Anticonvulsants - Classification, Definition & Mechanism

Anticonvulsants (also called antiepileptic drugs or AEDs) are agents used to prevent or reduce the frequency and severity of seizures in epilepsy and other seizure disorders.
They work by modulating the activity of excitatory and inhibitory neurotransmitters or ion channels in the brain.

Anticonvulsants work mainly by:
Modulating voltage-gated sodium channels (e.g., Phenytoin, Carbamazepine)
Enhancing GABAergic inhibition (e.g., Benzodiazepines, Barbiturates)
Inhibiting T-type calcium channels (e.g., Ethosuximide)
Inhibiting glutamate receptors or release (e.g., Felbamate)
Altering synaptic transmission via GABA or voltage-gated channels (e.g., Valproic acid)

Barbiturates
- Enhance GABAergic activity at GABA-A receptors.
- Also inhibit voltage-gated calcium channels at higher concentrations.
- Examples:
  - Phenobarbitone
  - Metharbital
Hydantoins
- Block voltage-gated Na⁺ channels, stabilizing neuronal membranes.
- Examples:
  - Phenytoin
  - Mephenytoin
  - Ethotoin
Oxazolidine Diones
- Suppress T-type Ca²⁺ currents in thalamic neurons.
- Primarily used in absence seizures.
- Examples:
  - Trimethadione
  - Paramethadione
Succinimides
- Selectively block T-type calcium channels in thalamic neurons.
- Effective for absence (petit mal) seizures.
- Examples:
  - Phensuximide
  - Methsuximide
  - Ethosuximide
Ureas and Monoacylureas
- Block sodium channels, reduce repetitive neuronal firing.
- Examples:
  - Phenacemide
  - Carbamazepine
Benzodiazepines
- Potentiate the action of GABA at GABA-A receptors, increasing chloride influx and neuronal inhibition.
- Example:
  - Clonazepam

Class	Mechanism	Examples	Uses	Common Side Effects
1. Barbiturates	Enhance GABA-A activity; at high doses, inhibit Ca²⁺ channels	Phenobarbitone, Metharbital	Generalized tonic-clonic & partial seizures	Sedation, respiratory depression, dependence
2. Hydantoins	Block voltage-gated Na⁺ channels	Phenytoin, Mephenytoin, Ethotoin	Generalized tonic-clonic & partial seizures	Gingival hyperplasia, ataxia, hirsutism
3. Oxazolidine Diones	Suppress T-type Ca²⁺ currents in thalamus	Trimethadione, Paramethadione	Absence seizures	Visual disturbances, sedation, teratogenicity
4. Succinimides	Block T-type calcium channels in thalamic neurons	Ethosuximide, Phensuximide, Methsuximide	Absence seizures (petit mal)	GI upset, lethargy, headache
5. Ureas & Monoacylureas	Block Na⁺ channels; stabilize membranes	Carbamazepine, Phenacemide	Partial & generalized tonic-clonic seizures	Dizziness, rash, hyponatremia, aplastic anemia
6. Benzodiazepines	Potentiate GABA-A activity; increase Cl⁻ influx	Clonazepam	Absence, myoclonic seizures, status epilepticus	Sedation, tolerance, withdrawal, dependence