Antiepileptics Part 1: Silencing the Electrical Storm
A seizure is an electrical storm — millions of neurons firing together in a runaway chain reaction that convulses the body. Anti-seizure drugs are storm-breakers, and they work in only three basic ways: dampen the 'go' signal, boost the 'stop' signal, or steady the wires themselves. Learn those three strategies and the classic drugs fall into place — along with why one wrong choice can make certain seizures worse.
A man collapses in a supermarket, his body stiffening then jerking rhythmically, eyes rolled back — a generalized tonic-clonic seizure. In his brain, a small group of neurons has begun firing abnormally and recruited its neighbours, until a wave of synchronized electrical discharge sweeps across the cortex, overwhelming the normal balance of excitation and inhibition. Ninety seconds later it stops on its own and he slowly comes round, confused. To stop this from happening again, we need drugs that break the storm before it spreads — and there are only three ways to do it.
The three ways to stop a seizure
Every antiepileptic uses one or more of three strategies. First, block voltage-gated sodium channels: these channels let neurons fire repeatedly, so blocking them (specifically the over-firing ones) stops the runaway repetitive discharge without silencing normal activity. Second, boost GABA inhibition: strengthen the brain's 'stop' signal so the storm can't build. Third, block T-type calcium channels in the thalamus: these drive the rhythm behind one special seizure type (absence), covered in Part 2. This article focuses on the first strategy and the broad-spectrum drug valproate.
The sodium-channel blockers
Phenytoin is the classic. It blocks sodium channels to control focal and tonic-clonic seizures — but it's a demanding drug: it follows zero-order kinetics near therapeutic levels (from the pharmacokinetics series — a small dose rise can send the level soaring), so it needs monitoring, and it carries memorable side effects: gum overgrowth (gingival hyperplasia), excess hair, a rash, and it's a strong liver-enzyme inducer and a teratogen. Its IV form (fosphenytoin) is used in prolonged seizures. Carbamazepine also blocks sodium channels and is a first-line focal-seizure drug (and treats trigeminal neuralgia and bipolar disorder); watch for low sodium (SIADH), bone-marrow suppression, a dangerous rash (Stevens–Johnson, linked to a specific gene in some Asian populations), and enzyme induction — including inducing its own metabolism.
Valproate is broad-spectrum: it blocks sodium channels, dampens calcium channels, AND enhances GABA — so it works across focal, tonic-clonic, absence, and myoclonic seizures (and treats migraine and bipolar disorder). But two things stand out: it is a liver-enzyme INHIBITOR (opposite of phenytoin and carbamazepine, which induce), and it is one of the most teratogenic antiepileptics — causing neural-tube defects — so it is avoided in women who could become pregnant. It also risks liver toxicity, pancreatitis, weight gain, and tremor.
- A seizure = excessive, synchronized neuronal firing (excitation overwhelms inhibition).
- Three strategies: block Na⁺ channels, boost GABA, block T-type Ca²⁺ channels.
- Phenytoin & carbamazepine block Na⁺ channels → focal & tonic-clonic seizures; both induce enzymes.
- Valproate is broad-spectrum but an enzyme INHIBITOR and highly teratogenic.
- Phenytoin's zero-order kinetics demand careful dose titration and monitoring.
Enzyme induction is the theme that ties the classic antiepileptics together — and makes them interaction machines. Phenytoin, carbamazepine, and phenobarbital all INDUCE liver enzymes, so they speed up the metabolism of many other drugs, famously weakening oral contraceptives (a woman on one of these may need backup contraception). Valproate is the exception — it INHIBITS enzymes, so it can raise the levels of drugs given alongside it (like lamotrigine). Always ask what an antiepileptic does to the liver's enzymes.
- Titrating phenytoin in big steps. Its zero-order kinetics can jump the level into toxicity.
- Forgetting valproate INHIBITS enzymes while phenytoin/carbamazepine INDUCE them.
- Prescribing valproate to a woman who could become pregnant. It's strongly teratogenic.
- Ignoring drug interactions — enzyme-inducing antiepileptics weaken contraceptives and more.
Which antiepileptic is broad-spectrum, INHIBITS liver enzymes, and is strongly teratogenic?
- Seizures are runaway synchronized firing; drugs block Na⁺ channels, boost GABA, or block T-type Ca²⁺.
- Phenytoin & carbamazepine (Na⁺ block) treat focal/tonic-clonic and INDUCE enzymes.
- Valproate is broad-spectrum, INHIBITS enzymes, and is highly teratogenic.
- Phenytoin's zero-order kinetics demand slow titration and level monitoring.
- Katzung BG. Basic & Clinical Pharmacology — Antiseizure Drugs (sodium-channel blockers & valproate).
- Brunton LL, et al. Goodman & Gilman's The Pharmacological Basis of Therapeutics — Pharmacotherapy of the epilepsies.
- Rang HP, Dale MM, et al. Rang & Dale's Pharmacology — Antiepileptic drugs.
- Whalen K. Lippincott Illustrated Reviews: Pharmacology — Antiepileptic drugs.
- NICE / ILAE guidelines — Antiepileptic drug choice & teratogenicity of valproate.
