List the 3 main stages of epileptogenesis following the brain insult. Give an example of cellular events that take place on each stage
Early phases= Occuring imediately after insult that causes seizuring cause network dysfunction and thus seizuring.
- The is a gross dysregulation of ion channels and thus the ion gradients.
- Along side gross dysfunction induces ROS release realted to the toxic Glutamate release and action on NMDA>Ca2+ influx
Mid- Phases- during this phase the resultant effects of early dysfunction are seen.
- Neuronal and glial death resulting from glutamate linked excitiotoxicity and mitochondrial dysfunction.
- PRIMARILY associate with the brain attempting to fix the situation.
- Growth hromone release
- inflamation ( WE can observe the break down of the BBB and the leakage of proteins into the brain, micro glial ACTIVATION)
- Transcriptional events in response to immadite early gene release. This is an attempt to fix damge.
Late-phase- In this stage we see recovery of damage but to detrimental effect.
- Inorder to compensate for damage the neuronal input into the area is increased which will increase its excitability and so this is matched with GABA input to control this excitable region.
- In particular changes have been seen in the mossy fibre projection of the dentate gyrus to the CA3- late satge changes shown the creation of local excitatory circuits in which these now project back into the DG rather than out.
- These longterm circuit changes can take weeks in animal models and so explains the delay ssee between insult and epilepsy. (Can be years in humans)
- However, the threshold has now been lowered and so normal excitatory activity has a greater chance pf inducing SPONTANEOUS EPILEPTIC SEIZURE.
define focal and generalised seizures, explain the two mechanisms by which focal seizures can spread.
Focal seizures-
- This refers to seizures originating in focal ragion of one hemisphere and thus showing varying symotoms based on the area of dysfunction.
- This means they are commonly asscoiated with AURAs which are predictive expereinces of seizures. for example if it strats in a motor are might get limb jerking, or memory flashbacks if in the temporal lobe.
- Seizures can be met with a loss of awarenss or with awareness.
- Can be related to Tonic or myoclonic seizures but the expereince will remain local. (Tonic paralysis of 1 limb or rhythmic spasm of 1 limb)
Generalised seizures
- This refers to seizures orgination from bilateral spreading and thus effecting both lobes.
- The Best associated are Tonic-Clonic seizures starting showing signs of first tonic tensing and loss of conciousness and them rhythmic convulsions associated with clonic seizure.
- Absent seizures- These involve the transient loss of awarenss often mistook for day dreaming but distinguishable my the pertial myoclonic aspect of flickering eyelids. (related to disruption in the thalmo-cortical circuits) (in atypical cases this can last longer and be accompanied by aspects like chewing or rubbing fingers)
note: Generalsised seixures are largely associated with being idiopathic, genetic, as a system wide vulnerbaility to seizure is seen.
- Focal are more associated with brain insult as their is suceptibility in a defined region.
- This is shown in MZ and DZ condordance studies , this is 0.77vs 0.33 in PGE this is not seen in focal.
FOCAL seizure spread.
- In both cases we seen TRANGENTIAL SPREAD of activity. This means incorporporating neigbhouring ares and spread between connected regions of the brain. In focal this spread in uni-hemispehric.
- Focal seizures can access subcortical white matter and thus transition to the other hemisphere causing bilateral spreading. At this point it is refered to as a SECONDARY GENERALISED SEIZURE.
(Permissive spreading between interconnected regions along more distant reaches via white matter tracts is also seen. This is related to the progressive nature of dysfunction seen in epilepsy.)
Define a seizure. explain How does activity cause a seizure?
The brain transitions between mental states through arteration in natural oscillation in activity reffered to as brain waves. This is what would confer a switch between the concious awake state and the unconcious sleeping state.
- A seixure is a transient brain state much like this.
- The ILAE define it as a trasient brain state characterised by excessive or snchronous neuronal activity.
The synchronous activity increases the amplitude of the brain wave and thus causes the excessive activity.
How can we mimic seizures in brain tissue samples?
1 way to mimic a seizure is to lower the threshold for seizure much like in real life by increasing the exitability of the sample.`
This can be achieved by removing Mangnesium and these often block the calcium permeable NMDA channels, thus activity here now will induce excessive synchronous activity and seizure.
How can we record seizures in the brain?
We record seizures using an electroencephalogram.
Indicators of a seizure are transient bursts of spiking activity called an ICTUS. Interictal period where spikes vary in size. A pre ica period in which there is frequent activity that grows insize u=until the ictal period. Seen on EEG.
you can often observe what is reffered to as the PYROXYMAL DEPOLARISING SHIFT
- This is a shift in activity to a maintained suprathreshold level preventing hyperpolarisation relating to K+ activity. Hence, this leads to the maintained excessive activity in seizure.
- simmilarly in severe cases a constant state of maintained seizure activity can be observed called STATUS EPILEPTICUS, these seizures can last around 30 minutes and cause severe damage to neural ciruits, it is these prolonged seixures that we asscoiate with epileptigenesis following brain insult.
EEG is important when indentifying regions for surgical intervention intherpay resistant refractory epilepsy. This can be used to seen all impicated regions by observing the strat and end sites of activity.
EEG has been shown to have some predicitive ability.
- investigating the EEG of children in ICU they found that 47% of those showing symptomatic SE developed epilepsy VS 11% of those who didnt (Wagenman, 2014)
How do seizures spread?
Seizures spread trangentially, i.e from incorporating neighbouring regions of neural tissue.
Permissive spreading between interconnected regions along more distant reaches via white matter tracts is also seen. This is related to the progressive nature of dysfunction seen in epilepsy.
Outline the main types of generalised seizure.
Outline Tonic and Conic seizures
Generalised seizure-
Tonic-Clonic:
- This incorporates both tonic and myoclonic fetaures.
- Patients can often appear blue/ashy in the face. This is likely due to the Tonic contrcation seen early on preventing respiration.
- Patients will then likley lose conciousness.
- Following this their WHOLE body will undergo rhythmic myoclonic convulsions.
- When over the patients will have little or no recolection of the event.
- These can aslo stem from secondary generalised seizures
Absent-
- Ver common in yound children with 90% levaing before adultho0d.
- Largely asscoiated with idiopathic causes.
- This is charcterised by brief absences of awareness often mistook fro day dreams but distinguishable by the myonclonic fetaures of flickering eye lids.
- The root is thought to be renetrant excitation in the thalmo-cortical networks, which have been associated with conciousness and switiching brain states
- In atypical forms This can last longer and be seen alongside other motor abnormalities like chewing or rubbing fingers.
Tonic=
- this solely involves he tonic contraction of muscles.
- This can be caused by both genral and focal seixures differentiating between on whther the contaction is general or local.
- The can be seen in some genetic conditions like LENNOX-GASTAUT SYNDROME in which it can be associated with mutations in the voltage gated Na channels (SCN1A,2A,8A) (channel opathy)
Myoclonic
- just involves th rhymic jerking
- much rarer then tonic-clonic
- again caused by both seixure forms differing on the same aspects as tonic seixure.
What is status epilepticus
Staus epileptic is a hyperexcitable state of maintained seizure actvity with no reocvery.
The prolonged seizures can last around 30 minutes and have been associated with the longterm damage stemming from brain insult in epileptogenisis.
What are genetic forms of seizure? which form of seizure are they best associated with?
Most primary generalised froms of seizure are associated with idiopathic causes due to them confering a system wide vulnerbaility to seizure.
This has been demonstarted in the concordance of twin studies. MZ 0.76, DZ 0.33
A GENTIC Form of tonic seizure is LENNOX-GASTAUT syndrome associated with mutations in the voltage gated Na channels (SCN1A,2A,8A)
Why do focal seizures originate in the same area?
focal seizures are associated with a focal region of of increased porpensity to seizure. a localised reduction in seizure threshold, hence this region is commonly the source ofspontaneous seizure activity in epilepsy cases.
What risk factors can be associated with increased propensity to have epileptic seizures?
Genetic factors- alterations in ion channles e.t.c. These are ofetn polygenic cases although rare mednelian inherited cases exist.
Environemental- Alcohol ( fast withdrawlfrom alcohol, has been seen to cause seizure). Brain insult ( assocuated with the localised dmage to reduce focal threshold)
often environment and predispoition are involved simultaneously.
What is the difference between epilepsy and normal seizures?
Seizure is a vulnerbailtiy to INDUCED SEIZURE.
Epilepsy is specifically the vulnerbaility of a region to SPONTANEOUS seizure.
How does the weight up risk of inherited vs aquired seizure change over life
Early on in childhood most epileptic cases are inherited, genetic.
between the 20-40s this is dominatly the result of TBI like bike crashes.
In the older generation this is linked to brain insult like stroke.
What electrochemical feaures can be associated with the epileptic brain?
increased Ca and Na activity, reduced K+ activity.
Altered synaptic function to favur EPSPS of IPSPS.
Pyroxymal depolarising shift- maintained suprathrashold activity to prevent hyperpolarisation and fasciliate recurrent seixure activity.
What is epileptogenesis?
Epileptogenis is the study of the formation of vulnerbailties in lowering the seizure threshold rsulting in epilepsy
This often involves inducing damge in brain regions to investigate the impact. A common method is the TBI method of Status epilepticus model (SE) which uses induced SE (achieved by the injection of kainic acid) to cause damge particuarly in the hippocampal DG and CA regions. They then develop regular epileptic atacks within several weeks.
Discuss the link between prolonged seizure role in brain damage and barin insult and epileptic seizure developement?
Studies inflicting insult in rats found that there was a delay in epilepsy formation of a few weeks, this can be years in humans. But this suggests that insult can cause epilepsy (kainic acid injection)
Studies by VESPA ET AL (2010) used MRI to image the brain of a patients with repeptive status epilepticus and reported Unilateral hippocampal sclerosis.
- This suggests that prolonged repetive seixures can cuase long lasting damage,
- This was replicated in mice in the SE model. they word inducing SE in the hippocampus was able to drive neruodegeneration in the CA3 neurons. Hnece this model could be used o investigate the effects follwoing this involved in epileptogeisis following brain insult.
- EEG has been shown to have some predicitive ability.
- investigating the EEG of children in ICU they found that 47% of those showing symptomatic SE developed epilepsy VS 11% of those who didnt (Wagenman, 2014)
- Ths now leads to the study of 3 key phases this covered immeadite dysfunction in metabolsim with the release of excessive glutamate, Ion gradient dyregulation, inflamation, leading to dysfunction and glial and neruonal excitiotocity mediated death. and consequelty network dysfunction.
- Also then involves the attamept of the body to repair damge and subsequent transcriptional changes that result in altered growth and longeterm changes in crcuitry and that result in the formation of a region that is vulnerable to the spontaneous seizures with normal excitation.
How can seizures following brain injury damage result in further seizuring and damge with delay.
The pemrissive spread of seixures has been related to the progressive spread of dysfunction.
rEPETITIVE SEIXURE WILL RESULT IN THE INCORPORATION OF NEW FUNCTIONING CIRCUITS.
-this was identified through the discovery of a phenomena call KINDLING,.
*They found stimulating a region would only causes a small reponse, but repetitive stimulation could drive te a cumjmualtive rempijng up of the repsonse until it could induce seixure. this is thought t be how new areas can become dysfunctional in epilepsy and thus why individuals who have had seixure for a longterm are often not seuceptible to treatemnt by surgery.
(original study was tring to mimick firing of memory and later studies showed stimulation on a daily basis could induce this. This mimics dail seizures.)
(longer individuals had surgery correlated with the the lack of efficay or surgical intervention. hence should be saved fro young individuals) JANSZKY ET AL 2005)
Give more specific information of targettable changes that occur in the early periods (first 2 phases) of epileptogenesis?
Early phase-A big issue is ROS formation
- This occurs early on and can be largely associated to the downstream cascade of activity following glutamate activation of NMDA.
- this involves the calcium influx actovtation of NOS and the calcium overload of mitochondrial causes depolarisation and dysfunction;.
- besides excitotoxicity and the caclium overloads role in driving the pathological state of the MPTP (mitochondrial permeability transition pore) and subsequent Cytochrom C release and the activation of the Apoptotit pathway through caspace cascade activation.
- NOS pottentiation of NO is dangerous- NOS ccan compete with O2 at the complex IV to disrupt the MRC to produce more ROS, but it can also intercat with ROS molecues to form Peroxynitrite- a toxic species involves in the breakdown of the lipid mebrane via lipid peroxidation and casues DNA damage.
Therapy- The aim in to remove ROS
- antioxidants alone do not work as they are quickly used up so insted therpaies have attempted to pottentiate the endogenous production of it.
- This has involved using transription factor called NRF2 which binds to the antioxidant reponse element on DNA ot turn on the cell defence.
- This is often inhibited by a protein calle KEAP1 so attempts have been made to inhibit this interaction with KEAP inhibitors.
- SHEKH-AHMED trialed RT-408 in mouse models and reported dose-dependant reductions in Hippocamapl CA1-CA3 death if given early after damage.
- This also Reduced thre number of seixures follwoing insult.
Middle phases
- Target RNA transcription changes.
- Changes in RNA transcription following insult can be associated with the eventual changes to the neuronal netowrk that are evetually detrimental.
- One of the key proteins in regulating RNA expression is microRNAs, short non coding RNA who asscoiated with the RISC complex to silence hundreds of RNA.
- Henshall et al *2012)reports the increased expression of miRNA-134 wich is a brain activity associted miRNA. increases seen in prolonged seizures is mice and human epilepsy. (anatgomirs caused a 95% reduction in mir=134 after 24hrs)
- They used inhibitors in healthy mice and found they reduced the density of spines in the CA3 and rendered mice resistant t status epileptica. (perhaps it is then an anti-convlsant in reducing the impacts of seizure(BUT)
- Using this in mice that has epielpsy resulting from status eplielptic, given 1 hour after SE, reduced later seizures BY 90%.
- However at their own admissio that do not know whether this action is anticonvulsant or antiepileptigenesis. his requires further investigation to see if this is a vaible primary line of treatment.
- target inflamation- Inflamationis eaully involved in the increased NO expression follwoing insult.
- Inflamation alone has been seen to cause seizures. this has been related t its downstream effects on caclium regulation through altering PI3K activity as well as glutamatergic and gabaergic transmission.
- Studies attempted using the anti inflamatory drug LOSARTIN, that targets TGF-beta, a cytokine and inflamatory mediator.
- The found that this could reduce seizure frquency in SE models. This effects was maintained.
- This is also supported by studies in TBI models using focal cooling to reduce the seizure number.
Neuronal death
- Studies have attempted to [prevent neuronal death earlu on follwoing insult.
- Khali et al carried this out and reported that using CAVACROL to reduce hippocampal death, had not impact on whether epilepsy formed ( This is because the changs in circuitry function is still occurin.)
Give more specific information of targettable changes that occur in the late period of epileptogenesis?
Late phases- This is dominatnly to stop changes in circuitry.
- LATE ON TO compensate fro deficits the brain will increased the exitatory inputs into the damged region, althoug this is matched by GABAergic inputs to ry and calm the exitabilty.
- However, this is only under control slightly.
- natural slight ncreases in the excitation of the region could then trip the balnace and induce a synchronous activity resulting in seizure.
Studies have shown this altered ciruity in the DG> CA3 projection of the hippocampus i SE models
- Instead of neurons projecting to the CA3 they now projecting back into the DG creating hyperexcitable micor circuits. Prevnting the mossy fibre growth could be effective.
- This has been larely associated with BDNF with an array of studie showing that the infusion of BDNF can promote granule ceel hypertrophy and mossy fibre branching. In addition to this electroconvulsive seizure induced branching is inhbited in BDNF KO mouse lines. And its levels increase in the DG after seizure.
- However, others counter this showing that oerexpression of BDNFdoes not cause mossy fbre sporuting.
- Much like the Jak/STAT pathway over activity f the MTORC1 pathway has been observed in rats following th induction of epilepsy in models. (inhibtion of STAT3 phosphorylation can reduce the severity of later epilepsy in rats)
- The have attempted this using Rapamycin to inhibit the mtor pathway but report that this does not benefit seizue frequency. (HENG ET AL 2013)
- it would seem LATE PHASE intervention is TOO LATE as tagetting this siingular change does no prevent the wide range of altered function occuring.
However
- BREWSTER ET AL- they showed that using rapamycin in the SE model to to inhibit motor and the abnormal growth was able to prevent some cognitive deficts in mice. With preventing MWM deficits.
Why might treating neuronal death be inssuficient to prevent epileptogenesis
Neuronal death
- Studies have attempted to [prevent neuronal death earlu on follwoing insult.
- Khali et al carried this out and reported that using CAVACROL to reduce hippocampal death, had not impact on whether epilepsy formed ( This is because the changs in circuitry function is still occurin.)
Is their evidnece that established Epilepsy can be reversed?
Wykes et al 2012
- Epilepsy was induced via cortical injection of tetanus toxin.
- This method has been investiagted in mice using lentiviral virus mediated gentic editing methods.
- The pathogenic units of the virus has been removed.
- The expression of Kv1.1 is attached to the CAMKII (calcium modulated kinase 2 only expressed in excitatory cells)
- Hence, this mediates a genetic change to drive over expression of the voltage gated calcium channles in the excitatory cells of the targetted region.
- This was shown to prevent epielpsy , when coinjected with tetanus toxin. and when injcetd in established SE this progressive reduce epilespy over a few weeks (time for exprresion) suggests it can reverse hyperexcitability of vulnerbale circuits.
This is promissing for refractory epilepsy where the only current method is surgical removal. which is damging and invasive,
Give 3 current anti epileptic drugs? what insight can phentytoin trials give us?
THESE ONLY TREAT THE SYMPTOMS tertiary treatment
Valproate- a inhibtor of GABA terminase and thus increae GABA activity, and Na channels inhibitor.
hence this works to reduce excitatory action and is often used in tonic-clonic seizures
Phenytoin- This uses use dependance to activate frequntly active sodium channels and thus traget hyperexcitable regions. This is ussed in focal seizures.
Diazepam- a BZ and thus pottentiator of GABA activity, this is ony use in extreme cases like staus epilepticus.
beyond seizures what deficits can be sen in epileptic patients?
Patients can also present wit cognitive defects
- 50% of epileptic patients present with a psychiatric condition
- depression is paritcuarly common in patients with temporal epilepsy (note: this is a bidirectional relationship, having either increases the risk of the other)
Discuss the concepts of progression of epilepsy and apply this to the viability of surgery as a treatement
Porgression can be related to the permissive nature of how seizure activity can spread.
This is demonstarted in JANSZKY et al studies identifiying a relationship in which the efficacy of surgical intervention reduced with the length of epilepsy.
This is because more regions are incorporated. (involves kindling)
