Characterization of vigabatrin-associated optic atrophy

Head Turn During Visual Field Testing to Minimize the Influence of Prominent Facial Anatomy

BACKGROUND

Facial contour naturally decreases the visual field. Peripheral visual field defects caused by facial anatomy and ocular pathology can be missed in a routine standard of care. Mathematically calculating the true angle for turning the head to optimize the peripheral visual field has not been studied to date. The purpose of this study was to explore the utility of turning the head during perimetry to maximize the testable visual field.

METHODS

Six healthy study participants aged 18-52 were enrolled, prospectively; the dominant eye of each participant was tested. In total, 60-4 visual fields were obtained from each participant's dominant eye with the head in primary position. Then, the 60-4 tests were repeated with the head turned prescribed degrees toward and away from the tested eye ("manual method"). Based on a photograph of the participant's face, a convolutional neural network (CNN) was used to predict the optimal head turn angle for maximizing the field, and the test was repeated in this position ("automated method").

RESULTS

Maximal visual field exposure was found at a head turn of 15° away from the tested eye using the manual method and was found at an average head turn of 12.6° using the automated method; maximum threshold values were similar between manual and automated methods. The mean of threshold in these subjects at the standard direction and the predicted optimum direction was 1,302, SD = 69.35, and 1,404, SD = 67.37, respectively ( P = 0.02).

CONCLUSIONS

Turning the head during perimetry maximizes the testable field area by minimizing the influence of prominent facial anatomy. In addition, our CNN can accurately predict each individual's optimal angle of head turn for maximizing the visual field.

Ocular examinations, findings, and toxicity in children taking vigabatrin

BACKGROUND

The antiepileptic medication vigabatrin has been associated with ocular toxicity, and close ophthalmic monitoring has been recommended; however, there is no clear consensus regarding the value and feasibility of such monitoring in children. We describe ophthalmic assessments in children in a real-world clinical setting, the incidence of vigabatrin-related ocular toxicity, and the utility of regular screening or ancillary testing in children taking vigabatrin.

METHODS

The medical records of children taking vigabatrin with one or more ophthalmic assessments at Children's Hospital of Philadelphia or University of California, San Francisco, between May 2010 and May 2021, were reviewed retrospectively. Abnormalities on ophthalmic examination, visual field (VF), electroretinogram (ERG), and optical coherence tomography (OCT) were reviewed and categorized as attributable to vigabatrin, possibly attributable to vigabatrin, or not attributable to vigabatrin.

RESULTS

A total of 1,281 assessments of 284 children (mean age, 2.09 years) were included. Of these, 283 (99.6%) had funduscopic examination(s), 37 (13.0%) had ERG, 19 (6.7%) had OCT, and 6 (2.1%) had formal VF. Rate of examinations and ERGs per child decreased over the 10-year study period. Two children (0.7%) had definite vigabatrin-related ocular toxicity, both identified on ERG. An additional 4 children (1.4%) had optic atrophy of unclear relation to vigabatrin, categorized as possible toxicity. The remaining 278 children did not have abnormal examination or testing findings attributable to vigabatrin.

CONCLUSIONS

The incidence of vigabatrin-related ocular toxicity in children was low in our cohort. Ocular and neurologic comorbidities and limited examinations in children make identification of such toxicity challenging and the value of screening is unclear.

β-alanine supplementation induces taurine depletion and causes alterations of the retinal nerve fiber layer and axonal transport by retinal ganglion cells

To study the effect of taurine depletion induced by β-alanine supplementation in the retinal nerve fiber layer (RNFL), and retinal ganglion cell (RGC) survival and axonal transport. Albino Sprague-Dawley rats were divided into two groups: one group received β-alanine supplementation (3%) in the drinking water during 2 months to induce taurine depletion, and the other group received regular water. After one month, half of the rats from each group were exposed to light. Retinas were analyzed in-vivo using Spectral-Domain Optical Coherence Tomography (SD-OCT). Prior to processing, RGCs were retrogradely traced with fluorogold (FG) applied to both superior colliculi, to assess the state of their retrograde axonal transport. Retinas were dissected as wholemounts, surviving RGCs were immunoidentified with Brn3a, and the RNFL with phosphorylated high-molecular-weight subunit of the neurofilament triplet (pNFH) antibodies. β-alanine supplementation decreases significantly taurine plasma levels and causes a significant reduction of the RNFL thickness that is increased after light exposure. An abnormal pNFH immunoreactivity in some RGC bodies, their proximal dendrites and axons, and a further diminution of the mean number of FG-traced RGCs compared with Brn3a⁺RGCs, indicate that their retrograde axonal transport is affected. In conclusion, taurine depletion causes RGC loss and axonal transport impairment. Finally, our results suggest that care should be taken when ingesting β-alanine supplements due to the limited understanding of their potential adverse effects.

Objective Derivation of the Morphology and Staging of Visual Field Loss Associated with Long-Term Vigabatrin Therapy

BACKGROUND

The morphology and between-eye symmetry of the visual field loss associated with the antiepileptic drug vigabatrin (VAVFL) has received little attention.

OBJECTIVE

Our objective was to model the appearance and ensuing staging of VAVFL derived with the European Medicines Agency-approved perimetric protocol.

METHODS

This was a retrospective, cross-sectional, observational study that identified 123 adults who had received vigabatrin for refractory seizures and who had no evidence of co-existing retino-geniculo-cortical visual pathway abnormality. A further 38 adults with refractory seizures and identical inclusion criteria but no exposure to vigabatrin acted as controls. For each group, the median outcome at each stimulus location in each eye (of absolute loss, relative loss or Pattern Deviation probability level, as appropriate) was derived for each successive ten pairs of fields, ranked for severity. Between-eye symmetry was quantified by an index that accounted for severity of loss and that was referenced to the likelihood of the occurrence of symmetry due to chance.

RESULTS

The modelled VAVFL was bilateral and highly symmetrical and was described by six stages that were all independent of the extent of vigabatrin exposure. The loss originated in the extreme temporal periphery and encroached centripetally along all meridians towards fixation. The initial appearance within the central field (Stage 2) occurred inferior-nasally. Subsequent stages exhibited increasing loss, which was greater nasally than temporally. Stage 6 described concentric loss extending to approximately 15° eccentricity from fixation.

CONCLUSION

The model exhibited a consistent pattern of VAVFL. The staging of the loss could assist the risk:benefit analysis of vigabatrin for the treatment of epilepsy.

Neuro-ophthalmological manifestations of tuberous sclerosis: current perspectives

Tuberous sclerosis complex (TSC) is a complex, multi-system disorder with a well-described underlying genetic etiology. While retinal findings are common in TSC and important in establishing the diagnosis, TSC also has many potential neuro-ophthalmology manifestations. The neuro-ophthalmology manifestations of TSC can have a significant impact on visual function and are sometimes a sign of serious neurological disease. The purpose of this review is to describe the neuro-ophthalmological manifestations of TSC. These manifestations include optic nerve hamartomas, elevated intracranial pressure, cranial nerve palsies, cortical visual impairment, visual field deficits, and ocular toxicity from vigabatrin treatment of infantile spasms. It is important to be aware of potential neuro-ophthalmological manifestations in these patients in order to detect signs of vision- or life-threatening disease and to optimize visual function and quality-of-life.

The role of optical coherence tomography in therapeutics and conditions, which primarily have systemic manifestations: a narrative review

Optical coherence tomography is designed to evaluate in vivo qualitative and quantitative changes of the anterior segment, optic nerve and the retina. Initial applications of this technology were confined mainly to ophthalmic diseases. However recently, numerous studies have evaluated its use in systemic conditions and in therapeutics where, optic nerve and retinal architecture can be assessed to monitor progression of systemic conditions and its response to treatment. This is a narrative review aimed at evaluating the debate surrounding the role of spectral domain optical coherence tomography, in systemic conditions where optic nerve affection can be measured and be used in the diagnosis, monitoring and assessment of treatment effect as a non-invasive, quick, novel technique.

The Topographical Relationship between Visual Field Loss and Peripapillary Retinal Nerve Fibre Layer Thinning Arising from Long-Term Exposure to Vigabatrin

BACKGROUND

The antiepileptic drug vigabatrin is associated with characteristic visual field loss (VAVFL) and thinning of the peripapillary retinal nerve fibre layer (PPRNFL); however, the relationship is equivocal.

OBJECTIVE

The aim of this study was to determine the function-structure relationship associated with long-term exposure to vigabatrin, thereby improving the risk/benefit analysis of the drug.

METHODS

A cross-sectional observational design identified 40 adults who had received long-term vigabatrin for refractory seizures, who had no evidence of co-existing retino-geniculo-cortical visual pathway abnormality, and who had undergone a standardized protocol of perimetry and of optical coherence tomography (OCT) of the PPRNFL. Vigabatrin toxicity was defined as the presence of VAVFL. The function-structure relationship for the superior and inferior retinal quadrants was evaluated by two established models applicable to other optic neuropathies.

RESULTS

The function-structure relationship for each model was consistent with an optic neuropathy. PPRNFL thinning, expressed in micrometres, asymptoted at an equivalent visual field loss of worse than approximately - 10.0 dB, thereby preventing assessment of more substantial thinning. Transformation of the outcomes to retinal ganglion cell soma and axon estimates, respectively, resulted in a linear relationship.

CONCLUSIONS

Functional and structural abnormality is strongly related in individuals with vigabatrin toxicity and no evidence of visual pathway comorbidity, thereby implicating retinal ganglion cell dysfunction. OCT affords a limited measurement range compared with perimetry: severity cannot be directly assessed when the PPRNFL quadrant thickness is less than approximately 65 µm, depending on the tomographer. This limitation can be overcome by transformation of thickness to remaining axons, an outcome requiring input from perimetry.

Increasing GABA reverses age-related alterations in excitatory receptive fields and intensity coding of auditory midbrain neurons in aged mice

A key feature of age-related hearing loss is a reduction in the expression of inhibitory neurotransmitters in the central auditory system. This loss is partially responsible for changes in central auditory processing, as inhibitory receptive fields play a critical role in shaping neural responses to sound stimuli. Vigabatrin (VGB), an antiepileptic agent that irreversibly inhibits γ-amino butyric acid (GABA) transaminase, leads to increased availability of GABA throughout the brain. This study used multi-channel electrophysiology measurements to assess the excitatory frequency response areas in old CBA mice to which VGB had been administered. We found a significant post-VGB reduction in the proportion of V-type shapes, and an increase in primary-like excitatory frequency response areas. There was also a significant increase in the mean maximum driven spike rates across the tonotopic frequency range of all treated animals, consistent with observations that GABA buildup within the central auditory system increases spike counts of neural receptive fields. This increased spiking is also seen in the rate-level functions and seems to explain the improved low-frequency thresholds.

Optical coherence tomography to monitor vigabatrin toxicity in children

PURPOSE

The antiepileptic drug vigabatrin is known to cause permanent loss of vision. Both visual field testing and electroretinogram are used to detect retinal damage. Adult data on optical coherence tomography (OCT) shows that retinal nerve fiber layer (RNFL) thinning may be an early indicator of vigabatrin-induced retinal toxicity. The purpose of this study was to investigate whether OCT can detect early vigabatrin-induced retinal toxicity in children.

METHODS

Pediatric patients (≤18 years of age) requiring vigabatrin for seizure control who were followed at our institution were invited to participate. Patients were examined according to manufacturer guidelines, with most examinations taking place under general anesthesia. RNFL thickness was measured by OCT (Stratus Model 3000, Zeiss) and compared to total cumulative dose of vigabatrin. In most cases, indirect ophthalmoscopy, fundus photography, and electroretinography were also performed.

RESULTS

OCT and complete dosing data was available for 19 patients. Patients with tuberous sclerosis (TS, n = 12) received higher cumulative doses (mean, 1463 g) than non-TS patients (mean, 351 g, P = 0.044). RNFL thinning was detected in the nasal (P < 0.01), superior (P < 0.01), and inferior (P < 0.05) quadrants in patients with TS, particularly once cumulative dose exceeded 1500 g.

CONCLUSIONS

In our study population of patients with TS, higher cumulative doses of vigabatrin were associated with RNFL thinning in the nasal, superior, and inferior quadrants. These findings were pronounced once cumulative dose exceeded 1500 g. This pattern of RNFL thinning is similar to what has been shown in adult patients taking vigabatrin.

Taurine: the comeback of a neutraceutical in the prevention of retinal degenerations

Taurine is the most abundant amino acid in the retina. In the 1970s, it was thought to be involved in retinal diseases with photoreceptor degeneration, because cats on a taurine-free diet presented photoreceptor loss. However, with the exception of its introduction into baby milk and parenteral nutrition, taurine has not yet been incorporated into any commercial treatment with the aim of slowing photoreceptor degeneration. Our recent discovery that taurine depletion is involved in the retinal toxicity of the antiepileptic drug vigabatrin has returned taurine to the limelight in the field of neuroprotection. However, although the retinal toxicity of vigabatrin principally involves a deleterious effect on photoreceptors, retinal ganglion cells (RGCs) are also affected. These findings led us to investigate the possible role of taurine depletion in retinal diseases with RGC degeneration, such as glaucoma and diabetic retinopathy. The major antioxidant properties of taurine may influence disease processes. In addition, the efficacy of taurine is dependent on its uptake into retinal cells, microvascular endothelial cells and the retinal pigment epithelium. Disturbances of retinal vascular perfusion in these retinal diseases may therefore affect the retinal uptake of taurine, resulting in local depletion. The low plasma taurine concentrations observed in diabetic patients may further enhance such local decreases in taurine concentration. We here review the evidence for a role of taurine in retinal ganglion cell survival and studies suggesting that this compound may be involved in the pathophysiology of glaucoma or diabetic retinopathy. Along with other antioxidant molecules, taurine should therefore be seriously reconsidered as a potential treatment for such retinal diseases.

Attenuation of the retinal nerve fibre layer and reduced retinal function assessed by optical coherence tomography and full-field electroretinography in patients exposed to vigabatrin medication

PURPOSE

To investigate the clinical value of assessment of peripapillary retinal nerve fibre layer (RNFL) thickness with OCT in addition to the evaluation of retinal function measured by full-field electroretinography (ff-ERG) in patients with suspected vigabatrin (VGB)-attributed visual field defects.

METHODS

Visual fields from adult patients in our clinical follow-up program for VGB medication were analysed. Twelve patients with suspected VGB-attributed visual field defects were selected for the study. They were re-examined with computerized kinetic perimetry, ff-ERG and OCT (2D circle scan).

RESULTS

Constricted visual fields were found in all patients. Comparative analysis of ff-ERG parameters showed reduced b-wave amplitudes for the isolated and the combined rod and cone responses (p < 0.0001). The a-wave, reflecting photoreceptor activity, was reduced (p = 0.001), as well as the summed amplitude of oscillatory potentials (p = 0.029), corresponding to inner retinal function. OCT measurements demonstrated attenuation of the RNFL in nine of 12 patients, most frequently superiorly and/or inferiorly. No temporal attenuation was found. Significant positive correlations were found between the total averaged RNFL thickness, superior and inferior RNFL thickness and reduced ff-ERG parameters. Positive correlations were also found between RNFL thickness and isopter areas.

CONCLUSION

OCT measurements can detect attenuation of the RNFL in patients exposed to VGB medication. RNFL thickness correlates with reduced ff-ERG parameters and isopter areas of constricted visual fields, indicating that VGB is retino-toxic on several levels, from photoreceptors to ganglion cells. The study also supports previous studies, suggesting that OCT measurement of the RNFL thickness may be of clinical value in monitoring patients on vigabatrin therapy.

Modelling the risk of visual field loss arising from long-term exposure to the antiepileptic drug vigabatrin: a cross-sectional approach

BACKGROUND

The antiepileptic drug vigabatrin has been used widely since 1989, but has only been approved for use in the US since 2009. The risk:benefit of vigabatrin is generally predicated upon an assumed frequency of associated visual field loss (VAVFL) of approximately 31 %. This estimate is based upon relatively short-term usage (up to 4-5 years) and it is essential to determine whether the frequency of VAVFL increases with longer-term usage.

OBJECTIVE

The aim of this study was to model, from cross-sectional evidence, over greater ranges of treatment duration and cumulative dose than previously evaluated, the risk (frequency) of VAVFL with increasing exposure to vigabatrin.

STUDY DESIGN AND SETTING

This was a retrospective cohort study undertaken in a regional hospital epilepsy clinic.

PATIENTS

The cohort comprised 147 consecutive patients treated with vigabatrin for refractory complex partial (focal) seizures, who had all undergone ophthalmological examination and who had undertaken perimetry, reliably, according to a standard and robust protocol. The visual field plots were evaluated masked to treatment duration and dose.

MAIN OUTCOME MEASURE

The risk (frequency) of VAVFL with increasing exposure to vigabatrin was modelled, from the cross-sectional evidence, by standard and plateau logistic regression.

RESULTS

The cohort comprised 80 females and 67 males (mean age 40.3 years, standard deviation 13.7). The median duration of vigabatrin exposure was 7.9 years (interquartile range 3.6-11.0, range 0.2-16.1 years); 46 patients (31 %) had received vigabatrin for over 10 years. Eighty-seven patients (59 %) exhibited VAVFL; the proportion with VAVFL was higher in males (66 %) than females (54 %). The plateau model for duration and for cumulative dose exhibited a better fit than the standard model (both p < 0.001). The modelled frequency of VAVFL increased with increasing exposure up to approximately 6 years duration and 5 kg cumulative dose, and plateaued at approximately 76 % (95 % CI 67-85) and 79 % (95 % CI 70-87), respectively. Severity of VAVFL, classified in terms of the visual field index Mean Deviation, was not significantly associated with either duration or cumulative dose of therapy.

CONCLUSION

Clinicians and patients, in enabling informed choice, should be alert to the possible substantial increased risk:benefit for VAVFL with increasing long-term exposure to vigabatrin and the ensuing increased cost:benefit resulting from the necessary additional visual assessments.

Vigabatrin prevents seizure in swine subjected to hyperbaric hyperoxia

Oxygen is the most widely used therapeutic strategy to prevent and treat decompression sickness (DCS). Oxygen prebreathe (OPB) eliminated DCS in 20-kg swine after rapid decompression from saturation at 60 feet of seawater (fsw). However, hyperbaric oxygen (HBO) has risks. As oxygen partial pressure increases, so do its toxic effects. Central nervous system (CNS) oxygen toxicity is the most severe side effect, manifesting as seizure. An adjunctive therapeutic is needed to extend OPB strategies to deeper depths and prevent/delay seizure onset. The Food and Drug Administration-approved anti-epileptic vigabatrin has prevented HBO-induced seizures in rats up to 132 fsw. This study aimed to confirm the rat findings in a higher animal model and determine whether acute high-dose vigabatrin evokes retinotoxicity symptoms seen with chronic use clinically in humans. Vigabatrin dose escalation studies were conducted 20-kg swine exposed to HBO at 132 or 165 fsw. The saline group had seizure latencies of 7 and 11 min at 165 and 132 fsw, respectively. Vigabatrin at 180 mg/kg significantly increased latency (13 and 27 min at 165 and 132 fsw, respectively); 250 mg/kg abolished seizure activity at all depths. Functional electroretinogram and histology of the retinas showed no signs of retinal toxicity in any of the vigabatrin=treated animals. In the 250 mg/kg group there was no evidence of CNS oxygen toxicity; however, pulmonary oxygen toxicity limited HBO exposure. Together, the findings from this study show that vigabatrin therapy is efficacious at preventing CNS oxygen toxicity in swine, and a single dose is not acutely associated with retinotoxicity.

Evaluation of the taurine concentrations in dog plasma and aqueous humour: a pilot study

In the 70s, the amino acid taurine was found essential for photoreceptor survival. Recently, we found that taurine depletion can also trigger retinal ganglion cell degeneration both in vitro and in vivo. Therefore, evaluation of taurine levels could be a crucial biomarker for different pathologies of retinal ganglion cells such as glaucoma. Because different breeds of dog can develop glaucoma, we performed taurine measurements on plasma and aqueous humour samples from pet dogs. Here, we exposed results from a pilot study on normal selected breed of pet dogs, without any ocular pathology. Samples were collected by veterinarians who belong to the Réseau Européen d'Ophtalmologie Vétérinaire et de Vision Animale. Following measurements by high-performance liquid chromatography (HPLC), the averaged taurine concentration was 162.3 μM in the plasma and 51.8 μM in the aqueous humour. No correlation was observed between these two taurine concentrations, which exhibited a ratio close to 3. Further studies will determine if these taurine concentrations are changed in glaucomatous dogs.

Taurine provides neuroprotection against retinal ganglion cell degeneration

Retinal ganglion cell (RGC) degeneration occurs in numerous retinal diseases leading to blindness, either as a primary process like in glaucoma, or secondary to photoreceptor loss. However, no commercial drug is yet directly targeting RGCs for their neuroprotection. In the 70s, taurine, a small sulfonic acid provided by nutrition, was found to be essential for the survival of photoreceptors, but this dependence was not related to any retinal disease. More recently, taurine deprivation was incriminated in the retinal toxicity of an antiepileptic drug. We demonstrate here that taurine can improve RGC survival in culture or in different animal models of RGC degeneration. Taurine effect on RGC survival was assessed in vitro on primary pure RCG cultures under serum-deprivation conditions, and on NMDA-treated retinal explants from adult rats. In vivo, taurine was administered through the drinking water in two glaucomatous animal models (DBA/2J mice and rats with vein occlusion) and in a model of Retinitis pigmentosa with secondary RGC degeneration (P23H rats). After a 6-day incubation, 1 mM taurine significantly enhanced RGCs survival (+68%), whereas control RGCs were cultured in a taurine-free medium, containing all natural amino-acids. This effect was found to rely on taurine-uptake by RGCs. Furthermore taurine (1 mM) partly prevented NMDA-induced RGC excitotoxicity. Finally, taurine supplementation increased RGC densities both in DBA/2J mice, in rats with vein occlusion and in P23H rats by contrast to controls drinking taurine-free water. This study indicates that enriched taurine nutrition can directly promote RGC survival through RGC intracellular pathways. It provides evidence that taurine can positively interfere with retinal degenerative diseases.

Patterns of peripapillary retinal nerve fiber layer thinning in vigabatrin-exposed individuals

PURPOSE

To explore the relationship of peripapillary retinal nerve fiber layer (ppRNFL) thinning in individuals exposed to the antiepileptic drug vigabatrin with respect to 2 separate variables: cumulative vigabatrin exposure and severity of vigabatrin-associated visual field loss (VAVFL).

DESIGN

Cross-sectional observational study.

PARTICIPANTS

Subjects were older than 18 years, 129 with vigabatrin-treated epilepsy (vigabatrin-exposed group) and 87 individuals with epilepsy never treated with vigabatrin (nonexposed group).

METHODS

All subjects underwent ppRNFL imaging using spectral-domain optical coherence tomography. Eighty-four vigabatrin-exposed individuals underwent Goldmann kinetic perimetry. The visual field examined from the right eye was categorized as normal (n = 47), mildly abnormal (n = 18), or moderately to severely abnormal (n = 19). In 91 vigabatrin-exposed individuals, the cumulative vigabatrin exposure could be ascertained: 41 subjects received 1000 g or less, 23 subjects received more than 1000 g but equal to or less than 2500 g, 16 subjects received more than 2500 g but equal to or less than 5000 g or less, and 11 subjects received more than 5000 g.

MAIN OUTCOME MEASURES

Differences in ppRNFL thickness across the twelve 30° sectors: (1) among all nonexposed individuals and all vigabatrin-exposed individuals, (2) between each vigabatrin-exposed group, according to cumulative vigabatrin exposure, and the nonexposed group, (3) among different vigabatrin-exposed subjects grouped according to cumulative vigabatrin exposure, and (4) among vigabatrin-exposed subjects grouped according to severity of VAVFL.

RESULTS

The ppRNFL was significantly thinner in vigabatrin-exposed compared with nonexposed individuals in most 30° sectors (P<0.004). The temporal, temporal superior, and temporal inferior 30° sectors, as well as the nasal 30° sector, were not affected. There was a trend for increasing ppRNFL thinning with increasing cumulative vigabatrin exposure. The nasal-superior 30° sector was significantly thinner in group 1 (≤1000 g) compared with nonexposed individuals (P<0.05) and in vigabatrin-exposed individuals with normal visual fields compared with nonexposed individuals (P<0.05).

CONCLUSIONS

After vigabatrin exposure in individuals receiving cumulative doses of 1000 g or less or in the presence of normal visual fields, ppRNFL thinning in the nasal superior 30° sector may occur. With higher cumulative doses of vigabatrin exposure, additional ppRNFL thinning was observed. The temporal aspects of the ppRNFL are spared, even in individuals with large cumulative vigabatrin exposures and moderate or severe VAVFL.

Primer on visual field testing, electroretinography, and other visual assessments for patients treated with vigabatrin

Vigabatrin, an irreversible inhibitor of γ-aminobutyric acid transaminase, is an antiepileptic drug indicated in the United States as adjunctive therapy for adult patients with refractory complex partial seizures who have responded inadequately to several alternative treatments and for monotherapy treatment of infantile spasms in patients 1 month to 2 years of age. Approval of vigabatrin in the United States was contingent on the implementation of a Risk Evaluation and Mitigation Strategy (REMS) to manage the threat of a progressive, permanent bilateral concentric peripheral visual field defects (pVFDs) that may occur in patients treated with vigabatrin. The REMS is designed to promote compliance with evidence-based recommendations for baseline (within 4 weeks of the start of treatment) ophthalmologic evaluations and ongoing vision monitoring in all patients treated with vigabatrin. In view of the challenges associated with visual field testing in patients with epilepsy and in infants, clinicians must understand the qualitative (pattern of damage), quantitative (degree of damage), electrophysiologic, and adjunctive techniques recommended for monitoring vigabatrin-treated patients. The objectives of ongoing research are to characterize the onset, progression, and risk of developing vision loss during the first year of vigabatrin treatment and to evaluate the potential of noninvasive imaging as a method for monitoring retinal changes corresponding to the pVFD. This article provides an overview of visual field testing procedures and electroretinography, summarizes the clinical characteristics of vigabatrin-associated pVFDs, and provides recommendations for visual field and visual electrophysiology testing relevant to both adult and infant patients treated with vigabatrin.

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.

Fragile X syndrome: the GABAergic system and circuit dysfunction

Fragile X syndrome (FXS) is a neurodevelopmental disorder characterized by intellectual disability, sensory hypersensitivity, and high incidences of autism spectrum disorders and epilepsy. These phenotypes are suggestive of defects in neural circuit development and imbalances in excitatory glutamatergic and inhibitory GABAergic neurotransmission. While alterations in excitatory synapse function and plasticity are well-established in Fmr1 knockout (KO) mouse models of FXS, a number of recent electrophysiological and molecular studies now identify prominent defects in inhibitory GABAergic transmission in behaviorally relevant forebrain regions such as the amygdala, cortex, and hippocampus. In this review, we summarize evidence for GABAergic system dysfunction in FXS patients and Fmr1 KO mouse models alike. We then discuss some of the known developmental roles of GABAergic signaling, as well as the development and refinement of GABAergic synapses as a framework for understanding potential causes of mature circuit dysfunction. Finally, we highlight the GABAergic system as a relevant target for the treatment of FXS.

Retinal nerve fibre layer attenuation: clinical indicator for vigabatrin toxicity

PURPOSE

To investigate whether persistent visual field defects among patients exposed once to the antiepileptic drug vigabatrin (VGB) were associated with peripapillary retinal nerve fibre layer thickness (RNFLT) attenuation.

METHODS

Nine individuals with partial epilepsy and VGB-attributed visual field loss (group 1; 18 eyes) and seven age- and gender-matched individuals with epilepsy and no previous VGB exposure (group 2; 14 eyes) were included in the study. Full-field 120 point screening perimetry out to 60 degrees from central fixation using the Humphrey Field Analyzer was performed. RNFLT was quantified by optical coherence tomography (OCT) using Fast RNFLT protocol, Stratus OCT (3.0) after pupillary dilation. The results from the right eye are presented in this article.

RESULTS

Among the patients with VGB-attributed visual field loss, five patients had only peripheral field defect (group 1a) and the remaining four had advanced field defects both in the periphery and within 30° from central fixation (group 1b). None of the patients in the control group had manifest visual field loss. The mean RNFLT among the patients with VGB-attributed visual field loss was significantly attenuated compared to the controls [mean total RNFLT: group 1: 75.6 ± 12.7 μm, group 2: 103.5 ± 9.7 μm, mean difference 27.9 μm, (CI 15.9-39.9; p < 0.001)]. RNFLT values classified as borderline according to normative database (Stratus OCT) occurred more frequently among individuals with VGB-attributed visual field loss than in controls (frequency in group 1: 6/9; group 2: 0/7, p = 0.011). The nasal, superior and inferior quadrants of RNFLT in individuals with VGB-attributed visual field loss were significantly attenuated, while no difference was detected in temporal quadrants compared to controls. Both individuals with peripheral and those with advanced visual field losses in the VGB group had attenuated mean total RNFLT compared to controls (p = 0.006, p = 0.002, respectively). Occurrence of borderline classification of total RNFLT ≤5th percentile was more frequent among individuals with advanced visual field loss than among controls (p = 0.048).

CONCLUSION

Persistent visual field loss attributed to VGB is associated with reduced peripapillary RNFLT and was detected both among patients with advanced and among patients with only peripheral visual field defects. Measurements of RNFLT with OCT might be considered as a diagnostic supplement in the follow-up of patients exposed to vigabatrin.

Recommendations for visual evaluations of patients treated with vigabatrin

PURPOSE OF REVIEW

Although vigabatrin has been used for many years in Europe, this antiepileptic medication was approved for use in the USA only 1 year ago because of concerns for irreversible peripheral visual field loss that can result in functional visual disability.

RECENT FINDINGS

Visual function testing involves both subjective and objective techniques, some of which can be standardized across multiple centers. With these subjective and objectives issues as a reference, the current literature about the pathogenesis of vigabatrin visual toxicity is reviewed in order to develop a practical and reliable testing program for detecting peripheral visual field changes early in their course; thereby, hopefully limiting disabling visual field loss.

SUMMARY

Based upon the existing literature as well as the currently available data about the evaluation of visual function, an algorithm has been developed to serve as a starting point for clinicians to evaluate patients treated with vigabatrin for refractory epilepsy.

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.

Taurine deficiency damages photoreceptors and retinal ganglion cells in vigabatrin-treated neonatal rats

The anti-epileptic drug vigabatrin induces an irreversible constriction of the visual field, but is still widely used to treat infantile spasms and some forms of epilepsy. We recently reported that vigabatrin-induced cone damage is due to a taurine deficiency. However, optic atrophy and thus retinal ganglion cell degeneration was also reported in children treated for infantile spasms. We here show in neonatal rats treated from postnatal days 4 to 29 that the vigabatrin treatment triggers not only cone photoreceptor damage, disorganisation of the photoreceptor layer and gliosis but also retinal ganglion cell loss. Furthermore, we demonstrate in these neonatal rats that taurine supplementation partially prevents these retinal lesions and in particular the retinal ganglion cell loss. These results provide the first evidence of retinal ganglion cell neuroprotection by taurine. They further confirm that taurine supplementation should be administered with the vigabatrin treatment for infantile spasms or epilepsy.

Visual impairment at large eccentricity in participants treated by vigabatrin: visual, attentional or recognition deficit?

A relationship between peripheral visual field loss and vigabatrin (VGB) has been reported in several studies but with inconsistent results. We investigated the level of visual processing at which the impairment occurs: attentional or cognitive (recognition) deficit. A simple reaction time task was used as a baseline condition. A spatial attention task measured the benefit and cost for the detection of a target appearing at a cued or at an uncued location. A rapid categorization task assessed object recognition. Performance was tested at eccentricities varying from 30 degrees to 60 degrees on a panoramic screen covering 180 degrees. Participants were patients with epilepsy treated with VGB, patients treated with other drugs and healthy controls. In the VGB group 9 patients exhibited a mild visual field constriction. We observed a general slowing down of response times in participants treated by VGB, especially at 60 degrees eccentricity but their performance remained above chance at large eccentricity in the most complex categorization task. The slowing down of visual processing at large eccentricity for flashed stimuli suggests that VGB treated patients might be impaired at detecting moving objects in the periphery and this may have consequences in behavioural tasks like driving.

Reduced grating acuity associated with retinal toxicity in children with infantile spasms on vigabatrin therapy

PURPOSE

To determine whether visual functions are decreased in children with infantile spasms and vigabatrin-attributed retinal toxicity.

METHODS

Contrast sensitivity and grating acuity were measured by using sweep visual evoked potential (VEP) testing in 42 children with infantile spasms (mean age, 29.23 +/- 18.31 months). All children had been exposed to vigabatrin (VGB) for a minimum of 1 month. These children were divided into retinal toxicity and no toxicity groupings based on 30-Hz flicker amplitude reductions on the full-field electroretinogram. A multivariate analysis of variance (MANOVA) compared visual functions between children with and without retinal toxicity.

RESULTS

The MANOVA showed that visual function was significantly affected by VGB retinal toxicity. Further univariate analysis revealed that grating acuity was significantly reduced in children with toxicity. No differences in contrast sensitivity were found between children with toxicity and those without.

CONCLUSIONS

Reduced visual functions from VGB-attributed retinal toxicity can be detected in children with infantile spasms with the sweep VEP.

Nasal retinal nerve fiber layer attenuation: a biomarker for vigabatrin toxicity

PURPOSE

To investigate whether nasal peripapillary retinal nerve fiber layer (RNFL) attenuation is associated with visual field loss attributed to the anti-epileptic drug vigabatrin.

DESIGN

Prospective cross-sectional observational study.

PARTICIPANTS

Twenty-seven individuals with focal-onset epilepsy exposed to vigabatrin and 13 individuals with focal-onset epilepsy exposed to non-GABAergic anti-epileptic drug monotherapy.

METHODS

At one visit, suprathreshold perimetry of the central and peripheral field (3-zone, age-corrected Full Field 135 Screening Test) and threshold perimetry of the central field (Program 30-2 and the FASTPAC strategy) were undertaken using the Humphrey Field Analyzer (Carl Zeiss Meditech, Dublin, CA). At a second visit, ocular coherence tomography was undertaken for the right eye using the 3.4 RNFL thickness protocol of the StratusOCT (Carl Zeiss Meditech).

MAIN OUTCOME MEASURES

The magnitude, for each individual, of the RNFL thickness, averaged across the 4 oblique quadrants, and for each separate quadrant.

RESULTS

Of the 27 individuals exposed to vigabatrin, 11 (group I) exhibited vigabatrin-attributed visual field loss, 15 exhibited a normal field, and 1 exhibited a homonymous quadrantanopia (group II). All 13 individuals exposed to non-GABAergic therapy had normal fields (group III). All individuals in group I exhibited abnormal average and nasal quadrant RNFL thicknesses in the presence of a normal temporal quadrant thickness. Most also exhibited additional RNFL attenuation in either the superior or inferior quadrant, or both. Four individuals in group II exhibited an identical pattern of RNFL attenuation suggesting that nasal RNFL thinning is a more sensitive marker for vigabatrin toxicity than visual field loss. None of the 13 individuals in group III exhibited nasal quadrant RNFL attenuation.

CONCLUSIONS

Vigabatrin-attributed visual field loss is associated with a characteristic pattern of RNFL attenuation: nasal quadrant thinning and normal temporal quadrant thickness with, or without, superior or inferior quadrant involvement. Nasal attenuation may precede visual field loss. Ocular coherence tomography of the peripapillary RNFL should be considered in patients previously exposed to vigabatrin. It should also be considered at baseline and follow-up in those commencing vigabatrin for treatment of epilepsy or in trials for anti-addiction therapy. The pattern of RNFL thinning seems to be a useful biomarker to identify vigabatrin toxicity.

Visual system involvement in patients with Friedreich's ataxia

Optic neuropathy is common in mitochondrial disorders, but poorly characterized in Friedreich's ataxia (FRDA), a recessive condition caused by lack of the mitochondrial protein frataxin. We investigated 26 molecularly confirmed FRDA patients by studying both anterior and posterior sections of the visual pathway using a new, integrated approach. This included visual field testing and optical coherence tomography (OCT), pattern visual evoked potentials (P-VEPs) and diffusion-weighted imaging. The latter was used to study optic radiation by calculating water apparent diffusion coefficients (ADC). All patients suffered optic nerve involvement with their disorder. Different patterns of visual field defects were observed and a variably reduced retinal nerve fiber layer thickness was seen by OCT in all cases. P-VEPs were abnormal in approximately half of the patients. Decreased visual acuity and temporal optic disc pallor were present in advanced stages of the disease, but only five patients were symptomatic. Two of these patients suffered a sudden loss of central vision, mimicking Leber's hereditary optic neuropathy (LHON), and of the other three symptomatic patients two were noted to be compound heterozygotes. ADC values of optic radiations in patients were significantly higher than controls (P < 0.01). Retinal nerve fiber layer thickness at OCT and P-VEPs correlated with age at onset and ICARS total score. ADC values correlated with age at onset, disease duration, GAA triplet expansion size, ICARS total score and P-VEPs. Visual pathway involvement is found consistently in FRDA, being previously underestimated, and we here document that it also involves the optic radiations. Occasional LHON-like cases may occur. However, optic neuropathy in FRDA substantially differs from classic mitochondrial optic neuropathies implying a different pathophysiology of visual system degeneration in this mitochondrial disease.

Current role of vigabatrin in infantile spasms

Vigabatrin (VGB), a selective irreversible inhibitor of gamma-aminobutyric acid transaminase, has proved to be effective against cryptogenic and symptomatic infantile spasms (IS). Unfortunately, reports of serious visual field defects have led to a drastic reduction in the use of the drug. This review is based on a systematic search in the literature for evidence regarding efficacy and safety of VGB in IS. Based on a specific mechanism of action, there is a solid evidence of clinical efficacy of VGB in children with Tuberous Sclerosis. Similarly, VGB could represent a potential effective therapy also for spasms due to focal cortical dysplasia. In infants with spasms due to other causes, the risk of ophthalmologic toxicity should be carefully weighted against the benefit of controlling spasms.

Vigabatrin

Refractory epilepsies such as infantile spasms (IS) and complex partial seizures (CPS) can have a severe negative impact on the neurological integrity and quality of life of affected patients, in addition to drastically increasing their risk of premature mortality. Early identification of potentially effective pharmacotherapy agents is important. Vigabatrin has been shown to be a generally well tolerated and effective antiepileptic drug (AED) in a wide variety of seizure types affecting both children and adults, particularly those with IS and CPS. A bilateral, concentric constriction of the peripheral visual field characterizes the visual field defect (VFD) associated with vigabatrin, well characterized by numerous studies. This peripheral VFD presents in 30-50% of patients with exposure of several years; however, most of these patients are asymptomatic. In well-controlled studies, the earliest onset in patients with CPS is 11 months and at 5 months in infants, with average onsets being more than 5 years and 1 year, respectively. Patients with a peripheral VFD retain an average 65 degrees of lateral vision (normal, 90 degrees). The fact that many patients never develop the vigabatrin-related peripheral VFD, despite long-term exposure at high doses, may support the hypothesis that the injury is an idiosyncratic adverse drug reaction (as opposed to a strict dose- or duration-dependent toxicity). Effective testing methods are available to aid in the early detection and management of the peripheral VFD. This article discusses issues of importance to clinical decision-making in the use of vigabatrin to assist the physician and patient in assessing the benefits of vigabatrin therapy and understanding the potential risks of the VFD and uncontrolled seizures.

Vigabatrin Retinopathy in an Irish Cohort: Lack of Correlation with Dose

PURPOSE

The anticonvulsant vigabatrin (VGB) causes irreversible visual-field constriction in 19-92% of patients. It is unclear whether this correlates with dosing, and the natural history of the retinopathy remains obscure. We conducted a retrospective analysis of patients receiving long-term VGB to examine whether toxicity is related to the daily dose, duration of therapy, or cumulative dose.

METHODS

Information from 93 patients taking long-term, stable VGB therapy was analyzed. We recorded data on patient demographics, VGB dosing, and all visual-field assessments. We used the mean redial degrees (MRD) from the right eye to compare the amount of constriction with the dose of VGB.

RESULTS

The mean number of assessments was two (range, 1-6). Of patients having more than one assessment (n = 65), the mean follow-up time was 2.4 years (range, 0.7-5.6 years); in 52.7%, visual-field constriction developed. Male and female patients were affected equally. We found no correlation between the average MRD and either the maximum dose of VGB taken, the duration of exposure, or the cumulative dose. The shortest exposure time to development of constriction was 1.1 years. All patients with normal fields on initial assessment continued to have normal fields on follow-up. Most patients who had evidence of constriction on initial assessment and remained taking VGB showed no progression on follow-up. One patient had a substantial recovery of vision after discontinuation of VGB.

CONCLUSIONS

Development of visual constriction in patients receiving prolonged, standard doses of VGB does not depend on the daily dose, duration of exposure, or cumulative dose. Other contributing factors were not identified. Our data suggest that field defects may develop within the first few years of therapy and possibly remain stable thereafter.

Ocular complications of neurological therapy

Treatments used for several neurological conditions may adversely affect the eye. Vigabatrin-related retinal toxicity leads to a visual field defect. Optic neuropathy may result from ethambutol and isoniazid, and from radiation therapy. Posterior subcapsular cataract is associated with systemic corticosteroids. Transient refractive error changes may follow treatment with acetazolamide or topiramate, and corneal deposits and keratitis with amandatine. Intraocular pressure can be elevated in susceptible individuals by anticholinergic drugs, including oxybutynin, tolterodine, benzhexol, propantheline, atropine and amitriptyline, and also by systemic corticosteroids and by topiramate. Nystagmus, diplopia and extraocular muscle palsies can occur with antiepileptic drugs, particularly phenytoin and carbamazepine. Ocular neuromyotonia can follow parasellar radiation. Congenital ocular malformations can result from in utero exposure to maternally prescribed sodium valproate, phenytoin and carbamazepine. Neurologists must be aware of potential ocular toxicity of these drugs, and appropriately monitor for potential adverse events.

Vigabatrin-associated loss of vision: rarebit perimetry illuminates the dose-damage relationship

PURPOSE

The utility of vigabatrin in the treatment of epilepsy is partially offset by its retinal toxicity. The relationship between dosage and damage is obscure. This may be due to perimetric shortcomings. The new technique of rarebit ('microdot') perimetry might be more informative.

METHODS

Twelve patients who had been treated with vigabatrin for various durations were examined by manual, kinetic perimetry and by rarebit perimetry.

RESULTS

Rarebit results differed significantly between patients and normal controls and rarebit deficits were directly proportional to cumulated vigabatrin doses (correlation coefficients were - 0.92 in the nasal field and - 0.82 in the temporal field). Manual perimetry results were less clearly related to dosage (r = - 0.54 and r = - 0.73, respectively).

CONCLUSION

Rarebit perimetry indicates that each treated subject will develop visual loss and that visual loss will be proportional to the accumulated dose. Conventional perimetry is less well suited to detecting and quantifying vigabatrin-associated visual loss.