Neurobiological and physiological mechanisms of fever-related epileptiform syndromes

Familial Mesial Temporal Lobe Epilepsy: Clinical Spectrum and Genetic Evidence for a Polygenic Architecture

OBJECTIVE

Familial mesial temporal lobe epilepsy (FMTLE) is an important focal epilepsy syndrome; its molecular genetic basis is unknown. Clinical descriptions of FMTLE vary between a mild syndrome with prominent déjà vu to a more severe phenotype with febrile seizures and hippocampal sclerosis. We aimed to refine the phenotype of FMTLE by analyzing a large cohort of patients and asked whether common risk variants for focal epilepsy and/or febrile seizures, measured by polygenic risk scores (PRS), are enriched in individuals with FMTLE.

METHODS

We studied 134 families with ≥ 2 first or second-degree relatives with temporal lobe epilepsy, with clear mesial ictal semiology required in at least one individual. PRS were calculated for 227 FMTLE cases, 124 unaffected relatives, and 16,077 population controls.

RESULTS

The age of patients with FMTLE onset ranged from 2.5 to 70 years (median = 18, interquartile range = 13-28 years). The most common focal seizure symptom was déjà vu (62% of cases), followed by epigastric rising sensation (34%), and fear or anxiety (22%). The clinical spectrum included rare cases with drug-resistance and/or hippocampal sclerosis. FMTLE cases had a higher mean focal epilepsy PRS than population controls (odds ratio = 1.24, 95% confidence interval = 1.06, 1.46, p = 0.007); in contrast, no enrichment for the febrile seizure PRS was observed.

INTERPRETATION

FMTLE is a generally mild drug-responsive syndrome with déjà vu being the commonest symptom. In contrast to dominant monogenic focal epilepsy syndromes, our molecular data support a polygenic basis for FMTLE. Furthermore, the PRS data suggest that sub-genome-wide significant focal epilepsy genome-wide association study single nucleotide polymorphisms are important risk variants for FMTLE. ANN NEUROL 2023;94:825-835.

Genome-wide association study of febrile seizures implicates fever response and neuronal excitability genes

Febrile seizures represent the most common type of pathological brain activity in young children and are influenced by genetic, environmental and developmental factors. In a minority of cases, febrile seizures precede later development of epilepsy.

We conducted a genome-wide association study of febrile seizures in 7635 cases and 83 966 controls identifying and replicating seven new loci, all with P < 5 × 10⁻¹⁰.

Variants at two loci were functionally related to altered expression of the fever response genes PTGER3 and IL10, and four other loci harboured genes (BSN, ERC2, GABRG2, HERC1) influencing neuronal excitability by regulating neurotransmitter release and binding, vesicular transport or membrane trafficking at the synapse. Four previously reported loci (SCN1A, SCN2A, ANO3 and 12q21.33) were all confirmed. Collectively, the seven novel and four previously reported loci explained 2.8% of the variance in liability to febrile seizures, and the single nucleotide polymorphism heritability based on all common autosomal single nucleotide polymorphisms was 10.8%. GABRG2, SCN1A and SCN2A are well-established epilepsy genes and, overall, we found positive genetic correlations with epilepsies (r_g = 0.39, P = 1.68 × 10⁻⁴). Further, we found that higher polygenic risk scores for febrile seizures were associated with epilepsy and with history of hospital admission for febrile seizures. Finally, we found that polygenic risk of febrile seizures was lower in febrile seizure patients with neuropsychiatric disease compared to febrile seizure patients in a general population sample.

In conclusion, this largest genetic investigation of febrile seizures to date implicates central fever response genes as well as genes affecting neuronal excitability, including several known epilepsy genes. Further functional and genetic studies based on these findings will provide important insights into the complex pathophysiological processes of seizures with and without fever.

Occurrence of hyperventilation-induced high amplitude rhythmic slowing with altered awareness after successful treatment of typical absence seizures and a network hypothesis

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We show that typical absence seizures (AS) and hyperventilation-induced high amplitude rhythmic slowing (HIHARS) or HIHARS with Altered Awareness (HIHARSAA) can coexist in the same patient, but never at the same time.

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We found that alkalosis and dysfunction of the same neural network are involved in both AS and HIHARS.

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AS and HIHARS should be better recognized to avoid misdiagnosis and overtreatment.

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AS and HIHARS can coexist in the same patient, but never at the same time.

Background

Typical absence seizures (AS) are epileptic phenomena typically appearing in children 4–15 years of age and can be elicited by hyperventilation (HV). Hyperventilation-induced high-amplitude rhythmic slowing (HIHARS) represents a paraphysiological response during HV and may manifest with alteration of awareness (HIHARSAA). To date, HIHARSAA has mostly been described in patients without epilepsy.

Aim

To describe five patients with treatment-responsive typical AS who, after becoming seizure free, presented with HIHARSAA.

Methods

By using video-electroencephalographic recording (Video-EEG), we describe differential clinical characteristics and ictal electrophysiological patterns of both typical AS and HIHARSAA.

Results

We demonstrate that when HIHARSAA occurs in patients with typical AS there is a temporal window between the two phenomena. This suggests that the presence of typical AS precludes the appearance of HIHARSAA.

Conclusions

We hypothesize that alkalosis and dysfunction of the same neural network are involved in both typical AS and HIHARSAA and that their distinct electroclinic manifestations are due to the involvement of different ion channels.

Significance

A better understanding of the characteristics of typical AS and HIHARSAA and of the role of alkalosis in both, can help avoiding misdiagnosis and identifying more suitable therapies for typical AS.

TMEM16C is involved in thermoregulation and protects rodent pups from febrile seizures

As the most common convulsive disorder in infancy and childhood, affecting 2 to 5% of American children in their first 5 y of life, febrile seizures (FSs) are associated with genetic risk factors, including the Tmem16c (Ano3) gene. Whereas central neuronal hyperexcitability has been implicated in FSs, whether FSs may result from compromised body temperature regulation is unknown. To approach this question, we developed rodent models of FSs associated with deficient thermoregulation, including conditional knockout mice with TMEM16C eliminated from a hypothalamic neuronal population important for maintaining body temperature but not from most of the cortical and hippocampal neurons and sensory neurons. Our findings raise the possibility that impaired homeostatic thermoregulation could contribute to the risk of FSs.

Febrile seizures (FSs) are the most common convulsion in infancy and childhood. Considering the limitations of current treatments, it is important to examine the mechanistic cause of FSs. Prompted by a genome-wide association study identifying TMEM16C (also known as ANO3) as a risk factor of FSs, we showed previously that loss of TMEM16C function causes hippocampal neuronal hyperexcitability [Feenstra et al., Nat. Genet. 46, 1274–1282 (2014)]. Our previous study further revealed a reduction in the number of warm-sensitive neurons that increase their action potential firing rate with rising temperature of the brain region harboring these hypothalamic neurons. Whereas central neuronal hyperexcitability has been implicated in FSs, it is unclear whether the maximal temperature reached during fever or the rate of body temperature rise affects FSs. Here we report that mutant rodent pups with TMEM16C eliminated from all or a subset of their central neurons serve as FS models with deficient thermoregulation. Tmem16c knockout (KO) rat pups at postnatal day 10 (P10) are more susceptible to hyperthermia-induced seizures. Moreover, they display a more rapid rise of body temperature upon heat exposure. In addition, conditional knockout (cKO) mouse pups (P11) with TMEM16C deletion from the brain display greater susceptibility of hyperthermia-induced seizures as well as deficiency in thermoregulation. We also found similar phenotypes in P11 cKO mouse pups with TMEM16C deletion from Ptgds-expressing cells, including temperature-sensitive neurons in the preoptic area (POA) of the anterior hypothalamus, the brain region that controls body temperature. These findings suggest that homeostatic thermoregulation plays an important role in FSs.

Reduced Efficacy of the KCC2 Cotransporter Promotes Epileptic Oscillations in a Subiculum Network Model

Pharmacoresistant epilepsy is a chronic neurological condition in which a basal brain hyperexcitability results in paroxysmal hypersynchronous neuronal discharges. Human temporal lobe epilepsy has been associated with dysfunction or loss of the potassium-chloride cotransporter KCC2 in a subset of pyramidal cells in the subiculum, a key structure generating epileptic activities. KCC2 regulates intraneuronal chloride and extracellular potassium levels by extruding both ions. Absence of effective KCC2 may alter the dynamics of chloride and potassium levels during repeated activation of GABAergic synapses due to interneuron activity. In turn, such GABAergic stress may itself affect Cl- regulation. Such changes in ionic homeostasis may switch GABAergic signaling from inhibitory to excitatory in affected pyramidal cells and also increase neuronal excitability. Possibly these changes contribute to periodic bursting in pyramidal cells, an essential component in the onset of ictal epileptic events. We tested this hypothesis with a computational model of a subicular network with realistic connectivity. The pyramidal cell model explicitly incorporated the cotransporter KCC2 and its effects on the internal/external chloride and potassium levels. Our network model suggested the loss of KCC2 in a critical number of pyramidal cells increased external potassium and intracellular chloride concentrations leading to seizure-like field potential oscillations. These oscillations included transient discharges leading to ictal-like field events with frequency spectra as in vitro Restoration of KCC2 function suppressed seizure activity and thus may present a useful therapeutic option. These simulations therefore suggest that reduced KCC2 cotransporter activity alone may underlie the generation of ictal discharges.

SIGNIFICANCE STATEMENT

Ion regulation in the brain is a major determinant of neural excitability. Intracellular chloride in neurons, a partial determinant of the resting potential and the inhibitory reversal potentials, is regulated together with extracellular potassium via kation chloride cotransporters. During temporal lobe epilepsy, the homeostatic regulation of intracellular chloride is impaired in pyramidal cells, yet how this dysregulation may lead to seizures has not been explored. Using a realistic neural network model describing ion mechanisms, we show that chloride homeostasis pathology provokes seizure activity analogous to recordings from epileptogenic brain tissue. We show that there is a critical percentage of pathological cells required for seizure initiation. Our model predicts that restoration of the chloride homeostasis in pyramidal cells could be a viable antiepileptic strategy.

Forebrain-independent generation of hyperthermic convulsions in infant rats

Febrile seizures are the most common type of convulsive events in children. It is generally assumed that the generalization of these seizures is a result of brainstem invasion by the initial limbic seizure activity. Using precollicular transection in 13‐day‐old rats to isolate the forebrain from the brainstem, we demonstrate that the forebrain is not required for generation of tonic–clonic convulsions induced by hyperthermia or kainate. Compared with sham‐operated littermate controls, latency to onset of convulsions in both models was significantly shorter in pups that had undergone precollicular transection, indicating suppression of the brainstem seizure network by the forebrain in the intact animal. We have shown previously that febrile seizures are precipitated by hyperthermia‐induced respiratory alkalosis. Here, we show that triggering of hyperthermia‐induced hyperventilation and consequent convulsions in transected animals are blocked by diazepam. The present data suggest that the role of endogenous brainstem activity in triggering tonic–clonic seizures should be re‐evaluated in standard experimental models of limbic seizures. Our work sheds new light on the mechanisms that generate febrile seizures in children and, therefore, on how they might be treated.

Neuronal carbonic anhydrase VII provides GABAergic excitatory drive to exacerbate febrile seizures

Brain carbonic anhydrases (CAs) are known to modulate neuronal signalling. Using a novel CA VII (Car7) knockout (KO) mouse as well as a CA II (Car2) KO and a CA II/VII double KO, we show that mature hippocampal pyramidal neurons are endowed with two cytosolic isoforms. CA VII is predominantly expressed by neurons starting around postnatal day 10 (P10). The ubiquitous isoform II is expressed in neurons at P20. Both isoforms enhance bicarbonate-driven GABAergic excitation during intense GABAA-receptor activation. P13-14 CA VII KO mice show behavioural manifestations atypical of experimental febrile seizures (eFS) and a complete absence of electrographic seizures. A low dose of diazepam promotes eFS in P13-P14 rat pups, whereas seizures are blocked at higher concentrations that suppress breathing. Thus, the respiratory alkalosis-dependent eFS are exacerbated by GABAergic excitation. We found that CA VII mRNA is expressed in the human cerebral cortex before the age when febrile seizures (FS) occur in children. Our data indicate that CA VII is a key molecule in age-dependent neuronal pH regulation with consequent effects on generation of FS.

CARbon DIoxide for the treatment of Febrile seizures: rationale, feasibility, and design of the CARDIF-study

BACKGROUND

2-8% of all children aged between 6 months and 5 years have febrile seizures. Often these seizures cease spontaneously, however depending on different national guidelines, 20-40% of the patients would need therapeutic intervention. For seizures longer than 3-5 minutes application of rectal diazepam, buccal midazolam or sublingual lorazepam is recommended. Benzodiazepines may be ineffective in some patients or cause prolonged sedation and fatigue. Preclinical investigations in a rat model provided evidence that febrile seizures may be triggered by respiratory alkalosis, which was subsequently confirmed by a retrospective clinical observation. Further, individual therapeutic interventions demonstrated that a pCO2-elevation via re-breathing or inhalation of 5% CO2 instantly stopped the febrile seizures. Here, we present the protocol for an interventional clinical trial to test the hypothesis that the application of 5% CO2 is effective and safe to suppress febrile seizures in children.

METHODS

The CARDIF (CARbon DIoxide against Febrile seizures) trial is a monocentric, prospective, double-blind, placebo-controlled, randomized study. A total of 288 patients with a life history of at least one febrile seizure will be randomized to receive either carbogen (5% CO2 plus 95% O2) or placebo (100% O2). As recurrences of febrile seizures mainly occur at home, the study medication will be administered by the parents through a low-pressure can fitted with a respiratory mask. The primary outcome measure is the efficacy of carbogen to interrupt febrile seizures. As secondary outcome parameters we assess safety, practicability to use the can, quality of life, contentedness, anxiousness and mobility of the parents.

PROSPECT

The CARDIF trial has the potential to develop a new therapy for the suppression of febrile seizures by redressing the normal physiological state. This would offer an alternative to the currently suggested treatment with benzodiazepines. This study is an example of academic translational research from the study of animal physiology to a new therapy.

TRIAL REGISTRATION

ClinicalTrials.gov identifier: NCT01370044.

Cation-chloride cotransporters NKCC1 and KCC2 as potential targets for novel antiepileptic and antiepileptogenic treatments

In cortical and hippocampal neurons, cation-chloride cotransporters (CCCs) control the reversal potential (EGABA) of GABAA receptor-mediated current and voltage responses and, consequently, they modulate the efficacy of GABAergic inhibition. Two members of the CCC family, KCC2 (the major neuron-specific K-Cl cotransporter; KCC isoform 2) and NKCC1 (the Na-K-2Cl cotransporter isoform 1 which is expressed in both neurons and glial cells) have attracted much interest in studies on GABAergic signaling under both normal and pathophysiological conditions, such as epilepsy. There is tentative evidence that loop diuretic compounds such as furosemide and bumetanide may have clinically relevant antiepileptic actions, especially when administered in combination with conventional GABA-mimetic drugs such as phenobarbital. Furosemide is a non-selective inhibitor of CCCs while at low concentrations bumetanide is selective for NKCCs. Search for novel antiepileptic drugs (AEDs) is highly motivated especially for the treatment of neonatal seizures which are often resistant to, or even aggravated by conventional AEDs. This review shows that the antiepileptic effects of loop diuretics described in the pertinent literature are based on widely heterogeneous mechanisms ranging from actions on both neuronal NKCC1 and KCC2 to modulation of the brain extracellular volume fraction. A promising strategy for the development of novel CCC-blocking AEDs is based on prodrugs that are activated following their passage across the blood-brain barrier. This article is part of the Special Issue entitled 'New Targets and Approaches to the Treatment of Epilepsy'.

Network activity in hippocampal slice cultures revealed by long-term in vitro recordings

Organotypic hippocampal slice cultures (OHSCs) are widely used for anatomical, molecular and electrophysiological studies of the development of neuronal networks. Electrophysiological recordings are usually limited to a single time point during development, and recording conditions differ greatly based on culture conditions. Consequently, little is known about the maturation of neuronal network activity in vitro. Here, we describe a simple method that allows long-term electrophysiological recordings during culture maintenance in a CO2 incubator. We compared the occurrence of spontaneous network activity, including epileptiform activity, in OHSCs (maintained in Neurobasal/B27 serum-free medium) prepared at different postnatal days and investigated the effects of changes in osmolality and pH. Recordings over 48 h revealed spontaneous network activity culminating in seizure-like events (SLEs) in 65.4% of the OHSCs (n=78). SLE incidence peaked during the first 6h following implantation of the microelectrodes and a secondary increase in SLE-incidence began after 9h of recording and averaged 2.65SLEs/h. The initial peak was likely initiated by transient alkalosis induced by the low pCO2 during the positioning of the electrodes, whereas successive changes in the composition of the culture medium might explain the secondary increase in SLE incidence. Notably, changes in osmolality had no effect on SLE induction. In conclusion, long-term recordings in OHSCs will help to reveal changes in spontaneous network activity during maturation. The extent to which the axonal reorganization known to occur in OHSCs contributes to the susceptibility to epileptogenesis remains to be determined.

Fever and fever-related epilepsies

Febrile seizures are a common emergency faced by general pediatricians. They are mostly self-limiting, isolated events with no sequelae in later life. A minority are more complex. In the acute stage, there are a small number of underlying etiologies that are important to recognize in order to determine the prognosis accurately and to optimize management. There has been a long-standing debate about the relationship of early febrile seizures to the later development of epilepsy. It is now clear that this risk differs for simple and complex febrile seizures: complex febrile seizures may herald the presentation of a number of epilepsy syndromes of which febrile and illness-related seizures are part of the phenotype. This review examines the existing knowledge on febrile seizures and the various clinical phenotypes to which they are linked.

Neuronal injury and cytogenesis after simple febrile seizures in the hippocampal dentate gyrus of juvenile rat

PURPOSE

Although simple febrile seizures are frequently described as harmless, there is evidence which suggests that hippocampal damage may occur after simple febrile seizures. This study aimed to investigate possible neuronal damages as well as alterations in cytogenesis in the hippocampal dentate gyrus following simple febrile seizures.

METHODS

Simple febrile seizure was modeled by hyperthermia-induced seizures in 22-day-old male rats. The brains were removed 2 or 15 days after hyperthermia in all rats with (n=20) and without (n=10) occurrence of seizures as well as in control animals (n=10). The sections were stained with hematoxylin and eosin to estimate the surface numerical density of dark neurons. Ki-67 immunohistochemistry was performed to evaluate changes of cytogenesis following simple febrile seizures.

RESULTS

Hyperthermia induced behavioral seizure activities in 67 % of the rats. The numerical densities of dark neurons as well as the mean Ki-67 index (the fraction of Ki-67-positive cells) were significantly increased in dentate gyrus after induction of seizures by hyperthermia compared to both controls and rats without seizure after hyperthermia. Both the seizure duration and intensity were correlated significantly with numerical densities of dark neurons (but not with Ki-67 index).

CONCLUSION

The data indicate that simple febrile seizures can cause neuronal damages and enhancement of cytogenesis in the hippocampal dentate gyrus, which were still visible for at least 2 weeks. These findings also suggest the correlation of febrile seizure intensity and duration with neuronal damage.

A hot topic: temperature sensitive sodium channelopathies

Perturbations to body temperature affect almost all cellular processes and, within certain limits, results in minimal effects on overall physiology. Genetic mutations to ion channels, or channelopathies, can shift the fine homeostatic balance resulting in a decreased threshold to temperature induced disturbances. This review summarizes the functional consequences of currently identified voltage-gated sodium (NaV) channelopathies that lead to disorders with a temperature sensitive phenotype. A comprehensive knowledge of the relationships between genotype and environment is not only important for understanding the etiology of disease, but also for developing safe and effective treatment paradigms.

Five percent CO₂ is a potent, fast-acting inhalation anticonvulsant

PURPOSE

CO₂ has been long recognized for its anticonvulsant properties. We aimed to determine whether inhaling 5% CO₂ can be used to suppress seizures in epilepsy patients. The effect of CO₂ on cortical epileptic activity accompanying behavioral seizures was studied in rats and nonhuman primates, and based on these data, preliminary tests were carried out in humans.

METHODS

  In freely moving rats, cortical afterdischarges paralleled by myoclonic convulsions were evoked by sensorimotor cortex stimulation. Five percent CO₂ was applied for 5 min, 3 min before stimulation. In macaque monkeys, hypercarbia was induced by hypoventilation while seizure activity was electrically or chemically evoked in the sensorimotor cortex. Seven patients with drug-resistant partial epilepsy were examined with video-EEG (electroencephalography) and received 5% CO₂ in medical carbogen shortly after electrographic seizure onset.

RESULTS

In rats, 5% CO₂ strongly suppressed cortical afterdischarges, by approximately 75%, whereas responses to single-pulse stimulation were reduced by about 15% only. In macaques, increasing pCO₂) from 37 to 44-45 mm Hg (corresponding to inhalation of 5% CO₂ or less) suppressed stimulation-induced cortical afterdischarges by about 70% and single, bicuculline-induced epileptiform spikes by approximately 25%. In a pilot trial carried out in seven patients, a rapid termination of electrographic seizures was seen despite the fact that the application of 5% CO₂ was started after seizure generalization.

CONCLUSIONS

Five percent CO₂ has a fast and potent anticonvulsant action. The present data suggest that medical carbogen with 5% CO₂ can be used for acute treatment to suppress seizures in epilepsy patients.

Novel etiological and therapeutic strategies for neurodiseases: mechanisms and consequences of febrile seizures: lessons from animal models

Febrile seizures (FS) are the most common type of convulsive events in infancy and childhood. Genetic and environmental elements have been suggested to contribute to FS. FS can be divided into simple and complex types, the former being benign, whereas it is controversial whether complex FS have an association with the development of temporal lobe epilepsy (TLE) in later life. In the hippocampus of TLE patients, several structural and functional alterations take place that render the region an epileptic foci. Thus, it is important to clarify the cellular and molecular changes in the hippocampus after FS and to determine whether they are epileptogenic. To achieve this goal, human studies are too limited because the sample tissues are only available from adult patients in the advanced and drug-resistant stages of the disease, masking the underlying etiology. These facts have inspired researchers to take advantage of well-established animal models of FS to answer the following questions: 1) How does hyperthermia induce FS? 2) Do FS induce neuroanatomical changes? 3) Do FS induce neurophysiological changes? 4) Do FS affect the behavior in later life? Here we introduce and discuss accumulating reports to answer these questions.

Febrile seizures: current views and investigations

Febrile seizures (FSs) are seizures that occur during fever, usually at the time of a cold or flu, and represent the most common cause of seizures in the pediatric population. Up to 5% of children between the ages of six months and five years-of-age will experience a FS. Clinically these seizures are categorized as benign events with little impact on the growth and development of the child. However, studies have linked the occurrence of FSs to an increased risk of developing adult epileptic disorders. There are many unanswered questions about FSs, such as the mechanism of their generation, the long-term effects of these seizures, and their role in epileptogenesis. Answers are beginning to emerge based on results from animal studies. This review summarizes the current literature on animal models of FSs, mechanisms underlying the seizures, and functional, structural, and molecular changes that may result from them.