Evidence 3 - Epilepsy - Professional Practice

## Section 2 - Professional Practice ### Module 1 - Professional Knowledge ### EVIDENCE 3 Select a disease topic of your choice and discuss the following: clinical features/symptoms, causes, diagnostic criteria, prognosis/staging information, treatment and monitoring). ### Epilepsy Epilepsy is a...

## Section 2 - Professional Practice ### Module 1 - Professional Knowledge ### EVIDENCE 3 Select a disease topic of your choice and discuss the following: clinical features/symptoms, causes, diagnostic criteria, prognosis/staging information, treatment and monitoring). ### Epilepsy Epilepsy is a chronic neurological disease characterized by an increased susceptibility to seizures. These seizures are a spontaneous propagation of electrical activity in a specific brain region, which results in neuronal hyperexcitability. Due to frequent neuronal hyperexcitability, epilepsy can present as a sudden change in awareness, rapid involuntary movements, which may accompany a loss of consciousness (convulsions), issues with autonomous breathing, as well as a loss of bladder control, all of which will be due to altered neuronal function. #### Neuronal Signalling To understand the biological basis of epilepsy, we must first understand the principles of neuronal signaling. Normal ionic balance around neurons is: * Higher extracellular Na+ concentration compared to a higher intracellular K+ concentration * Relative permeability of the neuronal membrane to K+ at rest alongside the relative concentrations of other present ions (both intracellularly and extracellularly) such as Cl will establish an approximate resting membrane potential of -70mV. At this resting potential, there is no neuronal activity, and a stimulus will need to enable, initially, a small disturbance in this membrane potential in order to surpass a threshold voltage. This threshold is achieved via summation of slight local ionic disturbances, which are graded potentials. These graded potentials will be caused by a stimulus, such as an excitatory neurotransmitter, opening voltage-gated ion channels. The ionic movement through these opened voltage-gated ion channels that is associated with this graded potential cannot diffuse a significant distance intracellularly and as such, summation of multiple graded potentials to achieve the threshold transmembrane voltage of approximately -55mV is required to cause neuronal depolarisation. Once this -55mV voltage has been achieved, voltage-gated Na+ channels will open, which will vastly increase the permeability of the neuronal membrane to Na+, resulting in a large intracellular Na+ influx. This process of depolarisation results in the firing of action potentials. Action potentials can be described as the transmission of a wave of depolarisation across the entire axonal length to allow for a response at the pre-synaptic terminal. To maximise the efficiency of action potential propagation in the CNS, action potentials travel via saltatory conduction. An insulating layer of primarily lipids known as myelin will insulate the entire length of the axon aside from regular functional breaks in the myelin, these being the nodes of Ranvier. As previously established, ionic changes only affect the local transmembrane voltage and as such for the propagation of an action potential, this ionic disruption must be mimicked across the entire length of the axon. The nodes of Ranvier contain localised voltage-gated Na+ channels, which means that when the wave of depolarisation reaches these nodes, depolarisation will occur rather than depolarisation having to occur across the entire axonal length, as would be the case in the absence of myelin, overall increasing the speed of action potential conduction (Cohen et al, 2020). #### Depolarisation and Repolarisation Depolarisation and Na+ influx will occur until the transmembrane potential reaches approximately +30mV and at this point, voltage-gated Na+ channels close, and voltage-gated K+ channels open, this being repolarisation. The purpose of repolarisation is to enable the resting potential of -70mV to be re-achieved following an action potential. The exponentiated permeability of the neuronal membrane to K+ due to opening of the voltage gated K+ channels alongside the relative permeability of the neuronal membrane to K+ at rest leads to a large efflux of K+ from the neuron overall resulting in an achieved membrane potential that is slightly more negative than the intended -70mV. This slight deviation from the -70mV resting potential is necessary as to establish an absolute refractory period whereby irrespective of the stimulus that is applied, there will be a delay until the next action potential can be fired, which primarily ensures unilateral action potential propagation and also limits neuronal hyperexcitation. When the transmembrane voltage is more negative than 70mV, the neuron is hyperpolarised and the resting potential is re-established through the action of the Na+-K+ATPase, which will simultaneously transport 3 Na+ ions out of the neuron and 2 K+ ions into the neuron. #### Epilepsy: Hyperexcitability and Hypersynchrony Epilepsy is classified as high frequency action potentials leading to neuronal hyperexcitation, hypersynchrony of adjacent neurons, and a failure for this excitatory response to cease (Bromfield, Cavazos and Sirven, 2006). The underlying cause of this hyperexcitation is due to the action of glutamate, which is the primary excitatory neurotransmitter in the CNS. Once an excitatory action potential reaches the pre-synaptic terminal, depolarisation results in the opening of voltage-gated Ca2+ channels, which results in a large intracellular Ca2+ influx. Increased intracellular Ca2+ enables the formation of an active calcium-synaptotagmin complex, which mediates the fusion of glutamate-containing vesicles with the pre-synaptic membrane, and the subsequent exocytosis of glutamate into the synaptic cleft (Littleton et al, 2001). Once in the synaptic cleft, glutamate will bind to one of two key receptors associated with downstream neuronal excitation: * AMPA receptors * NMDA receptors Both AMPA and NMDA receptors are ionotropic receptors, which means that they are ligand-gated ion channels. * AMPA receptors allow for Na+ influx and K+ efflux. * NMDA receptors allow Na+ and Ca2+ influx and K+ efflux (Viscardi et al, 2021). NMDA receptors have a further level of inhibition due to the intracellular effects of increased intracellular Ca2+, and this inhibition is achieved through Mg2+ binding to the NMDA ion pore, which prevents initial Na+/K+/Ca2+ transport. Due to this inhibition glutamate in the synaptic cleft first binds AMPA receptors enabling neuronal Na+ influx and depolarisation. Depolarisation results in the transmembrane potential becoming more positive and due to the like positive charge of the inhibitory Mg2+ ion, this Mg2+ is released into the synaptic cleft, which allows the NMDA receptor to become active, allowing further depolarisation through Na+ influx once bound to glutamate and glycine, the latter of which like glutamate is also released from into the synaptic cleft from pre-synaptic vesicles (Rivadulla, Sharma and Sur, 2001). #### The Role of Astrocytes Following glutamate-induced post-synaptic excitation, the excessive glutamate remaining in the synaptic cleft has to be removed to prevent hyperexcitation of neurons. This is achieved through the action of neighbouring astrocytes in the CNS. These astrocytes will possess Kir4.1 transporters, which are able to uptake glutamate from the synaptic cleft as well as extracellular potassium, which prevents hyperexcitation and also prevents a prolonged absolute refractory period due to significantly elevated extracellular K+, which would adversely affect speed of neuronal signalling (Nwaobi et al, 2016). In the astrocytes, this glutamate is converted back into glutamine via glutamine synthase, this formed glutamine being released back into the synaptic cleft and taken up by the pre-synaptic neuron to enable re-formation of glutamate due to the action of glutaminase. #### Physiological Aberrations in Epilepsy Now that normal neuronal signalling and the action of glutamate in the CNS has been established, the biological basis of epilepsy can be discussed, which consists of 3 physiological aberrations. 1. **Abnormal fluctuation in neuronal membrane voltage** resulting in neuronal hyperexcitation. This is called a paroxysmal depolarising shift (PDS) (Kubista, Boehm and Hotka, 2019). PDS can be caused by a variety of factors, which center mainly on prolonged glutamate retention in the synaptic cleft. Take a normal process such as aging, as we age, there is a natural loss of functional astrocytes, and further to this, there is abnormal astrocytic distribution (Palmer and Ousman, 2018). Aging can therefore contribute to PDS as a loss of neighbouring astrocytes relative to the synaptic cleft means that the excess glutamate in the synaptic cleft will not be taken up by Kir4.1 transporters, so remains in the synaptic cleft for longer, and as such, is able to more frequently stimulate post-synaptic depolarisation through AMPA/NMDA receptor binding. PDS will occur across multiple neurons in epilepsy due to perhaps abnormal CNS astrocyte distribution and PDS leads into the 2nd physiological disturbance that is responsible for epilepsy: neuronal hypersynchrony. Mass PDS as expected, will lead to gross disturbances in the transmembrane voltage due to mass depolarisation. The main instigator of epilepsy being the raised extracellular K+ concentration. Higher extracellular K+ even in the absence of a stimulus such as glutamate will lead to the resting potential becoming more positive than -70mV, which in turn results in constant partial neuronal depolarisation (Takahashi, Shibata and Fukuuchi, 1997). Aside from mass depolarisation contributing to raised extracellular K+, the loss of astrocytic function/presence will also reduce excessive extracellular K+ uptake through Kir4.1 receptors. As hyperexcited neurons due to a PDS raised extracellular K+, the subsequently caused lower resting potential (which will be closer to the threshold value for an action potential to be reached) allows for more rapid depolarisation of adjacent neurons. As a consequence, these adjacent neurons, even if not directly influenced by glutamate, will fire more frequent action potentials. This is neuronal hypersynchrony. 2. **Failure of inhibition**. It is PDS and the resultant neuronal hypersynchrony that causes seizures that are associated with epilepsy, but it is the 3rd physiological abnormality, failure of inhibition, which will contribute to prolonged seizures. In terms of how failure to inhibit contributes to epilepsy, first, there is a shortened refractory period due to the excessive glutamate stimulation and excessive extracellular K+. In terms of the refractory period, there are two phases, the absolute and relative. The absolute period as established is where no stimulus, irrespective of magnitude, can stimulate a further action potential due to action of the Na+K+ATPase. The relative refractory period follows the absolute and only stimuli of sufficient magnitude, as would be these case due to the combinatory effects of excessive glutamate signalling and significantly elevated extracellular K+, would be able to stimulate an action potential, which in this context allows for more rapid post-synaptic depolarisation and resultant propagation of neuronal hyperexcitability (Dorn and Witte, 1995). Excessive glutamate signalling will also prevent neuronal inhibition. As established, the availability of neighbouring astrocytes as a consequence of aging may contribute to prolonged presence of glutamate in the synaptic cleft, but other factors, such as genetics, may contribute to raised cleft glutamate. For example, glutamine synthase deficiency can result in the conversion of less glutamate back into glutamine within astrocytes, even if there are a sufficient number of neighbouring astrocytes, resulting in the presence of more glutamate in the synaptic cleft and mass post-synaptic depolarisation (Eid et al, 2012). Finally, in conjunction with raised glutamate signalling, the final cause of inhibitory failure is due to the action of the NMDA receptor. As previously stated, unlike the AMPA receptor, when activated, the NMDA receptor is capable of stimulating a Ca2+ influx. Ca2+ is capable of acting as a potent secondary messenger due to its ability to form a variety of active calcium-protein complexes. One such protein that is activated that can contribute to epilepsy is calpain. Calpain is a calcium-activated protease so when intracellular Ca2+ concentration is increased due to glutamate opening NMDA ion channels, this Ca2+ can bind to and activate calpain. This calpain is a pro-apoptotic protein and cleaves both the nuclear and cytosolic membranes resulting in apoptosis (Moon, 2023). The action of calpain can cause both neurodegeneration of the excitatory neuron in which the Ca2+ was released, which does not prevent severity of a seizure due to the persistence of neuronal hypersynchrony, but also can induce apoptosis of inhibitory neurons, which primarily release the inhibitory neurotransmitter gamma-aminobutyric acid (GABA). This reduction in neuronal inhibition alongside continued neuronal excitation overall contributes to prolonged epileptic seizures. #### Causes of Epilepsy All causes of epilepsy, irrespective of the specific root cause, will centre on either abnormal neuronal hyperexcitability and PDS, neuronal hypersynchrony or failure to inhibit neuronal signal propagation. Aside from the already mentioned abnormal astrocytic distribution with age and glutamine synthase deficiency, there are other potential causes of epilepsy such as CDKL5 deficiency. #### CDKL5 Deficiency CDKL5 deficiency is caused by a mutation in the CDKL5 gene, which is found on the X-chromosome. CDKL5 deficiency is inherited in an X-linked dominant manner, and as such is much more prevalent in females although possession of this mutation itself is very rare (Rodak et al, 2022). The mechanism of CDKL5 deficient epileptic induction is not well understood, but it is theorised that the abnormal CDKL5 protein is not capable of adequate inhibition of Cav2.3, which is a protein responsible for neuronal Ca2+ transport (Yan, Guo and Xu, 2024). As such, intracellular Ca2+ influx is largely increased, which as mentioned can have adverse effects in the CNS such as neuronal apoptosis and resultant neurodegeneration through activation of calcium-dependent proteins such as calpain. This hypothesised mechanism of epilepsy in CDKL5 deficient patients will match with the clinical symptoms of CDKL5 deficiency such as the observed severely impaired motor coordination, may be due to Ca2+ induced neuronal degeneration, which will result in a much reduced frequency and speed of neuronal transmission, which itself can impair motor function. #### Environmental Factors that Cause Epilepsy Aside from normal biological and genetic factors, environmental factors can also be causative of epilepsy such as a stroke, physical head injury or a brain tumour, all of which can result in traumatic brain injury (TBI). Following an event that is causative of TBI, there will first be a neuroinflammatory response at the site of damage, which can result in, primarily with concern to latent epilepsy, astrogliosis. This increased presence of astrocytes can enable the release of a multitude of pro-neuroinflammatory cytokines, such as TNF-alpha, which is capable of upregulating proinflammatory gene expression, which will enable the recruitment of microglial cells to remove cell debris as a result of the sustained cranial trauma and TNF-alpha is also able to directly induce apoptosis of damaged cells, which can result in neurodegeneration (van Loo and Bertrand, 2023). Following on from TBI and the acute neuroinflammatory response, there will be neurogenesis and synaptogenesis to restore normal CNS function, and it is from this point that epilepsy can develop. One of the potential causes of epilepsy following TBI is cortical dysplasia. Due to TBI, neuronal repair and neurogenesis would be necessitated as to restore the damaged neuronal circuits. Following neurogenesis, there is a possibility that the subsequently formed synapse between adjacent pre and post-synaptic neurons is improperly aligned, which will result in inability for a signal to be received. This is cortical dysplasia (Nemes et al, 2016). Misalignment of these neurons could contribute to epilepsy if this synaptic junction primarily inhibitory through the release of GABA. This means that in the event of PDS and subsequent neuronal hypersynchrony, there is less inhibition through the action of GABA, leading to a prolonged epileptic seizure. #### Diagnosis of Epilepsy Epilepsy is initially diagnosed via electroencephalography (EEG). An EEG will be indicated if the individual presents with symptoms of epilepsy, most frequently following a seizure, the most overt presentation of epilepsy. An example generated encephalogram can be seen below in figure 1. The X-axis represents the time and the Y-axis the wave frequency. The different types of wave, being alpha, beta, delta and sigma are not relevant in diagnosis of epilepsy, and it is more so the morphology of the waves, which are a diagnostic indicator. An EEG will be able to diagnose epilepsy when a seizure is occurring due to the capability of the EEG to detect electrical abnormalities within the brain. Typically, an EEG in a non-epileptic patient will present as a steady frequency line, as can be seen in the highlighted box in figure 1. In those with epilepsy, however, during a seizure there will be abnormal paroxysmal depolarising activity, as has been established when discussing the pathophysiology of epilepsy, and this will result in a large spike in measured electrical activity on the EEG, which has been identified in figure 1. This sudden spike denotes paroxysmal depolarisation, which is suggestive of neuronal hyperexcitability. This spike being termed as epileptiform in morphology and is therefore suggestive of epilepsy (Hanke, Schindler and Seiler, 2022). Epilepsy can be confidently diagnosed from the EEG if it demonstrates an epileptiform morphology during a seizure and if following this presentation, there is a slowing of waves and return to normal EEG presentation (Maguire, 2022). There must as expected also be agreement with the EEG findings and the presented clinical symptoms for diagnosis of epilepsy. **Figure 1**: EEG demonstrating epileptiform activity and subsequent wave slowing, which is characteristic of epilepsy (Feyissa and Tatum, 2019). One of the limitations of the EEG in diagnosis of epilepsy, however, is that it can only detect epileptiform activity during a seizure and as such, even patients with epilepsy can have a normal EEG depending on the time that the test is performed. As such, an EEG will be likely repeated at a later date if epilepsy still suspected, but aside from this, the patient may also be offered an MRI scan. An MRI, unlike an EEG, cannot directly diagnose epilepsy and instead can identify potential causes of epilepsy, such as stroke or a tumour. This MRI can aid with the treatment of epilepsy, such as if a stroke detected from an MRI and deemed to be causative of the seizures, thrombosis can be treated through administration of tissue-type plasminogen activators such as alteplase, which due to their thrombolytic properties can induce thrombolysis of the clot responsible for the stroke, restoring blood flow to the brain and minimising the degree of ischemic damage sustained (Hill and Buchan, 2005). As such, the imminent seizure should cease, although depending on the degree of ischemic damage that is sustained, there may be recurrent epileptic episodes, with approximately 10% of latent epilepsy cases being due to previous strokes. #### Types of Epilepsy Following general diagnosis of epilepsy through EEGs, MRIs and consideration of clinical presentation, the specific type of epilepsy has to be diagnosed through determination of the type of seizure. In general, seizures are characterised by the location in which the seizure arises, how muscle tone is affected and whether there is an effect on consciousness. * **Focal onset seizures** will affect one part of the brain, this part being either a specific brain region of a large part of a single hemisphere. * **Bilateral seizures** are where multiple brain areas are affected and often develop following a focal onset seizure. * **Muscle tone** is also affected during a seizure, with sudden stiffening of muscles being *tonic* and a loss of muscle tone being *atonic*. * **Tonic seizures** unlike atonic seizures can also present with a loss of consciousness and often involve repetition of rhythmic jerking movements, which are characteristic of *clonic seizures*. Clonic seizures will occur depending on the brain localisation affected by the neuronal hyperexcitability. * **Awareness during the seizure** can be characterized dependant on the individual's degree of consciousness during the seizure. * **Aware seizures** are where the individual is conscious but may not be aware that they are having a seizure and instead often describe the feeling as simply strange. Often times individuals can remember aware seizures. * **Impaired awareness seizures** are where there is impairment in the consciousness of the individual and as such may have visual and auditory impairments. Impaired awareness seizures are often bilateral in origin and furthermore, due to the total loss of consciousness, clonic seizures can also be classified as impaired awareness (Epilepsy society, 2023). An overarching epilepsy diagnosis can be established, but discussion of the seizure presentation with a neurologist will enable specific diagnosis. The seizure, that is capable of causing the most adverse effects, are *bilateral-tonic-clonic seizures*. In these seizures, multiple brain areas are affected (bilateral), there is an increase in muscle tone (tonic) and a loss of consciousness accompanied by rapid rhythmic jerking movements (clonic). These bilateral-tonic-clonic seizures due to their intensity are more likely to have adverse implications, such as temporary arrest of breathing due to the rapid involuntary contraction of muscles. #### Treatment of Epilepsy Due to the perhaps life-threatening consequences of seizures, there has to be adequate treatment in place to reduce recurrence of these seizures. As previously established, MRI scans can identify potential causes of epilepsy, such as strokes and tumours, which are most often treated via tissue-type plasminogen activator thrombolytic drugs or surgery respectively, but in cases where epilepsy is not caused by an overt physiological factor and is more due to abnormal metabolic function, treatment often comes in the form of anti-epileptic drugs (AEDs). The effect of 5 AEDs in their treatment of bilateral-tonic-clonic seizures will be explored, these being valproate, carbamazepine, phenytoin, lamotrigine and levetiracetam. In terms of prescribing AEDs, a variety of factors have to be considered ranging from the seizure type, risks and benefits of treatment, the age and sex of the patient, as well as any other drugs that the individual is currently taking that could reduce the efficacy or promote toxicity. The dosage frequency of the indicated drug is often determined by the plasma-drug half-life, which is the time required for the concentration of the drug in the body to be reduced by 12. The approximate half-lives of the relevant AEDs are as follows: * Carbamazepine - 12-17hrs * Lamotrigine - 15-35hrs * Levetiracetam – 6-8hrs * Phenytoin – 7-42hrs * Valproate – 6-17hrs (Marvanova, 2016) AED half-life can be affected by a variety of factors, which is often depending on how they are cleared from the body. 5% of phenytoin is cleared unmodified in urine with the remaining 95% being modified by cytochrome-p450 to aid with clearance (Cuttle et al, 2000). Due to the predominance of cytochrome p450 in hepatocytes, a prolonged half-life >42hrs and increased risk of drug induced toxicity may be observed in patients with existing liver dysfunction and as such would unlikely be indicated in these patients. Most AEDs can be given twice daily, although those with longer half-lives, like phenytoin and levetiracetam, are typically given once daily. Higher doses of long-half-life AEDs, such as 3000mg of levetiracetam daily, would instead often require more frequent dosing due to the much higher risk of toxicity if administered solely at one time point (National Institute for Health and Care Excellence, 2024). #### Management of AED Therapy Following prescription of a specific AED, the patient has to be managed as to ensure that the drug is efficate and that there are no adverse side effects. Efficacy of the AED will be established if the serum concentration assayed in the efficate range and if there is a likewise reduction in frequency of seizures. In cases, where the drug lacks efficacy or if it accompanies undesirable symptoms/toxicity, then it may be prudent to change the indicated AED. If the individual on perhaps phenytoin, which shows little efficacy, then they would be weaned off of their dose as to avoid symptoms of rapid withdrawal such as recurrence of seizures or anxiety/depression, and following this, they would be started on another AED which is able to treat their specific type of seizure. As such, for bilateral-tonic-clonic seizures, levetiracetam may perhaps be subsequently prescribed following ineffective phenytoin therapy. It may be that adjunctive therapy instead indicated whereby two AEDs are prescribed, such as levetiracetam and valproate, to produce a desired level of efficacy. The risk with adjunctive AED therapy is, however, an increase in risk of side effects and as such, solitary AED therapy should be prescribed wherever possible. #### Mechanisms of Action of AEDs * **Carbamazepine, levetiracetam, lamotrigine, phenytoin and valproate** as established can all be used to treat bilateral-tonic-clonic seizures through two primary mechanisms. * **Carbamazepine, lamotrigine, phenytoin and valproate** will all bind to voltage-gated Na+ channels in the pre-synaptic neuron. This binding will result in a plugging of the pore that enables an intracellular Na+ influx during depolarisation and as such, depolarisation is prevented. In practice, when the action potential that has been generated from a PDS and propagated through neuronal hypersynchrony reaches the pre-synaptic terminal during the initial stages of a seizure, there is no depolarisation due to inhibition of Na+ influx due to AED binding. No depolarisation means that there is no subsequent Ca2+ influx, which is a crucial mediator of glutamate vesicle mobilisation and subsequent release into the synaptic cleft. Overall, glutamate release into the synaptic cleft is prevented, which due to the excitatory action of glutamate on the post-synaptic neuron, reduces downstream neuronal depolarisation, reducing the extent of neuronal hyperexcitability, which will prevent the seizures associated with epilepsy. * **Valproate**, additionally to pre-synaptic voltage-gated Na+ channel inhibition, is also capable of potentiating the effect of GABA, which works antagonistically to glutamate and as such induces neuronal inhibition. In terms of how GABA is produced and subsequently catabolised, alpha-ketoglutarate from the Krebs cycle is converted into glutamate via glutamate dehydrogenase, this glutamate being converted into GABA by glutamate decarboxylase (Ghit et al, 2021). This GABA can then elicit its neuronal inhibitory effects with excess GABA being converted into succinate semialdehyde, and then succinate due to the successive action of GABA transaminase and succinate semialdehyde dehydrogenase. *Valproate* is able to inhibit the action of GABA transaminase and succinate semialdehyde dehydrogenase, which will reduce the rate of GABA catabolism, which will increase the time in which GABA can elicit its effect (Johannessen, 2000). Greater GABA prevalence in the synaptic cleft due to reduced catabolism due to the effect of *valproate* will allow for more post-synaptic binding of GABA to GABAA receptors. Binding of GABA to GABAA receptors will induce a conformational change, which causes a post-synaptic influx of Cl- ions (Mihic and Harris, 1997). An influx of Cl- ions will cause neuronal hyperpolarisation by making the resting membrane potential more negative, which will require a greater summation of generator potentials for subsequent depolarisation and resultant action potential generation (Sorensen et al, 2017). In terms of how the two effects of *valproate* are used in conjunction to treat seizures, as established, the voltage-gated Na+ channel inhibition ultimately reduces pre-synaptic glutamate release, which inhibits post-synaptic depolarisation. Even if glutamate is still produced however, the ability of *valproate* to also maintain high levels of GABA within the synaptic cleft, enables post-synaptic neuron hyperpolarisation, which will further limit the effectiveness of any released glutamate, overall reducing downstream action potential propagation and as such alleviates the frequency and intensity of seizures. * **Levetiracetam**, unlike the previously mentioned AEDs, is poorly understood with respect to its mechanism of action, but it is suggested that it works by three mechanisms: 1. **Inhibition of pre-synaptic Ca2+ channels** 2. **Inhibition of vesicle protein SV2A** 3. **Activation of post-synaptic GABAA channels** (Contreras-Garcia, 2022). Initially, it is suggested that *levetiracetam* will bind to pre-synaptic Ca2+ channels, which will prevent a Ca2+ influx during neuronal depolarisation. As such, reduced intracellular Ca2+ concentrations will prevent mobilisation of glutamate containing vesicles so there will be no release into the synaptic cleft, and as such no post-synaptic glutamate binding and no downstream neuronal stimulation and subsequent hyperactivity. The second action of *levetiracetam*, which is better understood than the mechanism of Ca2+ channel inhibition, is inhibition of SV2A protein. Under normal conditions, following a Ca2+ influx during depolarisation in the pre-synaptic neuron, this Ca2+ will bind to SV2A proteins on the surface of glutamate containing vesicles, which will activate a complex of intracellular tethering proteins that ultimately result in the fusion of this vesicle with the neuronal membrane, enabling subsequent release of glutamate into the synaptic cleft (Rossi et al, 2022). Levetiracetam will compete with Ca2+ at the binding site of SV2A. Binding of levetiracetam will prevent the binding of Ca2+ and as such will inhibit vesicle mobilisation and subsequent release of glutamate into the synaptic cleft. Reduced glutamate release into the synaptic cleft will reduce binding to AMPA and resultant activation of NMDA receptors, which overall prevents post-synaptic neuronal depolarisation and with it, prevents propagation of the excitatory neuronal signal. The final proposed mechanism of levetiracetam is activation of GABAA receptors. Unlike valproate which will have an effect on these receptors by increasing availability of GABA through inhibition of its catabolism, *levetiracetam* will bind directly to the GABAA receptors. It is suggested that *levetiracetam* will act as an agonist to GABA, and as such binding will elicit the same effects as would be observed in the case of endogenous GABA binding. As such, *levetiracetam* binding to GABAA receptors will result in an influx of Cl, post-synaptic neuronal hyperpolarisation, and inhibition of depolarisation due to establishing a greater period of refractory which again prevents propagation of the excitatory signal during a seizure. In practice, *levetiracetam* will bind to pre-synaptic Ca2+ channels, preventing an intracellular Ca2+ influx during depolarisation and of the Ca2+ that does enter the neuron, its effect on SV2A protein to enable glutamate-vesicle mobilisation will be prevented due to competitive inhibition from *levetiracetam*. Furthermore, even if some glutamate is released into the synaptic cleft, much greater amounts would be required to depolarise the post-synaptic neuron due to the hyperpolarisation that has been caused by *levetiracetam* binding to GABAA receptors. #### Monitoring AED Therapy All 5 AEDs that have been discussed will have serum levels monitored at NUH for a variety of reasons ranging from establishing efficacy to ensuring no toxicity. * **Carbamazepine** is monitored to assess efficacy as serum levels should be within a narrow therapeutic range, but less frequently, serum carbamazepine is measured if toxicity is suspected. *Carbamazepine* is metabolised by hepatic enzyme CYP3A into its active form *carbamazepine-10-11-epoxide*, of which is thought to cause the toxic effects seen with higher doses of carbamazepine. This active product is then further metabolised in the liver into *carbamazepine-10,11-transdihydrodiol*, which is excreted in urine (Puranik et al, 2013). Due to this, it is suggested that high doses of carbamazepine will result in accumulation of the active, and suggested toxic, carbamazepine-10,11-epoxide, which primarily will induce hepatocellular damage. Damage to the hepatocytes due to accumulation of *carbamazepine-10,11-epoxide* at high doses will further reduce CYP3A activity due to less availability due to hepatocellular damage, propagating the toxic effects of carbamazepine. Therefore, it is suggested that carbamazepine monitored and kept within a narrow range as to ensure its efficacy whilst simultaneously preventing hepatotoxicity. Due to the primary role of CYP3A in *carbamazepine* metabolism, grapefruit juice should not be consumed when taking carbamazepine due to the grapefruit juice being a potent inhibitor of CYP3A, so could potentiate the toxic effects of carbamazepine. * **Valproic acid** is typically only measured when there is either poor adherence or suspected toxicity. Unlike carbamazepine, there is poor correlation between serum levels and efficacy and as such *serum valproate* would not be measured to assess efficacy. In terms of mechanism of *valproate* toxicity, levels >100mg/L are considered toxic. *Valproate* like *carbamazepine* is associated with hepatotoxicity but due to a separate mechanism. *Valproate* will inhibit the beta-oxidation of fatty acids because *valproate* itself is beta-oxidised to enable removal from the body and as such will consume oxidised coenzyme NAD+ in the process. Depletion of intracellular NAD+ stores will reduce rate of fatty acid beta-oxidation, resulting in accumulation of fatty acids (Grunig et al, 2020). It is then suggested that these fatty acids at high concentrations can disrupt the transmembrane mitochondrial proton gradient, reducing the production and subsequent availability of intracellular ATP, overall inducing cellular apoptosis (Berardi and Chou, 2014). Due to beta-oxidation of fatty acids occurring specifically in the liver, cellular apoptosis will occur in liver tissue, resulting in potential *valproate* induced hepatotoxicity at high doses. * **Phenytoin and lamotrigine** are like *carbamazepine* monitored to ensure that serum levels within a narrow therapeutic range and that levels not too high as too be toxic. *Phenytoin* and *lamotrigine*, like *carbamazepine* and *valproate*, will inhibit voltage-gated Na+ channels in neurons as an intended effect but *phenytoin* and *lamotrigine* at higher concentrations have a greater affinity for systemic voltage-gated Na+ channel binding, namely in cardiac tissue. *Phenytoin* and *lamotrigine* at higher concentrations can inhibit cardiac voltage-gated Na+ channels, and as with binding in pre-synaptic neurons, will inhibit depolarisation, which in cardiac tissue can cause cardiac arrhythmias (Awasthi et al, 2022). * **Levetiracetam** toxicity, unlike the aforementioned AEDs, does not have a specific indicator of toxicity and is often simply associated with observing multiple adverse side effects of *levetiracetam*. These side effects include vomiting, somnolence, flu-like symptoms, amnesia and widespread rashes (National Institute for Health and Care Excellence, 2024). As *levetiracetam* toxicity simply presents the same as levetiracetam side effects, *levetiracetam* toxicity is hard to overtly establish in the absence of serum measurements. #### Prognosis of Epilepsy To summarise, although epilepsy can be a debilitating condition, especially in those experiencing bipolar tonic-clonic seizures, treatment primarily through the use of AEDs can enable many individuals with epilepsy to live seizure free lives. It is often considered that combinatorial therapy with *levetiracetam* and *valproate* will have the best prognosis in individuals with bipolar tonic clonic seizures due to inhibition of pre-synaptic Na+ and Ca2+ influx, inhibition of glutamate vesicle mobilisation through SV2A binding, direct activation of GABAA receptors and prolonged GABAA action through an increase in synaptic cleft GABA availability. Not all forms of epilepsy require treatment via AEDs, nor do they require combinatorial therapy, especially if epilepsy caused by perhaps TBI, but in individuals that do require AEDs to manage their condition, they tend to have a very positive prognosis. #### Case Study A patient was admitted to A&E with a suspect Tegretol overdose. **Winpath results**: * Serum carbamazepine: 15mg/L * Sodium: 134mmol/L * Potassium: 4.4mmol/L * Urea: 4.4mmol/L * Creatinine: 99µmol/L * EGFR by MDRD: 82ml/min/1.73m2 * EGFR by CKD EFI: >60 * Total Bilirubin: 0.6030µmol/L * Total Alkaline Phosphatase: 144U/L * Albumin: 42g/L * Total Protein: 21g/L * Carbamazepine: 7mg/L Carbamazepine overdose can range from gastrointestinal discomfort, dizziness, muscle weakness, seizures, abnormal cardiac rhythm, and strokes (Al Khalili, Sekhon and Jain, 2023). The assayed carbamazepine value is 15mg/L, which is elevated beyond the reference range of 4-12mg/L. This supports the suspicion of a suspected Tegretol overdose. As stated on the supporting Winpath comment, toxic carbamazepine values are those greater than 15mg/L. An assayed value of 15mg/L is bordering on, if not, toxic, and as such would require urgent treatment. The most likely form of treatment would be through the use of activated charcoal, which would be administered orally and adsorbs the ingested carbamazepine in the gastrointestinal tract, preventing systemic absorption and further toxicity (Zellner et al, 2019). Following treatment, assayed carbamazepine was 7mg/L, which is within the reference range and is unlikely to exert toxic effects. LFTs were measured due to the primary adverse effect of carbamazepine toxicity being accumulation of carbamazepine-10, 11-epoxide due to limited CY3PA activity, which in turn can cause hepatotoxicity. From the LFTs, GGT and ALP were both slightly elevated, and ALT within the reference range, which meant that hepatotoxicity was not suspected. Likely, the activated charcoal would have adsorbed the carbamazepine prior to gastrointestinal absorption, preventing later conversion into carbamazepine 10,11-epoxide, reducing the risk of further toxicity Following presentation to A&E and attainment of the results in figure 2, the patient was referred to