ASM Classification & Selection

The antiseizure medication (ASM) armamentarium has grown to include over 30 agents with diverse mechanisms of action. Selection requires balancing efficacy for the seizure type, side-effect profile, drug interactions, patient comorbidities, pregnancy plans, and adherence considerations. This page covers the mechanism-based classification of ASMs, the principles of selection by seizure type, and the framework for choosing among similar agents. (See the dedicated Antiseizure Medications overview page for the original detailed treatment.)

Mechanism-Based Classification

Sodium Channel Modulators

Block voltage-gated Na⁺ channels, preferentially in their inactive state, reducing high-frequency neuronal firing:

  • Phenytoin / fosphenytoin
  • Carbamazepine
  • Oxcarbazepine
  • Eslicarbazepine
  • Lamotrigine (also affects glutamate release)
  • Lacosamide (enhances slow inactivation, different from classical Na⁺ blockers)
  • Cenobamate (also enhances GABA-A function)
  • Rufinamide
  • Topiramate (multiple mechanisms including Na⁺)
  • Zonisamide (multiple mechanisms including Na⁺)

Calcium Channel Modulators

  • Ethosuximide: T-type Ca²⁺ channel blocker (thalamic); first-line for absence epilepsy.
  • Gabapentin, pregabalin: α2δ subunit of voltage-gated Ca²⁺ channels; modulate presynaptic Ca²⁺ entry.
  • Valproate: T-type effects among other mechanisms.
  • Zonisamide: T-type effects.

SV2A Modulators

  • Levetiracetam: binds synaptic vesicle protein 2A (SV2A) → modulates neurotransmitter release.
  • Brivaracetam: more selective SV2A binding; similar efficacy with potentially better tolerability.

GABAergic

  • Benzodiazepines (clonazepam, clobazam, lorazepam): allosteric GABA-A modulators.
  • Phenobarbital: GABA-A modulator.
  • Tiagabine: GABA reuptake inhibitor.
  • Vigabatrin: GABA transaminase inhibitor.
  • Stiripentol: positive allosteric GABA-A; Dravet syndrome.

Glutamate Antagonists

  • Perampanel: AMPA antagonist.
  • Felbamate: NMDA antagonist + other actions; restricted use (aplastic anemia, hepatic failure).

Carbonic Anhydrase Inhibitors

  • Acetazolamide: rarely used as primary ASM; sometimes for catamenial seizures, episodic ataxia.
  • Zonisamide and topiramate have carbonic anhydrase inhibition contributing to side effects (renal stones, paresthesias, metabolic acidosis).

Mixed/Multiple Mechanisms

  • Valproate: GABA, Na⁺, T-type Ca²⁺.
  • Topiramate: Na⁺, AMPA/kainate, GABA, carbonic anhydrase.
  • Zonisamide: Na⁺, T-type Ca²⁺, carbonic anhydrase.
  • Cannabidiol (CBD): unclear mechanism; multiple targets including GPR55, TRPV1, adenosine reuptake; for Dravet, Lennox-Gastaut, tuberous sclerosis complex.
  • Cenobamate: Na⁺ inactivation + GABA-A; for focal seizures.

Newer/Targeted

  • Stiripentol: GABA-A allosteric; CYP inhibitor used in Dravet.
  • Fenfluramine: serotonergic; Dravet, Lennox-Gastaut.
  • Ganaxolone: neurosteroid; CDKL5 deficiency.
  • Soticlestat: cholesterol 24-hydroxylase inhibitor; Dravet/Lennox-Gastaut.

Selection by Seizure Type

Focal Seizures (with or without secondary generalization)

  • First-line: levetiracetam, lamotrigine, oxcarbazepine, carbamazepine.
  • Second-line: lacosamide, brivaracetam, eslicarbazepine, topiramate, zonisamide, perampanel, cenobamate.
  • Many older patients: levetiracetam or lamotrigine (lower side effects).

Generalized Tonic-Clonic Seizures

  • First-line: valproate (most effective; avoid in women of childbearing age), levetiracetam, lamotrigine.
  • Second-line: topiramate, zonisamide, perampanel.
  • Avoid Na⁺ blockers (carbamazepine, oxcarbazepine, phenytoin) in some generalized epilepsies (can worsen absence, myoclonus).

Absence Seizures

  • First-line: ethosuximide (childhood absence epilepsy).
  • Alternative: valproate (broader spectrum); lamotrigine (if absences + GTC).
  • Avoid: carbamazepine, phenytoin, oxcarbazepine (can worsen).

Myoclonic Seizures (including JME)

  • First-line: valproate (most effective).
  • Levetiracetam: also effective.
  • Lamotrigine: can worsen myoclonus in some.
  • Topiramate, zonisamide: alternatives.
  • Avoid: Na⁺ blockers (carbamazepine, phenytoin, oxcarbazepine), gabapentin.

Specific Syndromes

  • Lennox-Gastaut syndrome: rufinamide, cannabidiol, clobazam, felbamate, fenfluramine, lamotrigine, topiramate, valproate.
  • Dravet syndrome: valproate + clobazam + stiripentol; cannabidiol; fenfluramine. AVOID Na⁺ blockers (carbamazepine, lamotrigine, phenytoin).
  • Infantile spasms: ACTH (gold standard), vigabatrin (tuberous sclerosis), prednisolone.
  • Tuberous sclerosis complex: vigabatrin for infantile spasms; cannabidiol; everolimus.
  • CDKL5 deficiency: ganaxolone.

Special Considerations in Selection

Women of Childbearing Age

  • Levetiracetam, lamotrigine: lowest teratogenic risk.
  • Valproate: HIGHEST risk; AVOID unless absolutely necessary; use with contraception.
  • Topiramate: cleft lip/palate; reduce oral contraceptive efficacy.
  • Carbamazepine, phenytoin, phenobarbital: enzyme inducers; reduce OC efficacy.
  • Folic acid 4 mg daily.

Elderly

  • Levetiracetam, lamotrigine: well-tolerated.
  • Avoid: phenobarbital (sedation), phenytoin (multiple interactions, kinetics).
  • Start lower, titrate slower.

Comorbid Migraine

  • Topiramate, valproate: dual indication (migraine prophylaxis + ASM).

Comorbid Pain

  • Gabapentin, pregabalin, carbamazepine, lamotrigine: also useful for neuropathic pain.

Comorbid Mood Disorder

  • Valproate, lamotrigine: mood-stabilizing.
  • Levetiracetam, topiramate: can worsen mood/behavior.
  • Add pyridoxine for levetiracetam-induced irritability (anecdotal).

Renal Impairment

  • Gabapentin, pregabalin, levetiracetam, topiramate: renally cleared; dose adjustment.
  • Carbamazepine, valproate: hepatic clearance; less affected.

Hepatic Impairment

  • Levetiracetam, brivaracetam: non-hepatic clearance.
  • Avoid valproate (hepatotoxicity).
  • Lamotrigine: dose reduction.

Driving/Operating Machinery

  • State-specific driving laws.
  • Most states require seizure-free period of 6 months to 1 year before driving.
  • Document discussions.

Black Box / Severe Warnings

  • All ASMs: suicidality FDA warning (relative risk small, modest).
  • Carbamazepine: HLA-B*15:02 in Asians; SJS/TEN.
  • Lamotrigine: rash, especially with valproate co-administration.
  • Felbamate: aplastic anemia, hepatic failure — restricted.
  • Vigabatrin: visual field defects — restricted, monitoring.
  • Phenytoin: Stevens-Johnson with HLA-B*15:02; gingival hyperplasia.
  • Valproate: hepatic failure (young children with metabolic disease), pancreatitis, teratogenicity.

Drug Resistance / Pharmacoresistant Epilepsy

  • ~30% of patients fail to achieve seizure control with two adequately dosed ASMs.
  • Evaluation: surgical evaluation (epileptogenic zone), VNS, RNS, DBS, dietary therapy, careful re-evaluation for non-epileptic spells.
  • Newer adjuncts: cenobamate (notable efficacy in pharmacoresistant focal epilepsy), perampanel, lacosamide.
  • Cannabidiol for specific syndromes.

Combination Therapy Principles

  • Combine drugs with different mechanisms when monotherapy fails.
  • Avoid combinations with same mechanism (redundant side effects).
  • Watch for interactions: enzyme induction (carbamazepine, phenytoin, phenobarbital) reduces levels of other drugs; valproate inhibits glucuronidation of lamotrigine (rash risk).
  • Targeted combinations: levetiracetam + lamotrigine; brivaracetam + lacosamide.

🔍 Did You Know?

The “rational polytherapy” concept — combining ASMs with complementary mechanisms — has transformed how we treat pharmacoresistant epilepsy. The traditional approach was simply “add another drug” when monotherapy failed, which often produced cumulative side effects without proportional benefit. The modern approach emphasizes mechanistic diversity: combine a sodium channel blocker with a SV2A modulator (lamotrigine + levetiracetam), or a glutamate antagonist with a GABA modulator (perampanel + clobazam), to attack different pathophysiologic targets. Some specific combinations have shown particular synergy: levetiracetam + lamotrigine for generalized epilepsy; brivaracetam + lacosamide for focal epilepsy (both well-tolerated, complementary mechanisms); valproate + lamotrigine for myoclonic-absence patterns (but watch for rash). Conversely, combinations to AVOID include carbamazepine + oxcarbazepine (same mechanism, redundant side effects), or two enzyme-inducing drugs (cumulative reduction of other medications). The lesson generalizes: pharmacology is a science of mechanisms, not just drugs, and the highest-yield clinical decisions come from understanding which mechanisms a patient’s seizures involve and which complementary mechanisms can address them. For specialty epilepsy clinics, this mechanism-first thinking is increasingly applied through patient-specific algorithms incorporating EEG findings, MRI, and pharmacogenomic data.

Pitfalls and Pearls

  • Focal seizures: levetiracetam, lamotrigine, oxcarbazepine, carbamazepine first-line.
  • Generalized tonic-clonic: valproate, levetiracetam, lamotrigine; avoid Na⁺ blockers if mixed with myoclonus.
  • Absence: ethosuximide; avoid Na⁺ blockers.
  • Myoclonic/JME: valproate, levetiracetam; lamotrigine can worsen myoclonus.
  • Dravet: AVOID Na⁺ blockers (lamotrigine, carbamazepine).
  • Levetiracetam, brivaracetam: “clean” — minimal interactions.
  • Lacosamide: slow Na⁺ inactivation; well-tolerated.
  • Perampanel: AMPA antagonist; behavioral effects (black box).
  • Cenobamate: notable efficacy in pharmacoresistant focal epilepsy.
  • Cannabidiol: Dravet, Lennox-Gastaut, tuberous sclerosis.
  • Valproate: AVOID in women of childbearing age; teratogenic.
  • Lamotrigine titration with valproate: VERY slow; rash risk.
  • Levetiracetam: irritability; consider pyridoxine.
  • HLA-B*15:02 + carbamazepine: SJS in Asians.
  • Carbamazepine, phenytoin, phenobarbital: enzyme inducers; reduce OC efficacy.
  • Folic acid 4 mg/day: women on ASMs of childbearing age.
  • ~30% pharmacoresistant: surgical evaluation; alternative therapies.
  • Rational polytherapy: combine different mechanisms; avoid same-mechanism combinations.

References

  1. Brodie MJ, Sills GJ. Combining antiepileptic drugs—rational polytherapy? Seizure. 2011;20(5):369-375.
  2. Kanner AM, Ashman E, Gloss D, et al. Practice guideline update summary: Efficacy and tolerability of the new antiepileptic drugs I: Treatment of new-onset epilepsy. Neurology. 2018;91(2):74-81.
  3. Glauser T, Ben-Menachem E, Bourgeois B, et al. Updated ILAE evidence review of antiepileptic drug efficacy and effectiveness as initial monotherapy for epileptic seizures and syndromes. Epilepsia. 2013;54(3):551-563.
  4. Stafstrom CE, Carmant L. Seizures and epilepsy: an overview for neuroscientists. Cold Spring Harb Perspect Med. 2015;5(6):a022426.
  5. Patsalos PN, Berry DJ, Bourgeois BF, et al. Antiepileptic drugs—best practice guidelines for therapeutic drug monitoring. Epilepsia. 2008;49(7):1239-1276.