Question
A 30-year-old male was brought to the emergency department reporting a sudden onset of generalised tonic clonic convulsions and fall, in the nearby supermarket. His blood pressure and electrolytes were found to be normal. A diagnosis of generalised tonic clonic seizure was made.
- (a) Classify the drugs used in treatment of epilepsy 3 mark(s)
- (b) Describe the mechanism of action and adverse effects of phenytoin 3 mark(s)
Answer
a) Classification of antiepileptic drugs (by primary mechanism)
| Mechanism | Drugs |
|---|---|
| Na⁺ channel blockers | Phenytoin, Carbamazepine, Oxcarbazepine, Lamotrigine, Valproate |
| Ca²⁺ channel blockers (T-type) | Ethosuximide, Valproate |
| GABA-potentiating agents | Benzodiazepines (diazepam, clonazepam), Phenobarbitone, Vigabatrin |
| Newer/multiple mechanisms | Levetiracetam, Topiramate, Gabapentin |
For this patient’s generalized tonic-clonic seizure, first-line options include valproate, phenytoin, carbamazepine, or lamotrigine.
b) Phenytoin — mechanism of action and adverse effects Mechanism: phenytoin → binds and stabilizes the inactivated state of voltage-gated Na⁺ channels → use-dependent block of sustained high-frequency neuronal firing, without affecting normal single action potentials.
Adverse effects: gingival hyperplasia (a distinctive, characteristic effect), hirsutism and coarsened facial features with long-term use, cerebellar signs (nystagmus, ataxia) in toxicity, megaloblastic anemia, Stevens-Johnson syndrome/toxic epidermal necrolysis, teratogenicity (fetal hydantoin syndrome), and zero-order kinetics at therapeutic doses — a small dose increment can produce a disproportionate rise in plasma level and toxicity.