Understanding and interpreting vision safety issues with vigabatrin therapy

Vigabatrin is an irreversible inhibitor of γ-aminobutyric acid (GABA) transaminase. It is effective as adjunctive therapy for adult patients with refractory complex partial seizures (rCPS) who have inadequately responded to several alternative treatments and as monotherapy for children aged 1 month to 2 years with infantile spasms. The well-documented safety profile of vigabatrin includes risk of retinopathy characterized by irreversible, bilateral, concentric peripheral visual field constriction. Thus, monitoring of visual function to understand the occurrence and manage the potential consequences of peripheral visual field defects (pVFDs) is now required for all patients who receive vigabatrin. However, screening for pVFDs for patients with epilepsy was conducted only after the association between vigabatrin and pVFDs was established. We examined the potential association between pVFDs and epilepsy in vigabatrin-naïve patients and attempted to identify confounding factors (e.g., concomitant medications, method of vision assessment) to more accurately delineate the prevalence of pVFDs directly associated with vigabatrin. Results of a prospective cohort study as well as several case series and case reports suggest that bilateral visual field constriction is not restricted to patients exposed to vigabatrin but has also been detected, although much less frequently, in vigabatrin-naïve patients with epilepsy, including those who received treatment with other GABAergic antiepileptic therapy. We also reviewed published data suggesting an association between vigabatrin-associated retinal toxicity and taurine deficiency, as well as the potential role of taurine in the prevention of this retinopathy.

Is reduced ornithine-δ-aminotransferase activity the cause of vigabatrin-associated visual field defects?

BACKGROUND

A gabaergic antiepileptic drug, vigabatrin (VGB), is known to induce bilateral concentric visual field defects (VFD) in 30-40% of treated patients. Although the clinical and electrophysiological features of VFDs are well documented, the mechanism of retinal toxicity is still unclear.

PURPOSE

To determine if low basal ornithine-δ-aminotranspherase (OAT) activity is implicated in the etiology of VGB retinotoxicity, resulting in a phenotype of a mild form of gyrate atrophy.

METHODS

Assays of OAT activity in lymphocytes and GABA-transaminase activity in platelets were performed, and plasma levels of GABA, ornithine, lysine, glutamic acid and glutamine were measured, and visual fields were examined. A total of 47 subjects, aged 14-78 years, were examined. Twenty-one epileptic patients were off VGB more than 1 year; 11 patients with VGB-induced VFD and 10 with normal visual fields. Ten epileptic patients were on current VGB therapy more than 1 year; four patients with VGB-induced VFD and six with normal visual fields. The results were compared with those of 10 epilepsy patients taking tiagabine and six patients who suffered from gyrate atrophy (GA) or were obligate carriers of the disease.

RESULTS

In patients who had stopped VGB and who had VFDs, OAT activity was significantly reduced as compared with those who had normal visual fields (77.4pmol P5C/min/mgPro vs. 181.9pmol P5C/min/mgPro, p=0.002). In patients with ongoing VGB therapy, no difference was found between the patients with and without VFDs (149.4pmol P5C/min/mgPro vs. 159.1pmol P5C/min/mgPro).

CONCLUSIONS

: The results suggest that VGB retinotoxicity might be associated with elevated retinal ornithine mediated by low basal OAT activity.

Ocular Adverse Effects Associated with Systemic Medications

This article reviews several retrospective case series and reported adverse events regarding common ocular adverse effects related to systemic therapy. It is not intended as a comprehensive summary of these well described adverse drug reactions, nor is it intended to cover the complete spectrum of all ocular adverse effects of systemic therapy. Many systemic drugs may produce ocular toxicity, including bisphosphonates, topiramate, vigabatrin, isotretinoin and other retinoids, amiodarone, ethambutol, chloroquine and hydroxychloroquine, tamoxifen, quetiapine, cyclo-oxygenase (COX)-2 inhibitors, erectile dysfunction agents and some herbal medications. For this review, the certainty of the adverse effect profile of each medication was evaluated according to the WHO Causality Assessment Guide.A certain relationship has been established for pamidronate and alendronate as causes of scleritis, uveitis, conjunctivitis and blurred vision. Topiramate has been established as adversely causing symptoms consistent with acute angle-closure glaucoma, typically bilateral. Vigabatrin has been shown to cause bilateral irreversible visual field defects attributed to underlying medication-induced retinal pathology. Isotretinoin should be considered in the differential diagnosis of any patient with pseudotumour cerebri. Patients taking amiodarone and hydroxychloroquine should be monitored and screened regularly for development of optic neuropathy and maculopathy, respectively. Sildenafil has been reported to cause several changes in visual perception and is a possible, not yet certain, cause of anterior ischaemic optic neuropathy. Patients taking tamoxifen should also be monitored for development of dose-dependent maculopathy and decreased colour vision. COX-2 inhibitors should be included in the differential diagnosis of reversible conjunctivitis. Several herbal medications including canthaxanthine, chamomile, datura, Echinacea purpurea, Ginkgo biloba and liquorice have also been associated with several ocular adverse effects. It is the role of all healthcare professionals to detect, treat and educate the public about adverse reactions to medications as they are an important health problem.

Visual function in epilepsy patients treated with initial valproate monotherapy

PURPOSE

To investigate whether initial valproate (VPA) monotherapy for the treatment of epilepsy causes visual field defects and visual dysfunction.

METHODS

In a cross-sectional study, visual fields were examined with the kinetic Goldmann and automated Humphrey perimeters, contrast sensitivity function with the Pelli-Robson letter chart and colour vision with the Standard Pseudoisochromatic Plates Part 2 (SPP 2) and Farnsworth-Munsell 100 Hue test (FM 100) in eighteen epilepsy patients (aged 18--50 years, 30.2.+/-10 years, mean+/-S.D.) treated with initial valproate monotherapy for 2--20 years (8.4+/-5.1 years).

RESULTS

None had vigabatrin-type, concentric visual field defect with the kinetic Goldmann or automated Humphrey perimetries. In the Humphrey perimetry, the mean deviation for the group was within normal limits varying from -2.53 to 0.59 dB (-0.74+/-0.80 dB) in the right eye and from -2.66 to 0.67 dB (-0.78+/-0.82 dB) in the left eye. In the FM 100 test, acquired colour vision deficiency was found in two out of 18 patients (11%, 95% CI: 0--25%). However, the mean total error score was lower in the patient group than in the control group. All patients had normal contrast sensitivity function.

CONCLUSIONS

The use of VPA in the treatment of epilepsy is not associated with visual field defects similar to vigabatrin, but may induce abnormalities in colour vision.

Color vision and contrast sensitivity in epilepsy patients treated with initial tiagabine monotherapy

The purpose of the study was to determine whether the use of a GABAergic antiepileptic drug (AED), tiagabine, affects color vision and contrast sensitivity. Twenty newly diagnosed patients with partial epilepsy (aged 19-72 years), receiving tiagabine as their initial monotherapy for 5-41 months were examined. Color vision was examined with the Standard Pseudoisochromatic Plates 2 (SPP2), with the Farnsworth-Munsell 100 Hue Test (FM100) and with the Color Vision Meter 712 (CVM) anomaloscope. Contrast sensitivity was measured with the Pelli-Robson letter chart. Three patients excluded from the color vision evaluation for congenital red-green color vision defects. Seven out of 17 patients (41%) had acquired color vision deficit examined with the FM100. The CVM anomaloscope revealed minor defects in two patients. Contrast sensitivity function was within normal ranges. The present study suggests that AED therapy with tiagabine, like with other established and newer AEDs may interfere with color perception.

Progress report on new antiepileptic drugs: a summary of the Seventh Eilat Conference (EILAT VII)

The Seventh Eilat Conference on New Antiepileptic Drugs (AEDs) (EILAT VII) took place in Villasimius, Sardinia, Italy from the 9th to 13th May 2004. Basic scientists, clinical pharmacologists and neurologists from 24 countries attended the conference,whose main themes included advances in pathophysiology of drug resistance, new AEDs in pediatric epilepsy syndromes, modes of AED action and spectrum of adverse effects and a re-appraisal of comparative responses to AED combinations. Consistent with previous formats of this conference, the central part of the conference was devoted to a review of AEDs in development, as well as updates on second-generation AEDs. This article summarizes the information presented on drugs in development, including atipamezole, BIA-2-093, fluorofelbamate, NPS 1776, pregabalin, retigabine, safinamide, SPM 927, stiripentol, talampanel,ucb 34714 and valrocemide (TV 1901). Updates on felbamate, gabapentin, lamotrigine, levetiracetam, oxcarbazepine, tiagabine,topiramate, vigabatrin, zonisamide, new oral and parenteral formulations of valproic acid and SPM 927 and the antiepileptic vagal stimulator device are also presented.

The effect of antiepileptic drugs on visual performance

Visual disturbances are a common side-effect of many antiepileptic drugs. Non-specific retino- and neurotoxic visual abnormalities, that are often reported with over-dosage and prolonged AED use, include diplopia, blurred vision and nystagmus. Some anticonvulsants are associated with specific visual problems that may be related to the mechanistic properties of the drug, and occur even when the drugs are administered within the recommended daily dose. Vigabatrin, a GABA-transaminase inhibitor, has been associated with bilateral concentric visual field loss, electrophysiological changes, central visual function deficits including reduced contrast sensitivity and abnormal colour perception, and morphological alterations of the fundus and retina. Topiramate, a drug that enhances GABAergic transmission, has been associated with cases of acute closed angle glaucoma, while tiagabine, a GABA uptake inhibitor, has been investigated for a potential GABAergic effect on the visual field. Only mild neurotoxic effects have been identified for patients treated with gabapentin, a drug designed as a cyclic analogue of GABA but exhibiting an unknown mechanism while carbamazepine, an inhibitor of voltage-dependent sodium channels, has been linked with abnormal colour perception and reduced contrast sensitivity. The following review outlines the visual disturbances associated with some of the most commonly prescribed anticonvulsants. For each drug, the ocular site of potential damage and the likely mechanism responsible for the adverse visual effects is described.

Vigabatrin, but not gabapentin or topiramate, produces concentration-related effects on enzymes and intermediates of the GABA shunt in rat brain and retina

PURPOSE

The antiepileptic drug (AED) vigabatrin (VGB), which exerts its pharmacologic effects on the gamma-aminobutyric acid (GABA) system, causes concentric visual field constriction in >40% of exposed adults. This may be a class effect of all agents with GABA-related mechanisms of action. We compared the concentration-related effects of VGB in rat brain and eye with those of gabapentin (GBP) and topiramate (TPM), both of which have been reported to elevate brain GABA concentrations in humans.

METHODS

Adult male rats (n = 10) were administered 0.9% saline (control), VGB (250, 500, 1,000 mg/kg), GBP (50, 100, 200 mg/kg), or TPM (12.5, 25, 50, 100 mg/kg). At 2 h after dosing, animals were killed, a blood sample obtained, the brain dissected into eight distinct regions, and the retina and vitreous humor isolated from each eye. Samples were analyzed for several GABA-related neurochemical parameters, and serum and tissue drug concentrations determined.

RESULTS

VGB treatment produced a significant (p < 0.05) dose-related increase in GABA concentrations and decrease in GABA-transaminase activity in all tissues investigated. This effect was most pronounced in the retina, where VGB concentrations were 18.5-fold higher than those in brain. In contrast, GBP and TPM were without effect on any of the neurochemical parameters investigated and did not accumulate appreciably in the retina.

CONCLUSIONS

These findings corroborate a previously reported accumulation of VGB in the retina, which may be responsible for the visual field constriction observed clinically. This phenomenon does not appear to extend to other GABAergic drugs, suggesting that these agents might not cause visual field defects.

Progress report on new antiepileptic drugs: a summary of the Sixth Eilat Conference (EILAT VI)

The Sixth Eilat Conference on New Antiepileptic Drugs (AEDs) took place in Taormina, Sicily, Italy from 7th to 11th April, 2002. Basic scientists, clinical pharmacologists and neurologists from 27 countries attended the conference, whose main themes included dose-response relationships with conventional and recent AEDs, teratogenic effects of conventional and recent AEDs, update on clinical implications of AED metabolism, prevention of epileptogesis, and seizure aggravation by AEDs. According to tradition, the central part of the conference was devoted to a review of AEDs in development, as well to updates on AEDs, which have been marketed in recent years. This article summarizes the information presented on drugs in preclinical and clinical development, including carabersat (SB-204269), CGX-1007 (Conantokin-G), pregabalin, retigabine (D-23129), safinamide, SPD421 (DP-VPA), SPM 927, talampanel and valrocemide (TV 1901). Updates on fosphenytoin, gabapentin, lamotrigine, levetiracetam, oxcarbazepine, tiagabine, topiramate, vigabatrin, zonisamide, new formulations of valproic acid, and the antiepileptic vagal stimulator device are also presented.