Peripheral Refraction and Refractive Error in Singapore Chinese Children

Abstract

Purpose.: Peripheral hyperopia was hypothesized to stimulate axial elongation. This study describes peripheral refraction and its associations with central refractive error in young Singapore Chinese children.

Methods.: Two hundred fifty children aged 40 months or older recruited from the Strabismus, Amblyopia, and Refractive Error in Young Singapore Children study were included in this analysis. Peripheral refraction was measured after pupil dilation using an infrared autorefractor. A total of five measurements were captured: central visual axis and 15° and 30° eccentricities in the nasal and temporal visual fields.

Results.: The mean age of the participants recruited was 83 ± 36 months. There were 37 children with high and moderate myopia (≤−3 D; 14.8%), 81 with low myopia (−2.99 to −0.5 D; 32.4%), 84 with emmetropia (−0.49 to 1.0 D; 33.6%), and 47 with hyperopia (>1.0 D; 18.8%). Compared with the central axis, children with high and moderate myopia had relative hyperopia at all peripheral eccentricities (P < 0.001), whereas children with low myopia had relative hyperopia only at the temporal and nasal 30° (P < 0.001), but not at the nasal and temporal 15°. Children with emmetropia and hyperopia had peripheral relative myopia at all eccentricities (P < 0.001). A significant correlation between the nasal and temporal refractive error at 30° was noted (Spearson's correlation coefficient = 0.85, P < 0.001).

Conclusions.: Young myopic Singapore Chinese children had relative hyperopia in the periphery. This study substantiates previous studies in older children and in Caucasian subjects.

On-axis refraction (central refractive error) is typically regarded as the primary determinant of visual acuity and is the research topic of most studies on refractive errors. However, recent interest into understanding the influence of peripheral refraction on central refractive error has increased, and studies suggest that peripheral refractive error may play a key role in refractive eye development. ^{1 –6} Peripheral refraction was first implicated as a risk factor for the onset of myopia by Hoogerheide et al., ⁷ who observed that pilots who became myopic during their training had relative peripheral hyperopia before the onset of myopia. Subsequently, Mutti et al. ⁸ reported in the Orinda Longitudinal Study of Myopia that participants with myopia had peripheral relative hyperopia, compared with relative peripheral myopia for those with emmetropia and hyperopia. Relative peripheral hyperopia in myopic eyes is reflective of a prolate ocular shape, ^{8 –11} in which the axial length exceeds the equatorial diameter. 

The hypothesis that the peripheral retina has an effect on axial length growth and may participate in the process of emmetropization is further supported by animal studies. For example, peripheral form deprivation results in myopia whereas foveal ablation does not affect the normal emmetropization process. ^2,12 In one study, unilateral macular laser photocoagulation in monkeys demonstrated that the eye that was lasered recovered as quickly from vision deprivation or refractive-induced myopia as the nonlasered eyes. ²  

The prevalence of myopia is high among the Chinese, with >75% of Taiwanese schoolchildren being myopic at 18 years. ^13,14 Among Singaporean medical students, the rate of myopia is as high as 82%. ¹⁵ Reports on peripheral refraction in Asian populations, however, are few, with most investigations conducted in Caucasian populations. ^7,8,16 A study of 3618 children aged 6 to 14 years found that Asian Americans (n = 579) with myopia had the largest degree of relative peripheral hyperopia compared with myopic Hispanic (n = 1013), African (n = 724), and white (n = 1302) American children. ³ This suggests that relative peripheral hyperopia is not a universal feature of the myopic eye and that there may be ethnic differences. However, no distinction was made between the racial groups within the Asian Americans in this study. ³ Only one earlier published study has examined peripheral refraction in Chinese subjects, which found that relative peripheral hyperopia was associated with central myopia in Chinese eyes. ¹⁷ To the best of our knowledge, peripheral refraction in children younger than 5 years has not been investigated. 

The aim of this study was to describe measurements of peripheral refraction and its relationship to central refractive error in young Singapore Chinese children. 

Methods

Study Population

The present study is a substudy of children recruited through the Strabismus, Amblyopia and Refractive error in young Singaporean Children (STARS) study. Details of the STARS study have been described previously. ¹⁸ In brief, 3009 Chinese children were recruited from the southwest part of Singapore using a disproportionate age-stratified sampling strategy in 6-month age groups. For the present study, we recruited all children (n = 250) who were examined at one site (Jurong Medical Center in the western region of Singapore), 40 months or older, and cooperative with examination and autorefraction. Exclusion criteria were children who had ocular disease, chronic medical and mental conditions, and refused cycloplegic eyedrops. The study was conducted in accordance with the tenets of the Declaration of Helsinki, and written informed consent was obtained from the parents of all participants after explanation of the nature and possible consequences of the study. Ethical approval was obtained from the Institutional Review Board of the Singapore National Health Care Group. 

Eye Examinations

Eye examinations were performed by trained ophthalmologists, optometrists, and orthoptists to screen for ocular pathology. These included LogMAR visual acuity assessment, ocular motility tests, and evaluation of the anterior and posterior segments. 

Refraction was performed by a trained optometrist in a dimly illuminated room. Autorefraction was performed under cycloplegia approximately 30 minutes after topical instillation of three drops of 1% cyclopentolate and 2.5% phenylephrine, administered 5 minutes apart. A minimum pupil size of 6.0 mm was required, and mydriasis was adequate in all cases. 

Measurements of peripheral refraction were performed on the right eye of participants, while the left eye was occluded, with an open field, infrared autorefractor (Grand Seiko Autorefractor/Keratometer WAM-5500; Grand Seiko Co., Hiroshima, Japan), ¹⁹ which was calibrated once every fortnight. This autorefractor has been used previously for the measurement of peripheral refractive errors along the horizontal visual field. ^{20 –22} As many participants were very young and could not cooperate with refraction of both eyes, refraction was performed on only the right eye. 

The fixation target was placed at a distance of 33 cm from the patient's corneal vertex. This consisted of 5 LED targets in a horizontal arrangement: one located along the central visual axis, and two at either side of the central target. Each LED target was separated from the adjacent target by an angular distance of 15°, measured from the participant. Participants first fixated on the central LED target through an open view mirror, which allows the optometrist to monitor the participants' fixation. A total of 3 autorefractor measurements were made by a trained optometrist, and the average obtained. All measurements were within 0.25 D of each other. The participant was then instructed to fixate on each nasal and temporal target in turn by moving their eyes, with the head kept stationary. ²³ Fixation to the participant's right side corresponded to the nasal visual field, and fixation to the left side corresponded to the temporal visual field. Translation of the eye on rotation required realignment of the pupil along the instrument axis. The instrument was aligned such that the alignment mire was maintained in clear focus over the center of the pupil. Autorefractor measurements were then obtained in the same way as for the central target. The optometrist ensured that that the participant was fixating on the correct target through the open view mirror. 

A trained ophthalmic technician measured the axial length, radius of corneal curvature, and anterior chamber depth with the IOL Master (Carl Zeiss, Jena, Germany), which was calibrated once every fortnight. The averages of five consecutive readings taken for anterior chamber depth and three consecutive readings taken for axial length and corneal curvature were calculated. The signal-to-noise ratio for all readings was >2.0, which indicates that a clear signal was obtained when performing the measurement. Axial length, corneal curvature, and anterior chamber depth data were unavailable for 17, 21, and 28 participants, respectively. 

Definitions

The spherical and cylindrical refractive error obtained from the autorefractor was converted into power vectors using the equations ²⁴ :  

 

where SE is the spherical equivalent, C the cylindrical power, α the cylindrical axis, S the spherical power, and J180 and J45 the power of the two Jackson cross-cylinder components. All averaging was done in terms of these power vectors. 

The central SE was used to categorize the refractive status into groups groups: high and moderate myopia was defined as central SE ≤ −3.0 D; low myopia as central SE between −2.99 and −0.5 D, emmetropia as central SE between −0.49 and +1.0 D, and hyperopia as central SE > +1.0 D. 

The absolute peripheral refraction at a given eccentricity was the SE at that eccentricity. The relative peripheral refraction at a given eccentricity was calculated by the SE at that eccentricity with the central SE subtracted. 

Statistical Analysis

The associations between peripheral refraction (both absolute and relative) at the four eccentricities (temporal 30°, temporal 15°, nasal 15°, nasal 30°) and refractive status (high myope, low myope, emmetrope, hyperope), sex (male, female) and age (≤72 months, >72 months) were examined with repeated measures analysis. We also examined the association between axial length categorized into quartiles and SE at all five eccentricities (temporal 30°, temporal 15°, central, nasal 15°, nasal 30°) with repeated measures analysis. Multiple logistic regression analysis was performed for the absolute peripheral refraction at the two eccentricities (temporal and nasal 30°) with central myopia (central SE ≤ −0.5 D) as the dependent variable, adjusting for age, sex, anterior chamber depth, corneal curvature, and axial length. Multiple linear regression models were performed with absolute peripheral refraction in the nasal and temporal area as the dependent variables, and age, sex, axial length, anterior chamber depth, corneal curvature, and absolute SE at the temporal and nasal 30° as covariates. Statistical analysis was performed (SPSS, PASW Statistics 17; SPSS Inc., Chicago, IL). Statistical significance was set at P < 0.05. 

Results

Of the 902 eligible children who were >40 months old, 652 children (72.3%) were excluded because they refused cycloplegic eyedrops, were uncooperative with examination, or had incomplete data. We recruited 250 children (27.7%) with a mean age of 83 ± 36 months (range, 41.0 to 190.0 months). Of these, 147 participants (58.8%) were ≤72 months old, and 118 participants (47.2%) were male. There were 37 children with high and moderate myopia (14.8%), 81 with low myopia (32.4%), 84 with emmetropia (33.6%), and 47 with hyperopia (18.8%). The refractions in the center and periphery were fairly normally distributed. The SE at the central, temporal 15°, and nasal 15° eccentricities were more skewed toward the myopic values, compared with the temporal and nasal 30° eccentricities (Fig. 1). The association between the central refraction and the peripheral refraction at the nasal and temporal 30° was linear. Compared with the children who were included in the study, those who were excluded were younger (mean age = 59 ± 20 months, P < 0.001) and had a more hyperopic mean central SE (+0.46, 95% confidence interval [CI], +0.33 to +0.58, and −0.87, 95% CI, −1.15 to −0.60 D, respectively). There was no significant difference in the proportion of male and female children between those were included and those who were excluded from this study. 

Figure 1.

 

Histogram of spherical equivalent at the center, and at the nasal and temporal 15° and 30°. Arrows indicate the mean spherical equivalent at each meridian.

The mean SE was −0.87 ± 2.19 D at the center, −0.88 ± 1.79 D at temporal 30°, −1.09 ± 2.00 D at temporal 15°, −1.11 ± 2.07 D at nasal 15°, and −0.52 ± 1.70 D at nasal 30°. There was no significant difference in SE at all eccentricities between male and female participants. Participants who were >72 months old were significantly more myopic at all eccentricities compared with those who were ≤72 months old (P < 0.001; Table 1). 

Table 1.

 

Distribution of Spherical Equivalent

	Temporal Spherical Equivalent (30°)	Temporal Spherical Equivalent (15°)	Central Spherical Equivalent	Nasal Spherical Equivalent (15°)	Nasal Spherical Equivalent (30°)	P * (repeated measures analysis)
All children
n	250	247	249	250	250
Mean (SD) [95% CI]	−0.88 (1.79) [−1.1 to −0.66]	−1.09 (2.0) [−1.34 to −0.84]	−0.87 (2.19) [−1.15 to −0.6]	−1.11 (2.07) [−1.37 to −0.85]	−0.52 (1.7) [−0.73 to −0.3]	<0.001
Median (range)	−0.71 (−7.42 to 3.34)	−0.77 (−7.39 to 3.63)	−0.42 (−8.27 to 4.25)	−0.72 (−10.44 to 4.25)	−0.29 (−8.7 to 4.02)
Male
n	118	116	117	118	118
Mean (SD) [95% CI]	−0.96 (1.69) [−1.27 to −0.66]	−1.13 (1.9) [−1.48 to −0.78]	−0.94 (2.09) [−1.32 to −0.55]	−1.19 (2.04) [−1.56 to −0.81]	−0.63 (1.73) [−0.94 to −0.31]	<0.001
Female
n	132	131	132	132	132
Mean (SD) [95% CI]	−0.81 (1.88) [−1.13 to −0.48]	−1.06 (2.08) [−1.42 to −0.7]	−0.81 (2.28) [−1.21 to −0.42]	−1.04 (2.11) [−1.4 to −0.68]	−0.42 (1.68) [−0.71 to −0.13]	<0.001
P †	0.487	0.791	0.656	0.587	0.329
Age
≤72 months
n	147	144	146	147	147
Mean (SD) [95% CI]	−0.09 (1.32) [−0.3 to +0.13]	−0.08 (1.24) [−0.28 to +0.23]	0.27 (1.28) [+0.06 to +0.48]	−0.13 (1.32) [−0.34 to +0.09]	0.13 (1.2) [−0.06 to +0.34]	<0.001
>72 months
n	103	103	103	103	103
Mean (SD) [95% CI]	−2.01 (1.76) [−2.36 to −1.67]	−2.51 (1.99) [−2.9 to −2.12]	−2.5 (2.18) [−2.92 to −2.07]	−2.51 (2.15) [−2.93 to −2.09]	−1.45 (1.87) [−1.82 to −1.09]	<0.001
P †	<0.001	<0.001	<0.001	<0.001	<0.001

*   P for repeated measures analysis; means were adjusted for measures of five locations in the right eye.

†   P for independent sample t-test between categories.

In the nasal eccentricities, the cylindrical power in terms of astigmatic vectors J180 and J45 was generally higher in children with myopia compared with children with emmetropia. At the nasal 30° eccentricity, the J180 was −0.19 D (95% CI, −0.28 to −0.09 D) for children with emmetropia and 0.25 D (95% CI, 0.02 to 0.48 D) for children with high and moderate myopia (P < 0.001). At the nasal 15° eccentricity, the J180 was 0.02 D (95% CI, −0.10 to 0.13 D) for children with emmetropia and 0.40 D (95% CI, 0.23 to 0.57 D) for children with high and moderate myopia (P < 0.001), while the J45 was −0.02 D (95% CI, −0.08 to 0.03 D) for children with emmetropia and 0.17 D (95% CI, 0.06 to 0.27 D) for children with high and moderate myopia (P = 0.01). In most temporal eccentricities, there was no statistically significant difference in J180 and J45 between children with myopia and children with emmetropia. The distribution of J45 and J180 by central refractive status is shown in Table 2. 

Table 2.

 

Distribution of J45 and J180 by Central Refractive Status

Central Refraction (D)	Absolute J45 at T30		Absolute J45 at T15		Absolute J45 at N15		Absolute J45 at N30
	n	Mean (SD) [95% CI]	n	Mean (SD) [95% CI]	n	Mean (SD) [95% CI]	n	Mean (SD) [95% CI]
Total	249	0.03 (0.33) [−0.01 to 0.07]	246	−0.003 (0.21) [−0.03 to 0.02]	249	0.04 (0.31) [0.01 to 0.08]	249	0.07 (0.31) [0.03 to 0.11]
≤−3.0 (high and moderate myopia)	37	0.10 (0.36) [−0.01 to 0.23]	37	−0.03 (0.24) [−0.11 to 0.05]	37	0.17 (0.32) [0.06 to 0.27]	37	0.09 (0.35) [−0.03 to 0.21]
−2.99 to −0.5 (low myopia)	81	0.03 (0.38) [−0.05 to 0.11]	79	−0.04 (0.23) [−0.09 to 0.02]	81	0.07 (0.34) [−0.004 to 0.14]	81	0.10 (0.30) [0.04 to 0.17]
−0.5 to 1.0 (emmetrope)	84	0.03 (0.30) [−0.04 to 0.09]	84	0.03 (0.18) [−0.01 to 0.07]	84	−0.02 (0.26) [−0.08 to 0.03]	84	0.02 (0.33) [−0.05 to 0.10]
>1.0 (hyperope)	47	−0.01 (0.25) [−0.09 to 0.06]	46	0.01 (0.16) [−0.04 to 0.06]	47	0.02 (0.30) [−0.07 to 0.11]	47	0.07 (0.25) [−0.01 to 0.14]
P	0.45		0.18		0.01		0.41

Central Refraction (D)	Absolute J180 at T30		Absolute J180 at T15		Absolute J180 at N15		Absolute J180 at N30
	n	Mean (SD) [95% CI]	n	Mean (SD) [95% CI]	n	Mean (SD) [95% CI]	n	Mean (SD) [95% CI]
Total	249	−0.32 (0.70) [−0.41 to −0.24]	246	0.09 (0.55) [0.02 to 0.16]	249	0.15 (0.55) [0.08 to 0.22]	249	−0.03 (0.57) [−0.10 to 0.04]
≤−3.0 (high and moderate myopia)	37	−0.31 (0.76) [−0.56 to −0.06]	37	0.26 (0.54) [0.08 to 0.44]	37	0.40 (0.52) [0.23 to 0.57]	37	0.25 (0.70) [0.02 to 0.48]
−2.99 to −0.5 (low myopia)	81	−0.30 (0.75) [−0.47 to −0.14]	79	0.16 (0.68) [0.01 to 0.31]	81	0.24 (0.56) [0.11 to 0.36]	81	0.07 (0.66) [−0.08 to 0.22]
−0.5 to 1.0 (emmetrope)	84	−0.38 (0.69) [−0.53 to −0.23]	84	0.02 (0.45) [−0.07 to 0.12]	84	0.02 (0.54) [−0.10 to 0.13]	84	−0.19 (0.44) [−0.28 to −0.09]
>1.0 (hyperope)	47	−0.26 (0.57) [−0.43 to −0.09]	46	−0.05 (0.43) [−0.18 to 0.08]	47	0.03 (0.48) [−0.11 to 0.17]	47	−0.14 (0.38) [−0.25 to −0.03]
P	0.78		0.03		0.001		<0.001

Children with high and moderate myopia had relative hyperopia at all eccentricities (temporal 30° [mean = −3.7 D]; temporal 15° [mean = −4.63 D]; nasal 15° [mean = −4.68 D]; nasal 30° [mean = −3.00 D]) compared with the central axis (mean = −4.93 D; P < 0.001). This eccentric relative hyperopia was present to a lesser extent in children with low myopia at the temporal (mean = −1.54 D) and nasal 30° (mean = −1.12 D) compared with the central axis (mean = −1.62 D; P < 0.001), but was not seen at the nasal and temporal 15° (Table 3; Fig. 2). Children with emmetropia and hyperopia had peripheral relative myopia at all eccentricities (P < 0.001; Table 3). For hyperopic children (mean = +1.62 D), the SE increased at the nasal 30° (mean = +1.43 D) compared with the other three peripheral eccentricities (temporal 15°, temporal 30°, and nasal 15°; Fig. 2), hence there was less relative peripheral myopia at this eccentricity (Table 3). Compared with children with emmetropia and hyperopia, those with myopia had a more hyperopic peripheral relative SE at all eccentricities (P < 0.001; Table 3). 

Table 3.

 

The Distribution of Spherical Equivalent and Relative Spherical Equivalent by Myopia Status

Central Refractive Error (D)	Temporal Spherical Equivalent (30°)		Temporal Spherical Equivalent (15°)		Central Spherical Equivalent		Nasal Spherical Equivalent (15°)		Nasal Spherical Equivalent (30°)		P * (repeated measures analysis)
	n	Mean (SD) [95% CI]	n	Mean (SD) [95% CI]	n	Mean (SD) [95% CI]	n	Mean (SD) [95% CI]	n	Mean (SD) [95% CI]
≤−3.0 (high and moderate myopia)	37	−3.7 (1.3) [−4.13, −3.27]	37	−4.63 (1.27) [−5.06, −4.21]	37	−4.93 (1.27) [−5.35, −4.50]	37	−4.68 (1.75) [−5.26, −4.10]	37	−3.0 (1.85) [−3.62, −2.38]	<0.001
−2.99 to −0.5 (low myopia)	81	−1.54 (0.96) [−1.75, −1.32]	79	−1.82 (0.72) [−1.98, −1.66]	81	−1.62 (0.73) [−1.78, −1.46]	81	−1.79 (0.8) [−1.96, −1.61]	81	−1 .12 (0.9) [−1.32, −0.92]	<0.001
−0.5 to 1.0 (emmetrope)	84	−0.17 (0.84) [−0.36, +0.01]	84	−0.09 (0.61) [−0.23, +0.04]	84	+0.25 (0.44) [+0.15, +0.34]	84	−0.11 (0.82) [−0.29, +0.06]	84	+0.07 (0.7) [−0.08, +0.22]	<0.001
>1.0 (hyperope)	47	+1.18 (0.81) [+0.95, +1.42]	46	+1.17 (0.73) [+0.95, +1.38]	47	+1.62 (0.66) [+1.43, +1.81]	47	+1.06 (0.91) [+0.79, +1.32]	47	+1.43 (0.8) [+1.19, +1.66]	<0.001

Central Refractive Error (D)	T30SE - CenSE		T15SE - CenSE		N15SE - CenSE		N15SE - CenSE		P * (repeated measures analysis)
	n	Mean (SD) [95% CI]	n	Mean (SD) [95% CI]	n	Mean (SD) [95% CI]	n	Mean (SD) [95% CI]
≤−3.0 (high and moderate myopia)	37	+1.23 (0.89) [+0.93, +1.53]	37	+0.29 (0.84) [+0.02, +0.57]	37	+0.25 (0.72) [+0.01, +0.49]	37	+1.93 (1.28) [+1.5, +2.36]	<0.001
−2.99 to −0.5 (low myopia)	81	+0.09 (0.88) [−0.11, +0.28]	79	−0.18 (0.48) [−0.29, −0.08]	81	−0.16 (0.56) [−0.29, −0.04]	81	+0.50 (0.99) [+0.28, +0.72]	<0.001
−0.5 to 1.0 (emmetrope)	84	−0.42 (0.74) [−0.58, −0.26]	84	−0.34 (0.42) [−0.43, −0.25]	84	−0.36 (0.68) [−0.51, −0.21]	84	−0.18 (0.63) [−0.31, −0.04]	0.023
>1.0 (hyperope)	47	−0.43 (0.72) [−0.64, −0.22]	46	−0.44 (0.49) [−0.59, −0.3]	47	−0.56 (0.6) [−0.74, −0.38]	47	−0.19 (0.66) [−0.38, +0.004]	0.016

*   P for repeated measures analysis; means were adjusted for measures of five locations in the right eye.

Figure 2.

 

Means of spherical equivalent as a function of visual field angle for high and moderate myopic children with central SE < = −3.0 D (n = 37), low myopic children with −3.0 D < central SE ≤ −0.5 D (n = 81), emmetropic children with −0.5 D < central SE ≤ 1.0 D (n = 84), and hyperopic children with central SE > 1.0 D (n = 47). Error bars indicate 95% confidence interval of means of spherical equivalent.

Children with the highest quartile of axial length were myopic at the center (−3.76 D) and had relatively more hyperopia at the periphery, whereas children with the lowest quartile of axial length were hyperopic in the center (+0.44 D) and had a relatively more myopic refraction in the periphery (Table 4). 

Table 4.

 

Distribution of Spherical Equivalent by Quartiles of Axial Length

Axial Length	Temporal Spherical Equivalent (30°)		Temporal Spherical Equivalent (15°)		Central Spherical Equivalent		Nasal Spherical Equivalent (15°)		Nasal Spherical Equivalent (30°)		P * (repeated measures analysis)
	n	Mean (SD) [95% CI]	n	Mean (SD) [95% CI]	n	Mean (SD) [95% CI]	n	Mean (SD) [95% CI]	n	Mean (SD) [95% CI]
1st quartile (AL: 20.74 to 22.15)	59	+0.14 (1.39) [−0.22 to +0.5]	58	+0.15 (1.26) [−0.18 to +0.49]	59	+0.44 (1.41) [+0.07 to +0.81]	59	+0.07 (1.38) [−0.29 to +0.43]	59	+0.35 (1.22) [+0.03 to +0.67]	0.001
2nd quartile (AL: 22.15+ to 22.58)	58	−0.45 (1.28) [−0.79 to −0.12]	57	−0.38 (1.22) [−0.71 to −0.06]	57	+0.04 (1.16) [−0.27 to +0.35]	58	−0.45 (1.31) [−0.79 to −0.11]	58	−0.21 (1.2 3) [−0.54 to +0.11]	<0.001
3rd quartile (AL: 22.58+ to 23.415)	58	−0.62 (1.34) [−0.97 to −0.27]	57	−0.78 (1.37) [−1.15 to −0.42]	58	−0.47 (1.36) [−0.83 to −0.11]	58	−0.74 (1.3) [−1.08 to −0.4]	58	−0.43 (1.33) [−0.78 to −0.08]	<0.001
4th quartile (AL: 23.415+ to 27.17)	58	−2.80 (1.62) [−3.23 to −2.38]	58	−3.55 (1.79) [−4.03 to −3.08]	58	−3.76 (1.91) [−4.26 to −3.26]	58	−3.59 (2.11) [−4.14 to −3.03]	58	−2.01 (2.02) [−2.54 to −1.48]	<0.001
P †		<0.001		<0.001		<0.001		<0.001		<0.001

Axial Length	T30SE - CenSE		T15SE - CenSE		N15SE - CenSE		N15SE - CenSE		P * (repeated measures analysis)
	n	Mean (SD) [95% CI]	n	Mean (SD) [95% CI]	n	Mean (SD) [95% CI]	n	Mean (SD) [95% CI]
1st quartile (AL: 20.74 to 22.15)	59	−0.3 (0.82) [−0.51 to −0.09]	58	−0.26 (0.48) [−0.38 to −0.13]	59	−0.37 (0.53) [−0.51 to −0.23]	59	−0.09 (0.61) [−0.25 to +0.07]	0.045
2nd quartile (AL: 22.15+ to 22.58)	57	−0.5 (0.7) [−0.69 to −0.31]	56	−0.44 (0.44) [−0.55 to −0.32]	57	−0.51 (0.69) [−0.69 to −0.32]	57	−0.25 (0.63) [−0.42 to −0.09]	0.03
3rd quartile (AL: 22.58+ to 23.415)	58	−0.15 (0.81) [−0.36 to +0.06]	57	−0.33 (0.49) [−0.46 to −0.2]	58	−0.27 (0.55) [−0.42 to −0.13]	58	+0.04 (0.79) [−0.17 to +0.25]	0.01
4th quartile (AL: 23.415+ to 27.17)	58	+0.95 (0.92) [+0.71 to +1.2]	58	+0.2 (0.69) [+0.02 to +0.38]	58	+0.17 (0.73) [−0.02 to +0.36]	58	+1.75 (1.12) [+1.46 to +2.05]	<0.001
P †		<0.001		<0.001		<0.001		<0.001

*   P for repeated measures analysis; means were adjusted for measures of five locations in the right eye.

†   P for analysis of variance (ANOVA) across categories.

In multiple logistic regression models with central myopia (central SE ≤ −0.5 D) as the dependent variable, the odds of having myopia increased by 0.16 for every 1 D unit increase in absolute temporal SE at 30°, and by 0.09 for every 1 D unit increase in absolute nasal SE at 30°, after adjustment for age, sex, axial length, anterior chamber depth, and corneal curvature (Table 5). 

Table 5.

 

Multiple Logistic Regressions for Peripheral Refraction with Central Myopia as the Dependent Variable

Variables in the Equation	Odds Ratio	95% CI for Odds Ratio		P	Odds Ratio	95% CI for Odds Ratio		P
		Lower	Upper			Lower	Upper
Age, months	1.01	0.99	1.04	0.23	1.01	0.98	1.02	0.66
Gender (ref. = boys)	0.53	0.2	1.39	0.2	0.76	0.27	2.08	0.59
Axial length, mm	4.09	1.37	12.22	0.01	9.83	2.97	32.59	<0.001
Anterior chamber depth, mm	0.22	0.02	2.9	0.25	0.17	0.01	2.10	0.17
Corneal curvature, mm	0.02	0.002	0.31	0.004	0.002	0.00	0.04	<0.001
Spherical equivalent at 30° periphery, D	0.16	0.09	0.29	<0.001	0.09	0.04	0.20	<0.001
	(temporal)				(nasal)

Multiple linear regression models of the effect of central SE on peripheral refraction, adjusting for age, sex, axial length, anterior chamber depth, and corneal curvature, were constructed (Table 6). For every 1 D increase in central SE, the temporal SE at 30° increased by 0.85 D (95% CI, 0.75 to 0.94 D), while for every 1 Diopter increase in central SE, the nasal SE at 30° increased by 0.91 D (95% CI, 0.82 to 1.01 D). 

Table 6.

 

Multiple Linear Regressions for Peripheral Refraction with Peripheral Refraction as the Dependent Variable

Variables in the Equation	Spherical Equivalent at Temporal 30° (D)				Spherical Equivalent at Nasal 30° (D)
	Regression Coefficients	95% CI for B		P	Regression Coefficients	95% CI for B		P
	B	Lower Bound	Upper Bound		B	Lower Bound	Upper Bound
Age, months	0.001	−0.004	0.005	0.79	0.002	−0.003	0.01	0.43
Gender (ref. = boys)	0.05	−0.15	0.25	0.65	0.11	−0.10	0.32	0.31
Axial length, mm	0.20	−0.003	0.41	0.06	0.50	0.28	0.72	<0.001
Anterior chamber depth, mm	0.23	−0.22	0.67	0.32	−0.42	−0.90	0.05	0.08
Corneal curvature, mm	−0.74	−1.26	−0.22	0.01	−1.30	−1.85	−0.75	<0.001
Central spherical equivalent, D	0.85	0.75	0.94	<0.001	0.91	0.82	1.01	<0.001

Discussion

The results of this study indicate that young Singapore Chinese children with central myopia had relative hyperopia in the periphery, at least for the horizontal meridian. Conversely, children with central hyperopia tended toward peripheral relative myopia. 

Our study provides new data on peripheral refraction measures in young preschool Chinese children. Previous studies on relative peripheral refraction have been largely confined to Caucasian subjects. ^8,16,20 Assessment of peripheral refractive error in 822 children aged 5 to 14 years participating in the Orinda Longitudinal Study of Myopia showed that myopic children had greater relative hyperopia in the periphery (+0.80 ± 1.29 D) compared with emmetropes (−0.41 ± 0.75 D) and hyperopes (−1.09 ± 1.02 D). ⁸ A recent study of 82 older Chinese subjects from Guangzhou, aged 8 to 25 years, also reported an association between relative peripheral hyperopia and central myopia (P = 0.017) but was limited by a small sample size. ¹⁷ Logan et al. ⁴ previously reported that ocular shape changes are larger in Chinese eyes than in Caucasian eyes with myopia, and the peripheral refraction profile was found to differ between East Asians and Caucasians with myopia between −2.50 and −5.25 D (Kang P, et al. IOVS 2009;50:ARVO E-Abstract 3940). Our results also indicate that hyperopic and emmetropic Chinese children have myopic peripheries. This is consistent with the findings of other studies of predominantly Caucasian subjects, ^8,20 though in some reports, peripheral refraction does not differ significantly from central refraction in emmetropic eyes. ^17,25,26  

Peripheral refraction was found to vary with the type of myopia, with axial myopia between −4 and −6 D having more hyperopic peripheries compared with refractive myopia. ²⁷ This is consistent with the results in our study: Relative peripheral hyperopia was found in children with the highest quartile of axial length, whereas those with the lowest quartile of axial length had peripheral relative myopia. Our results also indicate that relative hyperopia was a more prominent feature of eyes with a higher degree of myopia, which is similar to the findings of previous studies. ^17,20  

Our results suggest that in young Chinese children with myopia, ocular axial growth exceeds the equatorial growth, resulting in an elongated eye that has been distorted into a more prolate shape, ¹⁶ in which the axial length exceeds the equatorial diameter. In contrast, emmetropic eyes have an oblate shape, in which the equatorial diameter exceeds the axial length. ¹⁶ This is detectable on x-ray, ^10,11 interferometry, ²⁸ and magnetic resonance scanning. ^16,29 These differences in ocular shape are reflected as a change in the refractive error at the ocular periphery compared with the central axis. ^{30 –33} The association between relative peripheral hyperopia in the horizontal meridian and central myopia ^8,16,20 is consistent with a prolate ocular shape. ^{8 –11} However, the ocular dimensions may differ between the horizontal and vertical meridians. ¹⁶ Though earlier studies have reported no significant association between refractive status and peripheral refraction along the vertical meridian, ^17,20 these reports are few, and further studies are needed to investigate the peripheral refraction profile along the vertical meridian. 

As this was a cross-sectional study, we were not able to determine whether relative peripheral hyperopia was the cause or the consequence of myopia. ^5,6,32 However, our results show that the presence of hyperopic peripheries occurs early in young children. The Collaborative Longitudinal Evaluation of Ethnicity and Refractive Error study found that relative peripheral hyperopia preceded the onset of myopia by two years in 605 children between 6 and 14 years of age. ³ Hence, relative peripheral hyperopia is thought to play a causative role in the pathogenesis of myopia. The importance of peripheral vision on axial length growth has been demonstrated in rhesus monkeys, by obstructing the peripheral retina and sparing the central vision. ^1,2 Optically imposed peripheral hyperopic defocus also results in central axial myopia in infant monkeys, supporting the theory that peripheral relative hyperopia results in compensatory axial growth. ^1,12,34  

However, others argue that the more prolate shape of myopic eyes and the resultant relative peripheral hyperopia could be a consequence rather than a cause of axial growth. ⁸ The crystalline lens may be a source of equatorial restriction when ocular size exceeds the ability of the lens to stretch in response, ^35,36 resulting in a longer axial length compared with the equatorial diameter. In the Orinda Longitudinal Study of Myopia, Mutti et al. ⁸ found that thicker lenses are associated with more hyperopic relative peripheral refractions, and more myopic central refractive errors. This led them to propose that the failure of the lens to thin may have caused the eye to distort. However, the restrictive effect of the lens on ocular growth should be exerted equally in all directions, and hence this hypothesis is not supported by earlier reports of no significant association between refractive status and peripheral refraction along the vertical meridian. ^17,20 Additional studies evaluating the characteristics of the lens in myopic eyes, as well as the peripheral refraction profile in the vertical and other meridians, are indicated. 

The major strength of our study is the evaluation of peripheral refraction in young Chinese children in Singapore, in whom the prevalence of early-onset myopia is high. Other strengths include autorefraction under cycloplegia and the availability of information on possible confounders such as axial length, corneal curvature, and anterior chamber depth. However, some limitations should also be considered in the interpretation of our findings. First, we are unable to establish the temporal relationship between relative peripheral hyperopia and the onset of myopia in our cross-sectional study. Our results have defined the baseline characteristics of the emmetropic and hyperopic children in this cohort, whom we intend to follow up for the development of myopia, so as to determine how relative peripheral hyperopia and myopia are chronologically related. Second, we investigated only the peripheral refraction profile at limited positions along the horizontal meridian, and the vertical meridian was not assessed. 

In summary, our study shows that relative peripheral hyperopia was associated with central myopia in young Chinese children from Singapore, which has one of the highest rates of myopia in the world. These data substantiate previous studies in older children and in Caucasian subjects. Longitudinal studies will further establish the precise role of relative peripheral hyperopia in the development and progression of myopia. 

Footnotes

 Supported by Essilor International SA, as the Peripheral REfraction in Preschool children (PREP) study. The Grand Seiko autorefractor was on loan from Essilor International SA.

Footnotes

 Disclosure: C.C.A. Sng, Essilor International SA (F); X.-Y. Lin, Essilor International SA (F); G. Gazzard, Essilor International SA (F); B. Chang, Essilor International SA (F); M. Dirani, Essilor International SA (F); A. Chia, Essilor International SA (F); P. Selveraj, Essilor International SA (F); K. Ian, Essilor International SA (F); B. Drobe, Essilor International SA (F); T.-Y. Wong, Essilor International SA (F); S.-M. Saw, Essilor International SA (F)