Journal of Modern Optics Vol. 54, No. 9, 15 June 2007, 1325–1332
Contrast sensitivity with presbyopic contact lenses ARUNA S. RAJAGOPALANy, EDWARD S. BENNETT*z and VASUDEVAN LAKSHMINARAYANANx yUniversity of Illinois at Chicago, Chicago, Illinois, USA zUM-St Louis College of Optometry, St Louis, MO 63121, USA xUniversity of Waterloo, Waterloo, Ontario N2L 3GL, Canada (Received 13 January 2006; in final form 21 March 2006) The purpose of the work was to assess the contrast sensitivity function of individuals wearing gas permeable (GP) multifocal contact lenses, soft bifocal contact lenses, and GP monovision lenses. Twenty-six females and six males between the ages of 42 and 65 participated in this study. The study included subjects wearing monovision (N ¼ 8), the Acuvue Bifocal (Johnson & Johnson) (N ¼ 8), Essential GP Multifocal (Blanchard) lenses (N ¼ 8) and progressive addition spectacle lenses (PAL) (N ¼ 8), with PAL wearers forming the control group. Measurements of binocular contrast sensitivity were obtained using the VISTECH 6500 system. Thresholds for each spatial frequency were fit to the equation CS(k) ¼ ak exp(bk) [1 þ c exp(bk)]1/2, to describe the human contrast sensitivity function. The area under the contrast sensitivity function (CSF) curve was calculated for all four groups and compared. An index of performance was obtained, which was defined as the ratio of CSF with the contact lens correction to the CSF with spectacles. Of the contact lens wearing groups, GP multifocal contact lens wearers had the highest contrast sensitivity at all the spatial frequencies. Soft bifocal contact lens wearers exhibited higher contrast sensitivity than monovision wearers at all spatial frequencies. Subjects wearing GP multifocals had the largest area under the CSF; followed by those wearing soft bifocals, with monovision wearers having the smallest area. GP multifocals have the best visual function at 0.98, soft bifocals have an index of 0.65 and monovision has an index of 0.59. This study quantifies the visual performance of the three lens systems by measuring the area under the CSF curve. In addition, it provides indices of visual function with the contact lenses that will be helpful for analyses and comparisons in future studies.
1. Introduction Contrast sensitivity evaluates a range of visual performance under real-life conditions. It measures the least amount of contrast required to detect a visual stimulus and gives us a more complete quantification of patients’ visual
*Corresponding author. Email:
[email protected]
Journal of Modern Optics ISSN 0950–0340 print/ISSN 1362–3044 online ß 2007 Taylor & Francis http://www.tandf.co.uk/journals DOI: 10.1080/09500340600855452
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capabilities [1]. Contrast sensitivity is a sensitive measure with various visual problems like amblyopia, glaucoma, cataract neuro-ophthalmological and retinal diseases [2]. It has been used to evaluate the visual performance following various refractive and cataract extraction surgeries [3–5]. It has also been used previously for measurement of visual function with contact lenses. Early studies have monitored changes in contrast sensitivity immediately before, during, and after one hour of soft lens wear and their effect on contrast sensitivity during the first six months of wear [6, 7]. Collins et al. [8] measured contrast sensitivity for contact lens corrections of presbyopia and reported that the differences between various contact lens corrections were small and not statistically significant. Loshin and Loshin [9] reported loss of contrast sensitivity at all spatial frequencies for monovision with the loss being proportional to the add. In another study, front surface aspheric and diffractive bifocal contact lenses were evaluated and it was observed that the contrast sensitivity measures were better with the aspheric bifocal lenses [10]. Zandvoort et al. [11] recorded contrast sensitivity for aspheric soft multifocals and observed degradation in the contrast sensitivity at intermediate and high spatial frequencies. Although there have been major advancements in lens design and material technology, there have been no recent studies evaluating the contrast sensitivity with presbyopic contact lenses used in present day practice. Thus, in the current study contrast sensitivity function was assessed for individuals wearing GP monovision, soft bifocal, and GP multifocal contact lenses.
2. Materials and methods Thirty-two subjects between the ages of 42 and 65 years with an average of 51 6 years participated in this study. Out of the 32 subjects, eight wore progressive addition lenses (PAL) and formed the control group of the study. Eight subjects had monovision (GP spherical) correction using single vision gas permeable lenses. There were eight subjects wearing soft bifocal (Acuvue Bifocal, Johnson & Johnson Vision Care Inc. Jacksonville, FL, USA) contact lenses and eight subjects wearing gas permeable (GP Essentials Multifocal, Blanchard Laboratories, Manchester, NH, USA) bifocal contact lenses. The study was approved by the UM-St Louis Institutional Review Board and all the participants signed the informed consent after understanding the nature of the study. Each subject underwent a complete eye examination which included refraction, screening for ocular and systemic diseases, comprehensive slit lamp biomicroscopy and examination of the fundus. All subjects had best corrected distance visual acuity of 20/30 or better and near vision of N6 at 40 cm. The average refractive error in the right eye was 1.64 3.2D and in the left eye was 1.88 3.5D. The mean add was 2.02 0.5D. In the monovision group, five subjects used their right eye for near vision and three subjects used their left eye for near vision. Subjects wearing soft bifocals had less than 1.00D of refractive cylindrical correction and subjects wearing GP multifocals had less than 2.50D
Contrast sensitivity with presbyopic contact lenses
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of refractive cylindrical correction. All subjects had worn bifocal contact lenses for a period of one year. 25% of the participating subjects had subjective complaints of glare. Contrast sensitivity was measured using the Vistech VCTS 6500 system (Vistech Consultants, Inc., Dayton, OH, USA). It consists of circular stimuli arranged in five rows and nine columns. The circular stimuli consist of patches of sine wave gratings, with each patch subtending an angle of 1.4 at 10 ft. Each stimulus has sine wave gratings with different spatial frequencies and decreasing contrast across columns. The spatial frequency varies from 1.5 to 18 cpd. The contrast of each of the nine patches decreases from 33% to 0% in 0.2 log unit steps. Light reflected from the chart was measured with a photometer and found to be 130 cd m2. The subject was positioned at a distance of ten feet with their best optical (spectacles or contact lenses) correction in place. The subject was then instructed to look at the sample grating patches at the bottom. The four possible responses of left, right, up and down are reviewed, making it a four-alternative forced choice method. A detailed explanation of the testing procedure was given. With both eyes open, the patient was asked to report the orientation of the patches in the top row. The first incorrect response served as the end point for that row. Then the subject was asked to view the next row and perform the same procedure. The end point for each of the five rows was documented by marking the associated encircled number on the evaluation form. The numbers marked were connected to form a contrast sensitivity curve for the patient. Values of thresholds for each spatial frequency were noted in cpd with the help of the conversion chart available with the Vistech system. The values obtained were fit in the following equation [12] using Sigma Plot 8 (SPSS Inc., Chicago, IL, USA) CSðkÞ ¼ ak expðbkÞ½1 þ c expðbkÞ1=2 ; where k is the spatial frequency and a, b and c are defined parameters [12]. This equation helps describe the human contrast sensitivity function (CSF). The function has three parameters, a, influencing the low frequency behaviour of the function, b, describing the contrast sensitivity peak, and c, expressing the high frequency behaviour of the function. These coefficients a, b and c and the area under the CSF curve were calculated using a least-squares procedure (MatLab 5.3 The Mathworks Inc., Natick, MA, USA) for all four groups and compared. The area under the curve provided a quantitative measure of the visual performance for all four groups. Contrast sensitivity is a measure of visual function. An index of performance of the different modalities of presbyopic contact lens correction (soft bifocals, GP multifocals and monovision) can be obtained by comparing them to PAL. Thus, an index is obtained, which is the ratio of CSF with the contact lens correction to the CSF with spectacles. These values can range from zero to one, closer to one implying that the visual function with contact lens correction is similar to that of spectacle correction.
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3. Results Contrast sensitivity was measured at 1.5, 3.0, 6.0, 12.0 and 18.0 cpd (cycles/degree). Average contrast sensitivity values in cpd for monovision, soft bifocal, GP multifocal and PAL groups are presented in table 1. Of the contact lens wearing groups, GP multifocal contact lens wearers had the highest contrast sensitivity at all the spatial frequencies. The contrast sensitivity values were superior to that of the controls, which constitute PAL wearers, at all but 12 cpd and 18 cpd. PAL wearers exhibited better contrast sensitivity than soft bifocal wearers except at 3.0 cpd. Soft bifocal contact lens wearers showed improved contrast sensitivity than monovision wearers at all spatial frequencies. Figure 1 illustrates the mean contrast sensitivity for spatial frequencies of 1.5, 3.0, 6.0, 12.0 and 18.0 cpd. Table 1.
Mean contrast sensitivity values for monovision, soft bifocal, GP multifocal and PAL groups at 1.5, 3, 6, 12 and 18 cpd.
Monovision group Soft bifocal group GP multifocal group PAL group
1.5 cpd
3.0 cpd
6.0 cpd
12 cpd
18 cpd
40.88 40.00 54.38 50.63
68.38 101.13 116.88 95.63
63.75 64.38 119.38 108.13
39.50 43.38 61.63 67.75
10.75 11.88 15.13 22.38
140 Monovision Soft bifocals GP multifocals PAL
120
Contrast sensitivity
100
80
60
40
20
0 0.0
0.2
0.4
0.6 0.8 1.0 Log spatial frequency (c/d)
1.2
1.4
Figure 1. The contrast sensitivity functions at 1.5, 3, 6, 12 and 18 c/d is plotted for all four groups tested.
Contrast sensitivity with presbyopic contact lenses Table 2.
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Mean of coefficients a, b, c and mean area under the curve for monovision, soft bifocal, GP multifocals and PAL groups.
Monovision group Soft bifocal group GP multifocal group PAL group
a
b
c
Area
85.55 116.35 147.44 110.75
0.19 0.25 0.19 14.83
0.06 0.02 0.07 14.62
782.62 868.05 1297.06 1316.41
All four groups exhibited similar values for the low spatial frequencies. At the intermediate frequencies, GP multifocal wearers showed the highest peak. Soft bifocal wearers had an interesting curve with a peak at 3 cpd and a following dip at 6 cpd. Monovision wearers had the lowest contrast sensitivity function. At high spatial frequencies, PAL wearers had the highest contrast sensitivity while the monovision wearers had the lowest contrast sensitivity with the soft and GP multifocal groups in the middle. The spatial frequencies obtained were fit to the equation described above [12]. Among the three presbyopic contact lens correction modalities, subjects wearing GP multifocals had the largest area under the contrast sensitivity function; followed by those wearing soft bifocals, with monovision having the least area under the contrast sensitivity function (table 2). The differences between monovision and GP multifocals and between monovision and soft bifocals were statistically significant ( p50.01). The area under the curve is largest for GP multifocals and statistically similar to that of PAL spectacle wearers. Soft bifocal and monovision wearers had smaller areas under the curve. The plot of the mean area under the curve for all four groups is shown in figure 2. A comparison of the index of performance shows that GP multifocals have the best visual function at 0.98, soft bifocals have an index of 0.65 and monovision has an index of 0.59 (table 3).
4. Discussion The results from our study indicate that for the contact lens wearing groups, GP multifocal contact lens wearers have the highest contrast sensitivity at all the spatial frequencies and soft bifocal contact lens wearers exhibit higher contrast sensitivity than monovision wearers at all spatial frequencies. Superior performance with GP multifocals can be attributed to the good optical quality provided by the GP lenses [13] and the lens design. Monovision wearers in our study used GP lenses, but their performance was compromised by the inter-ocular differences. Subjects with monovision are unable to suppress blur [14] and have reduced stereoacuity [15] which decreases their binocular performance. Hence, GP multifocals perform better than the GP monovision modality.
Figure 2. The hatched area is the mean area under the contrast sensitivity function (CSF) curve for subjects in GP multifocals group (a), Soft bifocals group (b), Monovision group (c) and PAL group (d).
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Contrast sensitivity with presbyopic contact lenses Table 3.
Indices of visual function for GP multifocals, soft bifocals and monovision groups. Indices
Monovision/PAL Soft Bifocal/PAL GP Multifocal/PAL
0.59 0.65 0.98
Zandvoort et al. [11] recorded contrast sensitivity for an aspheric soft multifocal and observed degradation in the contrast sensitivity at intermediate and high spatial frequencies. In our study, a decrease in contrast sensitivity was observed in soft bifocals at 6, 12 and 18 cpd in agreement with his results. Collins et al. [8] compared the contrast sensitivities of different types of presbyopic contact lens correction which included distance contact lenses and lookover spectacles, progressive bifocal contact lenses, concentric bifocal contact lenses, monovision (with soft contact lenses), modified monovision (with soft contact lenses and soft bifocals) and hard bifocal contact lenses. Similar to the results obtained in our study, there were differences observed between the groups, but the differences were not statistically significant. In the same study it was observed that the performance of spectacles was similar to that of contact lenses at low spatial frequencies but better at high spatial frequencies, a trend confirmed in our study [8]. The area under the contrast sensitivity function curve was measured and an index of performance was derived from it. This index of performance will prove helpful for analyses and comparisons in future. This study also emphasizes the importance of the contrast sensitivity test as a measure of visual function in presbyopic contact lens wearers. Since it provides us with a large amount of information regarding the performance of the lenses, it can be beneficial to include contrast sensitivity measurements in presbyopic contact lens fitting and evaluating procedures.
Acknowledgements The authors would like to thank all the subjects who participated in this study. Presented in part at the Annual Meeting of the American Academy of Optometry, Dallas, Texas, December 2003 [Optom. Vis. Sci. 80 (12s) 194 (2003)].
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