Visual field constriction and electrophysiological changes associated with vigabatrin

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.

The diagnostic usefulness of the negative electroretinogram

The definition of the negative response of the full field electroretinogram is the presence of a b-wave with less amplitude than the a-wave (b/a ratio<1) in the combined response of cones and rods. The presence of this pattern reflects an alteration in the bipolar cells, the Müller cells, or in the transmission of the stimulus from the photoreceptors to the bipolar cells, with preserved photoreceptor function. This finding can be seen bilaterally and symmetrically in different hereditary conditions, such as congenital stationary night blindness, juvenile X-linked retinoschisis, and Duchenne and Becker muscular dystrophies. On the other hand, it can also be found unilaterally (or asymmetrically) in acquired pathologies, such as some types of immuno-mediated retinitis (Birdshot retinochoroiditis), autoimmune retinopathies, cancer/melanoma associated retinopathy, or retinal toxicity. The objective of this review is to summarise the characteristics of the pathologies in which this finding can be observed, in order to highlight its usefulness in the differential diagnosis of retinal conditions.

The Role of Multifocal Electroretinography in the Assessment of Drug-Induced Retinopathy: A Review of the Literature

Multifocal electroretinography (mfERG) is an objective, noninvasive examination for the assessment of visual function. It enables the stimulation of multiple retinal areas simultaneously and recording of each response independently, providing a topographic measure of retinal electrophysiological activity in the central 40-50° of the retina. A clinical application of mfERG represents the assessment of retinal toxicity associated with systemic medications. Drug-induced retinopathy represents a disease that, although not common, requires early recognition: if not detected early, it may progress and cause irreversible retinal dysfunction with subsequent visual impairment. This review aims to evaluate the use of mfERG in the assessment of retinal dysfunction associated with various systemic pharmacological agents based on the currently available literature. The most commonly recognized systemic medications affecting retinal function are included, such as chloroquine and hydroxychloroquine, vigabatrin, deferoxamine, ethambutol, interferon-α, tamoxifen, digoxin, sildenafil, canthaxanthin, amiodarone and nefazodone. The role of mfERG in the early diagnosis of retinal toxicity and the evaluation of disease severity is reviewed, as well as its clinical value in monitoring disease progression or recovery after drug cessation.

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.

Visual field and ocular safety during short-term vigabatrin treatment in cocaine abusers

PURPOSE

To evaluate the ocular safety of short-term vigabatrin treatment of cocaine abuse.

DESIGN

Multicenter, prospective, randomized, placebo-controlled, double-masked, parallel assignment study.

METHODS

Cocaine addicts were randomized to receive vigabatrin 3000 mg/day, cumulative dose 218 g (n = 92), or placebo (n = 94) for 12 weeks. Subjects underwent examination of visual acuity (ETDRS) and peripheral visual field (PVF) by Humphrey Field Analyzer (HFA) 60-4 program before and after treatment. Reliable PVF tests (fixation loss, false positive, and false negative <33%) for 103 subjects were included for the analysis. The threshold visual sensitivity (TVS) was analyzed by points, rings and zones. Main outcome measures included visual acuity decrease by 15 letters and/or significant PVF alteration, defined as 5 or more visual field location points having greater than or equal to 15 dB reduction in TVS or decline (≥33% loss) in posttreatment TVS for 1 or more rings.

RESULTS

Visual acuity decrease was detected in 1 eye of a subject receiving placebo and in none receiving vigabatrin. Posttreatment reduction in TVS more than 15 dB in 5 or more adjacent visual field location points combined with reduction in TVS greater than 33% in 1 or more of the rings was detected in 2 of 54 subjects (3.7%) from the vigabatrin group and in 1 of 49 subjects (2%) from the placebo group (P = .9, NS). None of the PVF changes were bilateral or concentric.

CONCLUSIONS

Short-term use of vigabatrin did not cause a decrease in visual acuity or significant peripheral visual field changes in cocaine abusers.

Electroretinographic (ERG) responses in pediatric patients using vigabatrin

The antiepileptic drug vigabatrin is known to cause retinal and visual dysfunction, particularly visual field defects, in some patients. Electroretinography (ERG) is used in an attempt to identify adverse effects of vigabatrin (VGB) in patients who are not candidates for conventional perimetry. We report data from 114 pediatric patients taking VGB referred for clinical evaluation; median age at test was 22.9 (2.4 to 266.1) months, and median duration of VGB use was 9.7 (0.3 to 140.7) months. Twenty-seven of them were tested longitudinally (3 to 12 ERG tests). ERG responses to full-field stimuli were recorded in scotopic and photopic conditions, and results were compared to responses from healthy control subjects. We found that abnormalities of photoreceptor and post-receptor ERG responses are frequent in these young patients. The most frequently abnormal scotopic parameter was post-receptor sensitivity, log σ, derived from the b-wave stimulus-response function; the most frequently abnormal photopic parameter was the implicit time of the OFF response (d-wave) to a long (150 ms) flash. Abnormal 30-Hz flicker response amplitude, previously reported to be a predictor of visual field loss, occurred infrequently. For the group as a whole, none of the ERG parameters changed significantly with increasing duration of VGB use. Four of the 27 patients tested longitudinally showed systematic worsening of log σ with duration of VGB use. In a subset of patients who underwent perimetry (N = 39), there was no significant association of any ERG parameter with visual field defects. We cannot determine whether the ERG abnormalities we found were due solely to the effects of VGB. We caution against over-reliance on the ERG to monitor pediatric patients for VGB toxicity and recommend further development of a reliable test of peripheral vision to supplant ERG testing.

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.

Electrophysiological evaluation of retinal function in children receiving vigabatrin medication

PURPOSE

To evaluate retinal function in children taking vigabatrin and to explore the influence of age and dose parameters on the results of full-field electroretinography (ff-ERG).

METHODS

The ff-ERGs from 14 children receiving vigabatrin were compared with ff-ERGs from healthy controls. Treated children were further grouped according to age (pre-school = 12-71 months; older = 72-228 months). Parameters of drug dosage were compared.

RESULTS

Treated children showed rod and cone dysfunction reflected by reduced b-wave amplitudes for the isolated rod response, the combined rod-cone response, and the 30-Hz flicker response. The a-wave amplitude and implicit time for the combined rod-cone response, reflecting photoreceptor function, were also altered. Further evaluation of age groups revealed similar findings in the pre-school group but not in the older group. Alterations in ff-ERG were seen in 57% of the treated children. Pre-school children had received significantly higher daily drug doses with start of medication at younger age. No differences were found concerning cumulative doses or duration of medication.

CONCLUSION

Alterations in ff-ERG are as frequent in children as in adults and the results indicate that exposure to high daily doses of vigabatrin may be associated with increased risk of retinal dysfunction, including photoreceptor damage, not previously shown in children. Thus, recommendations of careful follow-up for children receiving vigabatrin are supported.

Electroretinogram changes in a pediatric population with epilepsy: is vigabatrin acting alone?

Vigabatrin, a structural analogue of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA), is widely used as initial monotherapy in infantile spasms and add on therapy in partial onset seizures. Vigabatrin is associated with retinal toxicity causing constriction of the visual field. Our aim was to assess what effect add-on antiepileptic drug therapy has on the incidence of retinal toxicity in patients being treated with vigabatrin. Medication dosages, duration of treatment, and electroretinogram results were examined in a single center retrospective study. Retinal toxicity was detected in 18 of 160 patients (11.25%) over a 10-year period. A total of 14 (77%) were in the group treated with additional antiepileptic drugs, the other 4 received vigabatrin as monotherapy. We detected a significantly higher percentage of toxicity in the group of patients treated with vigabatrin and additional antiepileptic drugs. Our numbers were not sufficient to detect which drug or combination of drugs might be associated with higher risk.

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

AIM

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

The safety and tolerability of newer antiepileptic drugs in children and adolescents

The newer antiepileptic drugs (AEDs) provide more therapeutic options and overall improved safety and tolerability for patients. To provide the best care, physicians must be familiar with the latest tolerability and safety data. This is particularly true in children, given there are relatively fewer studies examining the effects of AEDs in children compared with adults. Since we now have significant paediatric literature on each of these agents, we provide a comprehensive and current literature review of the newer AEDs, focusing on safety and tolerability data in children and adolescents. Because the safety profiles in children differ from those in adults, familiarity with this literature is important for child neurologists and other paediatric caregivers. We have organized the data by organ system for each AED for easier reference.

Visual field loss in patients with refractory partial epilepsy treated with vigabatrin: final results from an open-label, observational, multicentre study

BACKGROUND

Use of the antiepileptic drug vigabatrin is associated with an elevated risk of visual field loss.

OBJECTIVE

To determine the frequency of, and risk factors for, vigabatrin-attributed visual field loss (VAVFL) in the setting of a large-scale, multinational, prospective, observational study.

STUDY DESIGN

A comparative, open-label, parallel-group, multicentre study.

SETTING

Hospital outpatient clinics at 46 centres in five countries.

PATIENTS

734 patients with refractory partial epilepsy, divided into three groups and stratified by age (8-12 years; >12 years) and exposure to vigabatrin. Group I comprised patients treated with vigabatrin for > or =6 months. Group II comprised patients previously treated with vigabatrin for > or =6 months who had withdrawn from the drug for > or =6 months. Group III comprised patients never treated with vigabatrin. Patients underwent perimetry at either 4- or 6-month intervals, for up to 36 months. Visual field outcome was evaluated masked to drug exposure.

INTERVENTION

Perimetry.

MAIN OUTCOME MEASURE

The visual field outcome at each of four analysis points: (i) at enrolment (i.e. baseline, all patients); (ii) for patients exhibiting a conclusive outcome at the initial visual field examination; (iii) for patients exhibiting at least one conclusive outcome to the visual field examinations; and (iv) at the last conclusive outcome to the visual field examinations.

RESULTS

Of the 734 patients, 524 yielded one or more conclusive visual field examinations. For Group I, the frequency of VAVFL at the last conclusive examination was 10/38 (26.3%) for those aged 8-12 years and 65/150 (43.3%) for those aged >12 years. For Group II, the respective frequencies were 7/47 (14.9%) and 37/151 (24.5%). One case resembling VAVFL was present amongst the 186 patients in Group III at the last conclusive examination. The frequency of VAVFL in Groups I and II combined was 20.0% for those aged 8-12 years and 33.9% for those aged >12 years. VAVFL was associated with duration of vigabatrin therapy (odds ratio [OR] up to 15.2; 95% CI 4.4, 51.7), mean daily dose of vigabatrin (OR up to 26.4; 95% CI 2.4, 291.7) and male gender (OR 2.51; 95% CI 1.5, 4.1). VAVFL was more frequently detected with static than with kinetic perimetry (OR up to 0.43; 95% CI 0.24, 0.75).

CONCLUSIONS

Since the probability of VAVFL is positively associated with treatment duration, careful assessment of the risk-benefit ratio of continuing treatment with vigabatrin is recommended in patients currently receiving this drug. All patients continuing to receive vigabatrin should undergo visual field examination at least every 6 months for the duration of treatment. We recommend two-level (three-zone), gradient-adapted, suprathreshold static perimetry of the peripheral field together with threshold perimetry of the central field out to 30 degrees from fixation. The frequency of ophthalmological and perimetric examinations should be increased in the presence of VAVFL.

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.

CNS adverse events associated with antiepileptic drugs

A variety of newer antiepileptic drugs (AEDs) are now available for treating patients with epilepsy in addition to the 'conventional' drugs that have been available throughout a large part of the last century. Since these drugs act to suppress the pathological neuronal hyperexcitability that constitutes the final substrate in many seizure disorders, it is not surprising that they are prone to causing adverse reactions that affect the CNS.Information on adverse effects of the older AEDs has been mainly observational. Equally, whilst the newer drugs have been more systematically studied, their long-term adverse effects are not clearly known. This is illustrated by the relatively late emergence of the knowledge of visual field constriction in the case of vigabatrin, which only became known after several hundred thousand patient-years of use. However, older drugs continue to be studied and there has been more recent comment on the possible effect of valproate (valproic acid) on cognition following exposure to this drug in utero.With most AEDs, there are mainly dose-related adverse effects that could be considered generic, such as sedation, drowsiness, incoordination, nausea and fatigue. Careful dose titration with small initial doses can reduce the likelihood of these adverse effects occurring. Adverse effects such as paraesthesiae are more commonly reported with drugs such as topiramate and zonisamide that have carbonic anhydrase activity. Weight loss and anorexia can also be peculiar to these drugs. Neuropsychiatric adverse effects are reported with a variety of AEDs and may not be dose related. Some drugs, such as carbamazepine when used to treat primary generalized epilepsy, can exacerbate certain seizure types. Rare adverse effects such as hyperammonaemia with valproate are drug specific. There are relatively very few head-to-head comparisons of AEDs and limited information is available in this regard.In this review, we discuss the available literature and provide a comprehensive summary of adverse drug reactions of AEDs affecting the CNS.

The negative ERG: clinical phenotypes and disease mechanisms of inner retinal dysfunction

Inner retinal dysfunction is encountered in a number of retinal disorders, either inherited or acquired, as a primary or predominant defect. Fundus examination is rarely diagnostic in these disorders, although some show characteristic features, and careful electrophysiological assessment of retinal function is needed for accurate diagnosis. The ERG in inner retinal dysfunction typically shows a negative waveform with a preserved a-wave and a selectively reduced b-wave. Advances in retinal physiology and molecular genetics have led to a greater understanding of the pathogenesis of these disorders. This review summarizes current knowledge on normal retinal physiology, the investigative techniques used and the range of clinical disorders in which there is predominantly inner retinal dysfunction.

Vigabatrin and Epilepsy: Lessons Learned

PURPOSE

The risk factors for visual field loss attributable to vigabatrin (VAVFL) are equivocal. This multinational, prospective, observational study aimed to clarify the principal/major factors for VAVFL.

METHODS

Interim analysis of three groups with refractory partial epilepsy, stratified by age (8-12 years; >12 years) and exposure to vigabatrin (VGB). Group I comprised participants treated with VGB for >or=6 months, Group II participants previously treated with VGB for >or=6 months who had discontinued the drug for >or=6 months and Group III those never treated with VGB. Perimetry was undertaken at least every six months, for up to 36 months; results were evaluated masked to drug exposure.

RESULTS

Based upon 563 participants in the locked data set, 432 yielded one or more Conclusive visual field examinations. For Group I, the frequency of VAVFL at the last Conclusive examination was 10/32 (31.2%) for those aged 8-12 years and 52/125 (41.6%) for those aged >12 years. For Group II, the proportions were 4/39 (10.3%) and 31/129 (24.0%). No cases resembling VAVFL manifested in Group III. VAVFL was associated with duration of VGB therapy (Odds ratio [OR] 14.2; 95% CI 5.0 to 40.5); mean dose of VGB (OR 8.5; 95% CI 2.2 to 33.2); and male gender (OR 2.1; 95% CI 1.2 to 3.7). VAVFL was more common with static than kinetic perimetry (OR 2.3, 95% CI 1.3 to 4.2).

CONCLUSIONS

The therapeutic benefit of VGB is counteracted by the progressive accrual of the risk of VAVFL with continued exposure and with increase in mean dose.

Antiepileptic drugs and visual function

Antiepileptic drugs are known to result in visual disturbances. A number of antiepileptic drugs have recently been reported to result in various abnormalities of vision, particularly deficiencies in visual fields and color vision. Moreover, there has been a marked improvement in the diagnosis and understanding of the pathophysiology of visual disturbance. This review collects evidence for visual adverse effects induced by the older antiepileptic drugs (barbiturates, benzodiazepine, carbamazepine, valproic acid, ethosuximide, and phenytoin) and the newer ones (vigabatrin, topiramate, tiagabine, levetiracetam, lamotrigine, gabapentin, felbamate, and oxcarbazepine).

The clinical applications of multifocal electroretinography: a systematic review

Multifocal electroretinography (mfERG) is an investigation that can simultaneously measure multiple electroretinographic responses at different retinal locations by cross-correlation techniques. mfERG therefore allows topographic mapping of retinal function in the central 40-50 degrees of the retina. The strength of mfERG lies in its ability to provide objective assessment of the central retinal function at different retinal areas within a short duration of time. Since the introduction of mfERG in 1992, mfERG has been applied in a large variety of clinical settings. This article reviews the clinical applications of mfERG based on the currently available evidence. mfERG has been found to be useful in the assessment of localized retinal dysfunction caused by various acquired or hereditary retinal disorders. The use of mfERG also enabled clinicians to objectively monitor the treatment outcomes as the changes in visual functions might not be reflected by subjective methods of assessment. By changing the stimulus, recording, and analysis parameters, investigation of specific retinal electrophysiological components can be performed topographically. Further developments and consolidations of these parameters will likely broaden the use of mfERG in the clinical setting.

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

The Eigth Eilat Conference on New Antiepileptic Drugs (AEDs)-EILAT VII, took place in Sitges, Barcelona from the 10th to 14th September, 2006. Basic scientists, clinical pharmacologists and neurologists from 24 countries attended the conference, whose main themes included a focus on status epilepticus (epidemiology, current and future treatments), evidence-based treatment guidelines and the potential of neurostimulation in refractory epilepsy. Consistent with previous formats of this conference, the central part of the conference was devoted to a review of AEDs in development, as well as updates on marketed AEDs introduced since 1989. This article summarizes the information presented on drugs in development, including brivaracetam, eslicarbazepine acetate (BIA-2-093), fluorofelbamate, ganaxolone, huperzine, lacosamide, retigabine, rufinamide, seletracetam, stiripentol, talampanel, valrocemide, JZP-4, NS1209, PID and RWJ-333369. Updates on felbamate, gabapentin, lamotrigine, levetiracetam, oxcarbazepine and new extended release oxcarbazepine formulations, pregabalin, tiagabine, topiramate, vigabatrin, zonisamide and new extended release valproic acid formulations, and the antiepileptic vagal stimulator device are also presented.

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.

Reduced visual function associated with infantile spasms in children on vigabatrin therapy

PURPOSE

To use visual evoked potential (VEP) testing to determine whether visual deficits are present in children with a history of vigabatrin use.

METHODS

Contrast sensitivity and visual acuity were assessed by visual evoked potential testing and compared between 28 children (mean age, 4.90 +/- 4.92 years) with seizure disorders who had taken vigabatrin and 14 typically developing children (mean age, 3.14 +/- 1.70 years). Exclusion criteria were heritable eye disease, suspected cortical visual impairment, nystagmus, and prematurity >2 weeks. The effects of the following factors on contrast sensitivity and visual acuity were examined: type of seizure (infantile spasms versus other), ERG result, duration of vigabatrin therapy, cumulative dosage of vigabatrin, and other seizure medications (other versus no other medication).

RESULTS

Contrast sensitivity and visual acuity were reduced in vigabatrin-treated children with infantile spasms compared with vigabatrin-treated children with other seizure disorders and typically developing control subjects. The other factors examined had no significant effect on contrast sensitivity or visual acuity, with adjustment for seizure type.

CONCLUSIONS

Children with infantile spasms on vigabatrin may have compromised visual function, even in the absence of suspected cortical visual impairment. The children tested in the present study have reduced vision, probably associated with infantile spasms rather than vigabatrin.

Characteristic retinal atrophy with secondary "inverse" optic atrophy identifies vigabatrin toxicity in children

OBJECTIVE

To describe the clinical pattern of retinal atrophy in children caused by the anticonvulsant vigabatrin.

DESIGN

An interventional case series report.

PARTICIPANTS

One hundred thirty-eight patients, mainly infants, were evaluated regularly for evidence of possible vigabatrin toxicity in the Eye and Neurology clinics at the Hospital for Sick Children, Toronto.

METHOD

Sequential clinical and electroretinographic (International Society for Clinical Electrophysiology of Vision standards) evaluations every 6 months.

MAIN OUTCOME MEASURES

Presence of recognizable retinal and optic atrophy in the presence of abnormal electroretinogram (ERG) and other clinical findings.

RESULTS

Three children being treated for seizures with vigabatrin showed definite clinical findings of peripheral retinal nerve fiber layer atrophy, with relative sparing of the central or macular portion of the retina and relative nasal optic nerve atrophic changes. Some macular wrinkling was evident in 1 case. Progressive ERG changes showing decreased responses, especially the 30-Hz flicker response, supported the presence of decreased retinal function.

CONCLUSIONS

A recognizable and characteristic form of peripheral retinal atrophy and nasal or "inverse" optic disc atrophy can occur in a small number of children being treated with vigabatrin. The changes in superficial light reflexes of the retina in children facilitate the clinical recognition of nerve fiber layer atrophy. The macula is relatively spared, although superficial retinal light reflexes indicating wrinkling of the innermost retina suggest early macular toxicity as well. Because these changes are accompanied by electrophysiologic evidence of retinal dysfunction, discontinuation of vigabatrin should be strongly considered.

The effect of GABA and the GABA-uptake-blocker NO-711 on the b-wave of the ERG and the responses of horizontal cells to light

BACKGROUND

The effects of GABA in the retina have now become of special interest because the anti-epileptic drug vigabatrin, a GABA analogue, can cause visual field loss in humans. Vigabatrin inhibits the GABA-aminotransferase, which finally results in GABA accumulation in the extracellular space. The b-wave of the electroretinogram (ERG), which originates partly in on-bipolar cells, is influenced by both GABAergic horizontal cells (HCs) and GABAergic amacrine cells (ACs). Their influences, however, are difficult to separate. In an attempt to isolate the effect of GABAergic ACs, use has been made of the specific effect of the GABA-uptake-blocker NO-711, which blocks only the GABA transporter GAT1 of GABAergic ACs.

METHODS

The ERG and the intracellular responses of HCs to light were recorded in the isolated rabbit retina, and the effects of GABA and NO-711, when added separately to the superfusate, were determined.

RESULTS

GABA reduced significantly both the light responses of HCs and the b-wave. NO-711 enlarged the b-wave drastically, but did not affect the responses of HCs to light.

CONCLUSIONS

An increase in the extracellular GABA concentration decreases the b-wave; an impairment of the function of ACs increases the b-wave. These conditions are discussed in the context of the lack of consistent changes to the b-wave during therapy with vigabatrin.

The effect of antiepileptic drugs on visual performance

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

Changes in the electroretinogram resulting from discontinuation of vigabatrin in children

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