in an FPBI model (Smith et al., 1995; Alsop et al., 1996). Another two experimental TBI studies evaluated in vivo the temporal evolution of DWI changes up to 7 days (closed head injury, Assaf et al., 1997) or 2 weeks (lateral FPBI, Albensi et al., 2000) and compared signal changes with histopathological and neurological outcome measures. They found a significant decrease in the apparent diffusion coefficients (ADC) in the injured area at 1–24 h post-injury, followed by abnormal increase in ADC as early as 7–14 days post-injury, which was in good correlation with infarct size as obtained by histology (Assaf et al., 1997; Albensi et al., 2000). Neurological assessment indicated that such changes were observed at the level of injury that produced moderate impairment 2 weeks after the insult (Albensi et al., 2000). No long-term studies have, however, been reported.
The hippocampus was selected as a ROI because it is located just beneath the trauma, and thus represents the ‘tissue at risk’ for primary and secondary damage (McCarthy, 2003). Previous studies have demonstrated substantial hippocampal damage after TBI both in experimental models and humans (Kotapka et al., 1993, 1994; Hicks et al., 1996; Tate and Bigler, 2000; Grady et al., 2003; Tomaiuolo et al., 2004; Wilde et al., 2007). In addition, the hippocampus ipsilateral to injury is a candidate focus for seizure initiation in PTE after lateral FPBI according to our long-term video-EEG monitoring studies (Kharatishvili et al., 2006) as in patients with PTE (Mathern et al., 1994; Diaz-Arrastia et al., 2000). We did not consider Dav measurements in the ipsilateral cortical ROI as reliable and informative as those of the hippocampus due to difficulties in unequivocally separating highly odematous cortex from CSF and progressive changes in the CSF volume fraction.
In the hippocampus ipsilateral to the lesion we observed an early (3 h) and transient diffusivity decrease in the animals subjected to lateral FPBI with little evidence of changes in T2-weighted MR images, which is in line with previous TBI studies (Assaf et al., 1997; Albensi et al., 2000). Similar early diffusivity drop was described in animals during and 24 hours after the kainic acid (KA)-induced status epilepticus and in the cortical electroshock model (Nakasu et al., 1995; Ebisu et al., 1996; Zhong et al., 1993, 1997). This early decrease is usually attributed to cellular swelling due to cytotoxic oedema (Wang et al., 1996). The ADC changes were closely correlated with assumed epileptogenic brain areas and the resulting histopathological changes such as neuronal pyknosis and neuropil vacuolization (Nakasu et al., 1995). Interestingly, peri-ictal and early post-ictal human studies using DWI showed transiently decreased local diffusivity in patients with status epilepticus (Kim et al., 2001) or with intractable focal epilepsy (Diehl et al., 1999, 2001; Hufnagel et al., 2003; Diehl et al., 2005) potentially in concordance with the epileptogenic zone.
Studies on the temporal evolution of diffusion changes have shown that decrease of ADC in the hippocampus, amygdala and piriform cortex after KA-induced status epilepticus was maximal at 24 h and resolved by 9 days. In our study, after 9 days post-injury we observed a consistent increase in diffusivity values in the hippocampus ipsilateral to injury, which may be explained by ongoing slowly progressive neurodegeneration resulting in expanded extracellular space over the time (Smith et al., 1997), eventually leading to hippocampal sclerosis described previously in lateral FPBI-induced PTE in rats (Lowenstein et al., 1992; Coulter et al., 1996; Hicks et al., 1996; Toth et al., 1997; Kharatishvili et al., 2006) as well as in human PTE (Diaz-Arrastia et al., 2000, Swartz et al., 2006).
In human PTE, the functional significance of early and chronic diffusion abnormalities has not been examined. Several diffusion studies have, however, demonstrated areas of increased diffusivity in the chronic phase in a proportion of patients with refractory partial epilepsies, including temporal lobe epilepsy (TLE), where diffusivity was invariably and significantly increased in the epileptogenic hippocampus (Hugg et al., 1999; Wieshmann et al., 1999a; Rugg-Gunn et al., 2001b, 2002; Kantarci et al., 2002; Yoo et al., 2002; Assaf et al., 2003; Lee et al., 2004; Sundgren et al., 2004; Thivard et al., 2005). Based on some of these studies and their own results, Hufnagel and colleagues suggested increased ADC as a potential surrogate marker to lateralize intractable epileptogenic seizure foci (Hufnagel et al., 2003).
Post-traumatic abnormalities in hippocampal quantitative diffusion-weighted imaging predict the development of post-traumatic hyperexcitability
Dav was not only shown to be a sensitive method to detect early brain damage, but more importantly this initial diffusivity drop correlated with the increased susceptibility to seizures as well as with mossy fibre synaptic reorganization 12 months later. Starting at the second week post-injury we observed a consistent increase in diffusivity values in the hippocampus ipsilateral to injury. At 23 days post-FPBI, and at all later time points hippocampal diffusivity values correlated with the EEG markers of the increased susceptibility to seizures in the PTZ test, including shortening of the latency to the first spike, total number of spikes and total number of epileptiform discharges.
The role of sprouting of granule cell axons (mossy fibres) in epileptogenesis and ictogenesis is under dispute (Elmer et al., 1997; Longo and Mello, 1998; Sloviter, 1999; Nissinen et al., 2001). However, several histopathological studies in tissue resected from patients who underwent surgery due to drug-refractory epilepsy caused by different aetiologies, including PTE (Sutula et al., 1989; Houser et al., 1990; Mathern et al., 1995a, b, 1996; Swartz et al., 2006) as well as in models of post-status epilepticus, post-traumatic or post-stroke epilepsy have demonstrated that mossy fibre sprouting is present in a large majority of epileptic subjects (Sutula et al., 1988; Golarai et al., 1992; Mello et al., 1993; Represa et al., 1993; Buckmaster and Dudek, 1997; Mathern et al., 1998; Bragin et al., 1999; Santhakumar et al., 2001; Sloviter et al., 2006; Kharatishvili et al., 2006; Pitkanen et al., 2007). We found invariably increased mossy fibre sprouting scores in the dentate gyrus of the hippocampus ipsilateral to injury in all animals in both experiments. The greater the mossy fibre sprouting score, the higher was the number of spikes and epileptiform discharges in the EEG after PTZ injection in animals with FPBI. Importantly, hippocampal Dav at both early (3 h) and chronic (2–11 months) time points predicted the severity of mossy fibre sprouting suggesting that hippocampal Dav can be used not only as a surrogate marker for increased excitability, but also for the development of circuitry reorganization characteristic to PTE.
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Conclusion and clinical relevance |
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Early diffusion drop has previously been described in humans after stroke (Marks et al., 1996
), SE (Diehl et al., 1999; Kim et al., 2001) and TBI (Liu et al., 1999; Schaefer et al., 2004) suggesting that it occurs after a variety of epileptogenic insults, and could actually be a general marker for potentially epileptogenic tissue damage caused by different aetiologies. The present study demonstrates that the alterations found in humans can be recapitulated in a clinically relevant model of closed head injury that in severe cases results in the development of PTE. Most importantly, the present study is the fist one to relate the early and subchronic changes in Dav with long-term functional (lowered seizure threshold) and structural (mossy fibre sprouting) outcome. Our study provides the first evidence that hippocampal Dav measured ipsilateral to TBI as early as 3 h post-injury shows significant associations with the increased susceptibility to seizures 1 year later. Importantly, quantitative measurements of Dav values correlated with functional and histological outcome also at later time points (23 days–11 months) which extends the diagnostic time window for MRI measurements beyond the acute phase that is more convenient in clinical practice. The data obtained provide a starting point for designing clinical studies searching surrogate markers using DWI for PTE in humans.
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Acknowledgements |
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This work was supported by Academy of Finland, Emil Aaltonen Foundation and Finnish Cultural Foundation of Northern Savo. We thank Dr Jari Nissinen for help in carrying out PTZ test, Ms Maarit Pulkkinen and Ms Merja Lukkari for technical assistance.
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