Vigabatrin

Side effects

Common side effects

• Somnolence/sedation (rarely Vigabatrin may cause encephalopathic symptoms)
• Weight gain
• Dizziness
• Double vision
• Fatigue
• Headaches
• Nausea and abdominal pain
• Behavioural disturbance (excitation/agitation/irritability)
• Cognitive change (thought disturbance/psychosis)

Other notable side effects

• Peripheral visual field loss: Vigabatrin is associated with a high incidence of visual field constriction. This is usually irreversible. Regular monitoring is required using ERG/perimetry according to age. The risk-benefit ratio of Vigabatrin usage must be discussed in this regard.
• MRI abnormality: Vigabatrin may cause prominent signal abnormality in basal ganglia and upper brain stem, especially in infants. These abnormalities are usually reversible and may even remit with continued drug usage.
• Rarely: Optic neuritis, rash, urticaria and angioedema.
• All anticonvulsants are potentially teratogenic and this is often dose related (see section: AED Prescribing - Pregnancy)

For a complete list of adverse effects, appropriate formularies should be consulted.

Dosing

The initiation and escalation dose depends upon age, weight, syndrome, seizure frequency and intensity, and side effect profile.
Unfortunately, a one dose regime does not fit all. A Paediatric Neurologist should be consulted if there is uncertainty.

Commonly used regime

• Initial dose (Infantile spasms): 25mg/kg bd. The dose may be increased by 25-50mg/kg/day at intervals (for example, every 2-3 days depending upon response). In the UKISS study, dosage was increased up to 150mg/kg/day.
• Initial dose (Adjunctive treatment for focal seizures): 10-20mg/kg/day given in two divided doses, then increase over 2-3 weeks to maintenance dose.
• Gradual escalation to maintenance dose depends on efficacy and tolerance. Aim is to achieve the lowest possible efficacious dose, particularly to avoid side-effect of peripheral field restriction.  Discussion with neurologist is suggested in usage for focal seizures, particularly above dosages of 40-50mg/kg/day. Regular visual field surveillance is mandatory.
• If the patient does not respond to Vigabatrin after a period of, for example, 3 months, withdrawal is suggested to avoid visual field abnormality.
• Vigabatrin can be taken with or without food
• Dosages per kilogram can only be used in children of weight approximately up to 30-40kgs. Consult appropriate formularies for higher weights and in the adult range.

These dosages are only a guideline and appropriate formularies should be consulted as needed.

Weaning

• When ceasing Vigabatrin it is recommended to reduce gradually (over a minimum of 2-4 weeks) to minimise the risk of increased seizure frequency or status epilepticus.

Preparations

• 500mg tablets
• 500mg sachets – dissolve in 10mL water.

Interactions | Precautions

Precautions

• Use with caution in patients with a history of behavioural problems, depression or psychosis.
• Vigabatrin may worsen absence and myoclonic seizures.
• Renal impairment: As Vigabatrin is renally excreted, consult formularies and neurology.

Pregnancy

• Usage in pregnancy needs to be discussed with a neurologist.
• There is limited data on the safety of Vigabatrin in pregnancy.
• All anticonvulsants are potentially teratogenic and this is often dose related (see section: AED Prescribing - Pregnancy)