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

Rarebit Perimetry in the Evaluation of Visual Field Defects in Idiopathic Intracranial Hypertension

Purpose.

Rarebit perimetry (RBP) is a new perimetric technique that has been reported as a sensitive test for the evaluation of the neural structure of the visual system. The aim of this study was to compare RBP with standard automated perimetry in detecting the visual field defects of patients with idiopathic intracranial hypertension (IIH).

Methods.

Twenty-five patients with IIH and 16 healthy controls were considered. Visual fields were analyzed with Marco automated static perimeter with threshold and central 30–2 programs (MP 30–2) and rarebit perimetry. A visual field was classified into 3 zones for both tests as central, paracentral, and peripheral zones. Mean sensitivity (MS) recorded by the MP 30–2 was compared with the mean hit rate (MHR) recorded by RBP for each zone.

Results.

MS recorded by MP 30–2 and MHR recorded by RBP for each zone were significantly lower in the IIH group than in healthy control eyes (p<0.001). Good correlation was present between the visual fields recorded by both tests. Although the sensitivity and specificity values for each zone were lower in the RBP group than the MP 30–2 group, RBP was found to be efficient to differentiate between the visual fields of normal and IIH-Affected eyes. The average RBP test duration was significantly shorter than the MP 30–2 test duration (p<0.001).

Conclusions.

RBP is a rapid, comfortable, and widely accessible perimetric test giving reliable results in detecting visual field defects in IIH.

Relationship between the area of isopters and Vigabatrin dosage during two years of observation

Background

The aim of the study was to evaluate the relationship between the area of isopters obtained using semi-automated kinetic perimetry (SKP) and Vigabatrin dosage in epilepsy patients with pretreatment baseline examination during 2-years of the follow-up.

Methods

29 epilepsy patients were included into the study, but 15 individuals were excluded due to cognitive impairment, intracranial pathologies or eye diseases. Finally, 14 patients were examined with SKP before VGB treatment and after 6, 12, 18, and 24 months. Reaction time (RT)-corrected areas of three isopters (III4e, I4e and I2e) were measured for each of five examinations and compared intra-individually during 2-years period. Additionally, six epilepsy patients on other antiepileptic drugs were examined five times with SKP as a control.

Results

There was a significant decrease of I2e, I4e and III4e isopters’ area during the follow-up of two years. Correlation was found between the I2e isopter’s area and both cumulative dose and mean daily dose of VGB. With increasing RT, there was decreasing of all isopters’ area in patients receiving VGB. In epilepsy patients who were not receiving VGB, there were no significance differences in isopters’ area during follow-up.

Conclusion

There was attenuation of area of III4e, I4e and I2e isopters obtained with SKP during a period of 2 years. RT, the cumulative dose and the mean daily dose of VGB influenced isopters' area obtained with SKP.

Rapid assessment of neurovisual integrity using multiple rarebits

OBJECTIVE

Rapid assessments of vision commonly rely on visual acuity testing alone. Although well suited for uncovering optical defects, visual acuity tests may fail to detect dysfunction of the neural substrate. There is a need for a rapid companion test sensitive to neural damage.

DESIGN

Evaluation of diagnostic tests.

PARTICIPANTS AND CONTROLS

Forty-seven patients with optic nerve or visual pathway lesions of low to moderate severities and 30 normal subjects.

METHODS

A new computer-based quick test of neurovisual integrity was developed using segmented digits defined by rarebits, that is, receptive field-size bright dots briefly presented on a dark background.

MAIN OUTCOME MEASURES

The test variable was the number of rarebits per segment; digit size was fixed. The test task was to call out verbally all digits that were seen during an 8-second presentation sequence. Test outcomes were contrasted with the results of an optotype acuity test and automated perimetry.

RESULTS

All subjects easily grasped the test task. The normal subjects read, on average, 5.6 (± 0.5 standard deviation) of the 6 digits contained in the test sequence, whereas the patients averaged 3.3 ± 1.8 digits. The numbers of digits read correlated modestly with the acuity and perimetry results. Analysis of receiver operating characteristic curves indicated that the multiple rarebit test provided the best discrimination.

CONCLUSIONS

The multiple rarebit test seemed to be highly capable of detecting neurovisual dysfunction. Its simplicity and uniquely short duration indicate a useful role in screening settings. The test is available free on the Internet.

FINANCIAL DISCLOSURE(S)

Proprietary or commercial disclosure may be found after the references.

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.

The relationship between Rarebit perimetry and OCT-derived retinal nerve fibre layer thickness in glaucoma

PURPOSE

To examine the association between measures of neuroretinal matrix integrity as determined with Rarebit perimetry and optical coherence tomography (OCT)-derived retinal nerve fibre layer thickness.

METHODS

One randomly selected eye of 30 White primary open-angle glaucoma patients (age: 60.9 +/- 11.7 years; MD: -3.2 +/- 5.1 dB) and 16 healthy White individuals (age: 33.2 +/- 6.4 years; MD: -0.8 +/- 0.8 dB) were included in the study. Participants underwent Rarebit perimetry testing (central field, software version 4) and an OCT fast retinal nerve fibre layer (RNFL) scan. Correlation was investigated between hemifield Rarebit scores and the corresponding RNFL values, as well as between global Rarebit scores and the respective RNFL measures.

RESULTS

Statistically significant correlations of average hit rate (HR) < 90 and mean hit rate (MHR) were detected with Max-Min and average thickness (Pearson's r ranging from 0.393 to 0.474). Number HR < 90 showed a moderate correlation only with Max-Min (r = -0.396, P = 0.030). Regarding the association between hemifield hit rates and the corresponding OCT thickness parameters, only inferior maximum correlated moderately with HR superior (r = 0.385, P = 0.035). A tendency was detected for the relationship of superior maximum with HR inferior (r = 0.345, P = 0.062). For the control group, no significant correlation was found for any of the global or hemifield indices and the corresponding thickness values.

CONCLUSION

Although Rarebit perimetry is based on a physiological principle distinctly different from conventional perimetry, it provides global indicators of neuroretinal matrix integrity that correlate with some OCT-derived RNFL thickness measures.

[Not Available]

Purpose of the study was to compare Rarebit Perimetry (RP) with Standard Automatic Perimetry (SAP) in patients with ischemic retrochiasmal lesions. Seventeen patients with homonymous hemianopia underwent SAP and RP. Correlation between the two techniques was evaluated, congruent defects were quantified in the spared hemifield and residual vision areas were investigated in the blind hemifield. A strong correlation was found between SAP and RP. In the spared hemifield, RP detected congruent defects in a higher percentage of cases. Residual detection function has been found at RP in 69,2 % of perimetrically blind visual field quadrants as assessed by SAP. In conclusion, RP appears to be useful to assess visual function in hemianopic patients. As compared to SAP, it seems able to detect small congruent defects in a higher percentage of cases. Finally, a certain residual function seems to be present at RP in the blind regions of the visual field, however, for this finding further demonstrations are required.

Rarebit perimetry and frequency doubling technology in patients with ocular hypertension

PURPOSE

To test the capability of rarebit perimetry (RP), a recent non-conventional perimetric technique, in detecting early functional damage in subjects with ocular hypertension (OHT) and to compare RP findings with those obtained by frequency-doubling technology (FDT) perimetry.

METHODS

Thirty patients with OHT were matched with 30 healthy subjects. All were tested with RP and FDT. Frequency-doubling technology mean deviation (MD) and pattern standard deviation (PSD), as well as RP mean hit rate (MHR), of the two groups were analyzed. The agreement between the two techniques was tested by Kappa analysis.

RESULTS

In the OHT group the mean (SD) FDT MD was 0.5 (2.1), the mean (SD) FDT PSD was 4.2 (1.6), and the mean (SD) RP MHR was 81.4 (6.7). In the control group, corresponding values were mean (SD) FDT MD 1.1 (1.4), mean (SD) FDT PSD 3.0 (0.3), mean (SD) RP MHR 96.2 (2.0). The differences between the two groups were not significant for the studied indexes. Eleven (36.6%) out of the 30 OHT eyes had abnormal RP results; 12 (40.0%) eyes had abnormal FDT results. Five (16.6%) eyes had abnormal RP and FDT findings. Only 1 eye (3.3%) in the control group had abnormal RP results and 3 eyes (10.0%) had abnormal FDT results. RP and FDT showed a moderate agreement (Kappa=0.43; 95% CI: 0.42 to 0.51).

CONCLUSIONS

RP and FDT showed VF defects not shown in standard automated perimetry in the OHT group. This may be indicative of an increased risk in developing glaucoma, even if a gold standard for detecting subtle defects is not currently available. RP has the additional advantage of not requiring any expensive device to be used. The poor agreement between these techniques in identifying eyes with early damage warrants further investigations. Large longitudinal studies are needed before defining the role of RP in early glaucoma diagnosis.

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.

Efficiency of Rarebit perimetry in the evaluation of homonymous hemianopia in stroke patients

AIMS

To compare the efficiency of Rarebit perimetry and the Humphrey field analyser (HFA) in detecting the homonymous hemianopia in stroke patients with occipital lobe infarcts.

METHODS

40 patients who suffered from visual complaints caused by acute occipital lobe infarcts underwent visual field analysis on the same day, in random order-first with either Humphrey perimetry 30-2, SITA standard program (Zeiss Humphrey Systems) or Rarebit perimetry. A visual field was classified into four quadrants for right and left eyes: superior temporal, superior nasal, inferior temporal, and inferior nasal. The entire mean hit rate numbers (MHR) and mean deviation and pattern standard deviation (PSD) values were compared for each quadrant of each eye.

RESULTS

The results of Rarebit MHR and HFA mean deviation values for each quadrant of the right and left eyes were highly correlated in all patients with homonymous hemianopia (Pearson's r correlation coefficients for superior temporal, superior nasal, inferior temporal and inferior nasal quadrants of right and left eyes were 0.827, 0.833, 0.843, 0851 and 0.746, 0821, 0882, 0.824, respectively (p<0.001 for all quadrants)). There was a strong correlation between Rarebit MHR and HFA PSD for each quadrant of both eyes.

CONCLUSIONS

Rarebit perimetry is rapid, reliable, and easily performed in patients with homonymous hemianopia. It can be done using a simple software program and simple hardware and it readily detects severe visual loss in patients with occipital lobe lesions.

Perception of very small visual stimuli in the fovea: normative data for the Rarebit Foveal Test

BACKGROUND

Conventional visual tests are not sensitive enough to detect low degree neural damage, as 6/6 (1.0) visual acuity can be upheld with less than two-thirds of the normal number of optic nerve axons. The aim of the current study was to evaluate the physiologic properties of a new computerised test, the Rarebit Fovea Test (RFT), using very small stimuli, by quantifying the effect of age and binocular summation in relation to stimulus luminance.

METHODS

The RFT relies on the perception of very small (less than 0.5 minutes of arc) bright stimuli. Two different experiments were performed. 1. Thirty-five subjects (age 19 to 63 years) were tested with five different stimulus luminances, 158, 64, 53, 41 and 33 cd/m(2). 2. Nineteen subjects (age 19 to 63 years) were tested using binocular stimulation to define the binocular summation.

RESULTS

Significantly reduced median hit rates were observed at luminances of 53 cd/m(2) or below. Age and mean hit rate correlated negatively at all luminance levels below 158 cd/m(2). The mean hit rate from binocular stimulation, compared to the highest value from monocular stimulation in the same subject, was increased by a factor of 1.54 +/- 0.45 (SD). No age effect was found regarding binocular summation.

CONCLUSIONS

The results in the current study indicate that RFT can identify some of the well-known features of the visual system, that is, the effects of age and binocular summation, provided that the stimulus luminance is adequately selected.

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.