Long term changes in the visual fields of patients with temporal lobe epilepsy using vigabatrin

OV329, a novel highly potent γ-aminobutyric acid aminotransferase inactivator, induces pronounced anticonvulsant effects in the pentylenetetrazole seizure threshold test and in amygdala-kindled rats

OBJECTIVE

An attractive target to interfere with epileptic brain hyperexcitability is the enhancement of γ-aminobutyric acidergic (GABAergic) inhibition by inactivation of the GABA-metabolizing enzyme GABA aminotransferase (GABA-AT). GABA-AT inactivators were designed to control seizures by raising brain GABA levels. OV329, a novel drug candidate for the treatment of epilepsy and addiction, has been shown in vitro to be substantially more potent as a GABA-AT inactivator than vigabatrin, an antiseizure drug approved as an add-on therapy for adult patients with refractory complex partial seizures and monotherapy for pediatric patients with infantile spasms. Thus, we hypothesized that OV329 should produce pronounced anticonvulsant effects in two different rat seizure models.

METHODS

We therefore examined the effects of OV329 (5, 20, and 40 mg/kg ip) on the seizure threshold of female Wistar Unilever rats, using the timed intravenous pentylenetetrazole (ivPTZ) seizure threshold model as a seizure test particularly sensitive to GABA-potentiating manipulations, and amygdala-kindled rats as a model of difficult-to-treat temporal lobe epilepsy.

RESULTS

GABA-AT inactivation by OV329 clearly increased the threshold of both ivPTZ-induced and amygdala-kindled seizures. OV329 further showed a 30-fold greater anticonvulsant potency on ivPTZ-induced myoclonic jerks and clonic seizures compared to vigabatrin investigated previously. Notably, all rats were responsive to OV329 in both seizure models.

SIGNIFICANCE

These results reveal an anticonvulsant profile of OV329 that appears to be superior in both potency and efficacy to vigabatrin and highlight OV329 as a highly promising candidate for the treatment of seizures and pharmacoresistant epilepsies.

Visual electrophysiology in the assessment of toxicity and deficiency states affecting the visual system

Visual disturbance or visual failure due to toxicity of an ingested substance or a severe nutritional deficiency can present significant challenges for diagnosis and management, for instance, where an adverse reaction to a prescribed medicine is suspected. Objective assessment of visual function is important, particularly where structural changes in the retina or optic nerve have not yet occurred, as there may be a window of opportunity to mitigate or reverse visual loss. This paper reviews a number of clinical presentations where visual electrophysiological assessment has an important role in early diagnosis or management alongside clinical assessment and ocular imaging modalities. We highlight the importance of vitamin A deficiency as an easily detected marker for severe combined micronutrient deficiency.

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.

Does the Swedish Interactive Threshold Algorithm (SITA) accurately map visual field loss attributed to vigabatrin?

Background

Vigabatrin (VGB) is an anti-epileptic medication which has been linked to peripheral constriction of the visual field. Documenting the natural history associated with continued VGB exposure is important when making decisions about the risk and benefits associated with the treatment. Due to its speed the Swedish Interactive Threshold Algorithm (SITA) has become the algorithm of choice when carrying out Full Threshold automated static perimetry. SITA uses prior distributions of normal and glaucomatous visual field behaviour to estimate threshold sensitivity. As the abnormal model is based on glaucomatous behaviour this algorithm has not been validated for VGB recipients. We aim to assess the clinical utility of the SITA algorithm for accurately mapping VGB attributed field loss.

Methods

The sample comprised one randomly selected eye of 16 patients diagnosed with epilepsy, exposed to VGB therapy. A clinical diagnosis of VGB attributed visual field loss was documented in 44% of the group. The mean age was 39.3 years ± 14.5 years and the mean deviation was -4.76 dB ±4.34 dB. Each patient was examined with the Full Threshold, SITA Standard and SITA Fast algorithm.

Results

SITA Standard was on average approximately twice as fast (7.6 minutes) and SITA Fast approximately 3 times as fast (4.7 minutes) as examinations completed using the Full Threshold algorithm (15.8 minutes). In the clinical environment, the visual field outcome with both SITA algorithms was equivalent to visual field examination using the Full Threshold algorithm in terms of visual inspection of the grey scale plots , defect area and defect severity.

Conclusions

Our research shows that both SITA algorithms are able to accurately map visual field loss attributed to VGB. As patients diagnosed with epilepsy are often vulnerable to fatigue, the time saving offered by SITA Fast means that this algorithm has a significant advantage for use with VGB recipients.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2415-14-166) contains supplementary material, which is available to authorized users.

Electroretinographic responses in epileptic children treated with vigabatrin

Vigabatrin is an antiepileptic drug that results in higher gamma-aminobutyrate levels in the brain and retina. Vigabatrin-induced visual field defects are usually asymptomatic and only detectable by perimetry. Further, perimetry requires good cooperation, and children aged under 10 years cannot do it. Electroretinogram response amplitude to full-field 30-Hz flicker shine has been offered to be more specific in predicting visual field defects. This study is scheduled to investigate the vigabatrin-associated visual complications in 67 epileptic children taking vigabatrin using full-field electroretinogram. Electroretinographic surveys showed normal range parameters despite 3 months of vigabatrin treatment, and just 3 (4.47%) children had been visually impaired at the end of 6-month treatment. Among these 3 cases, 1 patient had persistent electroretinogram abnormality despite vigabatrin discontinuation. Our study suggests that vigabatrin is secure for short-term pediatric antiepileptic treatment, with few cases of visual impairments and that are often reversible.

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.

Justification of vigabatrin administration in West syndrome patients? Warranting a re-consideration for improvement in their quality of life

West syndrome (WS) or infantile spasms (IS) is a severe epileptic syndrome associated with poor prognosis and increased morbidity. The exact etio-pathogenesis of the disorder still remains elusive ant therefore the management continues to pose a challenge to the clinicians. Currently, adreno-corticotrophic hormone (ACTH), steroids and vigabatrin (VGB) form the mainstay of its treatment. However, the recent detection of an irreversible visual field defect observed in as high as 30-50% of children treated with vigabatrin has raised concern over the drug's usage. This brief paper is intended to highlight the significance of the irreversible visual toxicity in an already existent mentally challenged state in WS patients, which can lead to a worsening in the disability status of such patients. Therefore, based on the enhancement of handicap by VGB administration it is recommended that a comprehensive review be performed on its continuation in WS patients in order to prevent further deterioration of their quality of life (QOL).

Treatment of refractory complex partial seizures: role of vigabatrin

Vigabatrin (VGB) is an antiepileptic drug that was designed to inhibit GABA-transaminase, and increase levels of gamma-amino-butyric acid (GABA), a major inhibitory neurotransmitter in the brain. VGB has demonstrated efficacy as an adjunctive antiepileptic drug for refractory complex partial seizures (CPS) and for infantile spasms (IS). This review focuses on its use for complex partial seizures. Although VGB is well tolerated, there have been significant safety concerns about intramyelinic edema and visual field defects. VGB is associated with a risk of developing bilateral concentric visual field defects. Therefore, the use of VGB for complex partial seizures should be limited to those patients with seizures refractory to other treatments. Patients must have baseline and follow-up monitoring of visual fields, early assessment of its efficacy, and ongoing evaluation of the benefits and risks of VGB therapy.

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 and visual field defects in pediatric epilepsy patients

We studied the prevalence, type and severity of vigabatrin (VGB)-attributed visual field defects (VFDs), and used these data to assess the associated risk factors in pediatric patients. Medical records were retrospectively reviewed for 67 pediatric patients who received VGB alone or in combination with other antiepileptic drugs, and who had undergone visual field examinations using a Humphrey visual field analyzer. Of the 67 patients, 15 had VGB-attributed VFDs: 13 had nasal arcuate type, 1 had nasal and temporal constricted type and 1 had nasal constricted type. In terms of severity, 7 patients had Grade I VGB-attributed VFDs, 5 had Grade II, 2 had Grade III, and 1 had Grade IV. Although there were no significant differences between the VFD and non-VFD groups with regards to all tested parameters, there were no cases of VGB-attributed VFDs in patients with total treatment durations <2 yr and cumulative doses <10 g/kg. In conclusion, the prevalence of VGB-attributed VFDs in VGB-treated pediatric epilepsy patients was 22%. The high frequency of VGB-attributed VFDs indicates that physicians should inform all patients of this risk prior to VGB treatment and perform periodic visual field examinations.

Acute vigabatrin retinotoxicity in albino rats depends on light but not GABA

PURPOSE

Vigabatrin (VGB) is an irreversible inhibitor of gamma-aminobutyric acid (GABA) transaminase. Its use as an antiepileptic drug (AED) has been limited because it causes retinal dysfunction, leading to visual field defects (VFDs). We performed this study to identify factors contributing to acute VGB retinotoxicity.

METHODS

In ex vivo experiments, Sprague-Dawley rat retinas were isolated and incubated with VGB or GABA in the presence or absence of light. In in vivo experiments, Sprague-Dawley rats were given intraperitoneal injections of VGB and then exposed to light or kept in the dark. The retinas were analyzed histologically by using both light and electron microscopy.

RESULTS

Incubating retinas with 50-500 microM VGB under 20,000 Lux white light for < or = 20 h caused a characteristic time- and dose-dependent degeneration limited to the outer retina. Incubating retinas with 500 microM VGB in darkness for 20 h caused no damage. Five hundred micromolar GABA and 50 microM tiagabine were not toxic in the presence or absence of light. Sprague-Dawley rats exposed to an intense white light for 20 h after a 1,000-mg/kg intraperitoneal injection of VGB showed damage in the outer retina, whereas those kept in the dark did not.

CONCLUSIONS

Direct exposure of the retina to VGB causes acute retinotoxicity that depends on light exposure rather than GABA accumulation.

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.

Characterization of vigabatrin-associated optic atrophy

AIMS

To report the discovery of a previously unknown form of optic atrophy associated with use of the anti-epileptic drug vigabatrin.

METHODS

We conducted a retrospective analysis of digitally enhanced ocular fundus photographs, kinetic visual field maps and treatment parameters for 25 patients, who were selected to represent a large spectrum of visual field defects.

RESULTS

In all, 21 patients (84%) evidenced subtle, diffuse atrophy of the retinal nerve fibre layer, in a pattern accessible to scoring. Atrophy scores correlated with visual field remains and cumulative vigabatrin doses. A pathophysiological model is proposed that involves the lengths of intraocular (unmyelinated) retinal ganglion cell axons.

CONCLUSION

Optic atrophy attests to the irreversible nature of vigabatrin's visual toxicity. Ocular fundus imaging should prove useful for objectively monitoring vigabatrin-treated subjects for visual toxicity.

Vigabatrin: longterm follow-up of electrophysiology and visual field examinations

BACKGROUND

To report the results of repeated electrophysiological and visual field examinations in patients with vigabatrin-associated visual field loss (VGB-VFL) and the relationship between these electrophysiological findings, the cumulative dose of vigabatrin and the extent of visual field loss.

METHODS

Twenty-two eyes of 11 patients with VGB-VFL were studied. All patients underwent surgery for therapy-resistant epilepsy. Repeated electro-oculograms (EOGs) and flash electroretinograms (ERGs) were made and the cumulative dose of vigabatrin and the visual field loss were recorded after a period of 37-47 months.

RESULTS

The visual field loss was stable in patients who had stopped vigabatrin at the time of the first examination. There was a slight increase in VFL in patients who continued vigabatrin. During the second EOG and ERG, abnormalities in scotopic and photopic a-wave latencies and in scotopic b-wave amplitude were found in more than 50% of patients. Only b-wave latency became normal, while EOG, a-wave latency, a-wave amplitude and b-wave amplitude stayed abnormal. The amount of VFL and the cumulative dose of vigabatrin were statistically correlated with the b-wave amplitude, mainly photopic, found during the first and second examinations.

CONCLUSION

After 4 years, EOG, flash ERG and visual field loss had not improved in patients with VGB-VFL. The statistically significant correlation found during the first examination between the amount of VFL and the cumulative dose of vigabatrin with the (mainly photopic) b-wave amplitude remained constant.

Vigabatrin-associated visual field constriction in a longitudinal series. Reversibility suggested after drug withdrawal

PURPOSE

To evaluate through a longitudinal study the effects on visual fields of long-term vigabatrin medication in patients with partial epilepsy and to discuss visual field screening strategies.

METHODS

A total of 26 patients aged 14-68 years with a mean history of vigabatrin medication of 8.5 years (range 2-14 years) were followed by manual kinetic Goldmann perimetry (objects IV,4 and I,4) for 6-26 months (mean value 12.3 months). At time zero and at follow-up, each patient was assigned a "pooled" averaged value, as a linear percentage of normal isopter position, for the two objects as tested nasally and temporally in the five most horizontal meridians on the Goldmann chart. Twelve eyes from nine adults (age 24-60 years) served as controls.

RESULTS

Constrictions were recorded in 24 of 26 patients at baseline. Averaged isopters ranged from 8% to 96% of the controls' averaged isopter positions. Median values of 71.5% and 60.5% for large and small objects, respectively, indicated that the smaller object was more sensitive to visual field constriction. There was no difference in the degree of constriction between nasal and temporal hemifields. Significant improvement in the visual field (mean gain 13.6% units) was seen in the eight patients who underwent full drug withdrawal. No similar improvement was seen in the 12 patients still on full dose or the six with reduced intake.

CONCLUSIONS

Most Danish patients on long-term vigabatrin medication have suffered some visual field loss. Contrary to most clinical evidence so far, the present follow-up study indicates some reversibility of visual field loss after drug withdrawal. Kinetic Goldmann perimetry appears to be a fair alternative to computerized static perimetry techniques for screening and following vigabatrin-treated patients.

Patients treated with vigabatrin exhibit central visual function loss

PURPOSE

To investigate visual function in the central 10 degrees in patients who have undergone vigabatrin (VGB) antiepileptic drug (AED) therapy with the aim of identifying a clinical regimen for assessing central visual function.

METHODS

The sample comprised 12 epilepsy patients (mean age, 38.6 +/- 11.7 years) who had been treated with VGB (either as monotherapy or polytherapy). A number of central visual-function tests were carried out for each eye, including high-contrast LogMAR visual acuity, short-wavelength automated perimetry (SWAP 10-2), spatial contrast sensitivity (CSV-1000), and Farnsworth-Munsell (FM) 100-hue colour discrimination.

RESULTS

The group mean cumulative VGB dose was 5,086 +/- 3,245 g. The average SWAP 10-2 mean deviation (MD) for the group was -3.24 +/- 3.23 dB; 14 eyes of eight patients showed defects (range, -1.62 to -9.46 dB). The square root of the group mean total error score for FM 100-hue was 7.42 +/- 3.84; nine eyes of five patients were classified as abnormal with an unsolved colour axis suggestive of complex drug interactions. For contrast sensitivity, 15 eyes of eight patients yielded abnormal results in one or more spatial frequencies. Defects were more prominent at higher spatial frequencies. Overall, four patients had defects in all three visual-function tests, six patients had mixed defects, and two patients were normal.

CONCLUSIONS

Visual-function deficits in epilepsy patients treated with VGB are present in the central 10 degrees of the retina. We recommend a battery of investigations, including SWAP 10-2 and spatial contrast sensitivity testing, to assess central visual function.

Detection of visual field defects in patients after anterior temporal lobectomy for mesial temporal sclerosis-establishing eligibility to drive

AIMS

The aim of this study is to quantify visual field defects after temporal lobectomy for mesial temporal sclerosis and to establish eligibility for driving.

METHODS

Automated static perimetry was performed on 14 patients who had undergone anterior temporal lobectomy for mesial temporal sclerosis. Perimetry consisted of monocular Humphrey Field Analyser (HFA) 30-2 test and a binocular Esterman 120 test.

RESULTS

Of the 14 patients, three had no loss or non-specific loss, eight had partial homonymous quadrantanopia, one had complete homonymous quadrantanopia and two had concentric loss attributable to vigabatrin, which may have masked any loss occurring due to surgery. Of these, only seven passed the standardised DVLA visual fields. Of the seven who failed DVLA visual field, one had complete quadrantanopia, four had partial quadrantanopia and two had concentric loss (due to vigabatrin).

CONCLUSIONS

Visual field defects contribute a great deal in the reduction of the quality of life in patients who have had surgery for mesial temporal sclerosis. Potential surgically induced visual field defects that could preclude driving need to be discussed with each patient preoperatively. In our study 50% of patients did not meet the required DVLA standards.

Neurotoxic effects of GABA-transaminase inhibitors in the treatment of epilepsy: ocular perfusion and visual performance

Vigabatrin is a GABA (gamma-aminobutyric acid) transaminase inhibitor that elicits an antiepileptic effect by enhancing inhibitory neurotransmission in the brain. Vigabatrin has been previously associated with concentric peripheral visual field loss and visual electrophysiological abnormalities. Recently, visual function deficits of the central retina have been identified in a proportion of patients receiving vigabatrin; these include disturbances in colour perception, contrast sensitivity and short-wavelength automated perimetry. Consequently, it is suggested that vigabatrin-associated retinal toxicity is diffuse inducing subtle central visual dysfunction and more severe peripheral visual defects. Reductions in cerebral blood flow and cerebral metabolic rate for glucose occur in epilepsy patients receiving antiepileptic drug therapy. Despite the known cerebral haemodynamic alterations in epilepsy and the visual consequences of vigabatrin therapy, ocular blood flow has only recently been investigated in this group. We present findings from a series of novel investigations that identify compromised retinal microvascular perfusion and pulsatile ocular blood flow (POBF) in epilepsy patients. The reduction in POBF was exacerbated in epilepsy patients treated with vigabatrin compared to conventionally treated epilepsy patients. A number of theories are presented to explain compromised ocular blood flow in vigabatrin treated epilepsy patients, and the possibility of a GABAergic mechanism of toxicity is discussed.

Vigabatrin associated visual field loss: a clinical audit to study prevalence, drug history and effects of drug withdrawal

PURPOSE

To survey clinical visual function including quantitative manual perimetry results in a group of patients taking vigabatrin; to assess the severity of any field defects; to tabulate cumulative and daily doses of medication and to assess possible changes in visual function over time.

METHOD

A prevalence study of 100 out of 183 patients currently attending a tertiary referral epilepsy centre who were taking or had recently discontinued vigabatrin (duration 83-3570 days; mean 1885 days) as part of combination anticonvulsant therapy. Complete neuro-ophthalmic examination including Goldmann kinetic perimetry was performed and monocular mean radial degrees (MRD) to the I/4e isopter calculated. Patients were followed up at 6-monthly intervals for not less than 18 months.

RESULTS

Acuity and colour vision remained stable in all patients regardless of changes in visual fields. Twenty per cent of patients had significant constriction of their visual field defined as a monocular MRD of 30 degrees or less. Males were significantly more likely to be severely affected than females (P < 0.01). Twenty one patients were followed after discontinuing vigabatrin treatment. Only three of these showed a change in MRD of 10 degrees or more with two deteriorating and one improving. No correlation between treatment duration or cumulative dosage/kg and the severity of defects could be demonstrated.

CONCLUSIONS

Earlier reports of a high prevalence of both moderate and more serious field defects were confirmed in patients taking vigabatrin but not in epileptic patients taking other anti-convulsants. We found no evidence of progression or resolution of visual field defects on discontinuing the drug, and no relationship between dose history and visual deficit field loss. An idiosyncratic drug reaction within the neurosensory retina may underlie the pathogenesis of the visual field loss in some patients.

Visual field constriction in 91 Finnish children treated with vigabatrin

PURPOSE

To study the prevalence and features of visual field constrictions (VFCs) associated with vigabatrin (VGB) in children.

METHODS

A systematic collection of all children with any history of VGB treatment in fifteen Finnish neuropediatric units was performed, and children were included after being able to cooperate reliably in repeated visual field tests by Goldmann kinetic perimetry. This inclusion criterion yielded 91 children (45 boys; 46 girls) between ages 5.6 and 17.9 years. Visual field extent <70 degrees in the temporal meridian was considered abnormal VFC.

RESULTS

There was a notable variation in visual field extents between successive test sessions and between different individuals. VFCs <70 degrees were found in repeated test sessions in 17 (18.7%) of 91 children. There was no difference in the ages at the study, the ages at the beginning of treatment, the total duration of the treatment, general cognitive performance, or neuroradiologic findings between the patients with normal visual fields and those with VFC, but the patients with VFC had received a higher total dose of VGB. In linear regression analysis, there were statistically significant inverse correlations between the temporal extent of the visual fields and the total dose and the duration of VGB treatment. The shortest duration of VGB treatment associated with VFC was 15 months, and the lowest total dose 914 g.

CONCLUSIONS

Because of a wide variation in normal visual-field test results in children, the prevalence figures of VFCs are highly dependent on the definition of normality. Although our results confirm the previous findings that VFC may occur in children treated with VGB, our study points out the need to reevaluate critically any suspected VFC to avoid misdiagnosis. Nevertheless, our study suggests that the prevalence of VFC may be lower in children than in adults, and that the cumulative dose of VGB or length of VGB therapy may add to the personal predisposition for developing VFC.

Visual field and electrophysiological abnormalities due to vigabatrin

The purpose of this study was to determine the electrophysiological changes in patients using the anti epileptic drug vigabatrin and to correlate these findings with the previously reported risk for visual field loss in these patients. In 1998 the neurologists of both involved hospitals referred all patients on vigabatrin medication for ophthalmological examination to the outpatients clinics. Of the 33 patients whom were referred to our outpatient clinics, four had to be dropped from the study because of disability to perform the examinations the remaining 29 patients were included in the study. Standard ophthalmological investigations were carried out, and contrast sensitivity, visual field (Humphrey 30-2 and Esterman or Octopus 32), colour vision (panel D15), ERG and EOG according to ISCEV standards were tested. 18 patients continued the medication and 11 stopped taking the drug during the study. Nine of the patients who stopped the drug were followed during at least half a year afterwards, this group will be described in the combined article 'Electro ophthalmic recovery after withdrawal from vigabatrin' (Graniewski and Van der Torren, this issue). The electro-ophthalmological findings in the group of 29 patients were correlated with the visual fields and the daily and cumulative dosages of vigabatrin. Of the patients, 32% showed no visual field constriction at all; from these patients 64% had EOG and/or ERG changes. Of the patients with slight to marked visual field constriction, 90% presented EOG and/or ERG changes. Significant correlation between daily dosages of vigabatrin and visual field defects was shown as well as between visual field defects and rod and cone b wave amplitude reductions. Cumulative vigabatrin dosages presented a significant correlation with EOG ratio and ERG rod b-wave amplitude. Conclusively EOG and ERG testing were found to be even an more accurate way to monitor the direct vigabatrin effect on the outer retina and is possible different from the visual field testing.

Vigabatrin and retinal changes

PURPOSE

Vigabatrin is an effective antiepileptic drug but visual field constriction (VFC) is found to be a severe side-effect. The aims have been to investigate whether visual field constriction (VFC) is related to changes in the electroretinography (ERG).

METHODS

Twenty patients with localisations related epilepsy of whom one half had received vigabatrin were subjected to examination without informing about the treatment given. The eye examination included Goldmann perimetry and ERG.

RESULTS

All the patients had normal visual acuity. A total of three patients (30%) in the vigabatrin group and none in the control group were found to have VFC. In the vigabatrin group ERG examination were normal in one case, in five cases there were changes scotopic, photopic and in the oscillatory potentials (OP), while the remaining four had changes in two of these parameters. OPs were abnormal in eight of 10 patients. Of the three patients with VFC all had changes in ERG. The four patients with the most severe abnormalities in ERG had received high daily doses of vigabatrin (4 - 6 mg) in a period. In the control group no abnormality was observed in five cases, and in the remaining five changes were present in one or two of the potentials.

CONCLUSION

It is found that 30% of patients treated with vigabatrin, develop VFC, and none in the control group. Similarly more patients in the vigabatrin group had changes in the ERG as compared to the control group, and the number of abnormal potentials are significantly higher among patients with VFC compared to those without. But the finding of abnormal ERG results is not synonymous with VFC, and this is important to bear in mind when examining patients that cannot cooperate to a VF examination. An individual sensitivity to vigabatrin is supposed, but severe ERG changes occurred in all patients having had high daily doses > or = 4 g.

Electro-ophthalmological recovery after withdrawal from vigabatrin

This study describes the effects of stopping of the anti-epileptic drug vigabatrin on the visual field and electrophysiological changes in one third of the group of patients which is described in the paper 'Visual field and electrophysiological abnormalities due to vigabatrin' (Van der Torren and Graniewski, 2002). In 1997 several reports described the possible oculotoxic effect of vigabatrin followed by a reconsideration of this medication in epileptic patients. Vigabatrin was discontinued in one-third of the patients on chronic medication (mean duration 4.8 years). The visual field and electrophysiological examinations were repeated every 3 months if possible, otherwise at 6-month intervals. The EOG Arden index and the ERG rod b-wave showed a significant improvement when vigabatrin was discontinued. Repeated examinations of visual fields and electrophysiology shortly after discontinuation of the drug (between 1 and 3 months) and later on after 6 months and 1 year showed a recovery of the EOG Arden index and the ERG rod b-wave during this period. The visual fields did not change in either direction.

CONCLUSION

the recovery effect is a strong argument for the hypothesis that the reduction in EOG and ERG b-wave is an oculotoxic effect. The electrophysiological improvement during 6 months or longer after discontinuation and the unchanged visual fields are an argument for the hypothesis that the visual field represents the irreversible intoxicating effect on the retina, whereas the electrophysiology represents a more direct effect on the retinal glial cells level.

Visual field defects and other ophthalmological disturbances associated with vigabatrin

Vigabatrin has been an important anticonvulsant drug for over 10 years with a reputation for high efficacy and excellent tolerability. However, since 1997, there have been over 25 reports in the literature of visual field defects attributable to the use of this agent. Most are case reports and many have only been reported as abstracts or posters or as letters or short communications. Only a small number of papers give details of patient characteristics. Typically, case reports detail ophthalmological tests such as visual acuity, funduscopic examination, integrity of colour vision, and the nature of the field cut. Many also include various electrophysiological tests which were performed in an attempt to further describe the nature of the visual changes. In order to shed light on the mechanism underlying these visual field changes, many investigators also tested various electrophysiological parameters. However, because electrophysiological testing requires considerable expertise on the part of the technician, this could be a source of variability in results and may also pose a challenge with data interpretation. The magnitude of the problem is difficult to assess. The manufacturer's estimate of incidence of visual field defects with vigabatrin was approximately 0.1%, but incidences estimated in the literature range from 6 to 30%. Since the majority of the published data are in case report form, proof of causation is also very difficult. Two papers that used proper scientific methodology to investigate this condition suggest that vigabatrin causes these changes; however, there needs to be further studies with larger populations to answer this question definitively. There is a lack of data on the dose-response characteristics of vigabatrin and the development of visual field defects. The only available data are reports of trends that implicate duration of therapy or cumulative dose. Perhaps the most important area to elucidate is whether or not the visual field defects are reversible. Data are scare on this subject, but we can hope that data will emerge as follow-up periods become more substantial. There is a need for more complete information regarding several aspects of the mechanistic basis of visual field defects associated with vigabatrin that will allow rational clinical decision making. The treatment choices, both pharmacological and nonpharmacological, for patients with refractory epilepsy have grown substantially in the last few years. Thus, it is doubtful that the clinical positioning of vigabatrin is likely to change in the future from that of a very valuable 'niche drug', with emphasis on paediatric usage.

Visual field defects with vigabatrin: epidemiology and therapeutic implications

Vigabatrin is an antiepileptic drug (AED) that acts as a selective irreversible inhibitor of gamma-aminobutyric acid (GABA) transaminase. In 1997, 3 cases of severe symptomatic and persistent visual field constriction associated with vigabatrin treatment were described. During 1997 to 1998, similar concentric visual field constrictions were described in patients with drug-resistant epilepsy who were receiving vigabatrin concurrently with other AEDs. However, a study of patients treated with vigabatrin monotherapy alone showed that there was a causal relationship between vigabatrin treatment and the specific bilateral concentric visual field constriction. The Marketing Authorisation Holders survey (involving 335 vigabatrin recipients aged >14 years) indicated that 31% of patients [95% confidence interval (CI) 26 to 36%] had a visual field defect attributable to vigabatrin, compared with a 0% incidence of visual field defects (upper 95% CI 3%) in an unexposed control group. Other studies in adults have given similar overall prevalences, with a total of 169 of 528 patients diagnosed with vigabatrin-associated field defects (32%, 95% CI 28 to 36%). Male gender seems to be associated with an increase in the relative risk of visual field loss of approximately 2-fold. The pattern of defect is typically a bilateral, absolute concentric constriction of the visual field, the severity of which varies from mild to severe. Data gathered so far suggest that the cumulative incidence increases rapidly during the first 2 years of treatment and within the first 2 kg of vigabatrin intake, stabilising at 3 years and after a total vigabatrin dose of 3 kg. The prevalence of vigabatrin-associated field defects seems to be lower in children, but there are also methodological problems and greater variability in the assessment of visual fields in children. There is particular concern that the increased risk of the visual field defects will outweigh the benefit of the drug in patients who could be controlled with other AEDs. Vigabatrin should currently be used only in combination with other AEDs for patients with resistant partial epilepsy when all other appropriate drug combinations have proved inadequate or have not been tolerated. Regular visual field testing should be performed before the start of treatment and at regular intervals during treatment. Patients with pre-existent visual field defects due to other causes should not be treated with vigabatrin. Currently, the benefits of treating infantile spasms with vigabatrin monotherapy seem to outweigh the risks, but further prospective studies and follow-up of children receiving treatment are needed to evaluate the place of vigabatrin in this indication.

The value of electrophysiology results in patients with epilepsy and vigabatrin associated visual field loss

PURPOSE

To determine the value of electrophysiological findings in patients with temporal lobe epilepsy and to relate these findings to the amount of concentric contraction of the visual field and the use of vigabatrin.

METHODS

Electro-retinograms and electro-oculograms were done on 30 patients, operated for temporal lobe epilepsy. The patients were divided into three groups: (A) concentric contraction of the visual field associated with a history of vigabatrin medication (15 patients), (B) normal visual field with vigabatrin use (11 patients) and (C) normal visual field without vigabatrin medication (4 patients).

RESULTS

Electrophysiological abnormalities were found in 50% of the patients in group A. The Arden ratio of the EOG was lowered in 57%. Abnormalities in the ERG were found: b-wave implicit time photopic F was prolonged (50%), b-wave amplitudes scotopic B (53%), C (73%) and G (50%) and photopic H (50%) were diminished. The amount of visual field loss and the total dose of vigabatrin used, showed only slight correlation with the ERG and EOG. The use of vigabatrin during the ERG and EOG recording in group A, gave a higher b-wave amplitude scotopic G in 64% of cases. The a-wave implicit times scotopic G (73%) and photopic G (59%) and H (73%) were shortened in group B.

CONCLUSION

EOG was abnormal in 57% in group A. ERG abnormalities could only be found in 50% of group A, mainly in the inner retina. Since also the total dose of vigabatrin and the amount of visual field loss did not really show a correlation with the electrophysiological findings and results of literature are not unanimous, electrophysiology does not appear at present to be a good method to detect patients with, or at risk of, vigabatrin associated visual field loss. Regularly performed visual field examination remains the cornerstone in screening.

Visual field loss associated with vigabatrin: quantification and relation to dosage

PURPOSE

To describe the correlation between visual field loss and the duration, dosage, and total amount of vigabatrin (VGB) medication in a group of patients with epilepsy. Co-medication of antiepileptic drugs (AEDs) and compliance were also studied.

METHODS

Ninety-two patients (53 male and 39 female) taking VGB medication in the past or the present, attending the Outpatient Epilepsy Clinic in Utrecht, were examined with the Goldmann perimeter. The amount of visual field loss was calculated by the Esterman grid method and by a new method, with which the percentage surface loss of the visual field is measured. A complete drug history was compiled, specifying the amount and duration of VGB medication. Concomitant AED medication was noted. Serum levels of AEDs were determined.

RESULTS

Linear regression showed the total amount of VGB as the most significant parameter to predict visual field loss (p < 0.001). Further, men were more affected than women (p = 0.026). Compliance was good, and other AEDs did not influence the results.

CONCLUSIONS

Because prolonged use of VGB medication is correlated with the amount of visual field loss, VGB should be prescribed only when there are no alternatives. In such cases, we recommend an examination of the peripheral visual field before starting therapy and a repeated examination every 6 months.