Evaluating loss of visual function in multiple sclerosis as measured by low-contrast letter acuity

INTRODUCTION

Disturbances of visual function are a common manifestation of multiple sclerosis (MS).1-3 In particular, acute demyelinating optic neuritis (ON) is a presenting symptom in approximately 15%–20% of patients with MS,2,4-6 and it may develop in up to 50% of patients during the course of the disease.6,7 In addition, up to 77% of patients with MS with no apparent visual symptoms or history of ON manifest subclinical changes in visual function, and these changes may involve the optic nerves or chiasm, or postchiasmal regions of the optic tract.8-10 A number of anatomic and functional assessments may be used to detect ophthalmic abnormalities in patients with MS, but the estimated prevalence of subclinical optic involvement among patients with MS may be lower than the true prevalence given that no single test can identify all lesions.8 For example, a small study of visually asymptomatic patients with relapsing-remitting MS (RRMS) reported that decrements in the estimated percentage of working neural channels across the visual field (measured by high-pass resolution perimetry) were universal, even in patients with no history of overt ON.9 This decrement remained after 5.5–10 years of follow-up, even though patients remained asymptomatic when evaluated by minimum angle of resolution, a conventional test of visual acuity based on the finely graduated letter chart.9 These findings suggest that visual impairments may remain undetected for long periods, depending on the sensitivity of the visual function test.

Visual acuity deficits reduce quality of life (QoL) in patients with MS.11-15 Compared with a published reference group of individuals without ocular disease, patients with MS (with or without a history of ON) demonstrated impairments in vision-specific health-related QoL assessed by the 25-item National Eye Institute Visual Function Questionnaire (VFQ-25).14 The effect on the VFQ-25 was similar to that caused by glaucoma or cataracts.14 Similarly, Ma et al.11 reported that patients with MS scored worse on 10 of the 12 VFQ-25 subscales compared with a control group. Vision-specific decreases in health-related QoL have also been observed using the 51-item National Eye Institute Visual Function Questionnaire (NEI-VFQ).12 In patients (45% with clinically definite MS) who had been treated for an acute episode of ON 5–8 years earlier, NEI-VFQ scores were lower on most subscales compared with a disease-free reference group.12 The majority of NEI-VFQ subscales showed greater dysfunction with increased neurologic disability.12 In a study evaluating QoL in patients with MS, Rudick et al.13 reported that the visual Functional System Score was the only Kurtzke Functional System Scale that significantly correlated with the total QoL score and all 4 QoL subscales of a 41-question modified version of the Farmer QoL Index.

Traditional tests of visual acuity (i.e., Snellen finely graduated letter chart) assess high-contrast visual acuity and may not identify all patients with MS with visual disturbances. In patients with MS with apparently normal high-contrast visual acuity, measurement of low-contrast letter acuity and visual evoked potentials may uncover previously undetected visual deficits.16-21 Low-contrast testing identifies the minimum size at which letters of a particular contrast level (i.e., shade of gray on white background) can be perceived.22 Low-contrast letter acuity has been found to be an informative measure of visual dysfunction in clinical trials of treatments for MS.17,23-25 This article reviews the types of visual dysfunction that occur in patients with MS, the importance and value of assessing low-contrast letter acuity, and the potential for low-contrast acuity to demonstrate treatment effects in MS clinical trials.

VISUAL DEFICITS IN MS

Acute demyelinating ON is characterized by a rapid decline in visual acuity, pain with eye movement, visual field defects, afferent pupillary defects, color vision impairment, delayed visual evoked potential, and optic nerve enhancement on orbital MRI.1,3 MRI studies have demonstrated that demyelinating lesions at any point along the afferent optic pathway may cause visual deficits. This includes defects in the chiasm, tracts, radiations, and striate cortex.3,26-29 The specific defects observed can vary widely depending on the stage at which the patient is examined, but the classic field loss is a central scotoma.1 The decline in visual acuity typically occurs over a 7–10-day period, and some recovery can be expected within 30 days of onset.3 Most patients (85%–90%) experience recovery of acuity over 1–3 months1; however, some visual deficits may persist.1,3 Chronic ON may also occur and is characterized by a gradual decline in vision accompanied by visual field loss, afferent pupillary abnormalities, and optic disc pallor.3 ON typically develops in 1 eye; however, subacute visual deficits may also be present in the other eye. The pattern of visual field defects in the other eye is variable, although diffuse loss and peripheral rim defects were the most common forms. Visual defects in the other eye are more likely in patients with a markedly depressed visual acuity in the affected eye.30

Optical coherence tomography (OCT) is capable of imaging the histologically identifiable layers of the retina in real time with high resolution, accuracy, and reproducibility. The technique allows the direct visualization and measurement of retinal nerve fiber layer (RNFL) thickness and macular volume.31,32 The RNFL is a structure that consists of isolated axons and some glia; therefore, measurement of its thickness reflects the burden of axons without the potential structural effects of myelin degeneration.33 The use of OCT in patients with MS and ON has demonstrated decreases in RNFL thickness and optic nerve thickness, indicating axonal loss in the anterior portion of the optic pathway of both affected and apparently unaffected eyes.24,31,34-39 Reductions in RNFL thickness were associated with optic nerve atrophy and impairments in visual acuity, visual field, and color vision.24,34,37 In patients who had experienced a single ON event, those whose RNFL thickness was less than 75 μm at time points 3 or more months after the acute ON event had less complete visual field recovery than those whose RNFL thickness was greater than 75 μm.34 These data suggest that decrements in RNFL thickness may predict persistent visual dysfunction after ON.34 The use of OCT in MS clinical trials may provide a meaningful outcome for assessing therapies.31

MEASUREMENT OF LOW-CONTRAST LETTER ACUITY DEFICITS IN MS

Low-contrast letter charts.

Low-contrast Sloan letter charts are readily available and provide a practical, quantitative, and standardized assessment of visual function (figure 1).17 Each chart consists of rows of gray letters (decreasing in size from top to bottom) on a white background. A set consists of 7 charts, each with a different level of contrast ranging from 100% to 0.6% (e.g., Precision Vision, LaSalle, IL).17 Letter scores indicate the number of letters identified correctly, and each chart is scored separately. Snellen visual acuity equivalent (e.g., 20/20) is also assessed in some cases and is based on the lowest line of the 100% contrast chart for which the patient is able to identify 3 of the 5 letters.17 Other types of low-contrast letter charts have been developed (e.g., low-contrast Snellen; Pelli-Robson; Smith-Kettlewell Institute Low Luminance; and Early Treatment Diabetic Retinopathy Study charts),40-43 but most studies of low-contrast letter acuity in patients with MS have used low-contrast Sloan letter charts (hereafter referred to as Sloan charts). Low-contrast letter acuity testing with Sloan charts is easy to administer and has been shown to have high interrater reliability in patients with MS and in healthy volunteers.17

Figure 1 Sloan letter charts17For example, the 100% and 5% contrast charts are shown. Balcer LJ, Baier ML, Pelak VS, et al. Mult Scler. 2000;6:163–171, copyright © 2000 by Sage Publications. Reprinted by permission of SAGE.

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Construct and predictive validity.

Several lines of evidence support the construct validity of Sloan charts for assessing visual acuity in patients with MS. Compared with healthy volunteers, patients with MS have worse low-contrast letter acuity scores, especially at lower contrast levels.17,23,24 In a substudy of the International Multiple Sclerosis Secondary Progressive Avonex Controlled Trial (IMPACT), mean letter scores were generally lower for patients with MS compared with healthy volunteers at all 4 of the contrast levels studied, with the greatest difference at the lowest contrast level (0.6%).17 This was despite similar median visual acuities based on the Snellen visual acuity equivalent (100% contrast level). Similar results were observed in patients from the Multiple Sclerosis Vision Prospective (MVP) cohort study.23 Although patients with MS and healthy volunteers had similar letter scores at 100% contrast, patients with MS had lower letter acuity scores for Sloan charts with contrast levels of 5%, 2.5%, and 1.25%.23

Studies in patients with MS also demonstrated correlations between low-contrast letter acuity and measures of disability such as the Expanded Disability Status Scale (EDSS) and the Multiple Sclerosis Functional Composite (MSFC). The EDSS assesses pyramidal, cerebellar, brain stem, bowel/bladder, sensory, and cerebral functions; scores increase with increasing disability.44 The MSFC is a composite of 3 quantitative tests of neurologic function (arm, leg, and cognitive); scores decrease with increasing disability.45 These disability scales are useful for assessing longitudinal changes in patients with MS. In patients with RRMS or secondary progressive MS participating in studies of IM interferon-beta-1a (IFNβ-1a), low-contrast letter acuity scores were significantly correlated with MSFC (positive correlation) and EDSS scores (negative correlation), with the correlation tending to be strongest with MSFC scores.25

The relationship between low-contrast letter acuity scores and brain MRI abnormalities in MS has also been demonstrated.25,29 In 1 study, patients with MS who had lower (worse) scores for low-contrast letter acuity had greater T2 lesion volumes on brain MRI scans and greater lesion volumes in visual pathway regions of the brain after adjusting for age and disease duration.29 On average, there was a 3-mm³ increase in T2 lesion volume within the whole brain for each 1-line (5-letter) worsening of low-contrast letter acuity score. A 1-line worsening in high-contrast acuity corresponded to a 5.5-mm³ increase in T2 lesion volume, suggesting greater sensitivity of low-contrast testing.29 Another study demonstrated stronger correlations between brain parenchymal fraction and low-contrast (i.e., 1.25% and 2.5%) visual acuity than for high-contrast acuity.25

Evidence from patients with MS suggests that lower (worse) scores for low-contrast letter acuity scores are associated with reduced RNFL thickness measured by OCT (table 1).24 A study compared 90 patients with MS with 36 disease-free controls, all with Snellen acuity equivalents of 20/20 or better.24 After adjusting for age, there was a 3- to 4-μm decrease in RNFL thickness for each 1-line reduction in low-contrast letter acuity score (table 1). There was a modest but highly significant correlation between mean RNFL thickness and visual function scores (p< 0.0001).24

Table 1 Association of worsening in visual function score and reduction in RNFL thickness in patients with MS (n = 180)24

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Baier et al.25 reported that low-contrast letter acuity scores were predictive for changes in MS disability and functionality. In this substudy of the IMPACT trial, 65 patients underwent low-contrast letter acuity testing. Change in low-contrast letter acuity scores from baseline to 1 year significantly predicted change in EDSS scores from year 1 to 2, after controlling for change in MSFC scores from baseline to 1 year (table 2).25 Thus, use of low-contrast letter acuity testing imparted additional value to the MSFC with respect to prediction of subsequent changes in the EDSS.25

Table 2 Predictive value of change in low-contrast letter acuity and MSFC score from baseline to year 1 for change in EDSS from year 1 to 2 in an IMPACT substudy25

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An analysis of data from an IMPACT substudy and patients enrolled in the observational MVP cohort study evaluated the inclusion of low-contrast letter acuity testing as a potential fourth component of the MSFC.23 In the MVP cohort study, low-contrast letter acuity at the 1.25% level was better in distinguishing between patients with MS and healthy controls than high-contrast (Snellen) acuity (p< 0.0001).23 In patients with MS in the MVP cohort and the IMPACT substudy, low-contrast letter acuity scores significantly correlated with MSFC and EDSS scores in patients with MS (table 3).23 Nevertheless, these correlations were moderate because low-contrast letter acuity measured aspects of neurologic disability were not captured by the MSFC or EDSS.23 When visual function as measured by low-contrast letter acuity was added to other components of the MSFC to compute a 4-component MSFC Z-score, each component, including low-contrast letter acuity, had similar correlations with composite scores, demonstrating that each component contributed similarly to the overall score.23

Table 3. Rank correlations of low-contrast Sloan letter scores (1.25% contrast level) and MSFC component scores with MSFC-3, MSFC-4, and EDSS scores in patients with MS from the MVP cohort and IMPACT substudy23

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VISUAL ACUITY TESTING IN CLINICAL TRIALS OF DISEASE-MODIFYING THERAPIES

With the exception of natalizumab, data relevant to the effects of disease modifying therapies (DMTs) on low-contrast letter acuity in patients with MS with or without ON are scarce. To our knowledge, no study to date has evaluated the effects of glatiramer acetate or IFNβ-1b on visual function in patients with MS. Low-contrast letter acuity was assessed in subsets of patients from a phase 3 study of IM IFNβ-1a in patients with RRMS and a study of IM IFNβ-1a in patients with secondary progressive MS; however, treatment effects on visual function were not reported.23,25 In a small (N = 27), noncontrolled study, patients with ON as the initial symptom of RRMS were treated with IV methylprednisolone followed by subcutaneous IFNβ-1a.31 An interim analysis conducted after ≤16 months of follow-up found that RNFL thickness in the ON-affected eye was less than that in the unaffected eye. However, these differences did not reach statistical significance.31 Furthermore, because the study had no controls, no conclusions can be drawn about the effects of subcutaneous IFNβ-1a on visual function.

The ability of low-contrast letter acuity testing to capture treatment effects was demonstrated for the first time in the phase 3 studies of natalizumab as monotherapy (AFFIRM study) and in combination with IM IFNβ-1a (SENTINEL study) in patients with RRMS.22 Changes in visual acuity were measured using low-contrast letter acuity charts at high contrast (black on white) and 2.5% and 1.25% low-contrast levels.22Figure 2 shows mean change from baseline in low-contrast (2.5%) letter acuity scores over time in the AFFIRM study.22 Patients in the placebo group showed a significant worsening of low-contrast letter acuity compared with patients in the natalizumab group as early as 12 weeks.22 There was no significant difference between treatment groups for visual acuity measured at the high-contrast (100%) level.22

Figure 2 Mean changes from baseline in low-contrast letter acuity scores (2.5% contrast level) in patients who received natalizumab vs placebo in the AFFIRM study22

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The cumulative probability of a sustained decrease in low-contrast letter acuity (defined as ≥2-line [10-letter] reductions in letter scores sustained over 12 weeks) was also reduced in the natalizumab group compared with placebo (figure 3). In the AFFIRM study, the risk of sustained worsening with natalizumab (vs placebo) was reduced by 47% at the 2.5% contrast level (p< 0.001) and by 35% at the 1.25% contrast level (p = 0.008).22 There was no significant difference between groups in sustained visual acuity worsening measured at the 100% contrast level.22 Differences were less pronounced in the SENTINEL study, in which placebo plus IFNβ-1a was compared with a natalizumab plus IFNβ-1a combination.22 The difference between treatment groups was most robust at the 1.25% contrast level at the end of the 2-year study. The cumulative probability of sustained low-contrast (1.25%) visual acuity loss was 28% lower in the natalizumab plus IM IFNβ-1a group compared with the placebo plus IM IFNβ-1a group (p = 0.038). The risk reduction at the 2.5% contrast level was not statistically significant.22 Overall, these data suggest that low-contrast letter acuity is a sensitive indicator of treatment effects in patients taking DMTs for MS.22

Figure 3 Kaplan-Meier plots of time to sustained worsening of visual acuity scores among patients receiving natalizumab compared with placebo in the AFFIRM study22

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CONCLUSION

Visual dysfunction is a unique aspect of neurologic status in patients with MS. Although acute ON is the most well-recognized ophthalmological manifestation of MS, patients may experience declining visual function in the absence of ON. Deficits in visual acuity have a marked effect on QoL in patients with MS. Measures of low-contrast letter acuity have a greater sensitivity to changes in visual function in patients with MS compared with assessments of high-contrast visual acuity. Low-contrast letter acuity is predictive for the presence of MS and is significantly correlated with other disease markers (e.g., disability scores, MRI findings, and RNFL thickness). Furthermore, low-contrast letter acuity scores are predictive for changes in MS disability and functionality. It has been suggested that low-contrast letter acuity testing may have utility as a fourth component of the MSFC. Despite the impact of visual abnormalities on daily functioning and QoL in patients with MS, the effects of DMTs on low-contrast letter acuity are not well studied. However, 2 phase 3 studies of natalizumab showed that low-contrast letter acuity testing, included as an exploratory outcome, can demonstrate treatment effects, and other trials have subsequently incorporated low-contrast acuity and OCT measures of RNFL thickness.

ACKNOWLEDGMENT

This supplement was supported by funding from Biogen Idec, Inc. and Elan Pharmaceuticals, Inc. The authors thank Paul Benfield, Maria D'Alessandro, Matthew Hasson, Natasha Kuchnir, and Michael Theisen of Scientific Connexions, Newtown, Pennsylvania, for editorial assistance in preparing this supplement. Mr. Benfield, Mr. Hasson, and Mr. Theisen were responsible for technical and mechanical editing of the manuscript for non-intellectual content and preparing electronic files for submission to the publisher. Ms. D'Alessandro provided word processing and copyediting for the manuscript, ensured compliance with Neurology journal style, and obtained copyright permissions on behalf of the authors where appropriate. Ms. Kuchnir provided medical writing on the first draft of the manuscript based on a content outline provided by the authors. Subsequent drafts and the final manuscript were reviewed, revised, and approved by the authors. This support was funded by Biogen Idec and Elan Pharmaceuticals.

DISCLOSURE

Dr. Balcer has served on a scientific advisory board for and received compensation for travel/honoraria from Biogen Idec; received honoraria from Bayer; and received research funding from the National Eye Institute and the National MS Society. Dr. Frohman has received compensation for travel/honoraria from Abbott, Biogen Idec, and Teva and served as a consultant and on speakers bureaus for Biogen Idec and Teva.

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