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Optometry & Vision Science:
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Visual Acuity in Contact Lens Wearers

BAILEY, MELISSA D. OD, MS; WALLINE, JEFFREY J. OD, MS, FAAO; MITCHELL, G. LYNN MAS; ZADNIK, KARLA OD, PhD, FAAO

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Author Information

The Ohio State University College of Optometry, Columbus, Ohio

Melissa D. Bailey

The Ohio State University

College of Optometry

338 West Tenth Avenue

Columbus, Ohio 43210-1240

e-mail: mbailey@optometry.ohio-state.edu

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Abstract

Purpose. The difference between high- and low-contrast visual acuity provides a sensitive indicator of vision loss in ocular disease; however, the effect of refractive error correction on this difference is still debated.

Methods. High- and low-contrast visual acuity was measured in 116 rigid gas permeable contact lens wearers, 51 spectacle wearers, and 50 soft contact lens wearers with habitual and best correction. Twenty-nine of the soft contact lens wearers reported that they wore disposable contact lenses (discarded on a monthly or more frequent basis), whereas the other 21 soft contact lens wearers wore traditional soft contact lenses.

Results. Rigid gas permeable contact lens wearers had statistically worse high-contrast habitual visual acuity than spectacle wearers (Tukey-Kramer, p = 0.0075). Traditional soft contact lens wearers had significantly worse low-contrast visual acuity compared with all other groups (Tukey-Kramer, p < 0.02 for each comparison). Traditional soft contact lens wearers had a significantly larger difference between high- and low-contrast visual acuity with best correction compared with rigid gas permeable wearers (Tukey-Kramer, p = 0.0099).

Conclusions. Rigid gas permeable contact lens wearers had statistically worse habitual high-contrast visual acuity compared with spectacle wearers, but no difference was present under best-corrected conditions. We hypothesize that rigid gas permeable contact lens wearers were not wearing their optimal correction habitually. Traditional soft contact lens wearers had significantly worse low-contrast visual acuity. They also had a larger difference between their best-corrected high- and low-contrast visual acuity scores compared with rigid gas permeable contact lens wearers.

Although visual performance measurements have been used for years in vision research, recently they have become important tools for assessing new modalities of refractive error correction. High- and low-contrast visual acuity measurements are used for studying diseases that affect the cornea, 1 investigating visual pathway dysfunction, 2,3 evaluating modes of correction for refractive error, 4–9 and evaluating refractive surgery outcomes. 6 Low-contrast visual acuity is sensitive to changes in vision in several ocular disease states. 1–3 Although some studies use an additional measurement of contrast sensitivity, the combination of high- and low-contrast visual acuity is reported to be sensitive enough in normal patients to provide the same information that would be gained by measuring contrast sensitivity. 10

There are many studies that have evaluated visual acuity across modes of refractive error correction, but the results of these studies are not consistent (Table 1). Some of the variability across these studies may be due to the constant improvement in contact lens design, various methods of visual acuity measurement, and/or small sample sizes. Only one study compared high- and low-contrast visual acuity in spectacle, soft contact lens, and rigid gas permeable (RGP) contact lens wearers. Lohmann et al. 6 reported that soft contact lens wearers had reduced low-contrast (5% contrast) visual acuity compared with spectacle wearers, rigid gas permeable contact lens wearers, and postoperative photorefractive keratectomy patients. They found no difference among refractive correction modalities at 100% or 20% contrast. Another recent study, however, did not find any difference for either high- or low-contrast (6% contrast) visual acuity between spectacle and Acuvue soft contact lens wearers. 4

Table 1
Table 1
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Several of the studies listed in Table 1 found a difference in contrast sensitivity across modalities. In all three of these studies, subjects wearing traditional, nondisposable contact lenses had significantly worse contrast sensitivity than subjects with other modes of refractive error correction. 5,7,8 In one study, where CibaSoft contact lenses were compared with Acuvue, Biomedics, and spectacles, only the CibaSoft contact lenses (nondisposable) reduced contrast sensitivity. 8

When considering visual acuity scores in studies evaluating refractive error correction, it has been reported that blur due to uncorrected refractive error did not affect the difference between a subject’s high- and low-contrast visual acuity scores. 11–13 Simpson 13 reported that placing a +1.25 D lens in front of a subject’s full manifest correction did not result in any additional difference between the high- and low-contrast visual acuity scores; both scores were reduced by an equal amount. Thus, in studies comparing the difference between high- and low-contrast visual acuity scores across modalities of refractive error correction, a statistically significant difference between modalities should be attributed to a difference in visual function rather than to differing abilities to correct refractive error.

Low-contrast visual acuity is a sensitive way to detect the presence of vision loss in disease states, and optical blur does not create any additional difference between high- and low-contrast visual acuity in normal patients. The effect of various modalities of refractive error correction (i.e., spectacles or contact lenses) on high- and low-contrast visual acuity in a normal population is less apparent. Such information would be useful for evaluating new modes of refractive error correction and comparing the effects of various ocular diseases on visual acuity. We have provided high- and low-contrast visual acuity data in spectacle, soft contact lens, and RGP contact lens wearers who are free of ocular disease.

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METHODS

Subjects

Two hundred seventeen subjects who were free of ocular disease and wore spectacles, soft contact lenses, or rigid gas permeable (RGP) contact lenses participated in the study. Of the 217 patients, 50 were soft contact lens wearers, 51 were spectacle-corrected, and 116 were corrected with RGP contact lenses. Of the 50 soft contact lens wearers, 29 subjects wore disposable soft contact lenses (i.e., disposed of monthly or more frequently as reported by the subjects). The other 21 soft contact lens wearers wore traditional soft contact lenses (i.e., they did not dispose of their lenses monthly or more frequently). Subjects were recruited via letters sent to patients at The Ohio State University College of Optometry and the Southern California College of Optometry, by word-of-mouth referral, or they were students, faculty, or staff at one of the colleges of optometry. All subjects were between the ages of 18 and 65 years, and they provided informed consent before examination. The study protocol was approved by the institutional review board at each college.

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Visual Acuity

High- and low-contrast visual acuity data were collected using Bailey-Lovie visual acuity charts. There were two different high-contrast and two different low-contrast charts. The low-contrast charts were 18% contrast by the Weber method of calculating contrast. 14 The charts were designed using a geometric progression of letter size with five letters per line.

Subjects were tested while standing 6 m from the chart, which was calibrated to a luminance of 75 to 110 cd/m2. Data with habitual correction were collected for each eye separately and then for both eyes, always beginning with the right eye first and changing charts between eyes and before the chart was read with both eyes. High-contrast charts were presented before the low-contrast charts. Subjects were instructed to start at the top of the chart and to read the letters on each line in order. They were allowed to stop when three or more letters were missed on a line after attempting to read that entire line. The total number of letters read correctly was recorded. The protocol for visual acuity testing used for this study followed the protocol of the Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study. 15 All examiners were certified in the visual acuity protocol to ensure consistency in examination.

After reading the visual acuity charts with habitual correction, retinoscopy and subjective over-refraction for contact lens wearers or subjective refraction for spectacle wearers were performed. The majority of refractions for RGP contact lens wearers were performed by one investigator (JJW), and the refractions for the spectacle and soft contact lens wearers were performed by another investigator (MDB). The results of the over-refraction or subjective refraction were placed in a trial frame, and the high- and low-contrast visual acuity charts were read with each eye separately using the same protocol described above. Data from the right eye only of all 217 subjects were selected for analysis.

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Analysis

Descriptive statistics were calculated using the SAS statistical software system (SAS, Inc, Cary, NC). The mean age of subjects in each group was compared using analysis of variance. Post hoc comparisons were performed using the Tukey-Kramer method of adjustment for multiple comparisons. 16 A χ2 test was used to compare the distribution of males and females between the groups.

Analysis of covariance was used to compare both the high- and low-contrast visual acuity between the groups of lens wearers while controlling for age and gender of the subjects. Separate analyses were performed for high- and low-contrast visual acuity with habitual and best correction. In addition, analysis of covariance was used to compare the difference in low- and high-contrast visual acuity with both habitual and best correction between the groups. The level of significance for all tests was set at 0.05. Post hoc comparisons of the group means (two at a time) were performed after adjusting each subject’s response for both age and gender. P-values for these comparisons, adjusted for multiple comparisons using the method described by Tukey and Kramer, are presented. 16

PASS software was used to calculate the power of detecting a difference as extreme or more extreme than what was observed in our data. In addition, this software was used to calculate the power of detecting a one-line difference between the means for each group. This calculation was performed using the spectacle lens group mean as the reference mean. That is, calculations were performed to determine our power to detect a one-line difference between the spectacle group and the other contact lens groups. All power calculations were performed assuming a two-sided test and α = 0.05.

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RESULTS

The mean (±SD) age of soft contact lens wearers was significantly different from rigid gas permeable (RGP) and spectacle wearers (29.4 ± 9.4 years vs. 36.0 ± 9.7 years and 38.4 ± 13.4 years, p < 0.05). There was no difference between RGP and soft contact lens wearers. The majority of subjects in the RGP contact lens group (75.4%) and the soft contact lens group (72.0%) were women. The proportion of men and women was almost equal among subjects in the spectacle lens group, with 51.0% female subjects. The χ2 test indicated a statistically significant difference in the male/female distribution among the three groups (p = 0.0073).

Mean (±SD) high- and low-contrast visual acuity scores are presented in Table 2. After controlling for age and gender of the subject, there was a significant difference in the mean high-contrast visual acuity with habitual correction (analysis of covariance, p = 0.01). There was a significant difference in the visual acuity of spectacle lens wearers compared with RGP contact lens wearers (Tukey-Kramer, p = 0.0075). The adjusted mean visual acuities for the RGP contact lens and spectacle lens groups were −0.01 logarithm of the minimum angle of resolution (logMAR) and −0.06 logMAR, respectively. This difference between the two groups corresponds to about a one-half line difference in visual acuity. No difference was detected between spectacle lens wearers and soft contact lens wearers or between RGP and soft contact lens wearers. There was no significant difference in the mean low-contrast visual acuity with habitual correction (analysis of covariance, p = 0.08). No significant difference was found between the groups for either low- or high-contrast visual acuity under best-corrected conditions (analysis of covariance, plow-contrast = 0.16, phigh-contrast = 0.18).

Table 2
Table 2
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The mean differences between high- and low-contrast visual acuity with habitual and best correction for each group are shown in Table 3. The distributions of mean difference scores are presented in Figs. 1 and 2. The plots provide a graphical representation of the variability in visual acuity measurements for patients who are free of ocular disease. After controlling for subject age and gender, there was a significant effect of lens type on mean difference with both habitual and best correction (p = 0.02 and 0.04, respectively). In our post hoc comparisons, significant results were found when comparing the mean difference in high- and low-contrast visual acuity with habitual correction of RGP contact lens wearers to soft contact lens wearers (Tukey-Kramer, p = 0.02). The same was also true when comparing the mean difference with best correction (Tukey-Kramer, p = 0.03). No other significant differences were found.

Table 3
Table 3
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Figure 1
Figure 1
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Figure 2
Figure 2
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Because previous studies have found differences in contrast sensitivity between subjects wearing traditional and disposable soft contact lenses (Table 1) and because the soft contact lens wearers had a significantly larger difference between high- and low-contrast visual acuity scores compared with RGP contact lens wearers, the above analyses were repeated to consider the type of soft contact lens. The soft contact lens group was divided into traditional and disposable soft contact lens wearers based on whether the subjects reported that they discarded their lenses on a monthly or more frequent basis. The results for power calculations for both sets of analyses are described below.

The mean (±SD) age of disposable soft contact lens wearers was significantly different from RGP contact lens wearers and spectacle wearers (27.1 ± 6.3 years vs. 36.0 ± 9.7 years and 38.4 ± 13.4 years, p < 0.05). There was no significant difference between the mean age of disposable soft contact lens wearers and traditional soft contact lens wearers (32.5 ± 12.0 years). As in the RGP group, women made up a majority of subjects in the disposable soft contact lens group (75.9%). Two-thirds of subjects in the traditional soft contact lens groups were women. The χ2 test indicated a statistically significant difference in the male/female distribution among the four groups (p = 0.02).

Mean (±SD) high- and low-contrast visual acuity scores are presented in Table 2. No differences were detected between the groups for high-contrast visual acuity with habitual correction other than the difference between spectacle and RGP contact lens wearers that is described above. There was no significant difference in the mean low-contrast visual acuity with habitual correction (analysis of covariance, p = 0.09). No significant difference was found between the groups in high-contrast visual acuity under best-corrected conditions (analysis of covariance, p = 0.22). However, there was a significant effect of lens type on low-contrast visual acuity under best-corrected conditions (analysis of covariance, p = 0.01). Traditional soft contact lens wearers had significantly worse visual acuity than each of the other three groups (Tukey-Kramer, p < 0.02 for each comparison).

The mean differences between high- and low-contrast visual acuity with habitual and best correction for each group are shown in Table 3. After controlling for subject age and gender, there was no significant effect of lens type on the mean difference with habitual correction (p = 0.05). There was a significant effect of lens type on the difference between high- and low-contrast visual acuity with best correction (p = 0.0201). In our post hoc comparisons, traditional soft contact lens wearers had a significantly larger difference between high- and low-contrast visual acuity compared with RGP contact lens wearers (Tukey-Kramer, p = 0.01).

With our sample size of 217, we had <50% power to detect differences as observed in the data for both high- and low-contrast visual acuity with best correction. This is not surprising given that the maximum difference between the adjusted means for the three groups was less than two letters for each contrast level. We also had <50% power to detect differences as observed in the data for low-contrast visual acuity with habitual correction. The maximum difference in adjusted mean visual acuity observed between the three groups was only slightly greater than two letters. The almost three-letter (one-half line) difference in mean high-contrast visual acuity with habitual correction translated to >80% power to detect some difference in the three observed means. Power calculations were then performed to determine our ability to detect a one-line difference between either of the two contact lens groups and the spectacle group. In all four cases (2 contrast levels × 2 correction types), the power exceeded 95%. Our sample size was more than adequate to detect clinically meaningful differences in the groups.

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DISCUSSION

The studies by Lohmann and co-workers 6 and Guillon and Schock 4 listed in Table 1 have study designs and results similar to our study. No difference was found in either of these two studies or in our study across modalities of refractive error correction for high-contrast visual acuity or for low-contrast visual acuity of around 20% contrast. The difference among the studies appeared at the 5% or 6% level of contrast.

Although we did not test our subjects using a 5% contrast chart, one of our findings may offer some insight as to why the results of past studies comparing modes of refractive error correction have not always agreed. We found that subjects wearing traditional soft contact lenses (i.e., they did not discard them on a monthly or more frequent basis) had significantly worse best-corrected, low-contrast (20%) visual acuity compared with all other groups, including subjects wearing disposable soft contact lenses. They also had a larger mean difference between high- and low-contrast visual acuity compared with RGP contact lens wearers. Guillon and Schock 4 compared Acuvue lenses (a 2-week disposable contact lens) with spectacles and found no difference between modalities for visual acuity at 6% contrast. Lohmann et al. 6 did not report what type of soft contact lenses their subjects were wearing, but it is possible that at least some of their 10 soft contact lens wearers were in traditional soft contact lenses in 1993. Studies have also consistently shown that traditional soft contact lenses reduce contrast sensitivity, 5,7,8 so the type of soft contact lenses, traditional vs. disposable, that were used in previous studies might explain some of the discrepancies in low-contrast visual acuity data.

Subjects in the present study averaged two lines (10 letters) of difference between their high- and low-contrast visual acuity scores (Table 3). We found that this difference was larger in traditional soft contact lens wearers compared with RGP contact lens wearers, 12,13 but some ocular diseases can also affect the difference between high- and low-contrast visual acuity. The Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study reports an average difference of 2.5 lines (13 letters) between best-corrected high- and low-contrast visual acuity scores. 1 This difference between high- and low-contrast visual acuity is greater than in patients who are free of eye disease other than refractive error, indicating slightly reduced visual function unrelated to uncorrected refractive error.

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CONCLUSION

The mean (±SD) high- and low-contrast visual acuities of subjects who are free of eye disease, except refractive error, was −0.06 ± 0.1 logMAR (∼20/17 Snellen acuity) and 0.15 ± 0.11 logMAR (∼20/29 Snellen acuity), respectively. The mean difference between high- and low-contrast visual acuity was 0.2 ± 0.08 logMAR, or approximately two lines. Although RGP contact lens wearers were found to have poorer high-contrast visual acuity with habitual correction than spectacle wearers, this difference was not present under best-corrected conditions. We hypothesize that RGP contact lens wearers were not wearing their optimal correction under habitual conditions, possibly due to a longer time elapsed since their last eye examination. Subjects wearing traditional, nondisposable soft contact lenses had a larger mean difference between high- and low-contrast visual acuity compared with RGP contact lens wearers. They also had significantly worse best-corrected, low-contrast visual acuity compared with all other groups. This difference between types of soft contact lenses (i.e., traditional vs. disposable) might explain some of the discrepancies between results of past studies that evaluate visual acuity in different modes of refractive error correction.

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ACKNOWLEDGMENT

This study was supported by grants U10-EY10419, T35-EY07151, and K23-EY00383 from the National Eye Institute, National Institutes of Health and by the Ohio Lions Eye Research Foundation.

Received November 20, 2000; revision received June 15, 2001.

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REFERENCES

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Keywords:

refractive error correction; visual acuity; contact lenses; spectacles

© 2001 American Academy of Optometry

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