A 7-year-old boy is brought to the physician because of spells of unresponsiveness and upward rolling of the eyes for 2 months. The episodes start abruptly and last a few seconds. During that time he does not hear anyone’s voice or make any purposeful movements. When the episodes end, he continues what he was doing before the spell. He does not lose his posture or fall to the ground. Episodes occur multiple times during the day. Physical examination shows no abnormal findings. An EEG following hyperventilation shows 3 Hz spike-and-slow-wave discharges. Which of the following is the most appropriate pharmacotherapy at this time?

No pharmacotherapy at this time

A 6-year-old boy is brought to the physician because of a 2-week history of frequent episodes of unresponsiveness. During these episodes, he stares blankly, rhythmically nods his head, and does not respond to verbal stimulation for several seconds. Hyperventilation for 30 seconds precipitates an episode of unresponsiveness and head nodding that lasts for 7 seconds. He regains consciousness immediately afterward. An electroencephalogram shows 3-Hz spikes and waves. Which of the following best describes the mechanism of action of the most appropriate pharmacotherapy for this patient's condition?

Blockade of thalamic T-type calcium channels

Inhibition of GABA reuptake into presynaptic neurons

Increased frequency of GABAA channel opening

Irreversible inhibition of GABA transaminase

Increased duration of GABAA channel opening

A neuroscientist is delivering a lecture on the electrophysiology of the brain. He talks about neuroreceptors which act as ion channels in the neurons. He mentions a specific receptor, which is both voltage-gated and ligand-gated ion channel. Which of the following receptors is most likely to be the one mentioned by the neuroscientist?

Nicotinic acetylcholine receptor

A 45-year-old homeless man presents to the emergency department with a 1-week history of an intensely pruritic, red rash on his hands, wrists, and finger webs. The itching is worse at night. Physical examination reveals small, erythematous papules and burrows. A topical drug with which of the following mechanisms of action is most likely to be effective in treating this condition?

Increase in keratinocyte turnover

Inhibition of histamine-1 receptors

Decrease in peptidoglycan synthesis

Inhibition of nuclear factor-κB

Sodium channel blocking antiepileptics for USMLE Step 2 CK: High-Yield Pharmacology Lessons

Mechanism of Action - The Inactive Gatekeepers

Voltage-gated Na⁺ channels cycle through three states: Resting (closed), Activated (open), and Inactivated (closed & refractory).
Antiepileptics in this class preferentially bind to the inactivated state, prolonging it and preventing the channel from returning to the resting state.
This creates use-dependence: the more frequently a neuron fires, the more its channels enter the inactivated state, thus enhancing the drug's blocking effect.

Voltage-gated sodium channel states

⭐ Use-dependence means these drugs selectively target rapidly firing epileptic neurons over neurons firing at a normal physiological rate, minimizing side effects on normal CNS function.

The Classics - Phenytoin & Carbamazepine

Both are potent CYP450 inducers used for focal and generalized tonic-clonic seizures.

Feature	Phenytoin	Carbamazepine
Unique Use	Status Epilepticus (2nd line)	Trigeminal Neuralgia, Bipolar Disorder
Pharmacokinetics	Zero-order elimination	Auto-induces own metabolism
Adverse Effects	* Gingival hyperplasia & Hirsutism * Nystagmus, Ataxia * Megaloblastic anemia * Fetal hydantoin syndrome	* SIADH & Agranulocytosis * Diplopia, Ataxia * Stevens-Johnson Syndrome (*HLA-B1502**)

Phenytoin: Follows zero-order kinetics, meaning a small dose increase can cause a large rise in plasma concentration, leading to toxicity (nystagmus, ataxia).
Carbamazepine: Can cause bone marrow suppression; monitor CBC.

⭐ Screen patients of Asian descent for the HLA-B*1502 allele before starting carbamazepine to prevent SJS/TEN.

The New Guard - Broader Spectrum Agents

These agents have multiple mechanisms of action, making them effective for a wider range of seizure types, including focal and generalized seizures.

Valproate / Valproic Acid:
- MoA: Broadest spectrum. Blocks Na+ channels, inhibits GABA transaminase (↑GABA), and blocks T-type Ca2+ channels.
- Side Effects: Major teratogen (neural tube defects), hepatotoxicity, pancreatitis, weight gain.
Lamotrigine (La-MOT-ri-jeen):
- MoA: Blocks voltage-gated Na+ channels and voltage-gated Ca2+ channels.
- Side Effects: ⚠️ Requires slow titration due to risk of Stevens-Johnson Syndrome (SJS) / Toxic Epidermal Necrolysis (TEN). 📌 LAMOtrigine = SLO-MO titration to avoid rash.
Topiramate (Toe-PYE-ra-mate):
- MoA: Multiple, including Na+ channel blockade, GABA-A receptor potentiation.
- Side Effects: Weight loss, cognitive slowing ("Dopamax"), kidney stones (carbonic anhydrase inhibition), paresthesias.
Lacosamide:
- MoA: Unique mechanism; enhances SLOW inactivation of voltage-gated Na+ channels, stabilizing the neuronal membrane.

Antiepileptic drug mechanisms of action

⭐ Valproate is a first-line agent for generalized epilepsy but is highly teratogenic and should be avoided in women of childbearing potential if possible.

High‑Yield Points - ⚡ Biggest Takeaways

Sodium channel blockers work by stabilizing the inactivated state of voltage-gated Na+ channels, thereby limiting the repetitive firing of neurons.

This class is highly use-dependent, meaning they preferentially bind to and block channels of rapidly firing neurons, such as those in an epileptic focus.

Key drugs include Phenytoin, Carbamazepine, Lamotrigine, and Valproate.

Major toxicities to remember: Phenytoin → gingival hyperplasia; Carbamazepine → agranulocytosis & SIADH; Lamotrigine → Stevens-Johnson syndrome.

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