Introduction
Anticonvulsants are medicines used to prevent or control seizures in epilepsy. Epilepsy occurs due to abnormal and repeated electrical activity in the brain. Seizures may involve loss of consciousness, jerky movements or brief staring episodes depending on the type. Anticonvulsants stabilise neuronal activity and prevent the spread of abnormal signals.
Types of Seizures
Generalized Seizures
- Generalized tonic–clonic seizures (GTCS): Loss of consciousness, body stiffening, jerks, followed by sleepiness.
- Absence seizures: Sudden staring, blank look, brief loss of awareness.
- Myoclonic seizures: Sudden, shock-like jerks of muscles.
Partial (Focal) Seizures
- Simple partial seizures: Motor or sensory symptoms without loss of consciousness.
- Complex partial seizures: Altered awareness, unusual behaviour, automatisms.
Classification of Anticonvulsants
Chemical Classification
- Hydantoins: Phenytoin, fosphenytoin
- Barbiturates: Phenobarbitone
- Iminostilbenes: Carbamazepine, oxcarbazepine
- Carboxylic acids: Valproic acid, divalproex
- Succinimides: Ethosuximide
- Benzodiazepines: Diazepam, lorazepam, clonazepam, clobazam
- Newer drugs: Lamotrigine, topiramate, gabapentin, pregabalin, levetiracetam, vigabatrin, tiagabine, lacosamide
Mechanism of Action of Anticonvulsants
These medicines prevent seizures by one or more mechanisms:
- Blocking sodium channels: Phenytoin, carbamazepine, valproate, lamotrigine
- Blocking T-type calcium channels: Ethosuximide, valproate
- Enhancing GABA activity: Benzodiazepines, phenobarbitone, valproate, vigabatrin, tiagabine
- Reducing glutamate activity: Topiramate, lamotrigine
Important Anticonvulsant Drugs
Phenytoin
Phenytoin stabilises neuronal membranes by delaying recovery of sodium channels. It prevents high-frequency neuronal firing.
Uses
- Generalized tonic–clonic seizures
- Partial seizures
- Neuralgias (trigeminal neuralgia)
- Status epilepticus (IV)
Adverse Effects (Remember the “H”s)
- Gum hypertrophy
- Hypersensitivity reactions
- Hirsutism
- Hyperglycaemia
- Hypocalcaemia
- Hepatic necrosis (rare)
- Fetal hydantoin syndrome
High levels cause ataxia, nystagmus and confusion. Requires dose monitoring due to zero-order kinetics at high doses.
Fosphenytoin
Prodrug of phenytoin; safer for IV/IM use. Preferred in status epilepticus.
Carbamazepine
Blocks sodium channels and reduces neuronal excitability.
Uses
- Drug of choice for partial seizures and GTCS
- Trigeminal neuralgia
- Bipolar disorder
Adverse Effects
- Drowsiness, blurred vision, dizziness
- Skin rashes and hepatitis
- Bone marrow depression (rare but serious)
- Water retention due to ADH action
Oxcarbazepine
Similar to carbamazepine but fewer drug interactions and less toxicity.
Phenobarbitone
Enhances GABA action; older anticonvulsant with strong sedative effect.
Uses
- GTCS and partial seizures
- Status epilepticus (when others fail)
- Febrile convulsions prevention
Adverse Effects
- Marked sedation
- Behavioural problems in children
- Megaloblastic anaemia
Ethosuximide
Blocks T-type calcium channels in thalamic neurons.
Uses
- Drug of choice for absence seizures
Side Effects
- Nausea, vomiting, anorexia
- Headache
- Bone marrow suppression (rare)
Valproic Acid (Sodium Valproate)
Broad-spectrum anticonvulsant with multiple actions: sodium channel block, T-type calcium block and increased GABA activity.
Uses
- Absence, myoclonic, partial and generalized seizures
- Bipolar disorder
- Migraine prophylaxis
Adverse Effects (VALPROATE mnemonic)
- Vomiting, anorexia, abdominal discomfort
- Ataxia, tremor
- Liver failure (serious)
- Pancreatitis
- Rashes, alopecia
- Teratogenic: neural tube defects
Benzodiazepines
Increase GABA activity. Used mainly for emergencies.
Uses
- Status epilepticus (diazepam, lorazepam)
- Absence and myoclonic seizures (clonazepam)
Side Effects
- Drowsiness, lethargy
- Respiratory depression (IV)
- Behavioural changes in children
Newer Anticonvulsants
Include lamotrigine, topiramate, gabapentin, pregabalin, vigabatrin, tiagabine, zonisamide, lacosamide and levetiracetam. They are used as add-on therapy or when older drugs fail. Most have fewer interactions and better tolerability.
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