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Mesopic Contrast Sensitivity and Ocular Higher-Order Aberrations after Overnight Orthokeratology
Mesopic Contrast Sensitivity and Ocular Higher-Order Aberrations after Overnight Orthokeratology TAKAHIRO HIRAOKA, CHIKAKO OKAMOTO, YUKO ISHII, TOMONORI TAKAHIRA, TETSUHIKO KAKITA, AND TETSURO OSHIKA ● PURPOSE:
To investigate mesopic contrast sensitivity and night driving ability in eyes undergoing overnight orthokeratology, and to analyze the relationship among mesopic contrast sensitivity, ocular higher-order aberrations, and myopic correction. ● DESIGN: Prospective, noncomparative, consecutive case series. ● METHODS: In 44 eyes of 22 subjects (mean age ⴞ standard deviation [SD], 24.0 ⴞ 3.2 years) with orthokeratology, ocular aberrations and mesopic contrast sensitivity were determined before and three months after commencement of the procedure. Mean spherical equivalent refraction ⴞ SD was ⴚ2.34 ⴞ 0.99 diopters at baseline. Mesopic contrast sensitivity with and without glare was assessed using the Mesotest II (Oculus, Wetzlar, Germany). ● RESULTS: Orthokeratology significantly reduced the log mesopic contrast sensitivity from 0.25 ⴞ 0.08 to 0.08 ⴞ 0.10 without glare (P < .0001, Wilcoxon) and from 0.21 ⴞ 0.11 to 0.07 ⴞ 0.10 with glare (P < .0001). The proportion of eyes that fulfilled the German standard recommendation level for night driving was 36%. The induced changes in log mesopic contrast sensitivity showed significant negative correlation with the changes in third-order (r ⴝ ⴚ0.490, P ⴝ .0013 without glare; r ⴝ ⴚ0.362, P ⴝ .0177 with glare; Spearman rank correlation coefficient) and fourth-order root mean square (r ⴝ ⴚ0.586, P ⴝ .0001 and r ⴝ ⴚ0.306, P ⴝ .0450, respectively). Furthermore, significant correlation was found between the amount of myopic correction and the induced changes in log mesopic contrast sensitivity (r ⴝ ⴚ0.442, P ⴝ .0038 without glare; r ⴝ ⴚ0.464, P ⴝ .0024 with glare). The induced changes in higherorder aberrations significantly correlated with the amount of myopic correction (P < .0001, Pearson correlation coefficient). ● CONCLUSIONS: Mesopic contrast sensitivity after overnight orthokeratology is deteriorated significantly as ocular higher-order aberrations increase, and these changes Accepted for publication Nov 27, 2007. From the Department of Ophthalmology, Institute of Clinical Medicine, University of Tsukuba, Ibaraki, Japan (T.H., C.O., Y.I., T.T., T.O.); and the Kakita Eye Clinic, Chiba, Japan (T.K.). Inquiries to Takahiro Hiraoka, Department of Ophthalmology, Institute of Clinical Medicine, University of Tsukuba, 1-1-1 Tennoudai, Tsukuba, Ibaraki, 305-8575 Japan; e-mail: [email protected] 0002-9394/08/$34.00 doi:10.1016/j.ajo.2007.11.021
©
2008 BY
depend on the amount of myopic correction. (Am J Ophthalmol 2008;145:645– 655. © 2008 by Elsevier Inc. All rights reserved.)
T
HE CURRENT GOAL OF ORTHOKERATOLOGY IS TO
provide the patient with good unaided vision during the day through the programmed application of specially designed rigid contact lenses, particularly in those with low to moderate myopia. As a result of this procedure, the corneal profile is reshaped by flattening of the central area, and a temporary correction of myopia is achieved.1– 4 With the development of higher gas-permeable lens materials, night therapy, where patients wear the lenses only during sleep, has become possible since the 1980s. This method, which is called overnight orthokeratology, allows satisfactory unaided vision during waking hours without spectacles and contact lenses.3,5 In the 1990s, the introduction of refined lens designs (reverse geometry design) enabled much quicker, more predictable achievement of corneal and refractive changes.3,5–7 Because of these innovations, the clinical application of overnight orthokeratology recently has grown popularity as a nonsurgical option for the correction of low to moderate myopia. Although a number of studies have demonstrated the success of overnight orthokeratology, the evaluation of this procedure has been performed mainly using high-contrast visual acuity.3,5,6,8 Standard high-contrast visual acuity, which relies on the patient’s recognition of familiar letters or Landolt rings with 100% contrast level, is just one feature of actual visual function. In daily life activities, we encounter various visual objects with different spatial frequencies and contrast levels. To evaluate quality of vision accurately, more sophisticated measurements of visual function other than standard high-contrast visual acuity are needed. In fact, we sometimes observe that patients report visual disturbances in several situations even though visual acuity measured with a high-contrast acuity chart is excellent. Contrast sensitivity is defined as the ability to detect differences in luminance between adjacent areas. A contrast sensitivity test is now considered to provide more information regarding visual performance, which may not be disclosed by standard visual acuity testing, and has been used widely as a representative of quality of vision in recent
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TABLE 1. Subject Data at Baseline and Three Months after Commencement of Overnight Orthokeratology
Manifest refraction (D) UCVA (logMAR) BCVA (logMAR) Third-order RMS (m) Fourth-order RMS (m) Total higher-order RMS (m) Mesopic log contrast sensitivity (without glare) Mesopic log contrast sensitivity (with glare)
Baseline, Mean ⫾ SD (Range)
After Treatment, Mean ⫾ SD (Range)
⫺2.34 ⫾ 0.99 (⫺4.00 to ⫺1.00) 0.75 ⫾ 0.31 (0.22 to 1.52) ⫺0.11 ⫾ 0.07 (⫺0.30 to 0.00) 0.074 ⫾ 0.028 (0.022 to 0.133) 0.039 ⫾ 0.020 (0.010 to 0.101) 0.085 ⫾ 0.032 (0.035 to 0.161) 0.25 ⫾ 0.08 (0.02 to 0.30) 0.21 ⫾ 0.11 (0.00 to 0.30)
⫺0.18 ⫾ 0.67 (⫺2.75 to 1.25)* ⫺0.04 ⫾ 0.15 (⫺0.18 to 0.40)* ⫺0.09 ⫾ 0.06 (⫺0.18 to 0.15) 0.263 ⫾ 0.152 (0.029 to 0.758)* 0.138 ⫾ 0.060 (0.037 to 0.275)* 0.303 ⫾ 0.152 (0.084 to 0.791)* 0.08 ⫾ 0.10 (0.00 to 0.30)* 0.07 ⫾ 0.10 (0.00 to 0.30)*
BCVA ⫽ best-corrected visual acuity; D ⫽ diopters; logMAR ⫽ logarithm of the minimum angle of resolution; RMS ⫽ root mean square; SD ⫽ standard deviation; UCVA ⫽ uncorrected visual acuity. *P ⬍ .0001, significant differences between pretreatment and posttreatment values.
years.9 –29 Numerous studies have shown that contrast sensitivity is compromised in eyes with various ocular pathologic features10 –15 and after refractive surgeries.16 –29 In addition, several studies have demonstrated that contrast sensitivity decreases in parallel with increases in higher-order aberrations after corneal refractive surgeries, such as radial keratotomy (RK),18 photorefractive keratectomy (PRK),24 and laser in situ keratomileusis (LASIK).27,29 As for orthokeratology, there have been two studies showing the reduction of contrast sensitivity after the procedure.30,31 Previous studies have shown that scotopic and mesopic vision can be affected by corneal refractive surgeries for myopia, such as RK,19,32 PRK,32–36 LASIK.32,37–39 These procedures are planned to correct defocus by means of surgical alteration of the corneal curvature, and the corneal asphericity accordingly changes from prolate to oblate profile. Similarly, it has been reported that orthokeratology also induces a nonphysiologic oblate-shaped cornea.6 In the clinical practice of orthokeratology, we have had a certain percentage of patients who gained good highcontrast visual acuity but still reported night vision disturbances, especially during driving at night. The impact of this procedure on mesopic vision or night driving ability, however, has not been investigated. Given that a significant proportion of traffic accidents happen at night and that most patients undergoing overnight orthokeratology are young and active, it is crucial to know how this procedure affects night driving ability. The relationship among mesopic vision, ocular higher-order aberrations, and myopic correction also is unknown. We conducted the current prospective study to evaluate mesopic contrast sensitivity with and without glare and night driving ability in eyes undergoing overnight orthokeratology and to analyze the relationship among changes in mesopic contrast sensitivity, changes in ocular higher-order aberrations, and amount of myopic correction. 646
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METHODS ● SUBJECTS:
Forty-four eyes of 22 subjects were enrolled in this prospective study. Inclusion criteria were spherical equivalent refraction between ⫺1.0 and ⫺4.0 diopters (D), astigmatism of 1.0 D or less, mean keratometry readings between 40.00 and 46.25 D, best-corrected visual acuity (BCVA) of 20/20 or better, the absence of ocular and systemic diseases, and age between 20 and 37 years. There were 11 males and 11 females, and the mean age ⫾ standard deviation (SD) was 24.0 ⫾ 3.2 years. Mean spherical equivalent refraction ⫾ SD was ⫺2.34 ⫾ 0.99 D, and mean uncorrected visual acuity (UCVA) ⫾ SD was 0.75 ⫾ 0.31 logarithm of the minimum angle of resolution (logMAR) units before treatment (Table 1). Contact lens users were required to remove their lenses for at least three weeks before the baseline examination. ● LENSES:
The orthokeratology lens used in this study was manufactured from fluorosilicone acrylate (BOSTON XO; Polymer Technology Corp, Wilmington, Massachusetts, USA) with an oxygen permeability (Dk) of 100 ⫻ 10–11 (cm2/sec)(ml O2/ml ⫻ mm Hg). The lens has a four-zone reverse geometry design with a base curve (central optical zone) diameter of 6.0 mm, reverse curve of 0.6 mm width, alignment curve of 1.0 mm width, and a peripheral curve of 0.4 mm width. The lenses were fitted according to the manufacturer’s recommended fitting guidelines. Total lens diameter of 10.0 mm was used initially in all cases. Subsequently, the parameters of the lens were varied to obtain appropriate centration, fluorescein pattern, and lens movement of approximately 1 mm on a blink. After a proper lens fit was accomplished, overrefraction was carried out to determine the final power. After lens dispensing, subjects were instructed to wear contact lenses every night in a consecutive manner. If the treatment effect was deemed inappropriate on the basis of corneal topographic changes and improvement of UCVA, the lens design was OF
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TABLE 2. Contrast Level, Threshold, and Log Sensitivity of the Mesotest II Optotypes That Were Used in the Current Overnight Orthokeratology Study Contrast Level
Percentage of Contrast Threshold
Log Contrast Sensitivity
1:23 1:5 1:2.7 1:2
95% 80% 63% 50%
0.02 0.1 0.2 0.3
modified throughout the course of the treatment. Detailed biomicroscopic evaluations of the anterior segment were performed at each visit to ensure continuing ocular health and to monitor any effects of lens wear. ● MEASUREMENTS:
pupil. This test is applied in Germany as a legal standard to evaluate the ability of night driving.34 According to the guidelines of the German Ophthalmic Society, the most important setting is the contrast level of 1:5, which is the critical limit for night driving.42 It is also known that lower contrast levels in this device are difficult to recognize, even in healthy young people.35,40 This implies that the device is able to detect subtle changes in mesopic contrast sensitivity because of no ceiling effects. Ocular wavefront aberrations for a 4-mm pupil were measured with the Hartmann-Shack wavefront analyzer (KR-9000 PW; Topcon Co, Tokyo, Japan). This system has been described in detail elsewhere,29,43,44 and the accuracy and repeatability have been validated.43,44 The obtained data were expanded with the normalized Zernike polynomials. From the Zernike coefficients, ocular higherorder aberrations were calculated and expressed as the root mean square (RMS; in micrometers). The RMS of the third-order Zernike coefficients was used to represent coma-like aberration, and the RMS of the fourth-order Zernike coefficients was used to denote spherical-like aberrations. Total higher-order aberrations were calculated as the RMS of the third- and fourth-order Zernike coefficients.28,29 To obtain well-focused and properly aligned Hartmann images, the measurements were repeated at least four times in each eye, and three better images were chosen and averaged. The averaged values were used for subsequent analyses. Pupil diameter in a mesopic condition (5 lux) was measured monocularly using the hand-held open-view type digital pupillometer (FP-10000; TMI Co, Saitama, Japan).45 The device was placed in front of one eye and patients were asked to look straight through a peek hole. Accommodation was controlled by instructing the subjects to fixate on a 1.5-m target. The pupillometer includes a charge-coupled device camera providing a continuous video-signal output to a personal computer. Using digital image software (ViewShot; TMI Co), which can recognize the pupillary edge, the largest horizontal and vertical pupil diameters were measured and then averaged. The averaged values were used for statistical analyses.
The baseline examination included manifest refraction, uncorrected and corrected high-contrast visual acuity, keratometry, slit-lamp microscopy containing fluorescein staining, dilated funduscopy, corneal topography, and wavefront aberrometry. The above examinations except for funduscopy were repeated at each visit after commencement of overnight orthokeratology. In addition, mesopic contrast sensitivity testing was performed before and three months after commencement of the treatment. Mesopic pupil diameter was evaluated at the three-month visit. To minimize the influence of diurnal variation, all measurements were conducted between 9 AM and 11 AM, and subjects were instructed to attend the examinations from two to four hours after lens removal. Mesopic contrast sensitivity with and without glare was assessed using the Mesotest II (Oculus, Wetzlar, Germany), which has been previously described.34,35,37,40 The test was performed monocularly with best-spectacle correction after dark adaptation for 10 minutes. The background luminance was 0.032 ⫾ 0.003 cd/m2 without glare and 0.1 ⫾ 0.01 cd/m2 with glare. The device presents the Landolt ring of 20/200 as the optotype, which can be displayed randomly in six directions. There are four contrast levels (1:23, 1:5, 1:2.7, and 1:2), and the contrast with respect to its background can be varied in a series of steps, decreasing by a factor of 0.10 log contrast sensitivity units. Each contrast level corresponds to log contrast sensitivity of 0.02, 0.1, 0.2, and 0.3, respectively (Table 2). First, the highest contrast level of 1:23 was examined. If the subject identified the Landolt ring in more than three different positions, the next lower contrast level was tested. The last contrast level that the subject recognized in more than three directions was defined as the mesopic contrast threshold of that eye. This value was converted to log contrast sensitivity41 and then was used for statistical analyses. Subsequently, glare source at a visual angle of 3 degrees to the left was introduced and the above testing was repeated. Glare illuminance was 0.35 ⫾ 0.03 lux at
Data obtained three months after commencement of orthokeratology were compared with the baseline measurements using the paired t test and Wilcoxon signed-rank test. The correlation among changes in log mesopic contrast sensitivity, changes in ocular higher-order aberrations, and amount of myopic correction were analyzed by Pearson and Spearman correlation tests. The amount of myopic correction was defined as the reduction in manifest refractive spherical equivalent at the three-month visit. All analyses were two-tailed and P ⬍ .05 was considered significant.
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● STATISTICAL ANALYSIS:
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FIGURE 1. Scatterplots demonstrating changes in log mesopic contrast sensitivity without glare and changes in ocular higher-order aberrations in eyes undergoing overnight orthokeratology. (Top left) Significant correlation between changes in log mesopic contrast sensitivity and third-order root mean square (RMS; r ⴝ ⴚ0.490; P ⴝ .0013, Spearman rank correlation coefficient). (Top right) Significant correlation between changes in log mesopic contrast sensitivity and fourth-order RMS (r ⴝ ⴚ0.586; P ⴝ .0001, Spearman rank correlation coefficient). (Bottom) Significant correlation between changes in log mesopic contrast sensitivity and total higher-order RMS (r ⴝ ⴚ0.548; P ⴝ .0003, Spearman rank correlation coefficient).
0.303 ⫾ 0.152 m (P ⬍ .0001). Log mesopic contrast sensitivity significantly decreased from 0.25 ⫾ 0.08 to 0.08 ⫾ 0.10 without glare (P ⬍ .0001, Wilcoxon signedrank test) and from 0.21 ⫾ 0.11 to 0.07 ⫾ 0.10 with glare (P ⬍ .0001) with the treatment (Table 1). The induced changes in log mesopic contrast sensitivity were analyzed in relation to the changes in ocular higherorder aberrations. The changes in log mesopic contrast sensitivity without glare had significant negative correlation with the changes in third-order RMS (r ⫽ ⫺0.490; P ⫽ .0013, Spearman rank correlation coefficient; Figure 1, Top left), fourth-order RMS (r ⫽ ⫺0.586; P ⫽ .0001, Spearman rank correlation coefficient; Figure 1, Top right), and total higher-order RMS (r ⫽ ⫺0.548; P ⫽
RESULTS OVERNIGHT ORTHOKERATOLOGY SIGNIFICANTLY REDUCED
manifest refraction from ⫺2.34 ⫾ 0.99 D at baseline to ⫺0.18 ⫾ 0.67 D at the three-month visit (P ⬍ .0001, paired t test) and improved UCVA from 0.75 ⫾ 0.31 to ⫺0.04 ⫾ 0.15 logMAR units (P ⬍ .0001). BCVA did not change significantly, with ⫺0.11 ⫾ 0.07 logMAR units before the procedure and ⫺0.09 ⫾ 0.06 logMAR units after procedure (P ⫽ .3465). The treatment resulted in significant increases in third-order RMS from 0.074 ⫾ 0.028 m to 0.263 ⫾ 0.152 m (P ⬍ .0001), fourth-order RMS from 0.039 ⫾ 0.020 m to 0.138 ⫾ 0.060 m (P ⬍ .0001), and total higher-order RMS 0.085 ⫾ 0.032 m to 648
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FIGURE 2. Scatterplot demonstrating changes in log mesopic contrast sensitivity with glare and changes in ocular higher-order aberrations in eyes undergoing overnight orthokeratology. (Top left) Significant correlation between changes in log mesopic contrast sensitivity with glare and third-order RMS (r ⴝ ⴚ0.362; P ⴝ .0177, Spearman rank correlation coefficient). (Top right) Significant correlation between changes in log mesopic contrast sensitivity with glare and fourth-order RMS (r ⴝ ⴚ0.306; P ⴝ .0450, Spearman rank correlation coefficient). (Bottom) Significant correlation between changes in log mesopic contrast sensitivity with glare and total higher-order RMS (r ⴝ ⴚ0.344; P ⴝ .0242, Spearman rank correlation coefficient).
.0003, Spearman rank correlation coefficient; Figure 1, Bottom). The changes in log mesopic contrast sensitivity with glare also showed significant negative correlation with the changes in third-order RMS (r ⫽ ⫺0.362; P ⫽ .0177; Figure 2, Top left), fourth-order RMS (r ⫽ ⫺0.306; P ⫽ .0450; Figure 2, Top right), and total higher-order RMS (r ⫽ ⫺0.344; P ⫽ .0242; Figure 2, Bottom). The induced changes in log mesopic contrast sensitivity and higher-order aberrations were analyzed in relation to the amount of myopic correction. Significant correlation was found between the amount of myopic correction and the induced changes in log mesopic contrast sensitivity (r ⫽ ⫺0.442; P ⫽ .0038 without glare; r ⫽ ⫺0.464; P ⫽ .0024 with glare; Figure 3). The amount of myopic correction demonstrated significant correlation with the induced changes in third-order RMS (r ⫽ 0.645; P ⬍ .0001, Pearson correlation coefficient; Figure 4, Top right),
fourth-order RMS (r ⫽ 0.576; P ⬍ .0001, Pearson correlation coefficient; Figure 4, Top right), and total higherorder RMS (r ⫽ 0.666, P ⬍ .0001, Pearson correlation coefficient; Figure 4, Bottom). The relationship between mesopic pupil diameter and log mesopic contrast sensitivity also was analyzed. There was no significant correlation between mesopic pupil diameter and log mesopic contrast sensitivity three months after the treatment (r ⫽ 0.037; P ⫽ .8091 without glare, r ⫽ ⫺0.015; P ⫽ .9196 with glare, Spearman rank correlation coefficient; Figure 5).
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DISCUSSION UNDER LOW LUMINANCE CIRCUMSTANCES, SUCH AS AT
night or in unclear weather conditions with rain or fog, 649
FIGURE 3. Scatterplots demonstrating changes in log mesopic contrast sensitivity and amount of myopic correction in eyes undergoing overnight orthokeratology. (Left) Significant correlation between changes in log mesopic contrast sensitivity without glare and amount of myopic correction (r ⴝ ⴚ0.442; P ⴝ .0038, Spearman rank correlation coefficient). (Right) Significant correlation between changes in log mesopic contrast sensitivity with glare and amount of myopic correction (r ⴝ ⴚ0.464; P ⴝ .0024, Spearman rank correlation coefficient). D ⴝ diopter.
mesopic contrast sensitivity both in the presence and absence of glare and has been used in several studies of PRK34,35 and LASIK.37 In these studies, the proportions of eyes that fulfilled the German standard recommendation level for night driving (contrast level of 1:5 or better) was analyzed. Schlote and associates reported that 45% of eyes after PRK using a 5.0-mm ablation zone were able to discriminate the 1:5 level in the absence of glare and that 33% of eyes were able to do so in the presence of glare.34 Nagy and associates examined the impact of the treatment zone size on mesopic contrast sensitivity in eyes after PRK and demonstrated that with a lager ablation zone of 6.5 mm, 87% of eyes under the without-glare lighting condition and 82% under the with-glare lighting condition met the criterion, whereas with a smaller ablation zone of 5.0 mm, only 34% and 32% of eyes satisfied the criterion, respectively.35 In eyes that have undergone LASIK with an ablation zone of 6.0 mm, Perez-Carrasco and associates showed that 70% of eyes without glare and 49% with glare recognized the critical level of 1:5.37 In the current orthokeratology study, the proportion of eyes passed the critical level was 36% in both the absence and presence of glare (Table 3). It is very surprising that nearly two-thirds of eyes undergoing orthokeratology did not qualify for night driving. These values were quite similar to those found in eyes after PRK using a 5.0-mm ablation diameter.34,35 Based on the study of Nagy and associates, better mesopic vision can be expected with a larger treatment zone than a smaller zone in eyes after PRK.35 Thus, it may be that a larger optical zone diameter of reverse-geometry contact lenses for orthokeratology is advantageous to reduce night vision disturbances, because most current
high-contrast visual acuity plays a less important role than the ability to discriminate dim contrasts.46 In subjects who have undergone corneal refractive surgery, it is well known that night vision disturbances and glare disability are frequent symptoms, even though high-contrast visual acuity is excellent.47–51 In the current study, we examined the influence of overnight orthokeratology on visual function in mesopic conditions. As shown in the results, we found that mesopic contrast sensitivity under both with-glare and without-glare conditions was decreased significantly by orthokeratology, although UCVA improved significantly and BCVA did not change as a result of the procedure. We also confirmed that orthokeratology significantly increased ocular higher-order aberrations in parallel with the amount of myopic correction. Furthermore, the induced changes in log mesopic contrast sensitivity correlated significantly with the changes in ocular higher-order aberrations and the amount of myopic correction. These findings indicate that the larger the amount of myopic correction is, the greater the changes in ocular higher-order aberrations are, leading to declines of mesopic vision. To the best our knowledge, this is the first study that elucidated the relationship among mesopic contrast sensitivity, ocular higher-order aberrations, and myopic correction in subjects undergoing overnight orthokeratology. There are several testers to assess scotopic or mesopic contrast sensitivity, and some of them can measure contrast sensitivity with a glare source that produces glare disability. Each tester, however, uses different optotype, luminance condition, and glare sources. Therefore, it is difficult simply to compare the results obtained by different testers. The Mesotest II used in this study is able to assess 650
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FIGURE 4. Scatterplots demonstrating changes in ocular higher-order aberrations and amount of myopic correction in eyes undergoing overnight orthokeratology. (Top left) Significant correlation between changes in third-order RMS and amount of myopic correction (r ⴝ 0.645; P < .0001, Pearson correlation coefficient). (Top right) Significant correlation between changes in fourth-order RMS and amount of myopic correction (r ⴝ 0.576; P < .0001, Pearson correlation coefficient). (Bottom) Significant correlation between changes in total higher-order RMS and amount of myopic correction (r ⴝ 0.666; P < .0001, Pearson correlation coefficient).
available orthokeratology lenses have an optical zone diameter of 6.0 or 6.2 mm. As for the relationship between mesopic vision and ocular higher-order aberrations, we are unaware of previous studies showing significant correlation between these parameters even in patients undergoing corneal refractive surgeries, although some researchers suggested that higherorder aberrations may be a promising parameter for predicting night vision symptoms after these procedures.25,52 In this study, we found a significant correlation between changes in mesopic contrast sensitivity and changes in total higher-order aberrations in patients undergoing orthokeratology. This finding indicates that ocular higherorder aberrations are good predictors for mesopic visual function. Furthermore, both third- and fourth-order aber-
rations significantly correlated with the reduction in mesopic contrast sensitivity. This suggests that both comalike and spherical-like aberrations are responsible for the deterioration of mesopic visual function in orthokeratology. Previously, we found that not only spherical-like aberrations but also coma-like aberrations deteriorate contrast sensitivity under photopic conditions after orthokeratology.31 We herein confirmed similar results under mesopic conditions. Theoretically, optical aberrations increase as pupil size enlarges. In addition, the effect of pupil dilation on optical aberrations is considered to be much greater after corneal refractive surgeries.53–55 Oshika and associates reported that before surgery, simulated pupillary dilation from 3 to 7 mm caused a five-fold to six-fold increase in the total
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FIGURE 5. Scatterplots demonstrating mesopic pupil diameter and log mesopic contrast sensitivity in eyes undergoing overnight orthokeratology. (Left) No significant correlation between mesopic pupil diameter and log mesopic contrast sensitivity without glare (r ⴝ 0.037; P ⴝ .8091, Spearman rank correlation coefficient). (Right) No significant correlation between mesopic pupil diameter and log mesopic contrast sensitivity with glare (r ⴝ ⴚ0.015; P ⴝ .9196, Spearman rank correlation coefficient).
orthokeratology. Berntsen and associates suggested that spherical aberration drove decreases in low-contrast bestcorrected visual acuity as pupil size increased after orthokeratology.30 Because there have been no other studies on the impact of pupil size on visual performance in orthokeratology, further studies are needed to elucidate this point. It is generally considered that glare can have negative influences on contrast sensitivity,40 but in our study, mesopic contrast sensitivity with glare was similar to that without glare in eyes after orthokeratology. As suggested by some researchers,37,40,58,59 this result may be explained by the pupillary miosis induced by glare possibly having a pinhole effect and may be offset by any loss in contrast sensitivity resulting from blur in glare conditions. We at present have no detailed data regarding the relation between pupil size and glare. Further studies are necessary to clarify this point. Until now, orthokeratology procedures have concentrated on the reduction of spherical defocus, which is one of the major optical aberrations, and much less attention has been paid to the higher-order aberrations. As shown in the current study, increases in ocular higher-order aberrations can play a significant role in the decline of mesopic contrast sensitivity after orthokeratology. For better quality of vision and life, further attention should be directed to optical quality of the eye and visual performance in detail, including mesopic visual function. In orthokeratology, the increase in spherical aberration results from the change in corneal asphericity to an oblate profile by the treatment, and the increase in coma aberration is attributed to contact lens decentration.60,61 Therefore, to avoid greater loss of mesopic visual function in current orthokeratology, large myopic correction should not be attempted, and the fitting and centration of the treatment lens should be performed strictly.
TABLE 3. Percentage of Eyes That Fulfilled the Contrast Level of 1:5 in Overnight Orthokeratology Baseline
After Treatment
Glare (⫺)
Glare (⫹)
Glare (⫺)
Glare (⫹)
98%
82%
36%
36%
Contrast level of 1:5 corresponds to log contrast sensitivity of 0.1. This level is the critical limit for night driving recommended by the German Ophthalmic Society.42
aberrations, but after surgery, the same dilation resulted in a 25-fold to 32-fold increase in PRK eyes and a 28-fold to 46-fold increase in LASIK eyes.54 Thus, pupil size has been the suspected highlighted variable of night vision disturbances.52 Haw and Manche, however, demonstrated that the effect of scotopic pupil size on scotopic contrast sensitivity was statistically insignificant in eyes that have undergone PRK.56 In eyes that have undergone LASIK, Lee and associates reported that no significant correlation was observed between the scotopic pupil size and nighttime contrast sensitivity with and without night glare.57 Moreover, Pop and Payette examined 1,488 eyes undergoing LASIK to clarify the preoperative risk factors for night vision symptoms and showed that there was no statistically significant contribution of scotopic pupil size to night vision symptoms.52 Similarly, in eyes undergoing orthokeratology, we could not find any significant correlations between mesopic pupil diameter and mesopic contrast sensitivity. This indicates that mesopic pupil size is not a primary factor responsible for the decline of mesopic visual function after orthokeratology. But, it cannot be denied that pupil size affects visual performance in patients undergoing 652
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One weakness of our study is that we evaluated mesopic contrast sensitivity at only one time point of three months. Although data on time-course changes in mesopic visual function are scarce, Holladay and associates reported that contrast threshold in darkness was worst immediately after LASIK and improved slightly thereafter, but had not returned to baseline by six months.23 At present, no information is available on long-term changes in mesopic visual function after orthokeratology. Further investigations are needed to know how mesopic visual function changes after orthokeratology.
In conclusion, we first found that regardless of the presence of glare light, mesopic contrast sensitivity after overnight orthokeratology deteriorated significantly as ocular higher-order aberrations increased, and these changes depended on the amount of myopic correction. In patients who undergo this treatment, ocular higher-order aberrations are good parameters to predict mesopic visual function. The indication for overnight orthokeratology must be discussed cautiously and sufficiently, especially in subjects with particular visual requirements for their profession, such as professional drivers, pilots, and military officers.
THE AUTHORS INDICATE NO FINANCIAL SUPPORT OR FINANCIAL CONFLICT OF INTEREST. INVOLVED IN CONCEPTION AND design (T.H., T.K., T.O.); analysis and interpretation (T.H., T.T., T.O.); writing the article (T.H.); critical revision of the article (T.O.); data collection (T.H., C.O., Y.I., T.K.); provision of materials (T.O.); statistical expertise (T.O.); literature search (T.H.); logistic support (T.O.); and final approval of the article (T.H., C.O., Y.I., T.T., T.K., T.O.). All patients were informed about the nature of the study and informed consent was obtained in accordance with Institutional Review Board of Tsukuba University Hospital and the tenets of the Declaration of Helsinki before their participation in the study.
REFERENCES 1. Coon LJ. Orthokeratology. Part II: evaluating the Tabb method. J Am Optom Assoc 1984;55:409 – 418. 2. Swarbrick HA, Wong G, O’Leary DJ. Corneal response to orthokeratology. Optom Vis Sci 1998;75:791–799. 3. Nichols JJ, Marsich MM, Nguyen M, Barr JT, Bullimore MA. Overnight orthokeratology. Optom Vis Sci 2000;77:252– 259. 4. Alharbi A, Swarbrick HA. The effects of overnight orthokeratology lens wear on corneal thickness. Invest Ophthalmol Vis Sci 2003;44:2518 –2523. 5. Rah MJ, Jackson JM, Jones LA, Marsden HJ, Bailey MD, Barr JT. Overnight orthokeratology: preliminary results of the Lenses and Overnight Orthokeratology (LOOK) Study. Optom Vis Sci 2002;79:598 – 605. 6. Mountford J. An analysis of the changes in corneal shape and refractive error induced by accelerated orthokeratology. ICLC 1997;24:128 –143. 7. Lui WO, Edwards MH. Orthokeratology in low myopia. Part 1: efficacy and predictability. Contact Lens Anterior Eye 2000;23:77– 89. 8. Cheung SW, Cho P. Subjective and objective assessments of the effect of orthokeratology—a cross-sectional study. Curr Eye Res 2004;28:121–127. 9. Jindra LF, Zemon V. Contrast sensitivity testing: a more complete assessment of vision. J Cataract Refract Surg 1989;15:141–148. 10. Elliott DB, Hurst MA. Simple clinical techniques to evaluate visual function in patients with early cataract. Optom Vis Sci 1990;67:822– 825. 11. Trobe JD, Beck RW, Moke PS, Cleary PA. Contrast sensitivity and other vision tests in the optic neuritis treatment trial. Am J Ophthalmol 1996;121:547–553. 12. Midena E, Degli AC, Blarzino MC, Valenti M, Segato T. Macular function impairment in eyes with early age-related macular degeneration. Invest Ophthalmol Vis Sci 1997;38: 469 – 477. 13. Bellmann C, Unnebrink K, Rubin GS, Miller D, Holz FG. Visual acuity and contrast sensitivity in patients with neo-
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vascular age-related macular degeneration: results from the Radiation Therapy for Age-Related Macular Degeneration (RAD) Study. Graefes Arch Clin Exp Ophthalmol 2003;241:968 –974. Stavrou EP, Wood JM. Letter contrast sensitivity changes in early diabetic retinopathy. Clin Exp Optom 2003;86:152– 156. Hawkins AS, Szlyk JP, Ardickas Z, Alexander KR, Wilensky JT. Comparison of contrast sensitivity, visual acuity, and Humphrey visual field testing in patients with glaucoma. J Glaucoma 2003;12:134 –138. Krasnov MM, Avetisov SE, Makashova NV, Mamikonian VR. The effect of radial keratotomy on contrast sensitivity. Am J Ophthalmol 1988;105:651– 654. Ginsburg AP, Waring GO III, Steinberg EB, et al. Contrast sensitivity under photopic conditions in the Prospective Evaluation of Radial Keratotomy (PERK) Study. Refract Corneal Surg 1990;6:82–91. Applegate RA, Howland HC, Sharp RP, et al. Corneal aberrations and visual performance after radial keratotomy. J Refract Surg 1998;14:397– 407. Ghaith AA, Daniel J, Stulting RD, Thompson KP, Lynn M. Contrast sensitivity and glare disability after radial keratotomy and photorefractive keratectomy. Arch Ophthalmol 1998;116:12–18. Verdon W, Bullimore M, Maloney RK. Visual performance after photorefractive keratectomy: a prospective study. Arch Ophthalmol 1996;114:1465–1472. Perez-Santonja JJ, Sakla HF, Alió JL. Contrast sensitivity after laser in situ keratomileusis. J Cataract Refract Surg 1998;24:183–189. Bullimore MA, Olson MD, Maloney RK. Visual performance after photorefractive keratectomy with a 6-mm ablation zone. Am J Ophthalmol 1999;128:1–7. Holladay JT, Dudeja DR, Chang J. Functional vision and corneal changes after laser in situ keratomileusis determined by contrast sensitivity, glare testing, and corneal topography. J Cataract Refract Surg 1999;25:663– 669. Mierdel P, Kaemmerer M, Krinke HE, Seiler T. Effects of photorefractive keratectomy and cataract surgery on ocular
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matic refractive keratectomy. J Cataract Refract Surg 2001;27:907–916. 57. Lee YC, Hu FR, Wang IJ. Quality of vision after laser in situ keratomileusis: influence of dioptric correction and pupil size on visual function. J Cataract Refract Surg 2003;29:769 – 777. 58. Elliot DB. Contrast sensitivity and glare testing. In: Benjamin WJ, editor. Borish’s clinical refraction. Philadelphia, Pennsylvania: Saunders, 1998:203–241.
59. Spraul CW, Roth HJ, Gackle H, Lang GE, Lang GK. Influence of special-effect contact lenses (Crazy Lenses) on visual function. CLAO J 1998;24:29 –32. 60. Joslin CE, Wu SM, McMahon TT, Shahidi M. Higher-order wavefront aberrations in corneal refractive therapy. Optom Vis Sci 2003;80:805– 811. 61. Hiraoka T, Matsumoto Y, Okamoto F, et al. Corneal higherorder aberrations induced by overnight orthokeratology. Am J Ophthalmol 2005;139:429 – 436.
REPORTING VISUAL ACUITIES The AJO encourages authors to report the visual acuity in the manuscript using the same nomenclature that was used in gathering the data provided they were recorded in one of the methods listed here. This table of equivalent visual acuities is provided to the readers as an aid to interpret visual acuity findings in familiar units. Table of Equivalent Visual Acuity Measurements Snellen Visual Acuities 4 Meters
6 Meters
20 Feet
Decimal Fraction
LogMAR
4/40 4/32 4/25 4/20 4/16 4/12.6 4/10 4/8 4/6.3 4/5 4/4 4/3.2 4/2.5 4/2
6/60 6/48 6/38 6/30 6/24 6/20 6/15 6/12 6/10 6/7.5 6/6 6/5 6/3.75 6/3
20/200 20/160 20/125 20/100 20/80 20/63 20/50 20/40 20/32 20/25 20/20 20/16 20/12.5 20/10
0.10 0.125 0.16 0.20 0.25 0.32 0.40 0.50 0.63 0.80 1.00 1.25 1.60 2.00
⫹1.0 ⫹0.9 ⫹0.8 ⫹0.7 ⫹0.6 ⫹0.5 ⫹0.4 ⫹0.3 ⫹0.2 ⫹0.1 0.0 ⫺0.1 ⫺0.2 ⫺0.3
From Ferris FL III, Kassoff A, Bresnick GH, Bailey I. New visual acuity charts for clinical research. Am J Ophthalmol 1982;94:91–96.
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Biosketch Takahiro Hiraoka, MD, is an Assistant Professor of Ophthalmology at Institute of Clinical Medicine, University of Tsukuba, Ibaraki, Japan. He graduated from University of Tsukuba School of Medicine in 1993 and completed postgraduate training in 1999. Dr Hiraoka main research interests include optical quality of the eye and quality of vision, especially in eyes undergoing corneal refractive surgery and orthokeratology.
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To investigate mesopic contrast sensitivity and night driving ability in eyes undergoing overnight orthokeratology, and to analyze the relationship among mesopic contrast sensitivity, ocular higher-order aberrations, and myopic correction. ● DESIGN: Prospective, noncomparative, consecutive case series. ● METHODS: In 44 eyes of 22 subjects (mean age ⴞ standard deviation [SD], 24.0 ⴞ 3.2 years) with orthokeratology, ocular aberrations and mesopic contrast sensitivity were determined before and three months after commencement of the procedure. Mean spherical equivalent refraction ⴞ SD was ⴚ2.34 ⴞ 0.99 diopters at baseline. Mesopic contrast sensitivity with and without glare was assessed using the Mesotest II (Oculus, Wetzlar, Germany). ● RESULTS: Orthokeratology significantly reduced the log mesopic contrast sensitivity from 0.25 ⴞ 0.08 to 0.08 ⴞ 0.10 without glare (P < .0001, Wilcoxon) and from 0.21 ⴞ 0.11 to 0.07 ⴞ 0.10 with glare (P < .0001). The proportion of eyes that fulfilled the German standard recommendation level for night driving was 36%. The induced changes in log mesopic contrast sensitivity showed significant negative correlation with the changes in third-order (r ⴝ ⴚ0.490, P ⴝ .0013 without glare; r ⴝ ⴚ0.362, P ⴝ .0177 with glare; Spearman rank correlation coefficient) and fourth-order root mean square (r ⴝ ⴚ0.586, P ⴝ .0001 and r ⴝ ⴚ0.306, P ⴝ .0450, respectively). Furthermore, significant correlation was found between the amount of myopic correction and the induced changes in log mesopic contrast sensitivity (r ⴝ ⴚ0.442, P ⴝ .0038 without glare; r ⴝ ⴚ0.464, P ⴝ .0024 with glare). The induced changes in higherorder aberrations significantly correlated with the amount of myopic correction (P < .0001, Pearson correlation coefficient). ● CONCLUSIONS: Mesopic contrast sensitivity after overnight orthokeratology is deteriorated significantly as ocular higher-order aberrations increase, and these changes Accepted for publication Nov 27, 2007. From the Department of Ophthalmology, Institute of Clinical Medicine, University of Tsukuba, Ibaraki, Japan (T.H., C.O., Y.I., T.T., T.O.); and the Kakita Eye Clinic, Chiba, Japan (T.K.). Inquiries to Takahiro Hiraoka, Department of Ophthalmology, Institute of Clinical Medicine, University of Tsukuba, 1-1-1 Tennoudai, Tsukuba, Ibaraki, 305-8575 Japan; e-mail: [email protected] 0002-9394/08/$34.00 doi:10.1016/j.ajo.2007.11.021
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2008 BY
depend on the amount of myopic correction. (Am J Ophthalmol 2008;145:645– 655. © 2008 by Elsevier Inc. All rights reserved.)
T
HE CURRENT GOAL OF ORTHOKERATOLOGY IS TO
provide the patient with good unaided vision during the day through the programmed application of specially designed rigid contact lenses, particularly in those with low to moderate myopia. As a result of this procedure, the corneal profile is reshaped by flattening of the central area, and a temporary correction of myopia is achieved.1– 4 With the development of higher gas-permeable lens materials, night therapy, where patients wear the lenses only during sleep, has become possible since the 1980s. This method, which is called overnight orthokeratology, allows satisfactory unaided vision during waking hours without spectacles and contact lenses.3,5 In the 1990s, the introduction of refined lens designs (reverse geometry design) enabled much quicker, more predictable achievement of corneal and refractive changes.3,5–7 Because of these innovations, the clinical application of overnight orthokeratology recently has grown popularity as a nonsurgical option for the correction of low to moderate myopia. Although a number of studies have demonstrated the success of overnight orthokeratology, the evaluation of this procedure has been performed mainly using high-contrast visual acuity.3,5,6,8 Standard high-contrast visual acuity, which relies on the patient’s recognition of familiar letters or Landolt rings with 100% contrast level, is just one feature of actual visual function. In daily life activities, we encounter various visual objects with different spatial frequencies and contrast levels. To evaluate quality of vision accurately, more sophisticated measurements of visual function other than standard high-contrast visual acuity are needed. In fact, we sometimes observe that patients report visual disturbances in several situations even though visual acuity measured with a high-contrast acuity chart is excellent. Contrast sensitivity is defined as the ability to detect differences in luminance between adjacent areas. A contrast sensitivity test is now considered to provide more information regarding visual performance, which may not be disclosed by standard visual acuity testing, and has been used widely as a representative of quality of vision in recent
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TABLE 1. Subject Data at Baseline and Three Months after Commencement of Overnight Orthokeratology
Manifest refraction (D) UCVA (logMAR) BCVA (logMAR) Third-order RMS (m) Fourth-order RMS (m) Total higher-order RMS (m) Mesopic log contrast sensitivity (without glare) Mesopic log contrast sensitivity (with glare)
Baseline, Mean ⫾ SD (Range)
After Treatment, Mean ⫾ SD (Range)
⫺2.34 ⫾ 0.99 (⫺4.00 to ⫺1.00) 0.75 ⫾ 0.31 (0.22 to 1.52) ⫺0.11 ⫾ 0.07 (⫺0.30 to 0.00) 0.074 ⫾ 0.028 (0.022 to 0.133) 0.039 ⫾ 0.020 (0.010 to 0.101) 0.085 ⫾ 0.032 (0.035 to 0.161) 0.25 ⫾ 0.08 (0.02 to 0.30) 0.21 ⫾ 0.11 (0.00 to 0.30)
⫺0.18 ⫾ 0.67 (⫺2.75 to 1.25)* ⫺0.04 ⫾ 0.15 (⫺0.18 to 0.40)* ⫺0.09 ⫾ 0.06 (⫺0.18 to 0.15) 0.263 ⫾ 0.152 (0.029 to 0.758)* 0.138 ⫾ 0.060 (0.037 to 0.275)* 0.303 ⫾ 0.152 (0.084 to 0.791)* 0.08 ⫾ 0.10 (0.00 to 0.30)* 0.07 ⫾ 0.10 (0.00 to 0.30)*
BCVA ⫽ best-corrected visual acuity; D ⫽ diopters; logMAR ⫽ logarithm of the minimum angle of resolution; RMS ⫽ root mean square; SD ⫽ standard deviation; UCVA ⫽ uncorrected visual acuity. *P ⬍ .0001, significant differences between pretreatment and posttreatment values.
years.9 –29 Numerous studies have shown that contrast sensitivity is compromised in eyes with various ocular pathologic features10 –15 and after refractive surgeries.16 –29 In addition, several studies have demonstrated that contrast sensitivity decreases in parallel with increases in higher-order aberrations after corneal refractive surgeries, such as radial keratotomy (RK),18 photorefractive keratectomy (PRK),24 and laser in situ keratomileusis (LASIK).27,29 As for orthokeratology, there have been two studies showing the reduction of contrast sensitivity after the procedure.30,31 Previous studies have shown that scotopic and mesopic vision can be affected by corneal refractive surgeries for myopia, such as RK,19,32 PRK,32–36 LASIK.32,37–39 These procedures are planned to correct defocus by means of surgical alteration of the corneal curvature, and the corneal asphericity accordingly changes from prolate to oblate profile. Similarly, it has been reported that orthokeratology also induces a nonphysiologic oblate-shaped cornea.6 In the clinical practice of orthokeratology, we have had a certain percentage of patients who gained good highcontrast visual acuity but still reported night vision disturbances, especially during driving at night. The impact of this procedure on mesopic vision or night driving ability, however, has not been investigated. Given that a significant proportion of traffic accidents happen at night and that most patients undergoing overnight orthokeratology are young and active, it is crucial to know how this procedure affects night driving ability. The relationship among mesopic vision, ocular higher-order aberrations, and myopic correction also is unknown. We conducted the current prospective study to evaluate mesopic contrast sensitivity with and without glare and night driving ability in eyes undergoing overnight orthokeratology and to analyze the relationship among changes in mesopic contrast sensitivity, changes in ocular higher-order aberrations, and amount of myopic correction. 646
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METHODS ● SUBJECTS:
Forty-four eyes of 22 subjects were enrolled in this prospective study. Inclusion criteria were spherical equivalent refraction between ⫺1.0 and ⫺4.0 diopters (D), astigmatism of 1.0 D or less, mean keratometry readings between 40.00 and 46.25 D, best-corrected visual acuity (BCVA) of 20/20 or better, the absence of ocular and systemic diseases, and age between 20 and 37 years. There were 11 males and 11 females, and the mean age ⫾ standard deviation (SD) was 24.0 ⫾ 3.2 years. Mean spherical equivalent refraction ⫾ SD was ⫺2.34 ⫾ 0.99 D, and mean uncorrected visual acuity (UCVA) ⫾ SD was 0.75 ⫾ 0.31 logarithm of the minimum angle of resolution (logMAR) units before treatment (Table 1). Contact lens users were required to remove their lenses for at least three weeks before the baseline examination. ● LENSES:
The orthokeratology lens used in this study was manufactured from fluorosilicone acrylate (BOSTON XO; Polymer Technology Corp, Wilmington, Massachusetts, USA) with an oxygen permeability (Dk) of 100 ⫻ 10–11 (cm2/sec)(ml O2/ml ⫻ mm Hg). The lens has a four-zone reverse geometry design with a base curve (central optical zone) diameter of 6.0 mm, reverse curve of 0.6 mm width, alignment curve of 1.0 mm width, and a peripheral curve of 0.4 mm width. The lenses were fitted according to the manufacturer’s recommended fitting guidelines. Total lens diameter of 10.0 mm was used initially in all cases. Subsequently, the parameters of the lens were varied to obtain appropriate centration, fluorescein pattern, and lens movement of approximately 1 mm on a blink. After a proper lens fit was accomplished, overrefraction was carried out to determine the final power. After lens dispensing, subjects were instructed to wear contact lenses every night in a consecutive manner. If the treatment effect was deemed inappropriate on the basis of corneal topographic changes and improvement of UCVA, the lens design was OF
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TABLE 2. Contrast Level, Threshold, and Log Sensitivity of the Mesotest II Optotypes That Were Used in the Current Overnight Orthokeratology Study Contrast Level
Percentage of Contrast Threshold
Log Contrast Sensitivity
1:23 1:5 1:2.7 1:2
95% 80% 63% 50%
0.02 0.1 0.2 0.3
modified throughout the course of the treatment. Detailed biomicroscopic evaluations of the anterior segment were performed at each visit to ensure continuing ocular health and to monitor any effects of lens wear. ● MEASUREMENTS:
pupil. This test is applied in Germany as a legal standard to evaluate the ability of night driving.34 According to the guidelines of the German Ophthalmic Society, the most important setting is the contrast level of 1:5, which is the critical limit for night driving.42 It is also known that lower contrast levels in this device are difficult to recognize, even in healthy young people.35,40 This implies that the device is able to detect subtle changes in mesopic contrast sensitivity because of no ceiling effects. Ocular wavefront aberrations for a 4-mm pupil were measured with the Hartmann-Shack wavefront analyzer (KR-9000 PW; Topcon Co, Tokyo, Japan). This system has been described in detail elsewhere,29,43,44 and the accuracy and repeatability have been validated.43,44 The obtained data were expanded with the normalized Zernike polynomials. From the Zernike coefficients, ocular higherorder aberrations were calculated and expressed as the root mean square (RMS; in micrometers). The RMS of the third-order Zernike coefficients was used to represent coma-like aberration, and the RMS of the fourth-order Zernike coefficients was used to denote spherical-like aberrations. Total higher-order aberrations were calculated as the RMS of the third- and fourth-order Zernike coefficients.28,29 To obtain well-focused and properly aligned Hartmann images, the measurements were repeated at least four times in each eye, and three better images were chosen and averaged. The averaged values were used for subsequent analyses. Pupil diameter in a mesopic condition (5 lux) was measured monocularly using the hand-held open-view type digital pupillometer (FP-10000; TMI Co, Saitama, Japan).45 The device was placed in front of one eye and patients were asked to look straight through a peek hole. Accommodation was controlled by instructing the subjects to fixate on a 1.5-m target. The pupillometer includes a charge-coupled device camera providing a continuous video-signal output to a personal computer. Using digital image software (ViewShot; TMI Co), which can recognize the pupillary edge, the largest horizontal and vertical pupil diameters were measured and then averaged. The averaged values were used for statistical analyses.
The baseline examination included manifest refraction, uncorrected and corrected high-contrast visual acuity, keratometry, slit-lamp microscopy containing fluorescein staining, dilated funduscopy, corneal topography, and wavefront aberrometry. The above examinations except for funduscopy were repeated at each visit after commencement of overnight orthokeratology. In addition, mesopic contrast sensitivity testing was performed before and three months after commencement of the treatment. Mesopic pupil diameter was evaluated at the three-month visit. To minimize the influence of diurnal variation, all measurements were conducted between 9 AM and 11 AM, and subjects were instructed to attend the examinations from two to four hours after lens removal. Mesopic contrast sensitivity with and without glare was assessed using the Mesotest II (Oculus, Wetzlar, Germany), which has been previously described.34,35,37,40 The test was performed monocularly with best-spectacle correction after dark adaptation for 10 minutes. The background luminance was 0.032 ⫾ 0.003 cd/m2 without glare and 0.1 ⫾ 0.01 cd/m2 with glare. The device presents the Landolt ring of 20/200 as the optotype, which can be displayed randomly in six directions. There are four contrast levels (1:23, 1:5, 1:2.7, and 1:2), and the contrast with respect to its background can be varied in a series of steps, decreasing by a factor of 0.10 log contrast sensitivity units. Each contrast level corresponds to log contrast sensitivity of 0.02, 0.1, 0.2, and 0.3, respectively (Table 2). First, the highest contrast level of 1:23 was examined. If the subject identified the Landolt ring in more than three different positions, the next lower contrast level was tested. The last contrast level that the subject recognized in more than three directions was defined as the mesopic contrast threshold of that eye. This value was converted to log contrast sensitivity41 and then was used for statistical analyses. Subsequently, glare source at a visual angle of 3 degrees to the left was introduced and the above testing was repeated. Glare illuminance was 0.35 ⫾ 0.03 lux at
Data obtained three months after commencement of orthokeratology were compared with the baseline measurements using the paired t test and Wilcoxon signed-rank test. The correlation among changes in log mesopic contrast sensitivity, changes in ocular higher-order aberrations, and amount of myopic correction were analyzed by Pearson and Spearman correlation tests. The amount of myopic correction was defined as the reduction in manifest refractive spherical equivalent at the three-month visit. All analyses were two-tailed and P ⬍ .05 was considered significant.
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FIGURE 1. Scatterplots demonstrating changes in log mesopic contrast sensitivity without glare and changes in ocular higher-order aberrations in eyes undergoing overnight orthokeratology. (Top left) Significant correlation between changes in log mesopic contrast sensitivity and third-order root mean square (RMS; r ⴝ ⴚ0.490; P ⴝ .0013, Spearman rank correlation coefficient). (Top right) Significant correlation between changes in log mesopic contrast sensitivity and fourth-order RMS (r ⴝ ⴚ0.586; P ⴝ .0001, Spearman rank correlation coefficient). (Bottom) Significant correlation between changes in log mesopic contrast sensitivity and total higher-order RMS (r ⴝ ⴚ0.548; P ⴝ .0003, Spearman rank correlation coefficient).
0.303 ⫾ 0.152 m (P ⬍ .0001). Log mesopic contrast sensitivity significantly decreased from 0.25 ⫾ 0.08 to 0.08 ⫾ 0.10 without glare (P ⬍ .0001, Wilcoxon signedrank test) and from 0.21 ⫾ 0.11 to 0.07 ⫾ 0.10 with glare (P ⬍ .0001) with the treatment (Table 1). The induced changes in log mesopic contrast sensitivity were analyzed in relation to the changes in ocular higherorder aberrations. The changes in log mesopic contrast sensitivity without glare had significant negative correlation with the changes in third-order RMS (r ⫽ ⫺0.490; P ⫽ .0013, Spearman rank correlation coefficient; Figure 1, Top left), fourth-order RMS (r ⫽ ⫺0.586; P ⫽ .0001, Spearman rank correlation coefficient; Figure 1, Top right), and total higher-order RMS (r ⫽ ⫺0.548; P ⫽
RESULTS OVERNIGHT ORTHOKERATOLOGY SIGNIFICANTLY REDUCED
manifest refraction from ⫺2.34 ⫾ 0.99 D at baseline to ⫺0.18 ⫾ 0.67 D at the three-month visit (P ⬍ .0001, paired t test) and improved UCVA from 0.75 ⫾ 0.31 to ⫺0.04 ⫾ 0.15 logMAR units (P ⬍ .0001). BCVA did not change significantly, with ⫺0.11 ⫾ 0.07 logMAR units before the procedure and ⫺0.09 ⫾ 0.06 logMAR units after procedure (P ⫽ .3465). The treatment resulted in significant increases in third-order RMS from 0.074 ⫾ 0.028 m to 0.263 ⫾ 0.152 m (P ⬍ .0001), fourth-order RMS from 0.039 ⫾ 0.020 m to 0.138 ⫾ 0.060 m (P ⬍ .0001), and total higher-order RMS 0.085 ⫾ 0.032 m to 648
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FIGURE 2. Scatterplot demonstrating changes in log mesopic contrast sensitivity with glare and changes in ocular higher-order aberrations in eyes undergoing overnight orthokeratology. (Top left) Significant correlation between changes in log mesopic contrast sensitivity with glare and third-order RMS (r ⴝ ⴚ0.362; P ⴝ .0177, Spearman rank correlation coefficient). (Top right) Significant correlation between changes in log mesopic contrast sensitivity with glare and fourth-order RMS (r ⴝ ⴚ0.306; P ⴝ .0450, Spearman rank correlation coefficient). (Bottom) Significant correlation between changes in log mesopic contrast sensitivity with glare and total higher-order RMS (r ⴝ ⴚ0.344; P ⴝ .0242, Spearman rank correlation coefficient).
.0003, Spearman rank correlation coefficient; Figure 1, Bottom). The changes in log mesopic contrast sensitivity with glare also showed significant negative correlation with the changes in third-order RMS (r ⫽ ⫺0.362; P ⫽ .0177; Figure 2, Top left), fourth-order RMS (r ⫽ ⫺0.306; P ⫽ .0450; Figure 2, Top right), and total higher-order RMS (r ⫽ ⫺0.344; P ⫽ .0242; Figure 2, Bottom). The induced changes in log mesopic contrast sensitivity and higher-order aberrations were analyzed in relation to the amount of myopic correction. Significant correlation was found between the amount of myopic correction and the induced changes in log mesopic contrast sensitivity (r ⫽ ⫺0.442; P ⫽ .0038 without glare; r ⫽ ⫺0.464; P ⫽ .0024 with glare; Figure 3). The amount of myopic correction demonstrated significant correlation with the induced changes in third-order RMS (r ⫽ 0.645; P ⬍ .0001, Pearson correlation coefficient; Figure 4, Top right),
fourth-order RMS (r ⫽ 0.576; P ⬍ .0001, Pearson correlation coefficient; Figure 4, Top right), and total higherorder RMS (r ⫽ 0.666, P ⬍ .0001, Pearson correlation coefficient; Figure 4, Bottom). The relationship between mesopic pupil diameter and log mesopic contrast sensitivity also was analyzed. There was no significant correlation between mesopic pupil diameter and log mesopic contrast sensitivity three months after the treatment (r ⫽ 0.037; P ⫽ .8091 without glare, r ⫽ ⫺0.015; P ⫽ .9196 with glare, Spearman rank correlation coefficient; Figure 5).
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FIGURE 3. Scatterplots demonstrating changes in log mesopic contrast sensitivity and amount of myopic correction in eyes undergoing overnight orthokeratology. (Left) Significant correlation between changes in log mesopic contrast sensitivity without glare and amount of myopic correction (r ⴝ ⴚ0.442; P ⴝ .0038, Spearman rank correlation coefficient). (Right) Significant correlation between changes in log mesopic contrast sensitivity with glare and amount of myopic correction (r ⴝ ⴚ0.464; P ⴝ .0024, Spearman rank correlation coefficient). D ⴝ diopter.
mesopic contrast sensitivity both in the presence and absence of glare and has been used in several studies of PRK34,35 and LASIK.37 In these studies, the proportions of eyes that fulfilled the German standard recommendation level for night driving (contrast level of 1:5 or better) was analyzed. Schlote and associates reported that 45% of eyes after PRK using a 5.0-mm ablation zone were able to discriminate the 1:5 level in the absence of glare and that 33% of eyes were able to do so in the presence of glare.34 Nagy and associates examined the impact of the treatment zone size on mesopic contrast sensitivity in eyes after PRK and demonstrated that with a lager ablation zone of 6.5 mm, 87% of eyes under the without-glare lighting condition and 82% under the with-glare lighting condition met the criterion, whereas with a smaller ablation zone of 5.0 mm, only 34% and 32% of eyes satisfied the criterion, respectively.35 In eyes that have undergone LASIK with an ablation zone of 6.0 mm, Perez-Carrasco and associates showed that 70% of eyes without glare and 49% with glare recognized the critical level of 1:5.37 In the current orthokeratology study, the proportion of eyes passed the critical level was 36% in both the absence and presence of glare (Table 3). It is very surprising that nearly two-thirds of eyes undergoing orthokeratology did not qualify for night driving. These values were quite similar to those found in eyes after PRK using a 5.0-mm ablation diameter.34,35 Based on the study of Nagy and associates, better mesopic vision can be expected with a larger treatment zone than a smaller zone in eyes after PRK.35 Thus, it may be that a larger optical zone diameter of reverse-geometry contact lenses for orthokeratology is advantageous to reduce night vision disturbances, because most current
high-contrast visual acuity plays a less important role than the ability to discriminate dim contrasts.46 In subjects who have undergone corneal refractive surgery, it is well known that night vision disturbances and glare disability are frequent symptoms, even though high-contrast visual acuity is excellent.47–51 In the current study, we examined the influence of overnight orthokeratology on visual function in mesopic conditions. As shown in the results, we found that mesopic contrast sensitivity under both with-glare and without-glare conditions was decreased significantly by orthokeratology, although UCVA improved significantly and BCVA did not change as a result of the procedure. We also confirmed that orthokeratology significantly increased ocular higher-order aberrations in parallel with the amount of myopic correction. Furthermore, the induced changes in log mesopic contrast sensitivity correlated significantly with the changes in ocular higher-order aberrations and the amount of myopic correction. These findings indicate that the larger the amount of myopic correction is, the greater the changes in ocular higher-order aberrations are, leading to declines of mesopic vision. To the best our knowledge, this is the first study that elucidated the relationship among mesopic contrast sensitivity, ocular higher-order aberrations, and myopic correction in subjects undergoing overnight orthokeratology. There are several testers to assess scotopic or mesopic contrast sensitivity, and some of them can measure contrast sensitivity with a glare source that produces glare disability. Each tester, however, uses different optotype, luminance condition, and glare sources. Therefore, it is difficult simply to compare the results obtained by different testers. The Mesotest II used in this study is able to assess 650
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FIGURE 4. Scatterplots demonstrating changes in ocular higher-order aberrations and amount of myopic correction in eyes undergoing overnight orthokeratology. (Top left) Significant correlation between changes in third-order RMS and amount of myopic correction (r ⴝ 0.645; P < .0001, Pearson correlation coefficient). (Top right) Significant correlation between changes in fourth-order RMS and amount of myopic correction (r ⴝ 0.576; P < .0001, Pearson correlation coefficient). (Bottom) Significant correlation between changes in total higher-order RMS and amount of myopic correction (r ⴝ 0.666; P < .0001, Pearson correlation coefficient).
available orthokeratology lenses have an optical zone diameter of 6.0 or 6.2 mm. As for the relationship between mesopic vision and ocular higher-order aberrations, we are unaware of previous studies showing significant correlation between these parameters even in patients undergoing corneal refractive surgeries, although some researchers suggested that higherorder aberrations may be a promising parameter for predicting night vision symptoms after these procedures.25,52 In this study, we found a significant correlation between changes in mesopic contrast sensitivity and changes in total higher-order aberrations in patients undergoing orthokeratology. This finding indicates that ocular higherorder aberrations are good predictors for mesopic visual function. Furthermore, both third- and fourth-order aber-
rations significantly correlated with the reduction in mesopic contrast sensitivity. This suggests that both comalike and spherical-like aberrations are responsible for the deterioration of mesopic visual function in orthokeratology. Previously, we found that not only spherical-like aberrations but also coma-like aberrations deteriorate contrast sensitivity under photopic conditions after orthokeratology.31 We herein confirmed similar results under mesopic conditions. Theoretically, optical aberrations increase as pupil size enlarges. In addition, the effect of pupil dilation on optical aberrations is considered to be much greater after corneal refractive surgeries.53–55 Oshika and associates reported that before surgery, simulated pupillary dilation from 3 to 7 mm caused a five-fold to six-fold increase in the total
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FIGURE 5. Scatterplots demonstrating mesopic pupil diameter and log mesopic contrast sensitivity in eyes undergoing overnight orthokeratology. (Left) No significant correlation between mesopic pupil diameter and log mesopic contrast sensitivity without glare (r ⴝ 0.037; P ⴝ .8091, Spearman rank correlation coefficient). (Right) No significant correlation between mesopic pupil diameter and log mesopic contrast sensitivity with glare (r ⴝ ⴚ0.015; P ⴝ .9196, Spearman rank correlation coefficient).
orthokeratology. Berntsen and associates suggested that spherical aberration drove decreases in low-contrast bestcorrected visual acuity as pupil size increased after orthokeratology.30 Because there have been no other studies on the impact of pupil size on visual performance in orthokeratology, further studies are needed to elucidate this point. It is generally considered that glare can have negative influences on contrast sensitivity,40 but in our study, mesopic contrast sensitivity with glare was similar to that without glare in eyes after orthokeratology. As suggested by some researchers,37,40,58,59 this result may be explained by the pupillary miosis induced by glare possibly having a pinhole effect and may be offset by any loss in contrast sensitivity resulting from blur in glare conditions. We at present have no detailed data regarding the relation between pupil size and glare. Further studies are necessary to clarify this point. Until now, orthokeratology procedures have concentrated on the reduction of spherical defocus, which is one of the major optical aberrations, and much less attention has been paid to the higher-order aberrations. As shown in the current study, increases in ocular higher-order aberrations can play a significant role in the decline of mesopic contrast sensitivity after orthokeratology. For better quality of vision and life, further attention should be directed to optical quality of the eye and visual performance in detail, including mesopic visual function. In orthokeratology, the increase in spherical aberration results from the change in corneal asphericity to an oblate profile by the treatment, and the increase in coma aberration is attributed to contact lens decentration.60,61 Therefore, to avoid greater loss of mesopic visual function in current orthokeratology, large myopic correction should not be attempted, and the fitting and centration of the treatment lens should be performed strictly.
TABLE 3. Percentage of Eyes That Fulfilled the Contrast Level of 1:5 in Overnight Orthokeratology Baseline
After Treatment
Glare (⫺)
Glare (⫹)
Glare (⫺)
Glare (⫹)
98%
82%
36%
36%
Contrast level of 1:5 corresponds to log contrast sensitivity of 0.1. This level is the critical limit for night driving recommended by the German Ophthalmic Society.42
aberrations, but after surgery, the same dilation resulted in a 25-fold to 32-fold increase in PRK eyes and a 28-fold to 46-fold increase in LASIK eyes.54 Thus, pupil size has been the suspected highlighted variable of night vision disturbances.52 Haw and Manche, however, demonstrated that the effect of scotopic pupil size on scotopic contrast sensitivity was statistically insignificant in eyes that have undergone PRK.56 In eyes that have undergone LASIK, Lee and associates reported that no significant correlation was observed between the scotopic pupil size and nighttime contrast sensitivity with and without night glare.57 Moreover, Pop and Payette examined 1,488 eyes undergoing LASIK to clarify the preoperative risk factors for night vision symptoms and showed that there was no statistically significant contribution of scotopic pupil size to night vision symptoms.52 Similarly, in eyes undergoing orthokeratology, we could not find any significant correlations between mesopic pupil diameter and mesopic contrast sensitivity. This indicates that mesopic pupil size is not a primary factor responsible for the decline of mesopic visual function after orthokeratology. But, it cannot be denied that pupil size affects visual performance in patients undergoing 652
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One weakness of our study is that we evaluated mesopic contrast sensitivity at only one time point of three months. Although data on time-course changes in mesopic visual function are scarce, Holladay and associates reported that contrast threshold in darkness was worst immediately after LASIK and improved slightly thereafter, but had not returned to baseline by six months.23 At present, no information is available on long-term changes in mesopic visual function after orthokeratology. Further investigations are needed to know how mesopic visual function changes after orthokeratology.
In conclusion, we first found that regardless of the presence of glare light, mesopic contrast sensitivity after overnight orthokeratology deteriorated significantly as ocular higher-order aberrations increased, and these changes depended on the amount of myopic correction. In patients who undergo this treatment, ocular higher-order aberrations are good parameters to predict mesopic visual function. The indication for overnight orthokeratology must be discussed cautiously and sufficiently, especially in subjects with particular visual requirements for their profession, such as professional drivers, pilots, and military officers.
THE AUTHORS INDICATE NO FINANCIAL SUPPORT OR FINANCIAL CONFLICT OF INTEREST. INVOLVED IN CONCEPTION AND design (T.H., T.K., T.O.); analysis and interpretation (T.H., T.T., T.O.); writing the article (T.H.); critical revision of the article (T.O.); data collection (T.H., C.O., Y.I., T.K.); provision of materials (T.O.); statistical expertise (T.O.); literature search (T.H.); logistic support (T.O.); and final approval of the article (T.H., C.O., Y.I., T.T., T.K., T.O.). All patients were informed about the nature of the study and informed consent was obtained in accordance with Institutional Review Board of Tsukuba University Hospital and the tenets of the Declaration of Helsinki before their participation in the study.
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59. Spraul CW, Roth HJ, Gackle H, Lang GE, Lang GK. Influence of special-effect contact lenses (Crazy Lenses) on visual function. CLAO J 1998;24:29 –32. 60. Joslin CE, Wu SM, McMahon TT, Shahidi M. Higher-order wavefront aberrations in corneal refractive therapy. Optom Vis Sci 2003;80:805– 811. 61. Hiraoka T, Matsumoto Y, Okamoto F, et al. Corneal higherorder aberrations induced by overnight orthokeratology. Am J Ophthalmol 2005;139:429 – 436.
REPORTING VISUAL ACUITIES The AJO encourages authors to report the visual acuity in the manuscript using the same nomenclature that was used in gathering the data provided they were recorded in one of the methods listed here. This table of equivalent visual acuities is provided to the readers as an aid to interpret visual acuity findings in familiar units. Table of Equivalent Visual Acuity Measurements Snellen Visual Acuities 4 Meters
6 Meters
20 Feet
Decimal Fraction
LogMAR
4/40 4/32 4/25 4/20 4/16 4/12.6 4/10 4/8 4/6.3 4/5 4/4 4/3.2 4/2.5 4/2
6/60 6/48 6/38 6/30 6/24 6/20 6/15 6/12 6/10 6/7.5 6/6 6/5 6/3.75 6/3
20/200 20/160 20/125 20/100 20/80 20/63 20/50 20/40 20/32 20/25 20/20 20/16 20/12.5 20/10
0.10 0.125 0.16 0.20 0.25 0.32 0.40 0.50 0.63 0.80 1.00 1.25 1.60 2.00
⫹1.0 ⫹0.9 ⫹0.8 ⫹0.7 ⫹0.6 ⫹0.5 ⫹0.4 ⫹0.3 ⫹0.2 ⫹0.1 0.0 ⫺0.1 ⫺0.2 ⫺0.3
From Ferris FL III, Kassoff A, Bresnick GH, Bailey I. New visual acuity charts for clinical research. Am J Ophthalmol 1982;94:91–96.
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Biosketch Takahiro Hiraoka, MD, is an Assistant Professor of Ophthalmology at Institute of Clinical Medicine, University of Tsukuba, Ibaraki, Japan. He graduated from University of Tsukuba School of Medicine in 1993 and completed postgraduate training in 1999. Dr Hiraoka main research interests include optical quality of the eye and quality of vision, especially in eyes undergoing corneal refractive surgery and orthokeratology.
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