Makindo Medical Notes

⚠️ Key safety warning: Vigabatrin can cause irreversible visual field constriction (peripheral vision loss). Patients require baseline and 6-monthly visual field testing throughout therapy. It should only be used when potential benefits outweigh this risk.

📘 About: Always check the BNF link here

• Vigabatrin is an antiepileptic drug and an analogue of the inhibitory neurotransmitter GABA.
• It acts as a GABA transaminase inhibitor, preventing GABA breakdown and thereby increasing its synaptic concentration in the CNS.
• Its action enhances inhibitory neurotransmission, reducing neuronal excitability and seizure propagation.

🎯 Indications

• Infantile spasms (West’s syndrome) — first-line treatment, especially in tuberous sclerosis.
• Focal seizures (partial seizures), with or without secondary generalisation, when other antiepileptic drugs have failed or are not tolerated.

⚙️ Mechanism of Action

• Irreversible inhibition of GABA transaminase (the enzyme that metabolises GABA).
• This increases central GABA concentrations, prolonging inhibitory postsynaptic potentials.
• Because the inhibition is irreversible, the effect persists until new enzyme is synthesised (≈ 5–6 days).

💊 Dose (Adults)

• Start 500 mg once daily, then increase gradually to 2–3 g per day in divided doses (BD).
• In infants with West’s syndrome: refer to the BNF for Children for age-specific guidance.

🔄 Interactions

• Plasma concentrations are reduced by carbamazepine and phenobarbital.
• Has minimal hepatic metabolism and few pharmacokinetic interactions, making it suitable in polytherapy when enzyme-inducing agents complicate management.

💥 Side Effects

• Somnolence — very common; may limit use in daytime dosing.
• Behavioural changes — agitation, aggression, irritability, depression, and (rarely) psychosis.
• Visual field defects — progressive bilateral concentric loss (“tunnel vision”), often irreversible.
• Other: weight gain, paraesthesia, headache, dizziness.

⚠️ Monitoring

• Baseline and 6-monthly ophthalmic assessment (perimetry or visual field testing).
• Assess mood and psychiatric status regularly.
• Monitor seizure control and review for any evidence of visual changes.

🧠 Teaching Point

Vigabatrin exemplifies how targeting neurotransmitter metabolism can achieve durable seizure control — but at a cost. By irreversibly inhibiting GABA transaminase, it amplifies inhibitory tone throughout the brain. However, retinal GABAergic neurons appear particularly vulnerable to this mechanism, explaining the characteristic visual field toxicity. For this reason, its use has shifted toward specific syndromes like West’s or drug-resistant focal epilepsy under close supervision.

📚 References

• BNF: Vigabatrin
• NICE CKS: Epilepsy
• MHRA Drug Safety Update: Vigabatrin Visual Field Defects