Optometry & Vision Science:
Ocular Monochromatic Aberrations in a Rural Chinese Adult Population
Wan, Xiu Hua*; Li, Shi-Ming*; Xiong, Ying†; Liang, Yuan Bo†; Li, Jing*; Wang, Feng Hua*; Li, Ji†; Jhanji, Vishal†; Wang, Ning Li*
Beijing Tongren Eye Center, Beijing Tongren Hospital, Capital Medical University, Beijing Ophthalmology and Visual Sciences Key Lab, Beijing, China (XHW, SML, YX, YBL, JL, FHW, NLW); Beijing Institute of Ophthalmology, Beijing Tongren Hospital, Capital Medical University, Beijing, China (XHW, NLW); Department of Ophthalmology and Visual Sciences, the Chinese University of Hong Kong, Hong Kong, China (YBL, VJ); Handan Eye Hospital, Handan, Hebei Province, China (JL); and Centre for Eye Research Australia, University of Melbourne, Melbourne, Victoria, Australia (VJ).
Ningli Wang Beijing Tongren Eye Center Beijing Tongren Hospital Capital Medical University Beijing Ophthalmology and Visual Sciences Key Laboratory No. 1 Dongjiaominxiang, Dongcheng District Beijing, China e-mail: firstname.lastname@example.org
Purpose: To investigate the distribution of monochromatic aberrations in a rural Chinese adult population and the possible effect of aberrations on the development of refractive error.
Methods: A total of 404 Chinese adults who grew up in rural Yongnian County, Handan City, Northern China, were included. Monochromatic aberrations of left eyes were measured using iTrace Dynamic Laser Refraction at 5.0-mm pupil size without cycloplegia.
Results: Mean age of all participants was 49.9 ± 10.5 years (range, 31 to 86 years). Mean spherical equivalent was 0.22 ± 1.14 diopters (D) (range, −7.06 to +3.62 D). With age, the refraction demonstrated a significant hyperopic shift (r2 = 0.25, p < 0.01). Oblique trefoil (C3−3), vertical coma (C3−1), horizontal coma (C13), and spherical aberration (SA) (C04) significantly differed from zero (−0.065 ± 0.133 μm, −0.043 ± 0.161 μm, +0.070 ± 0.115 μm, and +0.058 ± 0.082 μm, respectively). Total root mean square values of higher-order aberrations (HOAs) were 0.296 ± 0.147 μm, with predominant ones of coma (0.180 ± 0.115 μm), trefoil (0.151 ± 0.116 μm), and SA (0.081 ± 0.060 μm). Root mean square values of total HOAs, coma, trefoil, SA, and third- to seventh-order aberrations significantly increased with age (p < 0.01). Total HOAs, SA, coma, and trefoil were not significantly different between hyperopic, emmetropic, and myopic eyes after adjusting for age (p = 0.26, 0.15, 0.24, and 0.28, respectively). Zernike coefficient of SA showed a significant difference between hyperopic (0.076 ± 0.086), emmetropic (0.056 ± 0.079), and myopic (0.028 ± 0.088) eyes (p = 0.00).
Conclusions: Ocular refraction in rural Chinese adults showed significantly hyperopic shift with age. Magnitudes of HOAs in rural Chinese adults were similar to that of other populations and significantly increased with age but showed no differences in myopic, emmetropic, and hyperopic adults. The existence of HOAs is not, in itself, sufficient to account for the myopia epidemic in China.
Monochromatic aberrations consist of lower-order (defocus and astigmatism) and higher-order (e.g., coma, trefoil, and spherical aberration [SA]) anomalies of the eye that deteriorate visual performance of the human eye1 and may have an influence on myopia development and progression.2,3 Ocular higher-order aberrations (HOAs) have a large intersubject variability4 and are influenced by many factors such as pupil size,5 age,6,7 refractive error,8 accommodation,9 ocular surgery,10 and race.11 Although the lower-order aberrations can be corrected by optical correction, management of HOAs still requires other strategies.12,13
In clinical practice, the correction of monochromatic aberrations can be achieved through various modalities, such as customized refractive surgery,14 use of aspherical intraocular lenses (IOLs),15 and contact lenses.16 All of these applications are based on the description of monochromatic aberrations in a large population. Wei et al.17 have reported the characteristics of HOAs in young Chinese (mean age, 32.1 years) and found that only spherical aberration and primary vertical coma increased slightly with age. Carkeet et al.18 found that monochromatic aberrations in Singaporean schoolchildren were significantly higher in students of Chinese background than in their Malay counterparts, indicating that ethnic factors might affect the distribution pattern of monochromatic aberrations. Hu et al.19 reported that HOAs in Chinese myopic eyes increased with a higher refractive error. Based on the data of 50 emmetropic eyes, a Chinese generic eye model has been built.20 However, most of the aforementioned studies were performed in hospital-based populations, limiting the findings to potentially biased population groups. To date, there have been no population-based studies on the distribution of monochromatic aberrations in Chinese adults.
In recent decades, the prevalence of myopia in Chinese children has increased dramatically and has reached epidemic levels.21–25 On the contrary, the prevalence of myopia in rural Chinese adults was relatively much lower as reported in our previous Handan Eye Study.26 Chinese adults, especially those rural adults who grew up during the 1960s, were less likely to have had intense educational experiences and near work because of the social and political situation of the time, and this may have had a significant effect on eye growth. Whether HOAs in rural Chinese adults, older than 30 years, are different from those of other populations has not been reported. More knowledge in this field may give further insight into the development of refractive error.
In this study, we reported the distribution of monochromatic aberrations in a rural Chinese adult population aged 30 years or more based on a population-based cross-sectional study and investigated the relation between refraction, age, and HOAs.
Study Design and Population
In the Handan Eye Study,26–30 a population-based cross-sectional study conducted in rural Yongnian County, Handan City, Northern China, 6830 Han Chinese aged 30 years or older were randomly selected to be examined. Among them, the participants who had a pupil size less than 5.0 mm,4,17 ocular diseases,28,31–33 or those who could not cooperate with the examination were excluded from analysis. All included subjects did not have any history of previous ocular surgery or associated ophthalmic diseases. The study protocol was approved by the ethical committee of Beijing Tongren Hospital. The study adhered to the tenets of the Declaration of Helsinki. Written informed consent was obtained from each subject after the purpose of the study and protocols had been explained to all participants.
Detailed ophthalmic examinations were undertaken including visual acuity, slit lamp examination, autorefraction, and fundoscopy.27 All examinations were carried out by experienced examiners. An AutoRefractor-Keratometer (KR8800 Topcon, Tokyo, Japan) was used for measuring refraction. Subjective refraction was performed by a registered optometrist in participants who presented visual acuity of <20/20. Monochromatic aberrations were measured in the left eye before the instillation of any eye drops to increase measurement accuracy.34
The iTrace Dynamic Laser Refraction system (Tracey Technologies, Corp. EyeSys Vision, Inc.) is a ray-tracing aberrometry instrument that sequentially projects 256 near-infrared laser beams (785 nm) into the eye to measure ocular aberrations after point-by-point data processing. After dark adaptation for 10 minutes in a dim room, the left eye of each participant was measured at 5.0-mm pupil size without using any mydriatic agents. If the pupil size was less than 5.0 mm as measured with a circle on the screen, the measurement was cancelled. The participant was comfortably positioned with his or her forehead against the strap and the chin placed on the chin rest. The pupil was aligned to the circles exhibited in an independent computer display. The measurements were performed between 10:00 AM and 2:00 PM everyday. A total of three repeated measurements were obtained and averaged for each participant.
In the present study, root mean square (RMS) values of third- through seventh-order aberrations, coma, trefoil, SA, and total HOAs were calculated as square root of the sum of squared 6 to 35 Zernike coefficients. “Coma” was defined as the square root of the sum of squared coefficients of horizontal coma (C3−1) and vertical coma (C31), “trefoil” was defined as the square root of the sum of squared coefficients of C3−3 and C33. A paired-samples t test was performed to compare spherical equivalent (SE) measured by the wavefront sensor and the autorefractor. Linear regression was used to assess the relation between the RMS values of total HOAs, coma, trefoil, SA, and age. Analysis of covariance (ANCOVA) was used to evaluate the effect of SE and age on RMS values and Zernike coefficients of wavefront aberrations. “Myopia,” “emmetropia,” and “hyperopia” were defined as SE ≤ −0.50 diopters (D), −0.50 D < SE < 0.50 D, and SE ≥ 0.50 D, respectively. A significance level of α = 0.05 was used for all tests. Statistical analysis was performed using Statistical Analysis System Software (SAS 9.3).
A total of 404 participants (170 male; 234 female) were included in this study. Each participant had a best-corrected visual acuity (BCVA) of 20/25 or better in the left eye. Mean age was 49.9 ± 10.5 years (range, 31 to 86 years). Mean SE measured by the wavefront sensor and autorefractor were 0.08 ± 1.33 D and 0.22 ± 1.14 D, respectively. The difference was statistically significant (0.15 ± 0.80 D, p < 0.01). In the present study, we used the data from the autorefraction to report the distribution of refraction for all participants in each subgroup divided by age and SE (Table 1). The mean refraction ranged from −7.06 to +3.62 D, which became more hyperopic with increasing age (r2 = 0.25, p < 0.01) (Fig. 1).
Mean and SD of Zernike coefficients and RMS values of each mode from the third to seventh order are shown in Fig. 2 and Table 2. Oblique trefoil (C3−3), vertical coma (C3−1), horizontal coma (C31), and SA (C40) were significantly different from zero, with magnitudes of −0.065 ± 0.133 μm, −0.043 ± 0.161 μm, +0.070 ± 0.115 μm, and +0.058 ± 0.082 μm, respectively. Total RMS values of HOAs were 0.296 ± 0.147 μm with predominant ones of coma (0.180 ± 0.115 μm), trefoil (0.151 ± 0.116 μm), and SA (0.081 ± 0.060 μm). Root mean square values decreased with increasing Zernike orders (Table 3). The third- and fourth-order aberrations represented a large proportion of the total HOAs.
HOAs and Age
Table 4 shows the correlations between RMS values, signed Zernike coefficients of aberrations, and age. Root mean square values of total HOAs, coma, trefoil, SA, and third- to seventh-order aberrations significantly increased with age (p < 0.01). Of all the aberrations, coma had the highest r square (0.077), followed by trefoil (0.074) and SA (0.046). Only the Zernike coefficients of vertical coma (C3−1) and secondary astigmatism (C42) showed a significantly negative correlation with age (p < 0.01). The Zernike coefficient of SA (C40) showed a borderline correlation with age (p = 0.01).
HOAs and Refraction
After adjusting for age, total HOAs of hyperopic, emmetropic, and myopic eyes were 0.309 ± 0.148 μm, 0.283 ± 0.146 μm, and 0.300 ± 0.149 μm, respectively (p = 0.26; Fig. 3). SA was 0.089 ± 0.062 μm, 0.078 ± 0.053 μm, and 0.071 ± 0.061 μm, respectively (p = 0.15; Fig. 4, upper left). Coma was 0.194 ± 0.123 μm, 0.172 ± 0.109 μm, and 0.173 ± 0.114 μm, respectively (p = 0.24; Fig. 4, bottom left). Trefoil was 0.153 ± 0.123 μm, 0.142 ± 0.119 μm, and 0.166 ± 0.123 μm, respectively (p = 0.28; Fig. 4, bottom right). Only Zernike coefficient of SA showed a significant difference between hyperopic (0.076 ± 0.086), emmetropic (0.056 ± 0.079), and myopic eyes (0.028 ± 0.088) (p = 0.00; Fig. 4, upper right).
In the present study, we described the characteristics of monochromatic aberrations at 5.0-mm pupil size in 404 rural Chinese adults and investigated the relation between refraction, age, and HOAs. To the best of our knowledge, this is the first population-based study on monochromatic aberrations in a large sample of rural Chinese adults. It was found that coma, trefoil, SA, and third- and fourth-order aberrations were dominant among HOAs. Root mean square values of total HOAs, coma, trefoil, and SA significantly increased with age. However, there were no significant differences in HOAs among emmetropic, hyperopic, and myopic eyes after adjusting for age.
The present results showed that monochromatic aberrations varied widely among rural Chinese adults (0.077 to 1.254 μm), which was in agreement with previous studies.4 Mean RMS values of total HOAs were 0.296 ± 0.147 μm for all participants and 0.300 ± 0.017 μm for myopic participants in our study, which was smaller than that of younger myopic Chinese adults in Singapore (0.49 ± 0.16 μm).17 One important reason for the difference is that we measured aberrations at a smaller pupil size (5.0 mm) than they did (6.0 mm).17 It is well known that pupil size decreases with increasing age, thus reducing aberration magnitudes.35 The magnitudes of HOAs in our study were slightly higher than that reported by Wang et al.36 (0.229 μm, third- to sixth-order aberration at 5.0-mm pupil) on young myopic adults in Tianjin. Considering that aberrations increased with age6,7 and that our sample is composed of much older adults (49.9 years) than those of Wang et al.36 (21.9 years), our values of HOAs are consistent with theirs.
In the study by Wang and Koch,37 the mean RMS of total HOAs was reported to be 0.305 ± 0.095 μm at 6.0-mm pupil in subjects aged 41 ± 10 years (range, 20 to 71 years). Considering the effect of pupil size on aberrations and similar age distribution between their study and the present one, it seems that rural Chinese adults had approximate amounts of HOAs, which is similar to whites. In a review by Atchison,38 mean higher-order RMS for 6.0-mm pupil was approximately 0.3 μm, which was almost equal to that of the present study. Therefore, magnitudes of HOAs in rural Chinese adults were similar to that of other ethnic populations.
We found that RMS of total HOAs, coma, trefoil, SA, and third- to seventh-order aberrations significantly increased with age (Table 4), which is consistent with most previous studies.6,35,37,39–41 Atchison et al.42 only found a moderate effect of age on RMS of HOAs, which might be caused by a small refractive error range. Atchison and Markwell42 found that only the coefficient of horizontal coma demonstrated a negative correlation with age. In the present study, the coefficient of vertical coma (C3−1) and secondary astigmatism (C42) showed a significant negative correlation with age. However, only 2.7 and 3.7% of the variation are explained by vertical coma (C3−1) and secondary astigmatism (C42). The coefficient of spherical aberration (C40) showed a borderline correlation with age, which was in agreement with previous studies.6,35,37,41 Wei et al.17 found that only SA and vertical coma slightly increased with age. This may be accounted for by the fact that all their subjects were myopic (−5.23 ± 1.79 D, −0.75 to approximately −9.75 D) and had a relatively small age range (21.5 to approximately 52.8 years).
Interestingly, we found no significant differences in RMS of total HOAs, SA, coma, and trefoil between hyperopic, emmetropic, and myopic eyes after adjusting for age. This was consistent with some previous studies.43–45 Li et al.44 reported that ocular HOAs were similar among Chinese schoolchildren with different refractive errors (∼0.19 μm). Although these two studies focused on different Chinese populations, it seems that HOAs may play a weak role in the development of refractive error. However, some studies8,46,47 have reported higher HOAs in myopes than in emmetropes or higher HOAs in hyperopes than in emmetropes.48 We found that coefficients of SA were higher in hyperopic eyes than in emmetropic and myopic eyes (0.076 μm vs. 0.056 μm and 0.028 μm), which was in agreement with the findings of Hartwig and Atchison.48 The discrepancies among these studies may be explained by different subject groups, measurement techniques, and data analysis. In addition, it is not clear whether higher HOAs in these studies were merely the consequence of refractive error development. Therefore, longitudinal prospective studies on the same populations are necessary to investigate the effect of HOAs on refractive error development.
Strengths of the present study include a population-based survey among a large sample of rural Chinese adults. In addition, all participants grew up in villages and did farm work during their lives, which form an ideal condition to evaluate the effect of rural environments on refractive error. However, there are some limitations for this study. First, it is a cross-sectional analysis from which it is hard to establish a causative relation between monochromatic aberrations and refractive development. Second, other confounding variables such as time spent in outdoor activities and near work were not available for adjustment.
In conclusion, the current results demonstrated that ocular refraction in rural Chinese adults significantly increased with age. Higher-order aberrations in rural Chinese adults showed considerable variability and significantly increased with age. However, there was no difference in HOAs among myopic, emmetropic, and hyperopic adults. These findings suggest that HOAs may play a weak role in the development of refractive error.
Beijing Tongren Eye Center
Beijing Tongren Hospital
Capital Medical University
Beijing Ophthalmology and Visual
Sciences Key Laboratory
No. 1 Dongjiaominxiang, Dongcheng District
This work was supported by the Major State Basic Research Development Program of China (973 Program, 2011CB504601), the Major International (Regional) Joint Research Project of the National Natural Science Foundation of China (81120108807), and the Beijing Nova Program (Z121107002512055). We thank Dr. Michel Millodot (School of Optometry and Vision Sciences, Cardiff University, Cardiff, UK) for his help in revising this manuscript.
The authors declare that they have no competing financial interests.
Received November 2, 2012; accepted September 17, 2013.
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ocular monochromatic aberrations; refractive error; coma; spherical aberration; rural Chinese adult
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