Epilepsy-predictive magnetic resonance imaging changes following experimental febrile status epilepticus: Are they translatable to the clinic?

Abstract

Objective: A subset of children with febrile status epilepticus (FSE) are at risk for development of temporal lobe epilepsy later in life. We sought a noninvasive predictive marker of those at risk that can be identified soon after FSE, within a clinically realistic timeframe.

Methods: Longitudinal T₂ -weighted magnetic resonance imaging (T₂ WI MRI) of rat pups at several time points after experimental FSE (eFSE) was performed on a high-field scanner followed by long-term continuous electroencephalography. In parallel, T₂ WI MRI scans were performed on a 3.0-T clinical scanner. Finally, chronic T₂ WI MRI signal changes were examined in rats that experienced eFSE and were imaged months later in adulthood.

Results: Epilepsy-predicting T₂ changes, previously observed at 2 hours after eFSE, persisted for at least 6 hours, enabling translation to the clinic. Repeated scans, creating MRI trajectories of T₂ relaxation times following eFSE, provided improved prediction of epileptogenesis compared with a single MRI scan. Predictive signal changes centered on limbic structures, such as the basolateral and medial amygdala. T₂ WI MRI changes, originally described on high-field scanners, can also be measured on clinical MRI scanners. Chronically elevated T₂ relaxation times in hippocampus were observed months after eFSE in rats, as noted for post-FSE changes in children.

Significance: Early T₂ WI MRI changes after eFSE provide a strong predictive measure of epileptogenesis following eFSE, on both high-field and clinical MRI scanners. Importantly, the extension of the acute signal changes to at least 6 hours after the FSE enables its inclusion in clinical studies. Chronic elevations of T₂ relaxation times within the hippocampal formation and related structures are common to human and rodent FSE, suggesting that similar processes are involved across species.

Keywords: epileptogenesis; febrile seizures; febrile status epilepticus; magnetic resonance imaging; temporal lobe epilepsy.

Figure 1:. The trajectory in T₂ differences (48h – 2h) following eFSE is a better predictor of epileptogenesis than single early time point alone.

A) Representative pseudocolored 11.7 T₂ maps of a control rat, an eFSE rat that did not develop epilepsy, and an eFSE rat that went on to develop epilepsy. B) Whole brain T₂ values in individual animals of the three groups decreased significantly in control and eFSE-NoEpi, but not in eFSE-Epi animals, Control, eFSE-NoEpi, and eFSE-Epi (t=7.94, df=12, p<0.001; eFSE-NoEpi t=5.54, df=11, p<0.001; eFSE-Epi t=2.022, df=5, p=0.10) C) ROC curve of the predictive value of the delta T₂ between 2 and 48h of the BLA, MeA, and DMThal (BLA: AUC 0.99 ± 0.020, p=0.001; MEA: AUC 1.00 ± 0, p=0.001; DMThal: AUC 0.83 ± 0.098, p<0.05) (inset: original ROC of 2h time point alone in inset, as published in Choy, et al., [2014]; BLA: AUC 0.91 ± 0.08, p= 0.005; MEA: AUC 0.82 ± 0.10, p<0.05; DMThal: AUC = 0.87 ±0.092; p=0.011). The BLA, MEA, and DMThal are able to differentiate between the eFSE-NoEpi and eFSE-Epi groups (D, E, F), while the entorhinal cortex does not (G). One-way ANOVA; Bonferroni Multiple Comparison Test (BLA Ctrl vs eFSE-NoEpi p=0.89; Ctrl vs. eFSE-NoEpi p<0.001; eFSE-NoEpi vs. eFSE-Epi p<0.001) (MeA: Ctrl vs eFSE-NoEpi p>0.99; Ctrl vs. eFSE-NoEpi p<0.001; eFSE-NoEpi vs. eFSE-Epi p=0.001) (DMThal: Ctrl vs eFSE-NoEpi p<0.001; Ctrl vs. eFSE-NoEpi p<0.001; eFSE-NoEpi vs. eFSE-Epi p<0.05) (Entorhinal Cortex: Ctrl vs eFSE-NoEpi p>0.99; Ctrl vs. eFSE-NoEpi p=0.07; eFSE-NoEpi vs. eFSE-Epi p=0.11) *p<0.05, **p<0.01, ***p<0.001

Figure 2:. MRI T₂ values do not change significantly between 2 and 6 hours.

A) Representative T₂ maps of a rat at 2h and 6h after eFSE. The majority of rats remain in the same predictive group compared to controls at 2 and 6h after eFSE (C-E; Paired T-Test: BLA; p=0.45, t=0.78 df=17, correlation r=0.72, p<0.001; MeA: p=0.11, t=1.691, df=19, correlation r=0.81, p<0.001; DMThal: p=0.19, t=1.36, df=15, correlation r=0.89, p<0.001). F, G, H reveal the strong correlations between 2 and 6h time points for the BLA, MeA and DMThal

Figure 3:. Immature rat MRI 4 and 48h after eFSE in a human 3.0T scanner is able to differentiate groups of eFSE rats at a whole brain level but not in individual regions.

A) Representative 3.0T T₂ maps at 4 and 48h after eFSE. B) Whole brain 3.0T T₂ values revealed consistent reductions in all control rats between 4 and 48h, but eFSE rats exhibit increased variability in their trajectories. (Paired T-test: Ctrl: p<0.001, t=17.23, df=5; eFSE p=0.003, t=6.177, df=8) C) Delta T₂ (48h-4h) did not differentiate between the eFSE and control groups, but two clear groups within the eFSE emerge (T-Test, Ctrl vs. eFSE: p=0.12). By separating the eFSE animals into those with a similar trajectory as controls, and those with a smaller T₂ decrease, the two groups were statistically different from each other and eFSE-Resilient was different from controls (dividing line at control + 2SD; 2206≥19.5) (one-way ANOVA, eFSE-Vulnerable vs. eFSE-Resilient: p<0.01; eFSE-Vulnerable vs. Control p<0.001; eFSE-Resilient vs. Control p>0.99) D-G: In vivo imaging of immature rats in a human 3T scanner did not reveal regional differences between groups (two-way ANOVA, no significant interaction between treatment group and time, significant effect of time. BLA: F_(1,14) = 12.62, p<0.01; MEA: F_(1,14) = 39.54, p<0.0001). **p<0.01, ***p<0.001

Figure 4:. The 48h 11.7T MRI trajectories persist through 96h.

A) Representative 11.7T T₂ maps for controls and 48h and 96h after eFSE. B, C) Individual 11.7T whole brain T₂ values of eFSE and control rats revealed a decreased rate of change at 96h following eFSE. D, E, F) There was a significant difference in the trajectories from baseline for the basolateral amygdala (BLA) and medial amygdala (MeA) between control rats and rats that underwent eFSE, but no differences in the dorsal medial thalamus (DMThal). D’, E’) For the BLA and MeA, separating the eFSE rats into two groups those that followed a similar trajectory as control rats from 4–48h (eFSE-Resilient; n=6), and those with a small or no decrease (eFSE-Vulnerable; Δ48h-4h > control + 2SD; n=4), revealed two distinct trajectories (BLA: RM two-way ANOVA, Šidák Multiple Comparison test: significant interaction, F_(4,34) = 5.46, p<0.01; 48h Ctrl vs. eFSE-Vulnerable p<0.01, eFSE-Vulnerable vs. eFSE-Resilient p <0.001; 96h Ctrl vs. eFSE-Vulnerable p<0.01, Ctrl vs. eFSE-Resilient p<0.05) (MeA: RM two-way ANOVA, Šidák Multiple Comparison test: significant interaction, F_(4,34) = 6.60, p<0.001; 48h Ctrl vs. eFSE-Vulnerable p<0.01, eFSE-Vulnerable vs. eFSE-Resilient p <0.01; 96h Ctrl vs. eFSE-Vulnerable p<0.05, Ctrl vs. eFSE-Resilient p<0.001). *p<0.05, **p<0.01, ***p<0.001

Figure 5:. MRI of adult rats (ex vivo) that underwent eFSE revealed increased T₂ throughout the whole brain.

A) Group averaged images of control and eFSE rats demonstrate the globally elevated T₂ values in the eFSE group B) Heat map of brain regions compartmentalized by region type (white matter, cortex, limbic associated regions, other), revealing increased in T₂ relaxation times in FSE animals (n=4) relative to controls (n=9), particularly in limbic and cortical regions. A significant difference between the eFSE and control groups was observed when comparing all brain regions (two-way ANOVA, effect of eFSE, F_(1,11) = 7.17, p<0.05). ***p<0.001

Figure 6:. Limbic regions revealed increased T₂ in adult animals that underwent eFSE in early life.

There is a significant T₂ increase in the BLA, CA1, and CA3, with an increase trend across all limbic regions as shown in the heatmap (F). (Individual T-Test; BLA: p<0.05, t ratio = 2.63, df = 11; MeA: p=0.13, t ratio = 1.64, df = 11; CA1: p<0.05, t ratio = 2.80, df = 11, CA3; p<0.05, t ratio = 2.25, df = 11; Whole Brain: p = 0.057, t ratio =2.12, df = 11) (Group comparison: two-way ANOVA, effect of eFSE, F_(1,11) = 5.87, p<0.05). BLA - Basolateral amygdala, MeA - Medial Amygdala, CA1/3 - cornu ammonis of the hippocampus 1/3; MeDLTh – Medial Dorsal Thalamic Nuclei. *p <0.05. ***p<0.001