Chapter 20: Antiepileptic Drugs

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Welcome back to the Deep Dive.

You know, usually when we crack open these pharmacology texts, we're looking at maintenance, we're looking at cholesterol, or blood pressure, that are a slow burn, things you manage over decades.

Right.

The chronic, silent killers,

it's all about long -term management.

But today, today feels a little more high stakes.

We are strapping in for a journey into the electrical storm of the human brain.

That's a great way to put it.

We are looking exclusively at chapter 20 of Brenner and Stevens Pharmacology, the sixth edition.

And the topic on the table is anti -epileptic drugs.

It is a fascinating chapter.

It really bridges the gap between high level molecular biology, what's happening at those tiny ion channels in your nerve cells, and the very visible, sometimes dramatic clinical reality of epilepsy.

And honestly, reading through this, the sheer complexity of what we are trying to control here is kind of terrifying.

Oh, absolutely.

We're talking about the most complex structure in the known universe, the human brain, and what happens when its own communication system, you know, turns against it.

It's not just a medical issue.

It's a failure of the biological wiring.

And our mission today is to decode that pharmacology.

We want to understand what exactly a seizure is, biologically speaking.

Right.

And then look at the massive arsenal of chemical tools we've developed to quiet that storm.

And it is a massive arsenal.

I was actually surprised by how many there are.

We're going to see a lot of drugs today, some old classics and some cutting edge newcomers.

But let's start at the very beginning, setting the stage.

Okay.

When we talk about seizures, I think most people have a specific image in mind, someone shaking on the ground.

But the source material makes a distinction right away between a seizure and the actual disease of epilepsy.

It's a crucial distinction and one that changes a patient's life.

A seizure, by definition, is just an episode.

It's a transient occurrence of abnormal electrical activity in the brain.

So it's the event itself.

Exactly.

It results in involuntary movements, sensations or thoughts.

But having a single seizure doesn't automatically mean you have epilepsy.

Because there are plenty of things that can trigger a one -off seizure, right?

If I, you know, zap you with electricity, you're going to seize.

Exactly.

Or if we look at the list of proximal causes provided in the text, head trauma, for one.

Yeah.

A bad concussion can do it.

Right.

Stroke, brain tumors, severe hypoxia, which is a lack of oxygen.

Even things like high fever in children, hypoglycemia or chronic alcohol withdrawal can alter neuronal function enough to trigger a seizure.

So if you fix the low blood sugar or you get through the withdrawal, the seizures stop.

That is an epilepsy.

Correct.

Epilepsy is defined by recurrence.

It's recurrent seizures that cannot be attributed to a specific, immediate proximal cause like those we just mentioned.

It's a chronic condition where the brain has a lowered threshold for misfiring.

And does the chapter get into where that comes from?

Is it just bad luck?

Is it genetic?

It's often a mix.

There's frequently a genetic basis.

Yeah.

But it also points to environmental perturbations, things that happen perhaps during intrauterine development or complications right after birth.

These can set the stage, wiring the brain in a way that makes it prone to these electrical storms later in life.

In terms of scope, how big of a problem is this?

Are we talking about a rare condition here?

Not at all.

It's significant.

In the United States, epilepsy affects about one to two percent of the population.

One to two percent.

That's a lot of people.

It is.

To put that in perspective, the text says it is the second most common neurologic disease right after stroke.

So statistically, if you're in a crowded movie theater,

there are likely a couple of people there living with epilepsy.

This is something clinicians are going to encounter frequently.

Okay.

So that's the who and the what.

Let's get into the roadmap for today.

We are going to walk through this chapter exactly as it's laid out.

Sounds good.

We'll start with how we classify these seizures because apparently they aren't all the same.

Then we'll dive into the neurobiology, the actual why it's happening inside the cell.

Then we'll get into the drugs from the first line heavy hitters to the adjuncts.

And finally, how we manage it all.

Sounds like a plan.

Let's start with section one, classification.

The breakdown in table 20 .1 puts everything into two main buckets,

partial seizures and generalized seizures.

Help us draw the line between them.

Is it just about severity?

Not really severity, but geography.

The dividing line is anatomy and consciousness.

A partial seizure, also known as a focal seizure, originates in just one cerebral hemisphere.

So it's localized.

It's localized.

Think of it like a small fire starting in the kitchen.

And generalized.

Generalized seizures arise in both hemispheres simultaneously.

The whole house catches fire at once.

And the key clinical difference usually involves loss of Let's unpack the partial ones first.

The text splits them into simple and complex.

I assume simple doesn't mean it's easy to deal with.

No, not at all.

Simple refers to the patient's awareness.

In a simple partial seizure, consciousness is not altered.

The person is awake in a way that something is happening.

Okay.

So they know what's going on.

They do.

They might experience motor symptoms like a hand quitching or sensory changes like smelling burning rubber or seeing flashing lights.

But they are there.

They are the observer of their own seizure.

That sounds incredibly surreal to be trapped in your body watching it malfunction.

It is.

Patients describe it as being a passenger in their own body.

It can be terrifying.

Versus a complex partial seizure.

What's the difference there?

In a complex partial seizure,

consciousness is altered.

They aren't unconscious in the sense of being in a coma, but they aren't fully present.

The lights are on, but nobody's home.

They might exhibit repetitive behaviors called automatisms.

Automatisms like robots.

What does that mean?

Sort of.

It's an automatic purposeless behavior.

Examples would be lip smacking, chewing motions or fumbling with their clothes.

They're moving, but it's not purposeful and they won't remember doing it afterwards.

And usually these are coming from a specific part of the brain, right?

Yes.

Complex partial seizures often originate in the temporal lobe.

You'll sometimes hear this as temporal lobe epilepsy or psychomotor epilepsy.

Why psychomotor?

Because the temporal lobe deals with memory and emotion.

These can also come with weird psychic phenomena like feelings of deja vu or sudden unprovoked fear.

No, there's a specific description in the chapter that I've found really visual.

The Jacksonian march.

It sounds like a parade, but I'm guessing it's not festive.

No, not at all.

But it's a fascinating illustration of brain anatomy because a partial seizure is an electrical discharge spreading across the cortex.

It follows the map of the body on the brain.

The homunculus.

Homunculus.

Exactly.

The homunculus is that weird drawing where the hands and face are huge because they have more nerves.

That's the one.

So imagine the seizure starts in the area controlling the thumb.

The thumb starts jerking.

Then as the electrical discharge spreads physically along the motor strip of the brain,

it moves.

The jerking moves to the hand.

Then the wrist.

Then the elbow.

And finally the entire arm.

It marches up the limb because the electricity is marching across the brain.

That is wild.

It's literally tracing the wiring diagram of the body in real time.

Exactly.

But there's a danger here.

A partial seizure doesn't always stay partial.

The text calls this secondary generalization.

The fire in the kitchen spreads to the living room.

Precisely.

A seizure can start in one spot focal, but then the storm recruits enough neurons that spreads to the whole brain, becoming a generalized seizure.

Which brings us to the second big category.

Generalized seizures.

This is where consciousness is lost from the outset.

And the classic one, the one everyone knows from movies, is the tonic -clonic seizure.

Formerly known as grand mal.

This is the dramatic event.

It has two phases, as the name suggests.

The tonic phase comes first.

Tonic as in muscle tone.

Exactly.

That refers to tone.

Muscle tone.

There's a sudden massive increase in muscle tone.

The body goes rigid.

The patient might cry out as air is forced from their lungs, and they fall like a tree.

And the clonic phase.

That follows the rigidity.

It's the spasms, the rhythmic contraction and relaxation, the jerking.

This usually lasts three to five minutes.

And afterwards.

Do they just pop up and ask what happened?

No.

There's a specific state called the post -dictal period.

The brain has just run a marathon at a sprint pace.

It is exhausted.

So they're confused.

Very.

The patient is drowsy, confused.

Maybe they have a headache.

They need to recover.

They might sleep for hours.

On the other end of the spectrum, we have absent seizures.

These used to be called petit mal.

How do these look compared to the tonic -clonic?

Almost the opposite.

Instead of violent movement, there's a sudden stillness.

It's an abrupt loss of consciousness, but the muscle tone usually just decreases slightly.

The patient doesn't fall.

They just stare.

Like they're daydreaming.

Exactly.

There might be some blinking or mild twitching, but to an observer, it looks like they just zoned out.

Here's the thing.

If you snap your fingers at a daydreaming kid, they look at you.

Right.

If you snap your fingers at a kid having an absent seizure, they're gone.

They're offline.

There's no response.

But the EEG tells a very specific story here, doesn't it?

The diagram in figure 20 .1 shows the EEG signature for absent seizures, and it's striking.

It's pythognomonic, meaning if you see the diagnosis immediately.

It's a synchronous three -hertz spike and dome pattern.

What does that mean, spike and dome?

If you look at the tracing, you see a sharp, high -voltage spike, immediately followed by a slow, rounded dome wave.

That pattern repeats.

Spike, dome, spike, dome.

Three times per second.

Three hertz.

While the kid looks peaceful, their brain is firing in this very specific rhythmic three -hertz loop.

Exactly.

And distinguishing this from daydreaming or from something like ADHD is crucial for treatment, as we'll see later.

Before we leave classification, we have to mention the scary one.

Status epilepticus.

This is a medical emergency.

Status epilepticus is when you have recurrent tonic -clonic episodes without regaining consciousness in between, or a single seizure that just doesn't stop.

So the brain is just stuck in that seizure state.

Exactly.

We'll talk about how to treat it later, but biologically, this is the brain frying itself.

It's a race against time.

The chapter mentions a few others,

briefly.

Myoclonic and atonic.

What are those?

Right.

Myoclonic are rhythmic jerking spasms, like a single jolt or series of jolts.

Atonic is the opposite.

It's a sudden loss of all muscle tone.

So they just collapse.

Yeah.

They're sometimes called drop attacks because the person just collapses into a heap.

It's very dangerous because of the risk of injury from the fall.

Okay.

So we know what they look like.

Now we need to understand the why.

Section two.

The neurobiology of seizures.

The chapter talks about a seizure focus.

What is that, physically?

Imagine a choir.

Normally, everyone is singing their own part, harmonizing, maybe a little chaotic.

In a seizure focus, a group of neurons starts discharging synchronously, singing the exact same loud note at the exact same time.

This group is the focus.

And the problem is that they recruit others.

They're bad influences.

They are.

Once initiated, these abnormal discharges spread.

But the core question is, what starts it?

The text points to a few mechanisms, but the big one is the glutamate connection.

Okay.

Let's unpack the glutamate pathway.

We know glutamate is the brain's main excitatory neurotransmitter.

It's the gas pedal.

Right.

The theory is excessive excitatory neurotransmission.

Specifically, glutamate activates a receptor called the NMDA receptor.

We've talked about NMDA receptors and other deep dives.

They usually have a magnesium block, right?

Like a bouncer at the door.

That's the perfect analogy.

Magnesium ions, Mg2 plus wheat, sit in the channel like a plug.

Under normal conditions, they stop ions from flowing.

Even if glutamate knocks on the door, magnesium says, you can't come in.

So what happens during a seizure initiation?

How does the bouncer get moved?

If the neuron gets excited enough, depolarized, that magnesium bouncer gets displaced.

It gets kicked out of the channel.

And when the bouncer is gone, who rushes in?

Calcium, K2 plus sera.

Calcium floods into the neuron.

And calcium isn't just an electrical charge.

It's a powerful signaling molecule.

It kicks off a whole cascade of events inside the cell.

This is where the feedback loop comes in involving nitric oxide.

This seemed really important for how the seizure sustains itself.

It is.

It's a critical part of the pathophysiology shown in figure 20 .2.

The calcium influx activates the synthesis of nitric oxide, or NO.

Now, nitric oxide is a gas, so it can diffuse right through membranes.

It diffuses backward retrograde out of the neuron and back to the presynaptic neuron that sent the original signal.

And what does it tell that first neuron?

It tells it to release more glutamate.

Oh, wow.

So it's a vicious cycle.

Glutamate causes calcium influx, calcium makes nitric oxide, and nitric oxide goes back and calls for more glutamate.

Precisely.

It's a runaway train of excitation.

And this process leads to something called long -term potentiation, or LTP, which strengthens that excitatory pathway.

So it basically paves the road for the seizure to happen easier next time.

The brain learns how to seize.

That's a great way to put it.

It reinforces the faulty circuit.

The chapter also describes a specific electrical event called the depolarization shift.

Can you of a single neuron in that seizure focus?

Instead of a normal quick action potential, a blip,

you get an abnormally prolonged depolarization, a long wave of positive charge.

And there are spikelets on top of it.

Right.

Riding on top of this long wave are spikelets, little rapid -fire spikes.

This whole event, this depolarization shift, is so powerful that it recruits surrounding neurons, synchronizing them into the seizure.

Are there other mechanisms besides the glutamate storm?

Yes.

If glutamate is a gas pedal, GABA is the brake.

The main inhibitory neurotransmitter?

Exactly.

Seizures can also happen if there is a suppression of GABA.

If the brakes fail, the car goes out of control, regardless of how hard you press the gas.

And the text mentions one more specific mechanism for those absent seizures we talked about.

It's not the same as the others.

Right.

It's the T -type calcium channels.

These are specifically located in thalamic neurons.

Acts sort of like a pacemaker.

Excessive calcium influx through these T -type channels creates that rhythmic oscillating current that drives the three hertz spike and dome pattern.

So for absent seizures, the thalamus is the culprit.

It's the conductor of that particular pathological orchestra.

Yes.

Okay.

So we have the targets.

We have the why.

Now let's get to the how.

Section three.

Mechanisms of anti -epileptic drugs.

The chapter in table 20 .2 and figure 20 .3 breaks this down into three main levers we can pull to stop this.

Right.

And this aligns perfectly with the biology we just discussed.

Lever number one.

Ion channel inhibition.

This is primarily blocking sodium, the Na plus channels, or calcium, the K2 plus channels.

Conceptually, this is stopping the depolarization.

Yeah.

If you block the sodium channel, the neuron can't fire its action potential.

You're stopping the spark at the source.

Lever number two.

GABA augmentation.

Pressing the brake pedal harder.

Exactly.

Enhancing inhibitory neurotransmission so the neurons are less likely to fire.

Making the brain more resistant to that runaway excitation.

And lever number three.

Glutamate inhibition.

Taking the foot off the gas pedal.

Blocking those excitatory signals directly, either at the receptor or by reducing glutamate release.

Now wait a minute.

There is a crucial concept in this section called use dependent blockade.

I need you to explain this because my first thought was,

if we block sodium channels, don't we die?

Right.

My heart needs sodium channels.

My breathing muscles need them.

Why don't these drugs just shut everything down?

It's the million dollar question, isn't it?

Yeah.

And the answer is incredibly elegant.

It lies in the state of the channel.

Sodium channels aren't just open or closed.

They cycle through three states.

Resting, open, and inactivated.

Inactivated is like a refractory period.

It's just fired and can't fire again immediately.

Yes.

It's the recovery phase.

Now drugs like carbamazepine and phenytoin bind preferentially to the inactivated state.

They latch onto the channel when it's trying to reset.

Okay.

So how does that help with safety?

I'm still not quite seeing it.

Okay.

Think about a normal neuron.

It fires occasionally.

Pop, pop, pop, pop, pop.

It spends a lot of time in the resting state.

So the drug doesn't have much to bind to.

Got it.

It's just sitting there waiting.

But a seizure neuron.

It's going crazy.

It's firing like a machine gun.

Pop, pop, pop, pop, pop, pop.

It is cycling incredibly fast.

That means its channels are spending a huge amount of time in that inactivated state trying to recover between each shot.

So the drug sees the seizure neurons much more often than the normal neurons.

Exactly.

The drug preferentially binds to and blocks the hyperactive neurons because they are the ones constantly presenting the target.

That inactivated state.

It's like a speed trap that catches cars going over a hundred miles an hour.

So it selectively suppresses the high frequency firing of the seizure without completely wiping out normal brain activity.

That's the key.

It targets the troublemakers because they're making trouble.

That is elegant.

It's one of the most clever mechanisms in pharmacology.

It really is.

All right.

Let's get into the specific drugs.

Section four covers the drugs for partial and generalized tonic -clonic seizures.

These are the first line agents.

First up, carbamazepine and oxcarbazepine.

Carbamazepine is a classic, a real workhorse.

Its mechanism is exactly what we just described, blocking voltage -sensitive sodium channels in that use -dependent fashion.

But the chapter says it's not just a one -trick pony.

That's right.

It also blocks adenosine receptors and norepinephrine reuptake.

Which gives it a mood -elevating effect.

Is that why it's used for other things?

Yes.

That's part of it.

That action is similar to tricyclic antidepressants.

That's why you'll see it used in bipolar disorder or for things like trigeminal neuralgia, that excruciating facial pain condition.

But the big story with carbamazepine, the thing that trips up every medical student, is the pharmacokinetics.

Specifically, auto -induction.

This is huge.

And clinically, it's a major deal.

Carbamazepine is a potent inducer of CYP450 enzymes in the liver.

But it doesn't just speed up the metabolism of other drugs.

It speeds up its own metabolism.

Eats itself.

Essentially, the drug teaches the liver how to destroy it more efficiently.

So you start a patient on a standard dose.

It works great for a week or two, but two or three weeks later, their blood levels drop significantly, even though they are taking the exact same amount.

Because the liver has ramped up the demolition crew.

Exactly.

So the seizures might come back.

You have to anticipate this and adjust the dose upwards after a few weeks.

And because it ramps up those enzymes, it destroys other drugs too.

Like what?

Oral contraceptives and warfarin are the big ones mentioned in the text.

So a woman taking the pill starts carbamazepine, and suddenly her birth control is ineffective.

Exactly.

You have to warn patients about this.

It's a critical counseling point.

What about adverse effects?

The usual suspects for CNS drugs?

Drowsiness and ataxia, which is uncoordinated movement.

But there's a rare scary one mentioned.

A plastic anemia.

The bone marrow just stops making blood cells.

And briefly, oxcarbazazepine.

How is that different?

Think of it as carbamazepine's newer, slightly cleaner cousin.

Same mechanism,

but it's a prodrug.

It tends to have fewer drug interactions and is a less potent inducer of those liver enzymes.

There's also a new one mentioned, eslecarbazepine.

Yes, very closely related.

Same family, same sodium channel block.

Just a different formulation.

Also approved for partial onset seizures.

Moving on to the next heavy hitter.

The one that, I don't know, just feels like an old school drug.

Phenytoin and its partner phosphenytoin.

Phenytoin is another sodium channel blocker.

But it has some unique chemistry that makes it a real headache to handle.

It is poorly soluble in water.

So you can't just put it in an IV bag.

If you try to inject it too fast or mix it with the wrong thing, it precipitates, it turns into crystals in the vein.

That can cause severe tissue damage.

Hence, phosphenytoin.

Right, phosphenytoin is a clever solution.

It's a prodrug.

They added a phosphate group to make it water -soluble.

You can inject it safely.

And then enzymes in the body chop off the phosphate, and voila, you have phenytoin in the blood.

Much safer for for -you use.

Now, phenytoin has a very specific pharmacokinetic problem that we need to explain.

The zero -order problem.

This sounds complicated.

It is critical to understand this.

Most drugs follow first -order kinetics.

That means the liver eliminates a fixed percentage of the drug per hour.

It's linear.

You double the dose, you roughly double the blood level.

That makes sense.

It's predictable.

But phenytoin follows zero -order kinetics at therapeutic doses.

Imagine a bathtub.

The faucet is the drug going in.

The drain is the liver enzymes clearing it out.

Okay, I'm with you.

With phenytoin, the drain is very small.

It gets clogged easily.

The enzymes get saturated.

They are working at maximum speed and cannot go any faster.

So what happens if I turn the faucet up just a tiny bit more?

The tub overflows immediately.

Because the drain is maxed out, any extra drug you add just stays in the body.

So a tiny increase in dose causes a massive disproportionate spike in blood levels.

You go from a safe level to a toxic level and a heartbeat.

Exactly.

You can't just casually increase the dose.

That's why strict monitoring of blood levels is mandatory for this drug.

And if you do get toxic or even just with chronic use, the side effects list is quite a laundry list.

It is.

You have the neurological stud, astagmesotaxia, diplopia, double vision.

But then you have the really distinctive systemic effects.

Gingival hyperplasia.

That's the gums growing over the teeth.

Yes.

It's an issue with collagen metabolism.

If dental hygiene isn't perfect, the gums can swell up and literally cover the teeth.

It's a major cosmetic and dental problem.

That sounds horrific for a patient's self -esteem.

It is.

And then there's a suitism excessive hair growth in places you don't want it.

Imagine being a teenage girl dealing with epilepsy, and your medication causes gum overgrowth and facial hair.

Compliance becomes a huge battle.

The chapter also emphasizes fetal hydantoin syndrome.

Phenytoin is a known teratogen.

It interferes with folate metabolism.

It can cause a constellation of birth defects, cleft lip, cleft palate,

and microcyphaly in the fetus.

It's generally avoided in pregnancy, if at all possible.

One last specific warning for phenytoin involves an HLA allele in Asian populations.

What's that about?

Yes.

The HLA B452 allele.

It's a genetic polymorphism common in certain Asian populations.

And it significantly increases the risk of Stevens -Johnson syndrome, or SJS.

SJS, that's the skin reaction, right?

It's a severe, life -threatening skin reaction, where the skin blisters and peels off.

It's an absolute medical emergency.

So the recommendation is to screen for this gene before starting the drug in these populations.

Okay.

Next up, phenobarbital and primidone.

These sound old school.

Phenobarbital is the oldest AED still in use.

It's a barbiturate.

It works by enhancing GABA pressing the brake pedal.

Specifically, it makes the GABA receptor keep the chloride channel open longer, hyperpolarizing the neuron.

And primidone.

Primidone is interesting because it's metabolized into two active compounds, phenobarbital and Pima, which also has anticonvulsant activity.

So when you take primidone, you're effectively taking phenobarbital plus a kicker.

Why aren't these first line anymore?

They sound effective.

Thought effects.

Primarily sedation.

They cause significant cognitive impairment brain fog.

It's hard to function at work or school when you're on high dose phenobarbital.

And in high doses, they can cause respiratory depression.

They've been largely replaced by newer agents with better side effect profiles.

Now we come to what you could call the Swiss army knife of epilepsy drugs.

Valproit or Valproic acid.

It really does it all.

The chapter lists four distinct actions.

It blocks sodium channels.

It blocks T type calcium channels.

Wait, T type.

So it works on absence seizures too.

Yes, that's the key point.

And on top of that, it increases GABA synthesis and inhibits GABA degradation.

And it decreases glutamate synthesis.

It hits every single lever we talked about.

Gas, brake and the spark plugs.

It does.

And because of that, it has the broadest spectrum of activity.

It's effective for almost all seizure types.

Partial, generalized, tonic, clonic, absence, myoclonic.

If you aren't sure exactly what kind of epilepsy the patient has, Valproit is often the safest bet for efficacy.

But safety wise, what's the catch?

There's always a catch.

The liver again.

Hepatotoxicity.

It's rare, but it can be fatal.

Especially in children under two years old who are on multiple AEDs.

You have to monitor liver function tests.

It's a category D drug, a major teratogen.

It carries a high risk of neural tube defects like spina bifida.

And has been shown to impair cognitive development in the offspring.

It's one you really try to avoid in women of childbearing potential.

It also plays dirty with other drugs, right?

It does, but in the opposite way of carbamazepine, it inhibits metabolism.

So if you take Valproit with laminotriginy, for example,

the Valproit stops the liver from breaking down the lamatriginy.

So the lamatriginy levels skyrocket.

Exactly.

Which, as we'll see, dramatically increases the risk of that SJS rash.

Finally, for this section, a newer one.

Sinobamate.

Recently approved for partial onset seizures.

It's a dual action drug.

It blocks sodium channels and it potentiates GABA.

But it has a specific contraindication for anyone with familial short QT syndrome, a rare inherited heart rhythm issue.

Moving on to section five.

Adjunct drugs for partial seizures.

The chapter says these are mostly used in combination when the first line drugs don't work fully.

There's a long list here, so let's group them by mechanism to keep our head straight.

Good idea.

Let's start with the GABA modulators.

Okay, first we have tiagabine and vigabatrin.

Tiagabine blocks GABA reuptake.

It inhibits the transporter called GAT1.

Imagine a vacuum cleaner sucking GABA out of the synapse.

Tiagabine unplugs the vacuum.

So GABA stays around in the synapse longer to do its inhibitory job?

Precisely.

And vigabatrin.

It goes a step further.

It is an irreversible inhibitor of GABA transaminase.

That's the enzyme that breaks GABA down inside the cell.

It essentially breaks the machine that destroys GABA.

Then we have the pair that confuses everyone.

Gabapentin and progabalin.

Yes.

The big surprise.

Despite the name Gabapentin, they do not bind to GABA receptors.

Wait, really?

That seems like false advertising.

It was designed to mimic the structure of a GABA, but it turns out its actual mechanism is different.

It binds to the alpha -2 delta subunit of voltage -gated calcium channels.

So they are technically calcium channel blockers?

Effectively, yes, though they modulate the channel rather than block it outright.

By binding to that subunit, they reduce calcium influx, which in turn stops the release of excitatory neurotransmitters like glutamate.

Gabapentin does also increase GABA release somewhat, but the primary target is that calcium channel subunit.

And Gabapentin has weird absorption, right?

The text mentions that.

It does.

It's inversely related to dose.

There's a specific transporter in the gut that absorbs it, and that transporter gets saturated.

So if you take a huge dose, you actually absorb a smaller percentage of it than if you take a smaller dose.

These are used for more than just epilepsy.

I think more so now.

Oh, absolutely.

They have huge usage in neuropathic pain, fibromyalgia per goblins specifically, and post -therpetic neuralgia.

You see them in pain clinics more than neurology clinics these days.

Next group.

Glutamate antagonists.

Phelbamate and Parampanel.

Phelbamate blocks the glycine coactivation site on the NMDA receptor.

It looked promising, but it has some very serious side effects.

What are they?

A plastic anemia and acute hepatic failure.

Because of that, its use is severely restricted.

It's a last resort drug now.

And Parampanel.

It blocks AMPA receptors, another type of glutamate receptor, a different way of turning down the gas.

Now for the mixed mechanisms and channel blockers, let's talk about lamatrigine.

Lamatrigine blocks sodium channels, similar to carbamazepine.

It's a very common adjunct and sometimes a first -line agent.

But we have to talk about the rash again.

Stevens -Johnson syndrome.

Yes.

The risk is real.

It's why you have to titrate the dose up very slowly.

And remember when we said about Valprote.

If you combine them, the risk of SGS shoots way up because Valprote prevents the body from clearing the lamatrigine.

There's also a recent FDA warning mentioned for lamatrigine regarding aseptic meningitis.

Right.

Inflammation of the brain lining, the meninges, but not from an infection.

It presents like meningitis, stiff neck, headache, fever, but it's a drug reaction.

It's rare, but important to know.

This is another one with multiple actions.

It's got four actions listed in the text.

Block sodium channels, augments GABA activity, and blocks two types of glutamate receptors, Canadian AMPA.

It's a real shotgun approach.

And the side effects.

I've heard it called dopamine X because it makes people feel dopey.

The chapter politely calls it psychomotor impairment.

Cognitive slowing, word finding difficulties,

memory problems.

It can be significant for some patients.

It also has a cleft palate risk in pregnancy, making it a category D drug.

These are interesting because they have a totally unique target.

Yes, SV2A.

That stands for synaptic vesicle protein 2A.

What does that actually do?

It's not a channel or a receptor.

No, it's a protein on the vesicle, the little bubble that holds the neurotransmitters before they're released.

By binding to SV2A, these drugs somehow modulate the process of exocytosis, reducing the release of those chemicals.

We don't fully understand the molecular details, but it works, and it works differently than the other drugs.

A few more quick ones from the list.

Zoniefonide.

That one blocks both sodium channels and T -type calcium channels, so it has a broad spectrum.

The thing to watch out for is metabolic acidosis, especially in younger patients.

Licosamide.

That's a sodium channel blocker, but it works in a slightly different way.

It enhances the slow inactivation of the channel, which is another way to stabilize the membrane against that high frequency firing.

Azocabine.

This one has a unique mechanism involving potassium.

Yes, it's the only one in the chapter that works this way.

It opens potassium or K plus heat Azo channels.

And opening potassium channels?

What does that do to the neuron?

It causes potassium to leave the cell.

Positive charge leaves, so the inside of the cell becomes more negative.

It hyperpolarizes the neuron.

It cools it down, taking it further away from the firing threshold.

A very different way to apply the brakes.

And finally, rufinamide and clobazam.

These are specifically noted for their use in Lennox -Gastaut syndrome, or LGS, which is a severe childhood epilepsy syndrome that's very difficult to treat.

Clobazam is a benzodiazepine, and rufinamide modulates sodium channels.

Okay, that was the big list.

Now let's focus on a specific condition.

Section six, drugs for absence, myoclonic, and atonic seizures.

We talked about absence seizures earlier.

The Spacey Susie scenario.

And the specialist drug here, the drug of choice, is ethosuximide.

Why is it the specialist?

What makes it so specific?

Because it targets exactly that mechanism we discussed, the T -type calcium channels in the thalamus.

It selectively blocks the pacemaker current that generates that three hertz spike and dome rhythm.

Is it good for tonic -clonic seizures?

No.

And this is a key point.

It is not effective for tonic -clonic or partial seizures.

It can actually make them worse in some cases.

It is a precision tool for pure, uncomplicated absence seizures.

The case study in Box 20 .1 really brings this home.

Can you walk us through the story of Spacey Susie?

Sure.

Susie is a seven -year -old girl.

She's healthy, smart, but suddenly she's having trouble in school.

Her parents go to a parent -teacher conference, and the teacher says Susie is spacing out, not paying attention, maybe blinking her eyes a lot.

The parents didn't notice this at home.

The episodes are very brief, maybe 10 to 15 seconds.

It's easy to miss at home or to just think she's daydreaming.

But in a classroom where sustained attention is required, it becomes obvious she's checking out.

So they go to a neurologist.

Yes.

The neurologist suspects absence seizures.

They induce hyperventilation in the office, which can provoke an attack, hook her up to an EEG, and boom, there it is.

The classic three -hertz spike and dome pattern.

The diagnosis is confirmed.

And the treatment?

Ethosuximide.

It works very well.

And the prognosis, the text says, is good remission, often occurs by the time the child reaches age 18.

The text also mentions clonazepam for these types of seizures.

It's a benzodiazepine, so it enhances GABA.

It works, but it causes tolerance, meaning you need more over time to get the same effect in sedation.

Valproat is often used instead because it covers absence seizures.

And E protects against tonic -clonic seizures in case the diagnosis is mixed.

Now let's shift gears to the most dangerous situation.

Section seven, status epilepticus.

This is the red alert.

The definition is recurrent tonic -clonic seizures without recovery of consciousness in between them.

The chapter gives a strict timeline.

We have to control this within 60 minutes.

Why 60?

What happens after an hour?

Because after that, the risk of permanent brain damage skyrockets.

The combination of hypoxia, lack of oxygen to the brain, and the sheer metabolic demand of the seizure lead to neuromal death.

The neurons essentially burn out.

So what is the protocol?

The order of the drugs matters here.

It does.

It's a strict algorithm.

Step one, immediate FAV benzodiazepine, diazepam or lorazepam.

Why benzos first?

Why not jump straight to phenytoin?

Because benzos work instantly.

You need to stop the shaking now.

They flood the GABA receptors and slam on the brakes immediately.

Phenytoin takes time to load and start working.

The benzos don't last long, right?

Correct.

They're very lipid soluble, so they get into the brain quickly, but then they redistribute out into the body's fat tissues just as quickly.

So you stop the seizure, but you haven't prevented the next one from starting 20 minutes later.

That's step two.

You follow the benzo with a loading dose of IV phenytoin or phosphenytoin.

This provides the sustained long -term sodium channel blockade to keep the seizures from coming back once the benzo wears off.

And if that fails?

If the patient is still seizing?

Then you have refractory status.

You bring out the big guns.

You might give a loading dose of phenobarbital or in the worst cases move to general anesthesia with drugs like propofol.

You have to sedate the patient to the point where the brain simply cannot seize.

You induce a coma to save the brain.

We've got a brief section here, section eight on cannabinoid drugs.

This is a topic that gets a lot of press.

Right.

The text mentions Epidiolex.

It's a pharmaceutical grade purified cannabidiol or CBD.

Is this just medical marijuana?

No, and that's an important distinction.

It's a highly purified single component of the cannabis plant, but it is not THC.

It does not have psychoactive effects.

You don't get high from it.

What is it approved for?

Specifically for two rare and severe forms of childhood epilepsy, Lennox gas out and Dravid syndromes.

How does it work?

The chapter is very honest here.

The exact mechanism is unknown.

It does not seem to directly activate the main cannabinoid receptors, CB1 and CB2, like THC does.

It has other effects in the brain, but we're still figuring out precisely how it stops these seizures.

Finally, section nine, management of seizure disorders.

We have all these drugs.

How do we use them?

The chapter outlines some core principles.

The golden rule is monotherapy.

Start with one drug, start at a low dose, and titrate up gradually until the seizures are controlled or the patient has side effects.

Why not just hit it hard with two drugs from the start to be sure?

A few reasons.

First, to minimize side effects and improve compliance.

The more drugs you're on, the more side effects you're likely to have.

And second, scientifically, if you start two drugs and the patient gets better, which one worked?

If they get a rash, which one caused it?

You need to know what's doing what.

Combination therapy is only if monotherapy fails.

Correct.

And if you do add a second drug, the idea is to choose one with a different mechanism.

Don't use two sodium channel blockers.

Use a sodium blocker and a GABA booster.

Attack the problem from two different angles.

What about stopping the drugs?

Can you ever be cured and come off them?

If a patient is seizure -free for several years,

the text says usually two to five withdrawal can be considered, but it must be tapered very slowly over months.

Abrupt withdrawal can trigger rebound seizures or even status epilepticus.

And what's the relapse rate?

The book cites a relapse rate of about 25%, so it's a calculated risk that the doctor and patient have to discuss and decide on together.

Two major safety warnings to wrap up.

First, a class -wide warning about suicidality.

Right.

In 2008, the FDA issued a warning for all AEDs regarding an increased risk of suicidal thoughts and behaviors.

It's not one specific drug, it's the whole class.

So clinicians need to monitor patients for mood changes, depression, and anxiety when they start these medications.

And pregnancy.

We touched on this with Valpro and Finitoin, but it's a major concern overall.

It is.

Valpro and Finitoin are category D, the highest risk.

Folate antagonism seems to be a common theme, leading to neural tube defects.

The text also lists cleft palate and cardiac defects as potential teratogenic effects.

It seems like a terrible choice to have to make for a mother.

It is.

But you have to remember, uncontrolled tonic -clonic seizures are also very dangerous for the fetus.

There's a risk of hypoxia or trauma from the mother falling.

So it's a delicate balance.

Often doctors will try to switch to safer drugs if possible or use the lowest effective dose of the current drug and always supplement with high dose folate.

This has been a massive download of information.

Let's try to summarize the big picture before we go.

If you boil it all down, we're managing the brain's delicate electrical balance.

And we have three main levers we can pull.

We can block the channels that cause excitation like the sodium and calcium channels.

Right, stop the spark.

We can block the excitatory signal itself by antagonizing glutamate.

Or we can boost the brain's own inhibitory signal by augmenting GABA.

And the goal is to calm the storm without sedating the person to find that balance.

Exactly.

We want to use clever mechanisms like that used to pin a blockade to selectively stop the synchronous firing of the seizure focus.

Without shutting down the normal asynchronous firing that allows for thought and movement and life.

It's incredible when you think about it.

We are tossing these carefully designed chemicals into the most complex biological machine we know of.

All to tweak the opening and closing of microscopic gates on the surface of cells just to keep thoughts and movements flowing smoothly.

And the sheer variety of targets.

From the three hertz spike of an absent seizure originating in the thalamus to a protein on a synaptic vesicle like SV2A.

It really highlights how complex and how delicate the regulation of the brain's excitability truly is.

And on that note, we will sign off.

We hope this deep dive helps you navigate the complex pharmacology of anti -epileptic drugs.

Keep learning and keep questioning.

A warm thank you from the entire last minute lecture team.

See you on the next deep dive.