Visual field constriction associated with vigabatrin: retinal nerve fiber layer photographic correlation

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

BACKGROUND

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

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

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.

Retinal nerve fiber layer thickness in vigabatrin-exposed patients

OBJECTIVE

Vigabatrin-associated visual field loss (VAVFL) occurs in 25 to 50% of exposed patients and is routinely monitored using perimetry, which has inherent limitations. Using optical coherence tomography (OCT), retinal nerve fiber layer (RNFL) thinning has been described in a small number of vigabatrin-exposed patients. We explored the relationship between RNFL thickness and visual field size, to determine whether OCT is a suitable tool to use in patients exposed to vigabatrin.

METHODS

Two hundred one vigabatrin-exposed subjects with epilepsy, divided into 2 groups, and 90 healthy controls participated. Visual fields were obtained using Goldmann kinetic perimetry and quantified using mean radial degrees (MRD). RNFL imaging was performed using either spectral-domain (Group 1) or time-domain (Group 2) OCT.

RESULTS

Thirty-nine of 201 (19.4%) patients were unable to perform perimetry. Thirteen (6.5%) patients were unable to perform OCT. A total of 51.6% of patients showed VAVFL. Average RNFL thickness was significantly thinner in patients (77.9 μm) compared to healthy controls (93.6 μm) (p < 0.001). There was a strong correlation between MRD and average RNFL thickness for Group 1 (r = 0.768, p < 0.001) and Group 2 (r = 0.814, p < 0.001). OCT RNFL imaging showed high repeatability.

INTERPRETATION

OCT provides a useful tool to assess people exposed to vigabatrin, and can provide an accurate estimate of the extent of visual field loss in the absence of a reliable direct measure of the visual field. The strong linear relationship found between RNFL thickness and visual field size provides some evidence that irreversible VAVFL may be related to loss of retinal ganglion cell axons.

Recommendations for visual evaluations of patients treated with vigabatrin

PURPOSE OF REVIEW

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

RECENT FINDINGS

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

SUMMARY

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

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

PURPOSE

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

DESIGN

Prospective cross-sectional observational study.

PARTICIPANTS

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

METHODS

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

MAIN OUTCOME MEASURES

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

RESULTS

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

CONCLUSIONS

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

Retinal nerve fibre layer characteristics with vigabatrin-associated visual field loss—could scanning laser polarimetry aid diagnosis?

PURPOSE

To describe the retinal nerve fibre layer (RNFL) characteristics in patients suffering vigabatrin-associated visual field loss or behaviour consistent with field loss.

METHODS

Eight patients with visual field loss attributed to vigabatrin use had their RNFL evaluated by scanning laser polarimetry.

RESULTS

All eight patients managed to perform the test reliably. The mean length of vigabatrin treatment was 81 months with a mean cumulative dose of vigabatrin being 5.4 kg. All patients had significantly reduced RNFL parameters; mean TSNIT=36.5 microm, mean nerve fibre indicator=63.1, mean superior average=42.7 microm, and mean inferior average=39.2 microm. There was no correlation between cumulative dose or time on treatment and RNFL thickness.

CONCLUSION

Scanning laser polarimetry can be performed safely and reliably on patients with vigabatrin-associated field loss. RNFL thickness is reduced in these patients with vigabatrin-associated field loss. RNFL loss concentrates on the long nerve fibres, which explains the nasal pattern of field loss seen in these patients. This investigation shows promise in the screening of vigabatrin patients, especially in those patients that may not tolerate formal field testing well.

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.