Vigabatrin treatment in children

Effect of vigabatrin on seizure control and safety profile in different subgroups of children with epilepsy

OBJECTIVE

To evaluate the efficacy and safety of vigabatrin in pediatric epilepsy.

METHODS

We retrospectively reviewed patients with epilepsy treated with vigabatrin over a 2-year period at a pediatric tertiary center. We assessed the relationship between seizure frequency, etiology, vigabatrin dose, adverse events, medication discontinuation reasons, and electroencephalography (EEG) characteristics.

RESULTS

One hundred three patients followed at Boston Children's Hospital were treated with vigabatrin and had complete medical records. Within the follow-up interval, 69 (67%) of 103 patients had discontinued vigabatrin therapy. Two patients (1.9%) died during therapy for unknown reasons. Median age at vigabatrin initiation was 8 months (interquartile range [IQR] 5-15). Median starting dose was 48.1 mg/kg per day (IQR 29.8-52.3) with a median target of 100 mg/kg (IQR 81.9-107.9). Median treatment duration was 12.1 months (n = 89, IQR 5.0-22.9) overall, and 13.3 months (IQR 5.2-23.2) for patients who discontinued vigabatrin. The most common reasons for discontinuation were controlled seizures in 31 (43.7%) of 71 and unsatisfactory therapeutic effect in 23 (32.4%) of 71. Median percent seizure reduction from baseline to first follow-up was 83.3% (IQR 27.4-99.8) and 96.7% (IQR 43.3-100) to last follow-up. Twenty-four (38.7%) of 62 patients with a follow-up posttreatment remained seizure-free. Four patients who had initially achieved seizure freedom relapsed. Patients with structural/metabolic etiology had greater median percent seizure reduction at first follow-up than patients with genetic etiology (98.7% vs. 61.4%, respectively, p = 0.001). Hypsarrhythmia resolved after therapy in 18 of 20 (90%, 95% confidence interval [CI] 70-97) patients with pretreatment hypsarrhythmia, and 2 patients presented with hypsarrhythmia posttreatment. Risk of having hypsarrhythmia was reduced by 32% (95% CI 14.9-49.1) posttreatment.

SIGNIFICANCE

Vigabatrin is efficacious in all seizure types and resolved hypsarrhythmia in most patients. In this series with a median treatment duration of 12.1 months, vigabatrin had a good safety profile with a low rate of discontinuation due to nonophthalmologic and ophthalmologic adverse effects.

Changes in the electroretinogram resulting from discontinuation of vigabatrin in children

Electroretinograms (ERGs) have been recorded longitudinally in children before and during treatment with the antiepileptic drug vigabatrin for the past 3.5 years. Vigabatrin induced changes in ERG responses occur in children; the most dramatic changes occur in the oscillatory potentials. The purpose of this study was to identify changes in ERG responses associated with discontinuation of vigabatrin treatment. If vigabatrin-induced changes reverse after discontinuation of the drug we infer that the original change is not an indicator of toxicity. ERG data were analyzed from 17 children who discontinued vigabatrin therapy. The duration of treatment ranged from 5 to 52 months, the age for the first ERG ranged from 6 to 38 months (median 10 months). ERGs were tested using the standard protocol established by the International Society for Clinical Electrophysiology of Vision, with Burian-Allen bipolar contact-lens electrodes. In addition to standard responses we recorded photopic oscillatory potentials (OPs). During vigabatrin treatment OPs show a greater change than other ERG responses, with the early occurring wavelets from the photopic OPs showing the greatest change. With discontinuation of vigabatrin the amplitude of the early wavelets of the photopic OPs increased dramatically compared with amplitudes while taking the drug (paired t-test, p = 0.000075). The scotopic oscillatory potentials also show some recovery. Although changes in oscillatory potentials may occur with vigabatrin toxicity, a large change likely occurs with a non-toxic pharmacological effect of vigabatrin on GABAergic amacrine cells in the inner plexiform layer. Reduction of OPs in children on vigabatrin may not be related to toxicity.

The Hospital for Sick Children, Toronto, Longitudinal ERG study of children on vigabatrin

The purpose of this longitudinal study was to identify changes in ERG responses associated with vigabatrin treatment. We accomplished this by recording longitudinally ERGs in children before and during vigabatrin treatment and comparing results between children on vigabatrin monotherapy and those taking additional anticonvulsive medications. Thirty-three children on vigabatrin therapy were tested; the duration between visits was approximately 6 months. Thirteen children were assessed initially before starting vigabatrin therapy and seven were assessed soon after (age range 1.5-126 months, median 6 months). The remaining 13 patients were already on vigabatrin at the time of initial visit (age range 6.5-180 months, median 16 months). ERGs were tested using the standard protocol established by the International Society for Clinical Electrophysiology of Vision, with Burian-Allen bipolar contact-lens electrodes. In addition to standard responses we recorded photopic oscillatory potentials (OPs). All 33 patients were tested longitudinally on at least two occasions and 11 were tested on three occasions. For children whose only anticonvulsive drug was vigabatrin there was a significant curvature (quadratic function, p < 0.05) of the predicted cone b-wave amplitude with time; exhibited as increase in b-wave amplitude followed by subsequent decrease. Descriptive data demonstrated the same pattern in the group taking anticonvulsive medications in addition to vigabatrin. In most children the flicker amplitude declined between 6 months and 1 year of vigabatrin treatment. Our data demonstrated that rod responses, which may be abnormal before initiation of vigabatrin, did not change substantially with vigabatrin treatment.

Vigabatrin: a novel therapy for seizure disorders

OBJECTIVE

To review the pharmacology, pharmacokinetics, efficacy, and adverse effects of vigabatrin and its role in the management of seizure disorders.

METHODS

A MEDLINE search of English-language literature from January 1993 through January 1999 was conducted using vigabatrin as a search term to identify pertinent studies and review articles. Additional studies were identified from the bibliographies of reviewed literature. The manufacturer provided postmarketing surveillance data. Priority was given to randomized, double-blind, placebo-controlled studies.

FINDINGS

Vigabatrin is a selective and irreversible inhibitor of gamma-aminobutyric acid transaminase. In controlled clinical trials of vigabatrin add-on therapy in patients with uncontrolled partial seizures, 24-67% of patients achieved a < or =50% reduction in seizure frequency. Data from two comparative trials with carbamazepine monotherapy indicate that vigabatrin monotherapy reduces the frequency of partial seizures in patients with newly diagnosed epilepsy. Vigabatrin also controls infantile spasms, particularly those associated with tuberous sclerosis. Vigabatrin is more effective in patients with partial seizures than in those with generalized seizures. The drug is generally well tolerated. Headache and drowsiness were the most common adverse effects observed in controlled clinical trials; visual field defects, psychiatric reactions, and hyperactivity also have been reported. There are no known clinically significant drug interactions.

CONCLUSIONS

Vigabatrin improves seizure control as add-on therapy for refractory partial seizures and may produce therapeutic benefits in the treatment of infantile spasms. Vigabatrin is generally well tolerated, with a convenient administration schedule, a lack of known significant drug interactions, and no need for routine monitoring of plasma concentrations.

Vigabatrin in refractory childhood epilepsy. The Brazilian Multicenter Study

Children, 47, with various types of severe drug-resistant epilepsy were entered into a prospective, add-on, open trial with vigabatrin. Patients with West syndrome and idiopathic generalized epilepsies were excluded. Seven children had the drug withdrawn, five because of increase in seizure frequency and two because of adverse effects. Drug efficacy, measured according to seizure type, showed a 100% decrease in seizure frequency in 18.6% of partial seizures and 17.3% of the generalized seizures. There was a higher than 50% decrease in 39.5% of partial and 60.8% of generalized seizures, and less than 50% decrease or increase in seizure frequency in 41.8% and 21.8% of partial and generalized seizures, respectively. Vigabatrin mean dosage during phase 3 was 63.6 mg/kg per day (S.D. = 30.5), ranging from 19.3 to 110.5 mg/kg per day. Parametric statistical analysis (Student's t-test) of seizure frequency between phases 1 and 3 showed a significant decrease in seizure frequency for partial (P = 0.022), and generalized seizures (P < 0.0001). Drug-related adverse effects were observed in 18/47 cases (38.3%), consisting mainly of irritability, hyperactivity, dizziness, somnolence and gastrointestinal symptoms.

Vigabatrin

Vigabatrin is a structural analogue of gamma amino butyric acid (GABA), which binds irreversibly to GABA-transaminase causing increased brain levels of GABA. It is an important advance in the medical management of children with epilepsy. It appears to be particularly effective in the treatment of infantile spasms, especially when caused by tuberous sclerosis. It is also effective in the treatment of partial seizures and some generalized seizures including those of the Lennox-Gastaut syndrome. However, myoclonic seizures may be made worse by vigabatrin. It is not yet approved for use in the United States but it is approved throughout most of the rest of the world including Canada and Mexico. Release in the United States is expected in the near future.