Visual fields and tiagabine: a quandary

Research Status, Synthesis and Clinical Application of Antiepileptic Drugs

According to the 2017 ILAE's official definition, epilepsy is a slow brain disease state characterized by recurrent episodes. Due to information released by ILAE in 2017, it can be divided into four types, including focal epilepsy, generalized epilepsy, combined generalized, and focal epilepsy, and unknown epilepsy. Since 1989, 24 new antiepileptic drugs have been approved to treat different types of epilepsy. Besides, there are a variety of antiepileptic medications under clinical monitoring. These novel antiepileptic drugs have plenty of advantages. Over the past 33 years, there have been many antiepileptic drugs on the mearket, but no one has been found that can completely cure epilepsy. In this paper, the mentioned drugs were classified according to their targets, and the essential information, and clinical studies of each drug were described. The structure-activity relationship of different chemical structures was summarized. This paper provides help for the follow-up research on epilepsy drugs.

Ocular dysfunctions and toxicities induced by antiepileptic medications: Types, pathogenic mechanisms, and treatment strategies

INTRODUCTION

Ocular dysfunctions and toxicities induced by antiepileptic drugs (AEDs) are rarely reviewed and not frequently received attention by treating physicians compared to other adverse effects (e.g. endocrinologic, cognitive and metabolic). However, some are frequent and progressive even in therapeutic concentrations or result in permanent blindness. Although some adverse effects are non-specific, others are related to the specific pharmacodynamics of the drug. Areas covered: This review was written after detailed search in PubMed, EMBASE, ISI web, SciELO, Scopus, and Cochrane Central Register databases (from 1970 to 2019). It summarized the reported ophthalmologic adverse effects of the currently available AEDs; their risks and possible pathogenic mechanisms. They include ocular motility dysfunctions, retinopathy, maculopathy, glaucoma, myopia, optic neuropathy, and impaired retinal vascular autoregulation. In general, ophthalmo-neuro- or retino-toxic adverse effects of AEDs are classified as type A (dose-dependent), type B (host-dependent or idiosyncratic) or type C which is due to the cumulative effect from long-term use. Expert opinion: Ocular adverse effects of AEDs are rarely reviewed although some are frequent or may result in permanent blindness. Increasing knowledge of their incidence and improving understanding of their risks and pathogenic mechanisms are crucial for monitoring, prevention, and management of patients' at risk.

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.

Modelling the topography of absolute defects in patients exposed to the anti-epileptic drug vigabatrin and in normal subjects using automated static suprathreshold perimetry of the entire 80° visual field

AIM

(i) To map the spatial distribution of absolute visual field loss (AL) in patients exposed to vigabatrin and to compare the findings with AL in normal individuals, and (ii) to describe the relationship between the major risk factors for absolute loss (gender, age, cumulative dose), and the severity of field loss, expressed as the number of locations with AL per eye.

METHODS

Visual field plots were retrospectively reviewed from 428 individuals. Perimetry was done with the Tübingen Automated Perimeter (using a threshold-oriented, marginally supraliminal strategy, 80° eccentricity, 99 test locations). Three hundred and sixty-one individuals were ophthalmologically normal, and 67 were patients exposed to vigabatrin who had performed reliably during perimetry prior to neurosurgery for epilepsy. Two hundred and sixty-six of the 361 normals were included on the empirical basis that they manifested AL at no more than 19 stimulus locations per eye (the 74th percentile). The frequency per eye of AL at each of 99 stimulus locations was determined for the normals and patients. The effects of age, gender and cumulative dose of vigabatrin on the number of ALs per individual was assessed by an analysis of covariance. A stochastic model was developed to determine the 50% probability of AL at each location.

RESULTS

Thirty-nine of the 67 vigabatrin patients exhibited at least 20 locations with AL. The number of ALs was independent of age (p  =  0.7603). The frequency of AL was 14.8% in the peripheral field and 1.0% in the central field (odds ratio 16.7; 95% CI 15.0-18.6%; p  <  0.0001). Those exposed to vigabatrin exhibited a frequency of 17.1% ALs, compared to 5.2% for the normal individuals (odds ratio 3.77, 95% CI 3.6-4.0%; p  <  0.0001). According to the modelled data, males exhibited 20.9% (95% CI 1.3-44%; p  =  0.0360) more absolute losses than did females. The number of absolute losses per person doubled with an increase in cumulative dose of vigabatrin of 936 g (95% CI 775-1181 g).

CONCLUSIONS

According to this retrospective study, the spatial configuration of absolute defects attributable to vigabatrin indicates sparing of the temporal field up to approximately 60° eccentricity. Such a finding is likely to explain, at least in part, the initially asymptomatic nature of the defect. This study reconfirms a (cumulative) dose effect of vigabatrin on the extent of absolute field loss, with a greater risk for male gender.

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.

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.

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.

Adjunctive oxcarbazepine in comorbid anxiety and affective disorder with hyponatremic seizure: case analysis and literature review

Rational polypharmacy is required for management of refractory psychiatric disorders. Antiepileptic drugs are increasingly used as primary or adjunctive agents in the treatment of affective, schizoaffective, anxiety, and impulse control disorders. In this case report, the authors present the first case of hyponatremic seizure associated with oxcarbazepine in adjunctive treatment of comorbid affective/anxiety disorders.

Therapies with potential toxicity of neuro-ophthalmic interest

PURPOSE OF REVIEW

Several therapies have potential neuro-ophthalmic or ophthalmic complications. We reviewed the recent literature for relevant therapies that affect ocular function. In this review, we discuss a variety of toxic effects of antiepileptic drugs, endocrine drugs, and miscellaneous therapies (intravitreal silicone oil and TNF-alpha inhibitors).

RECENT FINDINGS

The pathogenesis of these side effects is quite heterogeneous affecting visual sensory function at the levels of the ciliary body, retina, optic nerve and chiasm, and central visual pathways.

SUMMARY

Appropriate management requires prompt recognition of these associations. Practitioners should be familiar with these neuro-ophthalmic manifestations because they may present in the course of daily practice.

Selection criteria for the clinical use of the newer antiepileptic drugs

In recent years, several new antiepileptic drugs (AEDs) have been licensed: felbamate, gabapentin, lamotrigine, levetiracetam, oxcarbazepine, tiagabine, topiramate, vigabatrin and zonisamide. These drugs have proven efficacy as add-on therapy in patients with difficult-to-treat partial epilepsy, as 20-50% of patients treated in add-on trials experienced a seizure reduction of >or=50%. Relatively few trials have been conducted to evaluate these drugs as monotherapy for patients with newly diagnosed epilepsy. In the monotherapy trials that have been conducted, the newer drugs were often as efficacious as conventional drugs, and their tolerability was often better. However, the methodology of these trials can be criticised. Because of the relative lack of robust data for the newer agents, the conventional drugs have thus far maintained their status as first-line monotherapy. However, when first-line monotherapy fails, an alternative drug has to be chosen from the available conventional and newer drugs. This article aims to give detailed background information on the newer AEDs in order to enable physicians to make a rational choice from the available drugs for individual patients. Data are provided for the different newer AEDs on mechanisms of action; efficacy in refractory partial epilepsy, newly diagnosed epilepsy in adults and generalised seizure types; adverse effects; pharmacokinetics; and use in special patient categories.

A controlled study comparing visual function in patients treated with vigabatrin and tiagabine

OBJECTIVE

Vigabatrin treatment is frequently associated with irreversible retinal injury and produces retinal electrophysiological changes in nearly all patients. Concern has been raised that tiagabine and other antiepilepsy drugs (AEDs) that increase brain gamma-aminobutyric acid (GABA) might produce similar electrophysiological and clinical changes in visual function. The study compared visual function between groups of patients with epilepsy treated long term with tiagabine, vigabatrin, and patients treated with other AEDs.

METHODS

A cross sectional study comparing visual acuity, colour vision, static and kinetic perimetry, and electroretinograms between groups of patients treated with tiagabine, vigabatrin, and other AEDs (control patients). Patients were adults receiving stable AED treatment for >6 months.

RESULTS

Vigabatrin treated patients had marked visual field constrictions in kinetic perimetry (mean radius 39.6 degrees OD, 40.5 degrees OS), while tiagabine patients had normal findings (mean 61 degrees OD, 62 degrees OS) (differences OD and OS, p=0.001), which were similar to epilepsy control patients (mean 60 degrees OD, 61 degrees OS). Vigabatrin patients had abnormal electroretinographic photopic B wave, oscillatory, and flicker responses, which correlated with visual field constrictions. These electroretinographic responses were normal for tiagabine patients and control patients. Patients were treated with vigabatrin for a median of 46 months compared with 29 months for tiagabine. Patients taking other AEDs that may change brain GABA had normal visual function.

CONCLUSION

Unlike vigabatrin, tiagabine treatment is associated with normal electroretinography and visual fields and ophthalmological function similar to epilepsy control patients. Differences between vigabatrin and other GABA modulating AEDs in retinal drug concentrations and other effects might explain why tiagabine increases in GABA reuptake do not cause retinal injury.