Sodium channel-blocking antiepileptics
Blocking neuronal sodium channels to stop rapid, repetitive firing.
Class card
- Drugs
- Phenytoin, Carbamazepine and Lamotrigine
- Used for
- Focal and generalized tonic-clonic seizures
- Status epilepticus (phenytoin, IV)
- Trigeminal neuralgia (carbamazepine)
- Bipolar disorder (lamotrigine, carbamazepine)
- Known for
- Unsteadiness (ataxia)
- Severe skin reactions (SJS, TEN)
- Name stems
- -toin
4 cards: the class, and each drug in it.
Mechanism of action
How do sodium channel-blocking antiepileptics work?
These drugs block voltage-gated sodium channels in neurons in a use- and voltage-dependent way: the faster a neuron fires, the more channels they block. Ordinary signaling carries on, but the sustained high-frequency bursts that spread a seizure are cut short. Phenytoin and carbamazepine are also strong inducers of liver enzymes, so they speed the breakdown of many other drugs.
The drugs
Sodium channel-blocking antiepileptics with a card
- PhenytoinIts kinetics are nonlinear: as the liver enzymes saturate, a dose rise of 10% or more can push levels into the toxic range.
- CarbamazepineHLA-B*15:02 is carried by over 15% of people in Thailand, Malaysia and Hong Kong and by about 2 to 4% of South Asians, including Indians.
- LamotrigineIts serious rash risk is highest in children and in the first 2 to 8 weeks, which is why the dose is built up over several weeks.
Uses
What are sodium channel-blocking antiepileptics used for?
- Focal and generalized tonic-clonic seizures
- Status epilepticus (phenytoin, IV)
- Trigeminal neuralgia (carbamazepine)
- Bipolar disorder (lamotrigine, carbamazepine)
Side effects
Side effects of sodium channel-blocking antiepileptics
- Unsteadiness (ataxia)Known for
- Severe skin reactions (SJS, TEN)Known forSerious
- Double or blurred vision
- Dizziness
- Drowsiness
- Rash
- Blood disorders, including agranulocytosisSerious
- Liver injurySerious
- Birth defects if taken in pregnancySerious
- Suicidal thoughtsSerious
What the class has in common. Each drug's card has its own, and Known for marks what an exam expects you to know.
Watch for
Cautions
- HLA-B*15:02 carriers, mostly of Asian ancestry, face a high risk of SJS/TEN with carbamazepine, and some with phenytoin and lamotrigine.
- Phenytoin and carbamazepine induce liver enzymes, lowering levels of hormonal contraceptives, DOACs and many other drugs.
- Stopping suddenly can bring on seizures or status epilepticus, so doses are tapered.
- Phenytoin and carbamazepine in pregnancy raise the risk of major malformations such as cleft lip and heart defects.
Remember them
A way to remember them
Sodium channel blockers let single shots through but jam the machine gun.
Their block grows with each firing, so ordinary signals pass while the rapid, repetitive bursts of a seizure are stopped.
Written for biofideX
Name stems
-toin means hydantoin antiepileptic (phenytoin type)
Phenytoin and fosphenytoin, its injectable prodrug, are hydantoins that block sodium channels. Nitrofurantoin, an antibacterial for urinary infections, shares the ending.
- Every drug name stem
Don't mix them up
Classes easily confused with these
Sources
Checked against
Written for biofideX in plain words, after checking these references:
- Extended phenytoin sodium capsules: label (DailyMed)
- Carbamazepine 200 mg prolonged-release tablets: SmPC (emc)
- Lamotrigine orally disintegrating tablets: label (DailyMed)
- Enzyme-inducing Antiepileptic Drug (NCI Thesaurus)
For learning pharmacology, not for treatment decisions: no doses are given, and a prescriber, pharmacist or the product's own label is the word on any one patient. Spotted a mistake? Tell us and we'll correct it.
Epilepsy and nerve pain
Practice sodium channel-blocking antiepileptics with the other classes in this group: what each drug is, how it works, what it treats and what it's known for.
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