Everything You Need to Know About Antiepileptic drugs for Step 1

Antiepileptic drugs (AEDs) are one of those Step 1 topics where mechanisms are testable, toxicity patterns are classic, and the “extras” (enzyme induction/inhibition, pregnancy, rashes, stones, sodium) show up everywhere—from OB to ID to psych. The fastest way to score points is to organize AEDs by what channel/receptor they hit, then memorize the high-yield adverse effects + drug interactions.

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Where AEDs Fit in Neuro (and Why They Work)

Seizures reflect abnormal, hypersynchronous neuronal firing—often driven by:

• Too much excitation (glutamate)
• Too little inhibition (GABA)
• Pathologic ion channel activity (Na$^+$, Ca$^{2+}$)

So AEDs mainly work by:

• Blocking voltage-gated Na$^+$ channels → suppress high-frequency firing
• Blocking T-type Ca$^{2+}$ channels → especially for absence seizures
• Increasing GABA activity → more inhibitory tone
• Decreasing glutamate release/signaling → less excitation

First Aid cross-reference: Neurology → Seizures; Pharmacology → Antiepileptic drugs; Pregnancy teratogens; CYP inducers/inhibitors.

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Quick Seizure Classification (Step 1 Version)

Focal (partial) seizures

• Focal aware (simple): no LOC
• Focal impaired awareness (complex): impaired awareness (often temporal lobe)
• Can become focal to bilateral tonic-clonic

Generalized seizures

• Absence: brief staring spells, 3-Hz spike-and-wave on EEG
• Tonic-clonic: LOC + tonic stiffening then clonic jerking
• Myoclonic: brief shock-like jerks
• Atonic: sudden loss of tone (“drop attacks”)

Clinical pearl: Treatment choices are often anchored to the seizure type; question stems usually give EEG clue + age + duration + triggers (e.g., hyperventilation for absence).

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Diagnosis & “When to Treat”

Typical workup concepts tested:

• EEG: supportive; seizure type patterns can be classic (absence = 3-Hz spike-and-wave)
• MRI brain: evaluate structural causes in new-onset focal seizures
• Labs: Na, Ca, Mg, glucose (metabolic provokers)

Status epilepticus definition (classic): continuous seizure activity or recurrent seizures without return to baseline (often taught as >5 minutes clinically).
Treatment algorithm is very Step-relevant (see below).

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AEDs by Mechanism (High-Yield Table)

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The “Big 6” You Must Know Cold

1) Phenytoin / Fosphenytoin

Mechanism: blocks voltage-gated Na$^+$ channels; use-dependent blockade.

Key uses

• Focal seizures
• Generalized tonic-clonic (not absence)
• Status epilepticus prevention after acute benzodiazepine control (often fosphenytoin IV)

High-yield adverse effects

• Nystagmus, ataxia, diplopia, sedation
• Gingival hyperplasia
• Hirsutism
• Megaloblastic anemia (↓ folate)
• Osteopenia (↑ vitamin D metabolism)
• Teratogen: fetal hydantoin syndrome (cleft palate, cardiac defects, growth retardation)

Drug interactions

• CYP inducer (classic Step trigger: reduces OCP effectiveness)

Kinetics pearl

• Zero-order kinetics at higher doses (saturation): small dose increase → big level jump

First Aid tie-in: enzyme induction; fetal hydantoin; gingival hyperplasia.

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2) Carbamazepine (and the close cousin: Oxcarbazepine)

Mechanism: blocks Na$^+$ channels (stabilizes inactivated state).

Key uses

• Focal seizures (often first-line)
• Trigeminal neuralgia
• Bipolar disorder (mood stabilizer)

High-yield adverse effects

• Agranulocytosis / aplastic anemia (watch for infections, sore throat)
• Hepatotoxicity
• SIADH → hyponatremia (more common with oxcarbazepine)
• Teratogen: neural tube defects (think “folate issues” across AEDs)

Drug interactions

• CYP inducer

Genetics pearl (Step 1 favorite)

• HLA-B*1502 (esp. Han Chinese, some SE Asian populations) ↑ risk of SJS/TEN with carbamazepine

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3) Valproate (Valproic acid)

Mechanism: increases GABA, blocks Na$^+$ channels, inhibits T-type Ca$^{2+}$ currents → broad spectrum.

Key uses

• Generalized seizures (tonic-clonic, absence, myoclonic)
• Bipolar disorder
• Migraine prophylaxis (also relevant to the “Headache” part of your block)

High-yield adverse effects

• Hepatotoxicity (box-warning vibe; especially in kids)
• Pancreatitis
• Tremor
• Weight gain
• Alopecia
• Thrombocytopenia
• Hyperammonemia (can cause encephalopathy)

Pregnancy

• Major teratogen: neural tube defects (spina bifida)

First Aid tie-in: valproate = “very problematic” in pregnancy; hepatotox/pancreatitis.

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4) Lamotrigine

Mechanism: blocks Na$^+$ channels; ↓ glutamate release.

Key uses

• Broad spectrum (focal + generalized)
• Bipolar disorder (maintenance)

High-yield adverse effect

• Stevens-Johnson syndrome / TEN
  • Risk increases with rapid titration and with valproate (valproate increases lamotrigine levels)

First Aid tie-in: rash/SJS with lamotrigine—classic.

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5) Levetiracetam

Mechanism: binds SV2A (synaptic vesicle protein) → modulates neurotransmitter release.

Key uses

• Broad spectrum; very common “default” AED clinically
• Useful when you want fewer drug interactions (Step relevance: it’s relatively “clean”)

High-yield adverse effects

• Behavioral symptoms: irritability, agitation, mood changes, depression (“Keppra rage”)

Pearl: minimal CYP interactions compared to older AEDs.

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6) Ethosuximide (Absence Seizures)

Mechanism: blocks T-type Ca$^{2+}$ channels in thalamus.

Key use

• Absence seizures (classic first-line)

Adverse effects mnemonic: EFGHIJ

• Ethosuximide
• Fatigue
• GI distress
• Headache
• Itching (urticaria)
• Johnson (SJS)

First Aid tie-in: 3-Hz spike-and-wave + ethosuximide.

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Status Epilepticus: The Treatment Flow You’ll Be Tested On

1. Acute stop: benzodiazepine (lorazepam IV is classic; diazepam also used)
2. Prevent recurrence: fosphenytoin/phenytoin (or levetiracetam/valproate in many modern protocols)
3. Refractory: phenobarbital, propofol, continuous EEG monitoring (depending on setting)

Benzodiazepines adverse effects: respiratory depression, sedation (synergistic with other CNS depressants).
First Aid tie-in: benzos increase GABA-A frequency; barbiturates increase duration.

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“Other” AEDs That Show Up in Vignettes

Topiramate

Mechanism: blocks Na$^+$, ↑ GABA-A, ↓ AMPA/kainate (glutamate), weak carbonic anhydrase inhibition.

Uses

• Seizures
• Migraine prophylaxis
• Weight-loss adjunct (clinical world)

High-yield adverse effects

• Cognitive slowing (“Dopamax”)
• Kidney stones (carbonic anhydrase inhibition → ↑ urine pH)
• Paresthesias
• Weight loss

Phenobarbital

Mechanism: ↑ GABA-A duration (Cl$^-$ channel opening time)

Adverse effects

• Sedation, respiratory depression
• CYP inducer

Gabapentin / Pregabalin

Mechanism: bind α2δ subunit of voltage-gated Ca$^{2+}$ channels → ↓ excitatory neurotransmitter release.

Uses

• Adjunct seizures
• Neuropathic pain (diabetic neuropathy, postherpetic neuralgia)
• Fibromyalgia (pregabalin)

AEs: sedation, dizziness, ataxia; weight gain/peripheral edema (esp. pregabalin)

Vigabatrin

Mechanism: irreversible inhibition of GABA transaminase → ↑ GABA.

HY toxicity: permanent visual field defects (retinal toxicity)

Tiagabine

Mechanism: inhibits GABA reuptake (GAT-1).

HY toxicity: CNS depression, confusion; can precipitate seizures in nonepileptics (less commonly tested)

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High-Yield Associations & “Buzzword” Triggers

Enzyme induction (decreases other drugs)

Inducers:

• Carbamazepine
• Phenytoin
• Phenobarbital (Also: primidone—less emphasized)

Step consequences

• ↓ oral contraceptive effectiveness → unintended pregnancy
• ↓ warfarin effect (lower INR)
• ↓ antiretroviral/antifungal levels (conceptual)

Teratogenicity (know the big ones)

• Valproate → neural tube defects
• Phenytoin → fetal hydantoin syndrome
• Carbamazepine → neural tube defects (less “iconic” than valproate, but tested)

Exam mindset: If the stem mentions pregnancy or “woman of childbearing age,” consider risk/benefit and the common switch toward agents with better pregnancy safety profiles (often individualized; Step typically just wants you to recognize the teratogens).

Rash/SJS/TEN

• Lamotrigine (big one)
• Carbamazepine (HLA-B*1502 association)
• Ethosuximide (rare but in mnemonic)

Hyponatremia (SIADH)

• Carbamazepine (and especially oxcarbazepine)

Hyperammonemia

• Valproate (encephalopathy risk)

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First Aid “Cross-Reference Map” (How to Connect Topics Fast)

• Neuro (Seizures): seizure types + EEG patterns
• Pharm (AED mechanisms): Na$^+$ vs T-type Ca$^{2+}$ vs GABA
• Biochem/Nutrition: folate deficiency (phenytoin)
• Endocrine/Renal: SIADH hyponatremia (carbamazepine)
• Repro/OB: teratogens (valproate, phenytoin) + OCP failure (CYP inducers)
• Derm: SJS/TEN (lamotrigine; carbamazepine + HLA)
• Headache: valproate/topiramate for migraine prophylaxis
• Sleep: sedating profiles (phenobarbital/benzos) and cognitive effects (topiramate) can be folded into “daytime somnolence” differentials

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A Mini “Which Drug for Which Seizure?” Cheat Table

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Rapid-Fire USMLE High-Yield Facts (Last-Minute Review)

• Absence seizure EEG: 3-Hz spike-and-wave → treat with ethosuximide (T-type Ca$^{2+}$ blocker).
• Phenytoin: zero-order kinetics; gingival hyperplasia; hirsutism; megaloblastic anemia; teratogen.
• Carbamazepine: agranulocytosis + SIADH; HLA-B*1502 → SJS/TEN.
• Valproate: hepatotoxicity, pancreatitis, weight gain, tremor; neural tube defects; hyperammonemia.
• Lamotrigine: SJS/TEN, especially with rapid titration or with valproate.
• Levetiracetam: “clean” interactions; behavioral side effects.
• Topiramate: kidney stones + cognitive slowing + weight loss.
• Status epilepticus: benzo first, then long-acting AED.