Category Archives: Pathophysiology of epilepsy

How to Understand Temporal Lobe Epilepsy

Pathophysiology of epilepsy, , ,

Introduction

In the previous blog (see blog 23), it was clear that not all animal models used for epilepsy research are of equal value.  Some are useful to develop antiseizure drugs.  Others, however, may uncover epileptogenesis, that is the pathological origins of epilepsy, plus the subsequent disease trajectory.  This blog focuses on one animal model of epilepsy with potential for a cure.  This is the kainic acid rodent model.  It exhibits many of the changes evident in humans with temporal lobe epilepsy.  Temporal lobe epilepsy is a common type of epilepsy.  It is generally drug-resistant.  Hence, the reason this model is a noteworthy animal model of epilepsy with potential for a cure. 

Temporal lobe epilepsy – brain site, manifestations, pathology

Location

Temporal lobe epilepsy is the general term for seizures originating from the temporal lobe.  The temporal lobe is one of several regions of the brain (see figure below).  The temporal lobes are located on either side of the brain (approximating the temples).  Temporal lobes control memory, facial recognition, language perception, and hearing.  Additionally, they have strong connections to the limbic system which affects behavior, emotion and motivation.

Symptoms

There are two subdivisions of temporal lobe epilepsy.  They are mesial temporal lobe epilepsy and lateral temporal lobe epilepsy.  The former is the most common.  Spontaneous seizures characterize temporal lobe epilepsy. They appear as a stare or automated muscle contractions and movements.  Additionally, memory loss and confusion occur post seizure.  Seizures originating from the limbic system may be preceded by an aura, strange smells/tastes or strong emotions.  Thus, specific manifestations depend on the brain region of origin.

Pathology

Data obtained in humans from a variety of sources (surgical procedures, EEGs, MRIs, autopsies) show consistent neurological damage.  Specifically, tissue injury (death/disappearance of neurons) appears in key sub regions of the temporal lobe.  Distinct structures such as the hippocampus and amygdala are clearly affected and scarred.   

Kainic Acid Animal Model 

Description

The kainic acid animal model is a rodent (mouse, rat) model.  A researcher injects kainic acid into the animal.  It is the initiator of  the development of epilepsy. This model mimics human temporal lobe epilepsy both in the underlying pathological and in seizure expression. 

Kainic acid is chemically similar to an excitatory neurotransmitter, glutamate.  Thus, kainic acid binds to a select group of glutamate receptors, concentrated in the temporal lobe and stimulates excessive electrical activity in the brain.  Within minutes to hours, severe seizures occur (termed status epilepticus).  Within days and months depending on the model protocol, chronic seizures indicative of temporal lobe epilepsy occur and persist.

As shown by extensive study results, many factors determine the exact outcome.  For example, some of these factors are a) the route of administration of kainic acid (into the abdominal cavity, directly into select brain sites, intranasal), b) the dose of  acid (and when given all at once or in small doses), c) the rodent choice (rat or mouse), d) rodent sex and age, and e) whether housed singly or in groups affect the precise seizure expression and its frequency.  However, this appears to be an asset of this model since the variety of outcomes (termed phenotype) approximates the variety of epilepsy expression in humans.

Key Mechanism

A significant change in the brain of the kainic acid model is the activation of a highly important and ubiquitous protein with the strange name of mTOR.  The name originated when this specific protein was discovered as the “mechanistic Target Of Rapamycin”.  Rapamycin (sirolimus), a well-known immunosuppressant strongly inhibits this protein.  mTOR influences just about all biological activities, e.g. metabolism, cell-cell communication, aging, cell death, protein formation, wound healing and immune function.  Additionally, mTOR in the brain influences generation and growth of neurons,  supports their well-being and enhances connections among them. 

Excessive stimulation of mTOR in the brain as in the induction of status epilepticus of the kainic acid model, and in humans following traumatic brain injury, stroke or genetic errors eventually morphs into temporal lobe epilepsy.  Rapamycin is obviously the drug of choice to block excessive activation of mTOR.  However, rapamycin exerts other effects, negatively affecting the outcome of treatment. 

Data show that undue activation of mTOR  is a reasonable cause of epilepsy.  Effort has been and continues to focus on  the role of  mTOR in epilepsy.  Specifically, it is necessary to understand just how mTOR causes neuronal cell damage and death, changes that perpetuate seizures.  Continuing research with the kainic acid model enables insights needed for an epilepsy cure.

Major Message

No animal model used to study a disease and develop a cure is perfect.  Animal brains are not human brains.  However, the kainic acid model is one of a few that has been characterized and shown to have many similarities, down to the cellular and mechanistic level, to human temporal lobe epilepsy.   Continued studies with this model have the potential to understand the epileptogenesis of seizures.  Rather than suppress seizures, the origin of seizures could be eliminated.

Brain Regions

References on request.

Oxidative Stress and Epilepsy

Pathophysiology of epilepsy, , , ,

Blog 12 – Oxidative Stress and Epilepsy

Oxidative stress is thought to be the underlying cause of major brain diseases including epilepsy.  In association with neuroinflammation (Blog 11 Sleep Disturbances and Epilepsy – Adversaries), oxidative stress is considered an important facilitator in epilepsy that eventually leads to brain damage.  On this point, there exist numerous reviews and animal studies supporting a causative role of oxidative stress in epilepsy.  Sadly, there are only a handful of clinical trials that have tested this role.

Scientists know very little about the development and progression of epilepsy.  It is clear that abnormal repetitive neuronal activity called seizures, are the major symptom of the disease.  A small percentage of  epilepsies are due to known genetic mutations.  The remainder, termed acquired epilepsies, have possible origins in known neurological insults e.g. trauma, illness. However, the cause of acquired epilepsy for most, remains unknown.  Therefore, studies that might identify a common biological pathway such as oxidative stress, for the majority of epilepsies, may lead to better therapies and a possible cure.

Recall that approximately 30-40 percent of patients with epilepsy do not respond to anti-seizure medication.   Those that do, however, must endure side effects, some of which cause patients to stop their medication.  Furthermore, anti-seizure medication treats the symptoms and not the underlying pathological processes.  Thus, it is vital that research into the basic mechanism(s) of epilepsy is front and center.  Evaluation of the role of oxidative stress in epilepsy is a reasonable place to start.

What is Oxidative Stress?

Metabolic processes in our cells, whether dependent on enzymes or not, constantly produce substances (small molecules, particles) that are highly reactive with nearby essential compounds.  These reactive species (RS) contain oxygen, nitrogen or carbon that promote oxidative changes.  Importantly, there is an abundance of RS in our cells and at high concentrations, all RS may directly harm neighboring indispensable compounds, such as proteins, DNA and fats.  Specifically, if the damaged compound is not repaired, these random oxidative changes hinder and subsequently prohibit normal cellular functions.  The outcome is that cells die prematurely and create a state of inflammation.

Fortunately, humans possess a wealth of anti-oxidative defense mechanisms to prevent random oxidative destruction.  These mechanisms include the well-known anti-oxidative compounds e.g. vitamin C and E, selenium, polyphenols, lycopene and flavonoids, all found in fruits and vegetables.  Lesser known mechanisms but of higher overall benefits are the anti-oxidative enzymes e.g. superoxide dismutase (SOD), catalase and glutathione peroxidase.  Additionally, sophisticated repair mechanisms exist that exclusively identify and repair oxidized DNA and proteins.

Oxidative stress occurs when the amount of RS overwhelms the antioxidative systems.  Therefore, key components are not repaired and become dysfunctional.  Fortunately, our anti-oxidative mechanisms are so effective that oxidative stress is minimal.  However, with age, illness, injury, extreme stress, and initial stages of chronic disease, oxidative stress is evident and harmful.

Role of Oxidative Stress in Epilepsy

Several recent reviews summarize the experimental data supportive of a role of oxidative stress in seizures and progression of epilepsy (Lin et al., 2020; Parsons et al., 2022).  Mainly, oxidative stress as defined above is considered “one of the early cellular events and a critical factor to determine the fate of neurons in epilepsy.” (Lin et al., 2020). 

Supportive Data that Oxidative Stress is a Key Mechanism in Epilepsy

1.  Epilepsies due to genetic mutations and genetic animal models of epilepsy show production of reactive species, deficiency of anti-oxidative mechanisms and subsequent neuronal damage (Pearson-Smith et al., 2017). 

2.  The amount of oxidative metabolites and antioxidative potential in children and adults with severe epilepsy correlate with seizure frequency (Morimoto et al., 2016).  Additionally, altered anti-oxidative systems have been identified in patients with temporal lobe epilepsy (Keskin et al., 2016).   Analysis of resected human brain tissue (those undergoing surgery) show evidence of oxidative damage and neuronal cell damage and death (Pecorelli et al., 2015).

3.  Results of numerous experiments in experimental models of epilepsy report that anti-oxidants e.g. N-acetyl-cysteine, melatonin, vitamin C and vitamin E inhibit seizure-induced oxidative damage.  Also, artificial activation of the genetic network that promotes anti-oxidative mechanisms prevents oxidative stress, preserves neurons and reduces seizures in animal models of epilepsy.

Clinical Trials

Armed with aforementioned findings, clinicians began to assess the efficacy of anti-oxidant therapy in patients with epilepsy.  Nineteen antioxidant compounds have been successfully evaluated in animal models of epilepsy (Lin et al.,2020).   Of these, 7 compounds (N-acetyl cysteine; ubiquinone; vitamin C, selenium/sildenafil; vitamin E and melatonin) have undergone clinical trials in man.  Despite the fact that many trials ended more than 7 years ago, results for only two of these compounds, vitamin E and melatonin, are publically available. 

Vitamin E

Vitamin E is a fat soluble anti-oxidant found in nuts, plant-based oils (e.g. olive, sunflower) and avocados, for example.  Two clinical trials using vitamin E as an add-on to anti-seizure medication produced contrasting results.  The first study of 12 patients with epilepsy reported that vitamin E, daily for six months significantly reduced seizure frequency in 10 of the 12 patients (Ogunmekan and Hwang, 1989).   In contrast, daily use of vitamin E for 3 months in 43 patients was without effect on seizure frequency or severity (Raju et al., 1994).  Both studies used what clinicians consider to be very high doses of vitamin E (more than 15 times the recommended daily allowance).

Melatonin

Melatonin is a sleep regulating hormone produced by the pineal gland in the brain.  It is commercially available in extended release form to enhance sleep duration and quality.  Additionally, due to its structure, it acts as an anti-oxidant.  Since epilepsy is associated with sleep disturbances (Sleep Disturbances and Epilepsy – Adversaries), the use of melatonin as an add-on therapy for drug resistant (intractable epilepsy) is reasonable.

There have been six published clinical trials evaluating melatonin in epilepsy.  Five of the 6 studies pertain to children and evaluated melatonin efficacy in 6-37 youngsters with epilepsy for generally a 3 month period.  Melatonin significantly improved sleep characteristics and improved seizure severity in 2 of the 5 studies.  Due to the small number of participants, it is impossible to know why positive results occurred in some but not all studies.  One study in adults with generalized epilepsy found melatonin added on to therapy of valproate for 8 weeks improved both sleep characteristics and reduced the seizure frequency and severity (Verma et al., 2021).  Although larger than trials with children, this trial evaluated only 52 patients.

Yin-Yang of Oxidative Stress

Conclusions

There is considerable experimental data supporting the hypothesis that oxidative stress is instrumental in creating an abnormal environment that continues to perpetuate seizures and maintain the disease state.  Clinical evaluations of more effective antioxidative compounds would be worthwhile.  The positive results of melatonin in adults is a good start that needs confirmation with a larger study.

Select References

Keskin Guler, S et al., Antioxidative-oxidative balance in epilepsy patients on antiepileptic therapy: A prospective case-control study. Neurol. Sci. 5:  763–767, 2016.

Lin T-K et al., Seizure-Induced Oxidative Stress in Status Epilepticus: Is Antioxidant Beneficial? Antioxidants 9: 1029, 2020

Morimoto M et al., Oxidative Stress Measurement and Prediction of Epileptic Seizure in Children and Adults With Severe Motor and Intellectual Disabilities. J Clin Med Res. 8(6):  437-444,2016.

Ogunmekaan AO, Hwang PA.  A randomized, double-blind, placebo-controlled, clinical trial of D-alpha-tocopheryl acetate (vitamin E), as add-on therapy, for epilepsy in children. Epilepsia. 30(1):  84-9, 1989.

Parsons ALM et al., The Interconnected Mechanisms of Oxidative Stress and Neuroinflammation in Epilepsy. Antioxidants  11:  157, 2022.

Pecorelli A et al., NADPH oxidase activation and 4-hydroxy-2-nonenal/aquaporin-4 adducts as possible new players in oxidative neuronal damage presents in drug-resistant epilepsy. Biochim Biophys Acta.  1852(3):  507-19, 2015..

Raju GB, Behari M, Prasad K, Ahuja GK.  Randomized, double-blind, placebo-controlled, clinical trial of D-alpha-tocopherol (vitamin E) as add-on therapy in uncontrolled epilepsy. Epilepsia. 35(2):  368-72,1994.

Verma N et al.,  Effect of add-on melatonin on seizure outcome, neuronal damage, oxidative stress, and quality of life in generalized epilepsy with generalized onset motor seizures in adults: A randomized controlled trial.  J Neurosci Res. 99(6):  1618-1631, 2021.

Assessment 8 – Nature of Epilepsy

Pathophysiology of epilepsy, ,

Introduction

The electroencephalogram known as the EEG is one of the main clinical tools used by neurologists to capture a seizure so as to make a  diagnosis of epilepsy (Assessment 7).  The EEG is a 20 minute procedure in which electrodes are placed on the scalp and brain wave activity is recorded and analyzed.  It has many flaws.

EEG limitations

The EEG  has serious limitations, chief of which is its short duration. Therefore, the EEG is unable to detect most seizures since most seizures, occur randomly.  In lieu of an EEG seizure, physicians rely on the patient and/or family observations of seizure activity for a diagnosis.  Thus, physicians make a selection of anti-seizure medication without hard evidence of seizure type and brain location.  Although this empirical approach may eventually lead to seizure suppression for the patient, albeit with side effects, it does not advance the field of epilepsy.  Treating the symptoms of epilepsy does not add to understanding the disease in order to find a cure.

Insights from intracranial EEG recordings

Cyclic Nature

Brain wave recordings gathered over a period of years in patients with epilepsy have revealed new and important insights into the nature of epilepsy.  Basically, these findings show that epilepsy creates a brain where abnormal brain waves (interictal epileptiform activity) occur in cycles of 7, 15, 20-30 days or more. 

What this means is that abnormal brain wave activity flows over the brain like ocean waves, rising to a peak and then falling to a baseline.  Cycle length is unique to each patient with epilepsy.  Importantly, these cycles are predictive of a seizure. This is because it is the rising number of abnormal brain waves of a cycle (the upswing of the cycle) that is associated with seizure onset.  Therefore, as the number of abnormal brain waves in each cycle increases to its peak so does the risk of seizures.  The waning number of abnormal brain waves of each cycle is not related to seizure risk. The section below describes the supportive experiments.

These are incredibly significant revelations. Firstly, cyclic brain wave activity exists in epilepsy. Secondly, the upward swing to the peak of each cycle is the time of greatest risk for seizures.  Therefore, cycle intervals are unique for each patient with epilepsy. Each cycle predicts the next round of seizures (since it follows the rise in wave number per cycle).  The knowledge of this relation between cycle and seizure could allow for adequate intervention and prevention.

Source of Insights

These findings appear in a publication by Baud MO et al. Under-sampling in epilepsy: Limitations of conventional EEG. Clinical Neurophysiology Practice 6 : 41–49, 2021.  Since copyright laws prevent reproduction of Baud’s figures, it is recommended that the reader obtain the free publication through PubMed (https://pubmed.ncbi.nlm.nih.gov/).  The representative EEGs with superimposed seizure events provide visual confirmation of these findings.

These findings are important. Two hundred patients with generalized epilepsy of unknown cause (idiopathic) resistant to antiseizure medication participated in Baud’s clinical study.  To suppress their uncontrolled seizures they chose an FDA approved intracranial neurostimulator (RNS_System.NeuroPace, Inc).  The device captures the abnormal brain waves (interictal epileptiform activity) described and can respond with a counter stimulation to neutralize the seizure.  

Sixty percent of the patients in Baud (2021)’s study exhibit cyclic rhythms as described above.  This is certainly a significant number of patients to propose cyclic brain waves rhythms as a basic characteristic of idiopathic generalized epilepsy.  However, in the future, it is important to evaluate patients with different types of epilepsy to determine if these cycles also occur across all epilepsies.

Future Challenges 

The hurdle to more wide spread research is how to obtain years long data without  implantation of electrodes directly into the brain.  Intracranial electrode implantation is major invasive surgery and hence, a choice of last resort.  Since these observations would never have been obtained in a 20 minute diagnostic EEG, there is clearly a need to develop external devices with the same sensitivity and accuracy as intracranial devices that would be suitable for long term use. 

Summary

The exciting findings of Baud et al (2021) uncovered significant novel characteristics in the brains of patients with epilepsy.  Results of years long brain recordings of patients with idiopathic generalized epilepsy show abnormal brain wave activity that continuously cycles over intervals of weeks or months.  As the number of abnormal waves increase so does the risk of seizures.  This is an incredible insight and if confirmed in other types of epilepsy, provides the first step toward understanding the disease of epilepsy.