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Show Journal of Neuro- Ophthalmology 21( 2): 124- 131, 2001. © 2001 Lippincott Williams & Wilkins, Inc., Philadelphia The SKILL Card Test in Optic Neuritis: Experience of the Optic Neuritis Treatment Trial Danielle T. Long, MSPH, Roy W. Beck, MD, PhD, Pamela S. Moke, MSPH, R. Clifford Blair, PhD, Kevin E. Kip, PhD, Robin L. Gal, MSPH, Barrett J. Katz, MD, MBA, and the Optic Neuritis Study Group Objectives: To assess the value of the Smith- Kettlewell Institute Low Luminance ( SKILL) Card test, designed to measure vision at reduced contrast and luminance, among patients with previous optic neuritis. Materials and Methods: The SKILL Card test was administered to 295 patients participating in the Optic Neuritis Treatment Trial ( ONTT) follow- up study, concurrent with measurement of visual acuity, visual field, contrast sensitivity, and color vision. Health- related quality of life ( HRQL) was also assessed in a subset of patients using the National Eye Institute Visual Function Questionnaire and an ONTT-developed questionnaire. Results: The SKILL Card difference score ( high- contrast acuity score minus low- contrast acuity score) was only weakly associated with the other measures of vision function ( rs absolute range, 0.05- 0.31) and with the HRQL measures ( rs absolute range, 0.02- 0.15). In contrast, the light and dark component scores of the SKILL Card test had higher associations with the other vision measures ( rs absolute range, 0.27- 0.54) and with the HRQL measures ( rs absolute range, 0.10- 0.40). Conclusions: The SKILL Card difference score is not a meaningful measure for patients with optic neuritis; however, the test appears to have clinical usefulness as a method to measure high- contrast and low- contrast acuity. Key Words: SKILL Card- High- contrast visual acuity- Low-contrast visual acuity- Optic neuritis- Health- related quality of life. Most cases of optic neuritis recover to normal or near normal visual acuity, with or without corticosteroid treatment ( 1,2). However, even when acuity returns to normal, patients often still report visual difficulties ( 3,4). Some patients with normal acuity and perceived visual deficits may have abnormal contrast sensitivity, color Manuscript received November 2, 2000; accepted April 8, 2001. Supported by cooperative agreement EY05435 from the National Eye Institute of the National Institutes of Health. From the Jaeb Center for Health Research ( DTL, RWB, PSM, RCB, KEK, RLG), Tampa, Florida; and the George Washington University Medical Center ( BJK), Washington, DC. Address correspondence and reprint requests to Roy W. Beck, MD, PhD, Jaeb Center for Health Research, 3010 E. 138th Ave. Suite 9, Tampa, FL 33613; e- mail: rbeck@ jaeb. org. vision, or visual field, whereas in other patients, these too may be normal. The Smith- Kettlewell Institute Low Luminance ( SKILL) Card test was designed to rapidly measure vision at reduced contrast and luminance. It has been reported to be a sensitive measure of optic nerve dysfunction that can detect visual abnormality in symptomatic post- optic neuritis patients ( 5). The test consists of two letter charts, mounted back- to- back and designed for a test distance of 40 cm. One side is a high- contrast black-on- white letter chart with letter sizes from 20/ 12.5 to 20/ 630; the other side is a low- contrast ( approximately 14%) chart comprised of black letters ranging in size from 20/ 20 to 20/ 630 on a dark gray background ( approximately 10% of the reflectance of white paper) designed to simulate low- luminance conditions. The background reflectance of the dark side results in a luminance approximately 1 log unit less than that of the light side. A difference score is computed by subtracting the number of letters correct on the dark chart from the number correct on the light chart. In a previous study of 15 post- optic neuritis patients ( with visual acuity of 20/ 40 or better), the SKILL Card difference score was abnormal in 73% of affected eyes and in 50% of eyes that had recovered to 20/ 20 or better acuity. In addition, 40% of unaffected fellow eyes had abnormal difference scores ( 6,7). The difference scores did not correlate with high- or low- contrast visual acuity, contrast sensitivity, color vision, or short- and middle-wavelength- sensitive cone increment thresholds, suggesting to the study's authors that it may be measuring a unique aspect of visual function. We had the opportunity to assess the value of the SKILL Card test in a much larger population by administering it to patients participating in the Optic Neuritis Treatment Trial ( ONTT). We describe the distribution of SKILL Card test scores and evaluate the relationship between SKILL Card test results and standard clinical measures of vision function. In addition, to evaluate whether the SKILL Card measures an aspect of vision function not measured by other vision tests, we assessed 124 SKILL CARD TEST IN OPTIC NEURITIS 125 the relationship between the SKILL Card results and two health- related quality- of- life ( HRQL) measures. METHODS The protocol, baseline characteristics of patients, and ONTT treatment trial results have been reported in previous publications ( 8- 12). Briefly, 457 patients between ages 18 and 46 years with acute unilateral optic neuritis and no indication of a causal systemic disease other than multiple sclerosis ( MS) were enrolled at 15 clinical centers. The primary visual outcome from treatment was assessed after 6 months. Patients continued to be monitored yearly to assess visual and neurologic courses. The study protocol was approved by the institutional review board at each clinical center. Written informed consent was obtained from each patient. From 1995 to 1997 ( 5- 8 years from study entry), the SKILL Card test was included as part of the testing at the annual examination of 311 patients. Because interpretation of the SKILL Card difference score requires good high- contrast acuity, patients with a visual acuity worse than 20/ 50 in one or both eyes were excluded from the analysis, reducing the sample size to 295. The HRQL analysis was conducted on a subset of this cohort, comprised of patients who had completed one or both of the HRQL measures. The National Eye Institute Visual Function Questionnaire ( NEI- VFQ) ( 13) was completed by 176 of the 295 patients, and a visual function questionnaire developed specifically for the ONTT ( ONTT questionnaire) ( 3) was completed by 89 of the 295 patients ( 55 of whom were among those who completed the NEI- VFQ and 34 of whom were not). SKILL Card and other visual function tests The SKILL Card test was administered according to the testing protocol provided to us by its developers. A difference score was then calculated by subtracting the dark side score from the light side score. Based on a previous study of 203 individuals whose ages were similar to those of patients in our study, and unpublished information from the authors of that study, the normal range was considered to be less than or equal to 28 letters for the difference score, greater than or equal to 76 letters ( logMar value = 0.07) for the light score, and greater than or equal to 56 letters ( logMar value = 0.63) for the dark score ( 5). The logMar values for the light and dark scores were used in the analyses. During the same examinations, the other vision tests, administered by standard protocols ( 14,15), included: 1) visual acuity with a retroilluminated Early Treatment Diabetic Retinopathy Study ( ETDRS) chart ( normal range: logMar value < 0.0 [ better than 20/ 20]), 2) contrast sensitivity with the Pelli- Robson chart ( normal range > line 15 [ approximately 1.75 log units]), 3) color vision with the Farnsworth- Munsell 100- hue test ( normal range: error score < 110), and 4) visual field with the Humphrey Field Analyzer ( normal range: mean deviation > - 3.00). Health- related quality- of- life measures From 1996 to 1997, the 51- item field test version of the NEI- VFQ was included as part of the testing at annual examinations. Written instructions preceded the self- administered test. Thirteen subscales, scored on a 0 to 100- point scale ( with 100 indicating highest function), were generated to assess areas of vision- related quality of life. For the analysis, only six subscales directly related to vision were used: general vision, near vision activities, distance vision activities, driving, color vision, and peripheral vision. A subscale was considered missing when one or more of its component items was missing. The question " At the present time, would you say your eyesight ( with glasses or contact lenses, if you wear them) is excellent, good, fair, poor, very poor, or are you completely blind?" was used to create a dichotomous variable: excellent/ good versus others. The ONTT questionnaire, originally used for a treatment group comparison after 6 months in the ONTT ( 3), was administered a second time after the fifth annual visit. The questionnaire consisted of 29 questions that rated particular aspects of visual function and daily visual activities on a scale of 1 ( excellent) to 5 ( very poor). For analysis, the number of questions was reduced to 24 by excluding five items that in an internal consistency analysis correlated less than 0.30 with the average score of all items. A continuous test score was created as the average score of the 24 remaining answered questions. When responses were missing for five or more applicable questions, the questionnaire was not scored ( this event applied to one patient). A dichotomous variable was created to distinguish patients who responded with " poor" or " very poor" on at least one question from those who did not. Statistical analyses Paired t tests and McNemar tests were used to compare the mean score and the proportion abnormal, respectively, on the SKILL Card and other vision tests between eyes with and without previous optic neuritis among the 230 patients with previous unilateral optic neuritis. Using data from the full cohort of 295 patients, a chi- squared test for trend was performed to assess whether the proportion of eyes with abnormal SKILL Card scores varied monotonically according to the number of other abnormal vision tests. Because of the skewed distributions of the vision test results, Spearman rank correlation coefficients, rs ( 16), were computed to assess the association of the SKILL Card difference scores with the other visual function measures, and the association of each of the visual function measures and the questionnaire scores. When both eyes of a patient were included in an analysis, the P value was adjusted according to the method of Gauderman and Barlow ( 17,18). To ensure conservative estimation, we corrected for the maximum intereye intraclass correlation across all of the vision measures ( r = 0.52). For the HRQL analysis, univaria t e logistic regression models were fit to estimate the odds ratio of an abnormal questionnaire score ( as previously defined) comparing patients whose best eyes were J Neuro- Ophthalmol, Vol. 21, No. 2, 2001 126 D. T. LONGETAL. TABLE 1. Distribution of SKILL Card and other vision measure scores inpatients with previous unilateral optic neuritis: a comparison of eyes with and without previous optic neuritis Measure of vision function Eyes with a history of optic neuritis Mean ( SD) [% abnormal] ( N = 230) Eyes without a history of optic neuritis Mean ( SD) [% abnormal] ( N = 230) P value* SKILL Card difference score SKILL Card light score SKILL Card dark score Visual acuity Contrast sensitivity Color vision Visual field 28 ( 10) [ 39] 0.02 ( 0.16) [ 30] 0.57 ( 0.25) [ 33] - 0.07 ( 0.14) [ 24] 14.2 ( 1.5) [ 48] 114.8 ( 131.1) [ 32] - 1.8 ( 3.2) [ 27] 25 ( 7) [ 30] - 0.04 ( 0.14) [ 19] 0.46 ( 0.18) [ 18] - 0.12 ( 0.11) [ 13] 15.4 ( 0.66) [ 8] 67.1 ( 56.3) [ 16] - 0.38 ( 2.3) [ 10] < 0.001/ 0.02 < 0.001/ 0.001 < 0.001/ 0.001 < 0.001/ 0.001 < 0.001/ 0.001 < 0.001/ 0.001 < 0.001/ 0.001 Missing observations: color vision ( 18), visual field ( 19). SKILL Card difference scores are reported in number of letters ( normal range £ 28 letters). SKILL Card light scores are reported in logMar conversions ( normal range £ 0.07 [ a 77 letters]). SKILL Card dark scores are reported in logMar conversions ( normal range £ 0.63 [ a 57 letters]). Visual acuity measured with a retroilluminated ETDRS chart ( normal range: logMar value < 0.0 [ better than 20/ 20]). Contrast sensitivity was measured with a Pelli- Robson chart ( normal range a line 15 [ approximately 1.75 log units]). Color vision was measured with the Farnsworth- Munsell 100- hue test ( normal range: color vision error score £ 110). Visual field was measured with the Humphrey Field Analyzer ( normal range: mean deviation a - 3.00). * The first P value is from a comparison of the means by a paired t test, and the second P value is from a comparison of the percent abnormal in each group by McNemar test. SD, standard deviation; SKILL, Smith- Kettlewell Institute Low Luminance. in the lowest quartile on a measure with the 75% of patients with a better score on the measure. Similar models were fit for each measure comparing patients with a normal value on the measure in both eyes with those patients with an abnormal value in at least one eye. All reported P values are two sided. Analyses were performed using SAS version 6.12 on a UNIX platform. RESULTS The 295 patients had an average age of 39 (± 7) years at the time of evaluation; 79% were women and 88% were white. Before the SKILL Card testing, 230 of the 295 patients had experienced optic neuritis in one eye and 65 had experienced optic neuritis in both eyes. The 176 patients who completed the NEI- VFQ were similar in age, sex, and race to the 119 patients who did not complete the NEI- VFQ. The 89 patients who completed the ONTT questionnaire were comparable to the 206 who did not with respect to age and sex, but a higher proportion of patients was white ( 97% vs. 85%; P = 0.003). Compared with the 162 ONTT patients who were not included in the study, the 295 patients included were slightly older on entry into the ONTT ( mean 32 ± 7 years vs. 31 ± 6 years, P = 0.03) and more likely to be white ( 88% vs. 79%, P = 0.009). Distribution of SKILL Card and other vision measure scores in eyes with and without optic neuritis Among the 230 patients with one affected and one unaffected eye, the SKILL Card difference scores ranged from 1 to 73 in the eyes with previous optic neuritis and from 5 to 46 in eyes without previous optic neuritis. The eyes with previous optic neuritis had a mean light score of 79 ± 8 ( logMar: 0.02 ± 0.16), mean dark score of 51 ± 13 ( logMar: 0.57 ± 0.25), and mean difference score of 28 ± 10, whereas eyes without previous optic neuritis had scores of 82 ± 7 ( logMar: - 0.04 ± 0.14), 57 ± 9 ( logMar: 0.46 ± 0.18), and 25 ± 7, respectively ( Table 1). As with the three SKILL Card measures, scores on each other visual function test were statistically worse in the eyes with previous optic neuritis than in the eyes without previous optic neuritis ( P = 0.02 or smaller for all comparisons). However, the degree of abnormality generally was slight ( Table 1). TABLE 2. Percentage of eyes with abnormal SKILL Card scores according to the number of abnormal other vision measures ( N = 550) Number of other vision tests abnormal* 0 1 2 3 4 P valuet Number of eyes 263 146 62 50 29 Eyes with abnormal SKILL Card difference score 76 ( 29) 55 ( 38) 28 ( 45) 31 ( 62) 19 ( 66) < 0.001 N (%) Eyes with abnormal SKILL Card light score N 25 ( 10) 43 ( 29) 17 ( 27) 26 ( 52) 23 ( 79) < 0.001 (%) Eyes with abnormal SKILL Card dark score N (%) 27 ( 10) 43 ( 29) 23 ( 37) 32 ( 64) 26 ( 90) < 0.001 Forty eyes did not have complete data for all vision tests and were excluded from the analysis. SKILL Card normal ranges- difference score: s 28 letters; light score: a 77 letters ( logMar value £ 0.07); dark score a 57 letters ( logMar value s 0.63). * Visual acuity, contrast sensitivity, color vision, and visual field. t Corrected P value for trend. SKILL, Smith- Kettlewell Institute Low Luminance. / Neuro- Ophthalmol, Vol. 21, No. 2, 2001 SKILL CARD TEST IN OPTIC NEURITIS 127 TABLE 3. Spearman correlation of conventional vision test scores with SKILL Card test scores ( N = 590) Measure of vision function Correlation with light score ( rs) Correlation with dark score ( rs) Correlation with difference score ( rs) Visual acuity Contrast sensitivity Color vision Visual field 0.54* 0.33* 0.30* 0.27* 0.43* 0.47* 0.36* 0.32* 0.05 0.31* 0.19* 0.16t Missing observations: color vision ( 18), visual field ( 23). Visual acuity was measured with a retroilluminated ETDRS chart. Contrast sensitivity was measured with a Pelli- Robson chart. Visual field was measured with the Humphrey Field Analyzer. Color vision was measured with the Farnsworth- Munsell 100- hue test. In order to avoid potential interpretative difficulties, the signs of the correlation coefficients have been adjusted where necessary so that all visual measures are scaled in such manner as to associate higher scale values with better visual performance and lower values with worse visual performance. * Corrected P value < 0.001. t Corrected P value = 0.002. Relationship of SKILL Card scores with other vision measures In an analysis including all eyes with complete data, abnormal SKILL Card difference, light, and dark scores were all positively related to the number of other abnormal vision tests ( all corrected P values for trend < 0.001; Table 2). However, among the 263 eyes that were in the normal range on each of the other four vision measures, 76 ( 29%) had an abnormal difference score. These 76 eyes were no more likely to have experienced previous optic neuritis ( 47% vs. 47%, P = 1.00) or to be from patients with MS ( 47% vs. 38%, P = 0.17) than the 187 eyes with a normal difference score. Correlation coefficients between the SKILL Card difference score and the other measures of vision function were modest, ranging from 0.05 to 0.31 ( absolute values) ( Table 3). Both the light and dark score components of the SKILL Card test had considerably higher correlation with the other vision measures ( absolute range, 0.27- 0.54) than did the difference score. Relationship of SKILL Card and other vision measure scores with health- related quality- of- life measures The SKILL Card difference score was weakly associated or not associated with the NEI- VFQ subscale scores. Higher correlations with the NEI- VFQ scores were observed for the SKILL Card light and dark scores and for all of the other vision measures ( Table 4). Of all vision tests, the SKILL Card difference score also was found to have the weakest association with the single question on the NEI- VFQ in which the patients provided an overall rating of their vision. This result was seen when, based on a patient's best eye on a given TABLE 4. Spearman correlation of the National Eye Institute Visual Function Questionnaire ( NEI- VFQ) subscale scores with visual measures in best eye and worst eye of each patient ( N = 176) * Measures of vision function SKILL Card difference score Best eye Worst eye SKILL Card light score Best eye Worst eye SKILL Card dark score Best eye Worst eye Visual acuity Best eye Worst eye Contrast sensitivity Best eye Worst eye Color vision Best eye Worst eye Visual field Best eye Worst eye General vision ( rs) 0.02 0.04 0.3 I t 0.40t 0.24t 0.32t 0.28t 0.34t 0.23t 0.30t 0.19t 0.24t 0.24t 0.29t Near activities « 0.07 0.15 0.30t 0.36t 0.32t 0.36t 0.28t 0.30t 0.21t 0.26t 0.21t 0.20t 0.11 0.15 NEI- VFQ subscales Distance activities ( rs) 0.05 0.08 0.20t 0.28t 0.20t 0.26t 0.16t 0.27t 0.25t 0.27t 0.17t 0.24t 0.18t 0.22t Driving ( rs) 0.11 0.07 0.18t 0.22t 0.21t 0.23t 0.16t 0.23t 0.19t 0.17t 0.07 0.10 0.13 0.15 Color vision ( rs) - 0.05 - 0.02 0.25t 0.23t 0.16t 0.18t 0.18t 0.18t 0.19t 0.14 0.19t 0.16t 0.22t 0.19t Peripheral vision ( rs) 0.08 0.14 0.10 0.21t 0.17t 0.25t 0.11 0.21t 0.19t 0.33t 0.34t 0.35t 0.24t 0.28t In order to avoid potential interpretative difficulties, the signs of the correlation coefficients have been adjusted where necessary so that all visual measures are scaled in such manner as to associate higher scale values with better visual performance and lower values with worse visual performance. * The effective n varies from 150 to 173 across correlations of vision tests and subscales. t P value < 0.05. J Neuro- Ophthalmol, Vol. 21, No. 2, 2001 128 D. T. LONGETAL. TABLE 5. Odds ratios for reported dysfunction on the National Eye Institute Visual Function Questionnaire ( NEI- VFQ) comparing the best 75% to the worst 25% of the distribution for the patients' better eyes on each visual function test Vision measure/ eye N 133 43 133 43 133 43 137 39 153 23 128 42 127 42 NEI- VFQ dysfunction* N (%) 17( 13) 6( 14) 9( 7) 14 ( 33) 11( 8) 12 ( 28) 14( 10) 9( 23) 14( 9) 9( 39) 11( 9) 11 ( 26) 15 ( 12) 7( 17) Odds ratio 1.00 1.11 1.00 6.65 1.00 4.29 1.00 2.64 1.00 6.38 1.00 3.77 1.00 1.49 95% confidence in ( 0.41- 3.01) ( 2.63- 16.86) ( 1.73- 10.65) ( 1.04- 6.66) ( 2.34- 17.38) ( 1.50- 9.52) ( 0.56- 3.96) P value SKILL Card difference score Best 75% of distribution Worst 25% of distribution SKILL Card light score Best 75% of distribution Worst 25% of distribution SKILL Card dark score Best 75% of distribution Worst 25% of distribution Visual acuity Best 75% of distribution Worst 25% of distribution Contrast sensitivity Best 75% of distribution Worst 25% of distribution Color vision Best 75% of distribution Worst 25% of distribution Visual field Best 75% of distribution Worst 25% of distribution 0.84 < 0.001 0.002 0.04 < 0.001 0.005 0.42 Missing observations: color vision ( 6), visual field ( 7). * NEI- VFQ dysfunction defined as a response of fair, poor, very poor, or completely blind on the question, " At the present time, would you say your eyesight ( with glasses or contract lenses, if you wear them) is excellent, good, fair, poor, very poor, or are you completely blind?" measure, the patients in the worst quartile of scores were compared with patients in the remaining 75% of the distribution ( Table 5). Unlike the SKILL Card difference score, the light and dark component scores were strongly associated with the responses on this global NEI- VFQ question. Results were similar for each vision test when patients with a normal score in both eyes were compared with those with an abnormal score in at least one eye. Each of the other four vision measures, and the light and dark SKILL scores, showed a strong association with the TABLE 6. Odds ratios for reported dysfunction on the National Eye Institute Visual Function Questionnaire ( NEI- VFQ) comparing normal and abnormal patients on each visual function test ( N = 176) Vision measure* NEI- VFQ dysfunctiont N (%) Odds ratio 95% confidence interval P value SKILL Card difference score Normal Abnormal Light side SKILL score Normal Abnormal Dark side SKILL score Normal Abnormal Visual acuity Normal Abnormal Contrast sensitivity Normal Abnormal Color vision Normal Abnormal Visual field Normal Abnormal 76 100 108 68 105 71 122 54 83 93 113 57 119 50 8( 11) 15 ( 15) 4( 4) 19 ( 28) 7( 7) 16 ( 23) 10( 8) 13 ( 24) 4( 5) 19 ( 20) 7( 6) 15 ( 26) 11( 9) 11 ( 22) 1.00 1.50 1.00 10.08 1.00 4.07 1.00 3.55 1.00 5.07 1.00 5.41 1.00 2.77 ( 0.60- 3.75) ( 3.26- 31.22) ( 1.58- 10.51) ( 1.45- 8.72) ( 1.65- 15.60) ( 2.06- 14.21) ( 1.11- 6.90) 0.39 < 0.001 0.004 0.006 0.005 < 0.001 0.03 Missing observations: color vision ( 6), visual field ( 7). * For vision measures, patients were considered normal when the test score was in the normal range in both eyes and abnormal when the test score in at least one eye was abnormal. t NEI- VFQ dysfunction defined as a response of fair, poor, very poor, or completely blind on the question " At the present time, would you say your eyesight ( with glasses or contact lenses, if you wear them) is excellent, good, fair, poor, very poor, or are you completely blind?" / Neuro- Ophthalmol, Vol. 21, No. 2, 2001 SKILL CARD TEST IN OPTIC NEURITIS 129 TABLE 7. Spearman correlation of visual measures in best and worst eye with Optic Neuritis Treatment Trial ( ONTT) questionnaire score Measure of vision function SKILL Card difference score SKILL Card light score SKILL Card dark score Visual acuity Contrast sensitivity Color vision Visual field Best eye ( rs) ( N = 89) 0.13 0.26* 0.28* 0.16 0.20 0.36* 0.21* Worst eye ( rs) ( N = 89) 0.04 0.33* 0.33* 0.23* 0.35* 0.38* 0.28* Missing observations: color vision ( 1). Visual acuity was measured with a retroilluminated ETDRS chart. Contrast sensitivity was measured with a Pelli- Robson chart. Visual field was measured with the Humphrey Field Analyzer. Color vision was measured with the Farns-worth- Munsell 100- hue test. In order to avoid potential interpretative difficulties, the signs of the correlation coefficients have been adjusted where necessary so that all visual measures are scaled in such manner as to associate higher scale values with better visual performance and lower values with worse visual performance. * P value < 0.05. NEI- VFQ global vision question, whereas the SKILL Card difference score did not ( Table 6). Replicating the analyses using the ONTT questionnaire rather than the NEI- VFQ, the results were very similar, particularly when the smaller sample size was considered ( Table 7 and Table 8). DISCUSSION The SKILL Card was developed to provide a measure of visual function at low contrast and reduced luminance to simulate a condition in which people often describe visual difficulties in day- to- day living ( 5). This development has particular potential relevance for patients with optic nerve disease who often have visual complaints even when visual acuity is normal or near normal ( 3). In this study, we found the SKILL Card difference score to be somewhat discriminating between eyes with and without previous optic neuritis in that eyes that had previous optic neuritis had worse scores on average than eyes that did not, although the magnitude of the difference ( approximately three letters) is not likely to be clinically relevant. The percentage of eyes with abnormal SKILL Card difference scores was higher than the abnormal percentage for each of the other measures, and even when all other vision measures were in the normal range, the SKILL Card difference score was abnormal in more than one quarter of the eyes. The SKILL Card difference score was poorly correlated with the other vision measures, whereas the individual light and dark component scores had higher correlation. Although our findings are consistent with the SKILL Card developers' contention that the difference score is a uniquely sensitive measure of subtle optic nerve dysfunction, the findings are also consistent with the possibility that the SKILL Card difference score has a high false- positive rate. We feel the latter is the more probable explanation. In support of this view is our finding that among eyes in which visual acuity, contrast sensitivity, color vision, and visual field were all normal, the eyes with an abnormal difference score were no more likely to have experienced previous optic neuritis or to be from patients with MS than the eyes with a normal difference score. Also, among all five vision tests, the SKILL Card difference score had the weakest association with the patient's self- reported visual impairment on the NEI-VFQ and the ONTT questionnaire. Because such HRQL measures could reasonably be expected to detect the kind TABLE 8. Odds ratios for reported dysfunction on the Optic Neuritis Treatment Trial ( ONTT) questionnaire comparing the best 75% to the worst 25% of the distribution for the patients' better eyes on each visual function test Vision measure/ eye N Abnormal on questionnaire* N (%) Odds ratio 95% confidence interval P value SKILL Card difference score Best 75% of distribution Worst 25% of distribution SKILL Card light score Best 75% of distribution Worst 25% of distribution SKILL Card dark score Best 75% of distribution Worst 25% of distribution Visual acuity Best 75% of distribution Worst 25% of distribution Contrast sensitivity Best 75% of distribution Worst 25% of distribution Color vision Best 75% of distribution Worst 25% of distribution Visual field Best 75% of distribution Worst 25% of distribution 68 21 69 20 70 19 68 21 75 14 66 22 67 22 34 ( 50) 11 ( 52) 32 ( 46) 13 ( 65) 31 ( 44) 14 ( 74) 33 ( 49) 12 ( 57) 36 ( 48) 9( 64) 28 ( 42) 16 ( 73) 29 ( 43) 16 ( 73) 1.00 1.10 1.00 2.15 1.00 3.52 1.00 1.41 1.00 1.95 1.00 3.62 1.00 3.49 ( 0.41- 2.93) ( 0.76- 6.04) ( 1.14- 10.85) ( 0.53- 3.79) ( 0.60- 6.37) ( 1.26- 10.42) ( 1.22- 10.04) 0.85 0.15 0.03 0.49 0.27 0.02 0.02 Missing observations: color vision ( 1). * The questionnaire was considered to be abnormal if the response to one or more questions was recorded as poor or very poor. J Neuro- Ophthalmol, Vol. 21, No. 2, 2001 130 D. T. LONGETAL. of real- world visual deficits that the SKILL Card was designed to measure, their lack of association with the SKILL Card difference score lends further support to the idea that the test has a high false- positive rate. Although we cannot completely rule out the possibility that the difference score is still measuring an aspect of visual function important to a patient with optic neuritis that is not being assessed in the questionnaires, this explanation seems unlikely, particularly in view of the consistency of the results using the two questionnaires. In addition, the fact that both the light and dark scores, from which the difference score is derived, correlated well with the other measures and with the questionnaires provides evidence that the test was administered properly. Thus, in this study, we did not find evidence that the SKILL Card difference score is likely to benefit the clinician in assessing optic nerve dysfunction in patients after optic neuritis. It is probable that this result would be true for other optic neuropathies as well. Despite the lack of apparent value of the SKILL Card difference score, the component high- contrast ( light) and low- contrast ( dark) scores did appear to relate to a patient's self-reported visual impairment as strongly as did the standard vision measures. Therefore, although the test may not be useful for its primary intended purpose, it may prove to be a useful test to obtain a rapid and easily administered assessment of high- contrast and low-contrast acuity. Acknowledgments: The authors thank Marilyn Schneck, PhD, who provided review of the manuscript. The following are all principal investigators involved in the ONTT study since its onset in 1988 and all other investigators and clinical center staff active in the study since 1992. Clinical centers University of Arkansas, Little Rock, AR M. Brodsky ( principal investigator [ PI]), W. Jay ( PI), P. Johnson ( clinic coordinator [ CC]), D. Lamey ( CC), S. Cain ( CC), B. Wheeler ( CC), B. Lam ( investigator [ I]), S. Nazarian ( I), J. Nichols ( I), A. Buckholtz ( technician [ T]), and R. Ford ( T). Baylor College of Medicine, Houston, TX S. Orengo- Nania ( PI) ( 1993- Present), J. McCrary ( PI) ( 1992), L. Rolak ( PI) ( 1992- 1993), B. Slight ( CC), S. Chung ( I), R. Gross ( I), A. Howard ( I), E. Lai ( I), and P. Frady ( T). California Pacific Medical Center, San Francisco, CA B. Katz ( PI), C. Foltos ( CC), T. Ambrosio ( CC), T. Cruz ( CC), N. Hawker ( CC), T. Swahn ( CC), D. Bareness ( T), J. Christ- Kuhn ( T), M. Gaffney ( T), and N. Loey ( T). Duke University, Durham, NC E. Buckley ( PI), M. Anderson ( CC), G. Valentine ( CC), E. Massey ( I), S. Pollock ( I), R. Radtke ( I), L. Duncan ( T), and D. Patterson ( T). University of Florida, Gainesville, FL J. Guy ( PI), S. Zam ( CC), R. Burke ( T), A. Francis ( T), and D. Shamis ( T). Georgetown University, Washington, DC G. Chrousos ( PI), E. Burt ( CC), and J. Kattah ( I). University of Illinois, Chicago, IL J. Goodwin ( PI), R. Asian ( CC), M. Lindeman ( CC), L. Sullivan ( CC), J. Nichols ( I), and P. Bobak ( T). University of Iowa, Iowa City, IA S. Thompson ( PI), J. Corbett ( PI), C. Musser ( CC), S. Donahue ( I), T. Farrell ( I), P. Johnston- McNussen ( I), R. Kar-don ( I), A. Mishra ( I), R. Neahring ( I), M. Wall ( I), and C. Fountain ( T). Wills Eye Hospital, Philadelphia, PA P. Savino ( PI), S. Ward ( CC), M. Devlin ( CC), T. Bosley ( I), C. Cantor ( I), L. McCloskey ( I), R. Sergott ( I), A. Davis ( T), R. Edwards ( T), and D. Morton ( T). Johns Hopkins University, Baltimore, MD N. Miller ( PI), M. Medina ( CC), C. Putzulo ( CC), D. Buch-holz ( I), S. Reich ( I), M. Repka ( I), and L. West ( T). University of Michigan, Ann Arbor, MI J. Trobe ( PI), R. Castillo ( CC), C. Caudill ( CC), W. Corn- Math ( I), and B. Michael ( T). Michigan State University, East Lansing, MI D. Kaufman ( PI), S. Holliday ( CC), S. Bickert ( CC), J. Froehlich ( CC), J. Kokinakis ( CC), E. Eggenberger ( I), S. My-int ( I), M. Barris ( T), and E. Rosick ( T). New York University, New York, NY M. Kupersmith ( PI), R. Langer ( CC), A. Addessi ( CC), J. Weinman ( CC), E. Frommer ( T), and S. Wahba ( T). Devers Eye Institute, Portland, OR W. Shults ( PI), L. Boly ( CC), C. Beardsley ( CC), R. Dreyer ( I), R. Herndon ( I), R. Wilson ( I), J. Arends ( T), and T. Beaudry ( T). Swedish Medical Center, Seattle, WA C. Smith ( PI), D. Kuder ( CC), G. MacKinnon ( T), S. Smith ( T), J. Sparenburg ( T), and E. Tran ( T). Data Coordinating Center: Jaeb Center for Health Research, Inc., Tampa, FL P. Moke ( Director), R. Beck, R. Gal, D. Long, R. Blair, and K. Kip. Visual Field Reading Center: University of California, Davis, CA J. Keltner ( Director), C. Johnson ( Associate Director), J. Spurr, L. Shapiro, H. Hakim, and D. Claunch. Regional coordinators M. M. Anderson ( Durham, NC) and L. Boly ( Portland, OR). Consultants P. Cleary ( Rockville, MD) and D. Paty ( Vancouver, BC). National Institutes of Health: National Eye Institute, Bethesda, MD R. Mowery and D. Everett. REFERENCES 1. Beck RW, Cleary PA. Optic neuritis treatment trial. One- year follow- up results. Arch Ophthalmol 1993; 111: 773- 5. / Neuro- Ophthalmol, Vol. 21, No. 2, 2001 SKILL CARD TEST IN OPTIC NEURITIS 131 2. Beck RW, Cleary PA, Backlund JC. The course of visual recovery after optic neuritis. Experience of the Optic Neuritis Treatment Trial. Ophthalmology 1994; 101: 1771- 8. 3. Cleary PA, Beck RW, Bourque LB, et al. Visual symptoms after optic neuritis. Results from the Optic Neuritis Treatment Trial. 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