5.4 Anticonvulsants and antiepileptic drugs

Anticonvulsant Mechanisms of Action

Anticonvulsants work by restoring the balance between excitation and inhibition in the brain. Seizures happen when neurons fire excessively or synchronously, and these drugs target the specific channels and neurotransmitters responsible for that runaway activity.

There are four main mechanism categories to know:

Sodium Channel Blockers

Drugs like carbamazepine, phenytoin, and lamotrigine prolong the inactivated state of voltage-gated sodium channels. Normally, sodium channels cycle rapidly between resting, open, and inactivated states to fire action potentials. These drugs bind preferentially to channels in the inactivated state, which means they selectively slow down neurons that are firing too fast (called use-dependent blockade). Neurons firing at normal rates are mostly unaffected.

Calcium Channel Modulators

Ethosuximide blocks T-type calcium channels, which are specifically involved in the thalamic oscillations that generate absence seizures. Gabapentin and pregabalin bind to the $\alpha_2\delta$ subunit of voltage-gated calcium channels, reducing calcium influx at nerve terminals and decreasing neurotransmitter release.

GABA Enhancers

These drugs boost the brain's main inhibitory system:

• Benzodiazepines (diazepam, lorazepam) bind to $GABA_A$ receptors and increase the frequency of chloride channel opening
• Barbiturates (phenobarbital) also act on $GABA_A$ receptors but increase the duration of chloride channel opening
• Vigabatrin irreversibly inhibits GABA transaminase, the enzyme that breaks down GABA, raising GABA concentrations in the synapse
• Tiagabine blocks GABA reuptake transporters, keeping GABA active longer

That frequency vs. duration distinction between benzodiazepines and barbiturates is a classic exam question.

Glutamate Antagonists

Topiramate blocks AMPA/kainate glutamate receptors (among other actions), while perampanel is a selective AMPA receptor antagonist. By reducing excitatory glutamate signaling, these drugs dampen excessive neuronal firing.

Multi-Mechanism Anticonvulsants

Some drugs don't fit neatly into one category. Valproic acid is the classic example: it increases GABA concentrations (by inhibiting GABA degradation), blocks voltage-gated sodium channels, and may also modulate T-type calcium channels. This multi-target action is why valproic acid has broad-spectrum efficacy across many seizure types. Topiramate similarly acts through multiple mechanisms, including sodium channel blockade, GABA enhancement, and glutamate antagonism.

Neurotransmitters and Epilepsy

Neurotransmitter Imbalance in Epilepsy

Epilepsy fundamentally comes down to a disrupted balance between excitation and inhibition in the brain.

Glutamate is the primary excitatory neurotransmitter driving seizure activity. It activates three main receptor types: AMPA, NMDA, and kainate receptors. When glutamate signaling becomes excessive, neurons become hyperexcitable and can fire in the synchronized bursts that produce seizures.

GABA is the main inhibitory neurotransmitter that normally keeps excitation in check. When GABA signaling is deficient, whether from reduced GABA synthesis, increased breakdown, or receptor dysfunction, the brain becomes more susceptible to seizures.

Ion Channels in Epilepsy and Drug Action

Voltage-gated sodium channels generate and propagate action potentials. Mutations or dysfunction in these channels can cause neurons to fire repetitively, and many anticonvulsants (phenytoin, carbamazepine, lamotrigine) target them directly.

Calcium channels contribute to excitability in different ways depending on the subtype:

• T-type calcium channels in thalamic neurons produce the rhythmic burst firing seen in absence seizures. Ethosuximide targets these specifically, which is why it works for absence seizures but not other seizure types.
• N-type and P/Q-type calcium channels at presynaptic terminals control neurotransmitter release. Drugs acting on these channels reduce the amount of excitatory neurotransmitter released into the synapse.

Many anticonvulsants affect multiple ion channel and neurotransmitter targets simultaneously, which is part of why they can control diverse seizure types.

Therapeutic Uses of Anticonvulsants

Common Anticonvulsants and Their Applications

Drug Interactions and Monitoring

Many older anticonvulsants are potent hepatic enzyme inducers (carbamazepine, phenytoin, phenobarbital) or inhibitors (valproic acid). This matters clinically because:

• Enzyme inducers speed up metabolism of other drugs, potentially making oral contraceptives, warfarin, and other medications less effective
• Valproic acid inhibits the metabolism of lamotrigine, raising lamotrigine levels and increasing toxicity risk

Therapeutic drug monitoring (checking blood levels) is essential for drugs with a narrow therapeutic index, particularly phenytoin, carbamazepine, and valproic acid. The gap between an effective dose and a toxic dose is small, so regular blood draws help keep patients in the safe range.

Newer anticonvulsants like levetiracetam generally have fewer drug interactions and wider therapeutic windows, though they still have their own adverse effects. Topiramate, for instance, can cause weight loss and cognitive slowing ("word-finding difficulty"), which matters for patient quality of life.

Seizure Management and Adherence

Principles of Seizure Management

The primary goal is seizure freedom with minimal side effects. Here's the general approach:

1. Start with monotherapy using a drug appropriate for the patient's seizure type
2. Titrate the dose gradually upward to minimize side effects and find the lowest effective dose
3. Maximize the first drug before switching. If the first drug fails due to side effects, try a different monotherapy
4. Consider polytherapy (adding a second drug) only after adequate trials of monotherapy have failed
5. Monitor regularly with follow-up visits to check drug levels (when applicable), liver function, blood counts, and side effects

Drug selection depends on several factors:

• Seizure type: Focal seizures, generalized tonic-clonic, absence, and myoclonic seizures each respond to different drugs. Ethosuximide works for absence but not tonic-clonic. Carbamazepine can actually worsen absence and myoclonic seizures.
• Patient characteristics: Age, sex, comorbidities, and pregnancy potential all influence the choice. Valproic acid is generally avoided in women of childbearing age due to teratogenicity.
• Existing medications: Enzyme-inducing anticonvulsants can reduce the effectiveness of other drugs the patient takes.

Importance of Medication Adherence

Consistent blood levels of anticonvulsants are what keep seizures suppressed. Missing doses allows drug levels to drop below the therapeutic threshold, which can trigger breakthrough seizures.

Sudden discontinuation is especially dangerous. Abruptly stopping an anticonvulsant can cause rebound seizures or even status epilepticus, a prolonged seizure lasting more than 5 minutes that is a medical emergency. If a drug needs to be stopped, it should always be tapered gradually.

For patients with refractory epilepsy (seizures not controlled by two or more appropriate drugs), non-pharmacological options can be added alongside medications:

• Ketogenic diet: A high-fat, very-low-carbohydrate diet that can reduce seizure frequency, particularly in children
• Vagus nerve stimulation (VNS): A surgically implanted device that delivers electrical impulses to the vagus nerve
• Epilepsy surgery: Removal or disconnection of the seizure focus, considered when seizures originate from a single identifiable brain region