Changes in ocular wavefront aberrations and retinal image quality with objective accommodation

ARTICLE

Changes in ocular wavefront aberrations and retinal image quality with objective accommodation Ying-Jun Li, MD, Jin A. Choi, MD, Hyojin Kim, PhD, Seung-Young Yu, MD, PhD, Choun-Ki Joo, MD, PhD

PURPOSE: To evaluate changes in ocular wavefront aberrations and retinal image quality with objective accommodation in normal human eyes. SETTING: Department of Ophthalmology and Visual Science, Seoul St. Mary’s Hospital, College of Medicine, The Catholic University of Korea, Seoul, Korea. DESIGN: Cohort study. METHODS: Healthy emmetropic eyes were measured with an iTrace wavefront aberrometer while the subjects fixated at far (6.0 m) and near (40 cm). Wavefront data were analyzed with 3.0 mm and 5.0 mm pupil diameters. The influence of higher-order aberrations (HOAs) on retinal image quality was simulated by computing the modulation transfer function (MTF) from the wavefront aberrations. RESULTS: Eighty-two eyes of 42 subjects were evaluated. The root mean square values of total HOAs before and after accommodation with 5.0 mm pupils were statistically significantly different (PZ.021). In particular, total spherical aberrations became more negative with accommodation, and the difference was statistically significant with 3.0 mm and 5.0 mm pupils (both P<.001). The MTF curves were significantly different before and after accommodation at 5 cycles per degree (cpd) and 10 cpd with 5.0 mm pupils (PZ.031 and PZ.045, respectively). CONCLUSIONS: Ocular wavefront aberrations and retinal image quality changed with accommodation in normal human eyes. Total spherical aberration changed more than other HOAs with accommodation. Financial Disclosure: No author has a financial or proprietary interest in any material or method mentioned. J Cataract Refract Surg 2011; 37:835–841 Q 2011 ASCRS and ESCRS

Wavefront aberrations blur the retinal image, reducing contrast sensitivity and visual acuity in a way that cannot be corrected by simple sphere and cylinder correction.1,2 Myopic laser in situ keratomileusis (LASIK) refractive surgery induces an increase in higherorder aberrations (HOAs), especially spherical aberrations.3 The increase in HOAs is one reason for the reduced contrast sensitivity after LASIK surgery.4,5 The recent development of wavefront-guided LASIK has reduced the occurrence of HOAs with refractive surgery.6 Precise measurement of preoperative HOAs is essential to maximize the effects of wavefront-guided custom ablation. Given that examination with a wavefront aberrometer is based on the supposition of Q 2011 ASCRS and ESCRS Published by Elsevier Inc.

a maximum nonaccommodative state, many aberrometers are designed to prevent accommodation by having a fixed distance target or by fogging the eye before measurement.7 However, ocular HOAs are not static phenomena and constantly change because of several factors, including age, pupil diameter, and accommodation.8 Accommodation not only alters the refractive power of the eye, it also affects HOAs.8–11 Ninomiya et al.9 and He et al.12 report that positive to negative changes in spherical aberrations occur with accommodation. Characterization of the changes in ocular wavefront aberrations with accommodation will also provide useful information to customize aberration correction using techniques such as refractive surgery and contact lenses.13 In this study, we evaluated 0886-3350/$ - see front matter doi:10.1016/j.jcrs.2010.11.031

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changes in ocular wavefront aberrations and retinal image quality according to objective accommodation using ray-tracing aberrometry in normal human eyes. SUBJECTS AND METHODS This study evaluated healthy emmetropic eyes with an uncorrected distance visual acuity of 20/20 or better, spherical ametropia in the range of 0.50 to C0.75 diopter (D), and astigmatism less than 0.50 D. All eyes had normal binocular function and a relatively large interpupillary distance (O6.0 cm). Exclusion criteria included ocular pathology, ophthalmic disorders, amblyopia, strabismus, previous intraocular surgery, laser treatment, and retinal complications. Informed consent was obtained from all subjects using a consent form approved by the Institutional Review Board, Catholic University of Korea. The measurement equipment used in this study was an iTrace wavefront aberrometer (Tracey Technologies, Inc.). Subjects viewed far or near targets through a 2.5 cm diameter wide, 20 cm long, open-field instrument housing. The aberrometer permits monocular viewing through the instrument. Monocular measurement involves projection of an infrared beam into the eye using laser ray-tracing technology and analysis of retinal spot patterns to determine wavefront aberration and the modulation transfer function (MTF).7 The distant target was a standard illuminated Snellen chart at 6.0 m. The near target was a high-contrast miniaturized Early Treatment Diabetic Retinopathy Study letter chart at 40 cm. The near chart was illuminated by a white light–emitting diode connected to a battery and rheostat. This setup maintained equal luminance between the far target and near target to avoid influencing the accommodative response. The letter size on the 40 cm chart was changed to keep the visual angle the same as that of the 6.0 m chart, and the examination room was maintained at a constant luminance of 0.1 lux. The use of cycloplegia was limited to achieve

ocular responses, and the eye not being measured was occluded with an eye patch. For each far or near target presentation, the objective refraction measurements were repeated at least 3 times. The best scan, defined as the image with the best quality peaks for individual points, was selected for the final analysis. This approach entailed using the software of the wavefront aberrometer to scroll through the 256 retinal-spot profiles to ensure that the spot peaks were high enough and that the software was correctly locating the horizontal and vertical peaks of each spot with a minimum of rejected points on the wavefront-verification display. The aberrations in the cornea were obtained with the Vista attachment of the wavefront aberrometer, and aberrations in the internal optics were calculated by subtracting the aberrations in the cornea from those in the entire eye measured by the ray-tracing aberrometer using a built-in program. Thus, each aberration of the cornea, internal optics, and total aberrations was evaluated. Wavefront data were analyzed using 3.0 mm and 5.0 mm pupil zones. The analyzed parameters included the following: (1) Zernike coefficients from the 3rd to the 6th orders; (2) root-mean-square (RMS) values of HOAs from the 3rd to the 6th orders; (3) RMS values of coma-like aberrations Z(3, 1), Z(3,1), Z(5, 1), and Z(5,1); and (4) RMS values of spherical-like aberrations Z(4,0) and Z(6,0). The wavefront aberrometer also provides imagequality metrics, such as the MTF, which displays the ratio of image contrast to object contrast for ocular optics as a function of the spatial frequency of a sinusoidal grating. For each subject, the MTF curves were analyzed from HOAs only to eliminate the effects of lower-order aberrations (eg, defocus and astigmatism). All statistical analyses were performed using SPSS for Windows software (version 12, SPSS, Inc.). The paired t test was used to compare the data before and after accommodation. A P value less than 0.05 was considered statistically significant.

RESULTS

Submitted: May 25, 2010. Final revision submitted: November 23, 2010. Accepted: November 23, 2010. From the Laboratory of Ophthalmology and Visual Science (Li, Joo), Korean Eye Tissue and Gene Bank Related to Blindness; the Departments of Ophthalmology and Visual Science, St. Vincent Hospital (Choi), and St. Mary’s Hospital (Joo), College of Medicine, The Catholic University of Korea; the Department of Visual Optics (Kim), Division of Health Science, Baekseok University; and the Department of Ophthalmology (Yu), Graduate School of Medicine, Kyung Hee University, Seoul, Korea. Ying-Jun Li, MD, and Jin A Choi, MD, contributed equally to this work. Supported by the Korea Healthcare Technology R&D Project, Ministry for Health Welfare & Family Affairs, Republic of Korea (A090573). Corresponding author: Choun-Ki Joo, MD, PhD, Department of Ophthalmology and Visual Science, Seoul St. Mary’s Hospital, College of Medicine, The Catholic University of Korea, #505, Banpo-dong, Seocho-Ku, Seoul 137-701, Korea. E-mail: ckjoo@ catholic.ac.kr.

The study enrolled 82 eyes of 42 subjects (22 men, 20 women). The mean age of the subjects was 21.3 years G 2.5 (SD) (range 19 to 25 years). The mean spherical ametropia was 0.45 G 0.33 D without accommodation and 2.61 G 0.59 D with accommodation (P!.001) and the mean astigmatism, 0.37 G 0.17 D and 0.45 G 0.22 D, respectively (PZ.631). The mean accommodative response was 2.92 G 0.84 D (range 0.95 to 3.91 D). Figure 1 shows the 2-dimensional wavefront maps of the entire eye, internal optics, and cornea for before and after accommodation in a 23-year-old subject. After accommodation, the wavefront maps of the entire eye and the internal optics showed a local myopic shift in the central pupillary area. Table 1 shows wavefront aberrations in the entire eye, internal optics, and corneal aberrations with 3.0 mm and 5.0 mm pupils before and after accommodation. There was no statistical difference in corneal aberrations between no accommodation and accommodation. The HOAs in the entire eye before

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Figure 1. Wavefront maps of the entire eye, the internal optics, and the corneal aberrations before and after accommodation.

Table 1. Entire eye, internal optics, and corneal aberrations before and after accommodation with 3.0 and 5.0 mm pupil diameters. 3.0 mm Pupil

5.0 mm Pupil

Mean G SD Parameter Entire eye (mm) RMS HOAs Coma SA Trefoil Internal optics (mm) RMS HOAs Coma SA Trefoil Cornea (mm) RMS HOAs Coma SA Trefoil

Far

Mean G SD Near

P Value

Far

Near

P Value

0.047 G 0.013 0.033 G 0.015 0.007 G 0.011 0.026 G 0.017

0.059 G 0.020 0.029 G 0.016 0.002 G 0.010 0.032 G 0.023

.342 .581 !.001* .517

0.196 G 0.075 0.137 G 0.091 0.032 G 0.021 0.101 G 0.071

0.258 G 0.073 0.131 G 0.061 0.096 G 0.075 0.118 G 0.088

.021* .798 !.001* .653

0.074 G 0.026 0.055 G 0.037 0.011 G 0.010 0.072 G 0.123

0.083 G 0.017 0.050 G 0.032 0.021 G 0.013 0.081 G 0.119

.421 .677 !.001* .785

0.271 G 0.101 0.198 G 0.137 0.103 G 0.052 0.216 G 0.129

0.389 G 0.084 0.203 G 0.101 0.225 G 0.085 0.202 G 0.145

!.001* .732 !.001* .714

0.053 G 0.025 0.027 G 0.015 0.018 G 0.006 0.033 G 0.026

0.050 G 0.021 0.025 G 0.011 0.019 G 0.007 0.030 G 0.019

0.313 G 0.167 0.142 G 0.094 0.135 G 0.071 0.152 G 0.120

0.301 G 0.147 0.141 G 0.101 0.129 G 0.062 0.139 G 0.097

.567 .452 .711 .766

RMS HOAs Z root mean square of higher-order aberrations; SA Z spherical aberration *Statistically significant

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.751 .523 .621 .529

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significantly at 5 cycles per degree (cpd) and 10 cpd with 5.0 mm pupils after accommodation (PZ.031 and PZ.045, respectively) (Figure 5). DISCUSSION

Figure 2. Total (top) and internal (bottom) HOAs before and after accommodation. Error bars represent the mean G SD (* Z P!.05; RMS Z root mean square).

accommodation and after accommodation with a 5.0 mm pupil were statistically significantly different (PZ.021) (Figure 2, top). The negative shift in spherical aberrations in the entire eye with accommodation was statistically significant with 3.0 mm and 5.0 mm pupils (both P!.001) (Figure 3). The internal optical HOAs before and after accommodation with 5.0 mm pupils were statistically significantly different (P!.001) (Figure 2, bottom). The negative shift in spherical aberrations in the entire eye with accommodation was statistically significant with 3.0 mm and 5.0 mm pupils (both P!.001) (Figure 4). The MTF curves before and after accommodation did not significantly differ with 3.0 mm pupils (PO.05), whereas the MTF curve decreased

Ocular wavefront aberrations are primarily created in the cornea and lens and are highly affected by various factors, including the accommodative state,12 pupil diameter,14 tear film,15 age,16 and entrance pupil decentration.17 This study examined the ocular HOAs caused by accommodation. Subjective accommodation testing, such as the routinely used clinical pushup test, cannot unequivocally show an accommodative optical change in the power of the eye.18 Subjective tests do not differentiate between a passive depth of field resulting from small pupils, ocular aberrations, or changes in the active accommodative power of the eye.19 Thus, objective assessment of accommodation is required. A previous study9 of accommodation and ocular wavefront aberrations evaluated in the range of emmetropia used a Hartmann-Shack wavefront analyzer; however, that study artificially fixed the power to 3.00 D. In contrast to aberrometers based on the Hartmann-Shack principle, the aberrometer in our study, the iTrace, uses a sequential ray-tracing method,20 which may have advantages, including a larger dynamic range to the measurement of accommodation. Another study21 comparing HartmannShack wavefront sensors and laser ray-tracing aberrometers produced similar results of reproducibility in healthy eyes. To our knowledge, our study is the first to measure the total, internal optics, and corneal aberrations in accommodative human eyes in the nonmydriatic state using an iTrace aberrometer. Moreover, this study assessed the impact of accommodation-induced HOAs on retinal image quality. The total and internal HOAs caused by accommodation were not significantly different when the pupil diameter was maintained at 3.0 mm; however, these HOAs increased with 5.0 mm pupils. In particular, in the dynamic accommodative state, the total and internal spherical aberrations in Z(4,0) and Z(6,0) were negatively shifted compared with those in the nonaccommodative state, regardless of pupil diameter. This finding is consistent with a study by Ninomiya et al.,9 in which total spherical aberrations shifted significantly in the negative direction with pupil diameters of 4.0 mm and 6.0 mm after the introduction of a 3.00 D accommodative stimulus in the context of emmetropia. These results show clear changes in the characteristics of ocular wavefront aberrations according to accommodation.

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Figure 3. Mean Zernike coefficients for total HOAs before and after accommodation (3rd- to 6th-order coefficient) with a 3.0 mm pupil (A) and a 5.0 mm pupil (B). Error bars represent the mean G SD (* Z P!.0012, corrected for multiple comparisons using the Bonferroni correction).

Applegate et al.22 report that HOAs near the center of the Zernike table (eg, coma, trefoil, and spherical) tend to have more significant effects on visual quality than those at the periphery of the table. In addition, Mester et al.23 report that spherical aberrations are

important in the prediction of not only visual acuity but also the visual quality at the retina because they influence contrast sensitivity. We also found that the change in spherical aberration Z(4,0) was important for visual quality in the accommodative state. The

Figure 4. Mean Zernike coefficients for internal optical HOAs before and after accommodation (3rdto 6th-order coefficient) with a 3.0 mm pupil (A) and a 5.0 mm pupil (B). Error bars represent the mean G SD (* Z P!.0012, corrected for multiple comparisons using the Bonferroni correction).

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before accommodation was 0.103 G 0.052 mm but changed to 0.225 G 0.085 mm with accommodation with 5.0 mm pupils, showing a 0.122 mm negative shift. Thus, wavefront-guided LASIK may decrease visual quality in near vision if all physiologic spherical aberrations in far vision are eliminated. In addition, leaving some positive spherical aberrations may be advantageous when the patient has a preference for near vision over far vision. Further studies are needed to evaluate the change in wavefront aberration as a function of objective accommodative response and clarify the optimum wavefront-guided correction after myopic laser refractive surgery for far and near vision. In conclusion, ocular wavefront aberrations and retinal image qualities were significantly changed with accommodation in normal emmetropic eyes. Of the ocular wavefront aberrations, changes in total and internal spherical aberrations were manifested and showed a negative shift. Further studies of HOA changes (especially spherical aberrations) according to the extent of accommodation may provide useful information in wavefront-guided LASIK surgery. REFERENCES

Figure 5. Modulation transfer function curves before and after accommodation for 3.0 mm pupil (top) and 5.0 mm pupil (bottom) at different spatial frequencies. Error bars represent the mean G SE (* Z P!.05; cpd Z cycles per degree).

optical quality of the eye is best described by the MTF, which shows the reduction in contrast as a function of spatial frequency.24 In this study, the MTFs used to evaluate visual quality in the accommodative and nonaccommodative states differed significantly at the low frequencies of 5 cpd and 10 cpd when the pupil diameter was fixed at 5.0 mm. Previous studies25,26 found that accommodative power was similar in emmetropia and myopia. Recently, wavefront-guided LASIK has been increasingly used to achieve better outcomes than those of conventional LASIK surgery. Wavefront-guided LASIK uses custom ablation, which is measured in the nonaccommodative state to correct preoperative aberrations. Therefore, after surgery, it may yield different wavefront results in the accommodative state. In our study, the mean internal spherical aberration

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First author: Ying-Jun Li, MD Laboratory of Ophthalmology and Visual Science (Li, Joo), Korean Eye Tissue and Gene Bank Related to Blindness, Seoul, Korea