The outcome of CSE in childhood depends mainly upon the cause but length of seizure may also be important
status epilepticus; children; epilepsy; outcome; mortality; morbidity
Children with epilepsy often present with pervasive cognitive and behavioral comorbidities including working memory impairments, attention deficit hyperactivity disorder (ADHD) and autism spectrum disorder. These non-seizure characteristics are severely detrimental to overall quality of life. Some of these children, particularly those with epilepsies classified as Landau-Kleffner Syndrome or continuous spike and wave during sleep, have infrequent seizure activity but frequent focal epileptiform activity. This frequent epileptiform activity is thought to be detrimental to cognitive development; however, it is also possible that these IIS events initiate pathophysiological pathways in the developing brain that may be independently associated with cognitive deficits. These hypotheses are difficult to address due to the previous lack of an appropriate animal model. To this end, we have recently developed a rat model to test the role of frequent focal epileptiform activity in the prefrontal cortex. Using microinjections of a GABAA antagonist (bicuculline methiodine) delivered multiple times per day from postnatal day (p) 21 to p25, we showed that rat pups experiencing frequent, focal, recurrent epileptiform activity in the form of interictal spikes during neurodevelopment have significant long-term deficits in attention and sociability that persist into adulthood. To determine if treatment with ACTH, a drug widely used to treat early-life seizures, altered outcome we administered ACTH once per day subcutaneously during the time of the induced interictal spike activity. We show a modest amelioration of the attention deficit seen in animals with a history of early life interictal spikes with ACTH, in the absence of alteration of interictal spike activity. These results suggest that pharmacological intervention that is not targeted to the interictal spike activity is worthy of future study as it may be beneficial for preventing or ameliorating adverse cognitive outcomes.
One of the most common and serious co-morbidities in patients with epilepsy is cognitive impairment. While early-life seizures are considered a major cause for cognitive impairment, it is not known whether it is the seizures, the underlying neurological substrate or a combination that has the largest impact on eventual learning and memory. Teasing out the effects of seizures from pre-existing neurological disorder is critical in developing therapeutic strategies. We therefore investigated the additional cognitive effects of seizures on rodents with malformations of cortical development induced with methylazoxymethanol acetate. Pregnant rats were injected with saline or methylazoxymethanol acetate at embryonic Day 15 or 17 to induce differing malformation severity. From the day of birth to 9 days of age, half the pups received 50 flurothyl-induced seizures. All rats underwent testing in the Morris water maze to test spatial memory at 25 days of age (immediate post-weaning) or during adolescence at 45 days of age. Post-weaning rats had severe spatial cognitive deficits in the water maze and seizures worsened performance. In contrast, in animals tested during adolescence, there was no longer an additional adverse effect of seizures. We also investigated whether the severity of the structural abnormality and seizures impacted brain weight, cortical thickness, hippocampal area and cell dispersion area. The mean brain weight in control animals was greater than in rats exposed to methylazoxymethanol acetate at embryonic Day 17, which was greater than rats exposed to methylazoxymethanol acetate at embryonic Day 15. Rats exposed to methylazoxymethanol acetate at embryonic Day 15 had a thinner cortical mantle compared with rats exposed at embryonic Day 17 and control animals. The hippocampal area was similar in rats exposed at embryonic Days 15 and 17 but was smaller compared with controls. Methylazoxymethanol at embryonic Day 17 caused dispersion of the CA1–4 cell layers in the hippocampus, whereas methylazoxymethanol at embryonic Day 15 caused focal nodules in or above the CA1 layer, but the CA1–4 layers were intact and similar to control. Early-life seizures did not have a significant impact on any of these parameters. These observations indicate that the major factor responsible for the cognitive impairment in the rats with cortical dysplasia was the underlying brain substrate, not seizures. These findings have significant implications for the understanding of cognitive impairments in childhood epilepsy and suggest that early aggressive therapy of seizures alone may not be an adequate strategy for minimizing cognitive effects.
epilepsy; cognitive impairment; cortical dysplasia; cortical malformations; spatial cognition
The ontogeny of neural substrates underlying episodic memory is not well described. Place cells are a surrogate for episodic memory and are important for spatial navigation in rodents. Although place cells are well described in mature brains, the nature of the maturation processes remains uncertain. We now report on the ontogeny of the place cell system in rats between P22 and P43, a time during which there is rapid improvement in spatial behavior. We found that place cells with adult like firing fields were observed at the earliest ages. However, at this age, adult like place cells were few in number and their place fields were not stable across multiple exposures to the same environment. Finally, independently of confounding factors such as the number of exposures to the environment, the proportion of adult-like place cells, their firing rate and their stability increased with age and the average spatial signal of all pyramidal cells improved. This finding could account for the poor spatial behavior observed at young ages (P20-P30) and suggests that a small number of adult-like place cells are insufficient to support navigation.
Children with malformations of cortical development (MCD) frequently have associated cognitive impairments which reduce quality of life. We hypothesized that cognitive deficits associated with MCD can be improved with environmental manipulation or additional training. The E17 methylazoxymethanol acetate (MAM) exposure model bears many anatomical hallmarks seen in human MCDs as well as similar behavioral and cognitive deficits. We divided control and MAM exposed Sprague-Dawley rats into enriched and non-enriched groups and tested performance in the Morris water maze. Another group similarly divided underwent sociability testing and also underwent Magnetic Resonance Imaging (MRI) scans pre and post enrichment. A third group of control and MAM rats without enrichment were trained until they reached criterion on the place avoidance task. MAM rats had impaired performance on spatial tasks and enrichment improved performance of both control and MAM animals. Although MAM rats did not have a deficit in sociability they showed similar improvement with enrichment as controls. MRI revealed a whole brain volume decrease with MAM exposure, and an increase in both MAM and control enriched volumes in comparison to non-enriched animals. In the place avoidance task, MAM rats required approximately 3 times as long to reach criterion as control animals, but with additional training were able to reach control performance. Environmental manipulation and additional training can improve cognition in a rodent MCD model. We therefore suggest that patients with MCD may benefit from appropriate alterations in educational strategies, social interaction and environment. These factors should be considered in therapeutic strategies.
Children with a history of a prolonged febrile seizure show signs of acute hippocampal injury on magnetic resonance imaging. In addition, animal studies have shown that adult rats who suffered febrile seizures during development reveal memory impairments. Together, these lines of evidence suggest that memory impairments related to hippocampal injury may be evident in human children after prolonged febrile seizures. The current study addressed this question by investigating memory abilities in 26 children soon after a prolonged febrile seizure (median: 37.5 days) and compared their results to those of 37 normally developing children. Fifteen patients were reassessed at a mean of 12.5 months after their first assessment to determine the transiency of any observed effects. We used the visual paired comparison task to test memory abilities in our group, as this task does not depend on verbal abilities and also because successful performance on the task has been proven to depend on the presence of functional hippocampi. Our findings show that patients perform as well as controls in the absence of a delay between the learning phase and the memory test, suggesting that both groups are able to form representations of the presented stimulus. However, after a 5-min delay, patients’ recognition memory is not different from chance, and comparison of patients and controls points to an accelerated forgetting rate in the prolonged febrile seizure group. The patients’ performance was not related to the time elapsed from the acute event or the duration of the prolonged febrile seizure, suggesting that the observed effect is not a by-product of the seizure itself or a delayed effect of medication administered to terminate the seizure. By contrast, performance was related to hippocampal size; participants with the smallest mean hippocampal volumes revealed the biggest drop in performance from the immediate to the delayed paradigm. At follow-up, children were still showing deficiencies in recognizing a face after a 5-min delay. Similarly, this suggests that the observed memory impairments are not a transient effect of the prolonged febrile seizures. This is the first report of such impairments in humans, and it is clinically significant given the links between mesial temporal sclerosis and prolonged febrile seizures. The persistence of these impairments a year onwards signals the potential benefits of intervention in these children who run the risk of developing episodic memory deficits in later childhood.
memory; hippocampus; prolonged febrile seizures
Dravet syndrome (DS) is a severe epileptic encephalopathy beginning in infancy in which children have difficult to control seizures and cognitive impairment. The majority of children with DS carry mutate,ons of the gene Scn1a, which codes for the alpha subunit of the type 1 voltage-gated sodium channel and is important for the function of interneurons. Interneurons have a critical role in the generation of brain rhythms involved in cognitive processing. We hypothesized that children with DS with Scn1a mutations would have abnormal oscillatory activity. To address this hypothesis, we used EEG power spectral analysis during the wakening to determine if frequency and power are altered in 23 EEGs from 12 children with DS compared to 18 age-matched controls. While there were few differences between the EEG power spectra in DS and controls in children under 2 years, in older children group differences were apparent. In DS children between 3-5 years there were significant decreases in percentage of alpha power compared to controls and in DS children over age 6 years there was a marked increase of theta and decrease of alpha compared to controls. Developmental status paralleled the power spectral analysis with an increasing likelihood of having severe cognitive problems with increasing age. These results demonstrate that Scn1a mutations result in an age-dependent alteration in oscillatory process. Such abnormalities in developmental progression of oscillations may play an important role in poor cognitive development in children with DS.
Dravet syndrome; Scn1a mutation; age-dependent alteration; brain oscillation; spectral analysis; Severe Myoclonic Epilepsy in Infancy
Dravet syndrome (DS) is a childhood disorder associated with loss-of-function mutations in SCN1A and is characterized by frequent seizures and severe cognitive impairment. Animal studies have revealed new insights into the mechanisms by which mutations in this gene, encoding the type I voltage-gated sodium channel (Nav1.1), may lead to seizure activity and cognitive dysfunction. In this review, we further consider the function of fast-spiking GABAergic neurons, one cell type particularly affected by these mutations, in the context of the temporal coordination of neural activity subserving cognitive functions. We hypothesize that disruptions in GABAergic firing may directly contribute to the poor cognitive outcomes in children with DS, and discuss the therapeutic implications of this possibility.
Scn1a; Nav1.1; Dravet syndrome; interneuron; oscillations; epileptic encephalopathy; EEG; epilepsy; sodium channel; cognitive impairment
Status epilepticus (SE) is a common neurological emergency, which has been associated with subsequent cognitive impairments. Neuronal death in hippocampal CA1 is thought to be an important mechanism of these impairments. However, it is also possible that functional interactions between surviving neurons are important. In this study we recorded in vivo single-unit activity in the CA1 hippocampal region of rats while they performed a spatial memory task. From these data we constructed functional networks describing pyramidal cell interactions. To build the networks, we used maximum entropy algorithms previously applied only to in vitro data. We show that several months following SE pyramidal neurons display excessive neuronal synchrony and less neuronal reactivation during rest compared with those in healthy controls. Both effects predict rat performance in a spatial memory task. These results provide a physiological mechanism for SE-induced cognitive impairment and highlight the importance of the systems-level perspective in investigating spatial cognition.
While extracellular somatic action potentials from freely moving rats have been well characterized, axonal activity has not. We report direct extracellular tetrode recordings of putative axons whose principal feature is a short duration waveform (SDW) with an average peak-trough length less than 179 μs. While SDW recordings using tetrodes have previously been treated as questionable or classified as cells, we hypothesize that they are representative of axonal activity. These waveforms have significantly shorter duration than somatic action potentials, are triphasic and are therefore similar to classic descriptions of microelectrode recordings in white matter and of in vitro action potential propagation along axons. We describe SDWs recorded from pure white-matter tracts including the alveus and corpus callosum. Recordings of several SDWs in the alveus exhibit grid-like firing patterns suggesting these axons carry spatial information from entorhinal cortical neurons. Finally, we locally injected the GABAA agonist Muscimol into layer CA1 of the hippocampus while simultaneously recording somatic activity and SDWs on the same tetrodes. The persistent activity of SDWs during Muscimol inactivation of somatic action potentials indicates that SDWs are representative of action potential propagation along axons projecting from more distal somata. This characterization is important as it illustrates the dangers of exclusively using spike duration as the sole determinant of unit type, particularly in the case of interneurons whose peak-trough times overlap with SDWs. It may also allow future studies to explore how axonal projections from disparate brain regions integrate spatial information in the hippocampus, and provide a basis for studying the effects of pharmaceutical agents on signal transmission in axons, and ultimately to aid in defining the potential role of axons in cognition.
axonal activity; short duration waveform; in vivo electrophysiology
Early-life seizures (ELS) are associated with long-term behavioral disorders including autism and ADHD, suggesting that frontal lobe structures may be permanently affected. We tested whether ELS produce structural alterations in the prefrontal cortex (PFC) and impair PFC-mediated function using an operant task of behavioral flexibility in rats. Adult rats that had been exposed to 75 flurothyl seizures during postnatal days 1–10 showed decreased behavioral flexibility in the task compared to controls over multiple behavioral sessions, measured as a lever preference asymmetry (p<0.001) and a decreased efficiency of attaining food rewards (p<0.05). ELS rats also showed an increased thickness of the PFC (p<0.01), primarily attributed to layer V (p<0.01) with no differences in cell density. These structural changes correlated with lever preference behavioral impairments (p<0.05). This study demonstrates that the consequences of ELS extend to the PFC, which may help explain the high prevalence of comorbid behavioral disorders following ELS.
Behavior; Epilepsy; Flexibility; Prelimbic
The risk of long-term mortality and its predictors following convulsive status epilepticus in childhood are uncertain. We report mortality within 8 years after an episode of convulsive status epilepticus, and investigate its predictors from a paediatric, prospective, population-based study from north London, UK. In the current study, we followed-up a cohort previously ascertained during a surveillance study of convulsive status epilepticus in childhood. After determining the survival status of the cohort members, we defined cause of death as that listed on their death certificates. We estimated a standardized mortality ratio to compare mortality in our cohort with that expected in the reference population. Multivariable Cox regression analysis was used to investigate any association between the clinical and demographic factors at the time of status epilepticus and subsequent risk of death. The overall case fatality was 11% (95% confidence interval 7.5–16.2%); seven children died within 30 days of their episode of convulsive status epilepticus and 16 during follow-up. The overall mortality in our cohort was 46 times greater than expected in the reference population, and was predominantly due to higher mortality in children who had pre-existing clinically significant neurological impairments when they had their acute episode of convulsive status epilepticus. Children without prior neurological impairment who survived their acute episode of convulsive status epilepticus were not at a significantly increased risk of death during follow-up. There were no deaths in children following prolonged febrile convulsions and idiopathic convulsive status epilepticus. A quarter of deaths during follow-up were associated with intractable seizures/convulsive status epilepticus, and the rest died as a complication of their underlying medical condition. On regression analysis, presence of clinically significant neurological impairments prior to convulsive status epilepticus was the only independent risk factor for mortality. In conclusion, there is a high risk of death within 8 years following childhood convulsive status epilepticus but most deaths are not seizure related. Presence of pre-existing clinically significant neurological impairments at the time of convulsive status epilepticus is the main risk factor for mortality within 8 years after the acute episode. The attributable role of convulsive status epilepticus on mortality remains uncertain, but appears less than is generally perceived.
status epilepticus; childhood; death; standardized mortality ratio; neurological impairment
Neurological insults during development are associated with later impairments in learning and memory. Although remedial training can help restore cognitive function, the neural mechanisms of this recovery in memory systems are largely unknown. To examine this issue we measured electrophysiological oscillatory activity in the hippocampus (both CA3 and CA1) and prefrontal cortex of adult rats that had experienced repeated seizures in the first weeks of life, while they were remedially trained on a delayed-nonmatch-to-sample memory task. Seizure-exposed rats showed initial difficulties learning the task but performed similar to control rats after extra training. Whole-session analyses illustrated enhanced theta power in all three structures while seizure rats learned response tasks prior to the memory task. Whilst performing the memory task, dynamic oscillation patterns revealed that prefrontal cortex theta power was increased among seizure-exposed rats. This enhancement appeared after the first memory training steps using short delays and plateaued at the most difficult steps which included both short and long delays. Further, seizure rats showed enhanced CA1-prefrontal theta coherence in correct trials compared to incorrect trials when long delays were imposed, suggesting increased hippocampal-prefrontal synchrony for the task in this group when memory demand was high. Seizure-exposed rats also showed heightened gamma power and coherence among all three structures during the trials. Our results demonstrate the first evidence of hippocampal-prefrontal enhancements following seizures in early development. Dynamic compensatory changes in this network and interconnected circuits may underpin cognitive rehabilitation following other neurological insults to higher cognitive systems.
Cognitive impairment is common in epilepsy, particularly in memory function. Interictal spikes are thought to disrupt cognition, but it is difficult to delineate their contribution from general impairments in memory produced by etiology and seizures. We investigated the transient impact of focal interictal spikes on the hippocampus, a structure crucial for learning and memory and yet highly prone to interictal spikes in temporal lobe epilepsy.
Bilateral hippocampal depth electrodes were implanted into fourteen Sprague-Dawley rats, followed by intrahippocampal pilocarpine or saline infusion unilaterally. Rats that developed chronic spikes were trained in a hippocampal-dependent operant behavior task, delayed-match-to-sample. Depth EEG was recorded during 5,562 trials among five rats, and within-subject analyses evaluated the impact of hippocampal spikes on short-term memory operations.
Hippocampal spikes that occurred during memory retrieval strongly impaired performance (p<0.001). However, spikes that occurred during memory encoding or memory maintenance did not affect performance in those trials. Hippocampal spikes also affected response latency, adding approximately 0.48 seconds to the time taken to respond (p<0.001).
We found that focal interictal spike-related interference in cognition extends to structures in the limbic system, which required intrahippocampal recordings. Hippocampal spikes seem most harmful if they occur when hippocampal function is critical, extending human studies showing that cortical spikes are most disruptive during active cortical functioning. The cumulative effects of spikes could therefore impact general cognitive functioning. These results strengthen the argument that suppression of interictal spikes may improve memory and cognitive performance in patients with epilepsy.
Seizures are common in children admitted with severe falciparum malaria and are associated with neuro-cognitive impairments. Prolonged febrile seizures are associated with hippocampal damage and impaired memory. It was hypothesized that severe malaria causes impaired everyday memory which may be associated with hippocampal damage.
An everyday memory battery was administered on 152 children with cerebral malaria (CM) (mean age, 7 y 4 months [SD 13 months]; 77 males) 156 children (mean age, 7 y 4 months [SD, 14 months]; 72 males) with malaria plus complex seizures (MS) and 179 children (mean age, 7 y 6 months [SD, 13 months]; 93 males) unexposed to either condition.
CM was associated with poorer everyday memory [95% CI, -2.46 to -0.36, p = 0.004] but not MS [95% CI, -0.91 to 1.16, p = 1.00] compared to unexposed children. Children with exposure to CM performed more poorly in recall [95% CI, -0.79 to -0.04, p = 0.024] and recognition subtests [95% CI, -0.90 to -0.17, p = 0.001] but not in prospective memory tests compared to controls. The health factors that predicted impaired everyday memory outcome in children with exposure to CM was profound coma [95% CI, 0.02 to 0.88, p = 0.037] and multiple episodes of hypoglycaemia [95% CI, 0.05 to 0.78, p = 0.020], but not seizures.
The findings show that exposure to CM was associated with a specific impairment of everyday memory. Seizures commonly observed in severe malaria may not have a causal relationship with poor outcome, but rather be associated with profound coma and repeated metabolic insults (multi-hypoglycaemia) that are strongly associated with impaired everyday memory.
Episodes of childhood convulsive status epilepticus (CSE) commonly start in the community. Treatment of CSE aims to minimise the length of seizures, treat the causes, and reduce adverse outcomes; however, there is a paucity of data on the treatment of childhood CSE. We report the findings from a systematic, population-based study on the treatment of community-onset childhood CSE.
We collected data prospectively on children in north London, UK, who had episodes of CSE (ascertainment 62–84%). The factors associated with seizure termination after first-line and second-line therapies, episodes of CSE lasting for longer than 60 min, and respiratory depression were analysed with logistic regression. Analysis was per protocol, and adjustment was made for repeat episodes in individuals.
182 children of median age 3·24 years (range 0·16–15·98 years) were included in the North London Convulsive Status Epilepticus in Childhood Surveillance Study (NLSTEPSS) between May, 2002, and April, 2004. 61% (147) of 240 episodes were treated prehospital, of which 32 (22%) episodes were terminated. Analysis with multivariable models showed that treatment with intravenous lorazepam (n=107) in the accident and emergency department was associated with a 3·7 times (95% CI 1·7–7·9) greater likelihood of seizure termination than was treatment with rectal diazepam (n=80). Treatment with intravenous phenytoin (n=32) as a second-line therapy was associated with a 9 times (95% CI 3–27) greater likelihood of seizure termination than was treatment with rectal paraldehyde (n=42). No treatment prehospital (odds ratio [OR] 2·4, 95% CI 1·2–4·5) and more than two doses of benzodiazepines (OR 3·6, 1·9–6·7) were associated with episodes that lasted for more than 60 min. Treatment with more than two doses of benzodiazepines was associated with respiratory depression (OR 2·9, 1·4–6·1). Children with intermittent CSE arrived at the accident and emergency department later after seizure onset than children with continuous CSE did (median 45 min [range 11–514 min] vs 30 min [5–90 min]; p<0·0001, Mann-Whitney U test); for each minute delay from onset of CSE to arrival at the accident and emergency department there was a 5% cumulative increase in the risk of the episode lasting more than 60 min.
These data add to the debate on optimum emergency treatment of childhood CSE and suggest that the current guidelines could be updated.
An anonymous donor to UCL Institute of Child Health; the Wellcome Trust; UK Department of Health National Institute for Health Research Biomedical Research Centres Funding Scheme; Medical Research Council.