Cross-sectional Sample of Peripheral Refraction in Four Meridians in Myopes and Emmetropes

Abstract

Purpose.: The study of peripheral refractive error is of growing interest as degradation of the retinal image quality in the periphery is known to affect central refractive error development and ocular shape in animal models. The purpose of this study was to measure peripheral refraction across the horizontal, vertical, and two oblique meridians in a group of myopic and emmetropic adults and to investigate retinal asymmetry in the human retina.

Methods.: Thirty-one myopes (spherical equivalent between −2.00 and −9.62 D) and 20 emmetropes (spherical equivalent between −0.50 and +0.50 D) with astigmatism less than −0.75 D, participated in the project. Noncycloplegic peripheral refraction measurements were captured with an autorefractor (NVision K-5001; Shin-Nippon, Tokyo, Japan) at the fovea and up to 30° eccentricity in the horizontal, vertical, and two oblique meridians in 10° steps. RPR was calculated by subtracting the foveal spherical equivalent refraction from that obtained at each eccentric location.

Results.: Along all measured meridians, myopic eyes showed a relative hyperopic shift in the periphery, with the superior–temporal portion of the retina exhibiting the smallest shift. Emmetropic eyes, however, exhibited a relatively consistent refractive profile across all meridians and eccentricities, confirming a spherical retinal shape for this group. In addition, off-axis astigmatism increased with eccentricity in all meridians.

Conclusions.: These results suggest that the myopic eye tends toward an ellipsoid shape, rather than the globular shape of an emmetropic eye.

The peripheral optics of the eye provide insight into changes in retinal shape associated with refractive error development and the process of emmetropization. ^{1 –4} This concept has been supported by animal studies in which peripheral form deprivation and imposed local retinal defocus appear to induce central axial myopia. ^2,5 It has also been hypothesized that relative hyperopic blur in the peripheral retina is a risk factor for the development of central myopia in both children and adults. ^6,7  

Understanding the peripheral refraction profile is informative for other areas of vision research such as psychophysical measurements of peripheral detection acuity, which may require the correction of the peripheral refractive errors. ^8,9 Previous studies have shown that detection acuity varies significantly with the amount of optical defocus, and improving the optics in the retinal periphery decreases detection thresholds. In contrast, peripheral resolution acuity is limited by neural sampling and hence insensitive to refractive blur. ^10,11 From a clinical perspective, in patients without central vision, the quality of their life can be improved significantly by optically correcting their peripheral vision. ¹²  

The history of peripheral refraction measurements dates back to the series of studies by Ferree et al. who reported refraction for the peripheral field of vision using a modified parallax refractometer (Carl Zeiss Meditec, Inc., Dublin, CA). ^{13 –15} Over the past few decades, several studies have investigated refractive status in the retinal periphery in relation to age, ^16,17 accommodation, ¹⁸ fixation distance, ¹⁹ impact of orthokeratology lenses on peripheral optics, ^20,21 and ethnicity ^22,23 and for the purposes of designing ophthalmic lenses to correct peripheral refractive errors. ²⁴ A wide range of techniques have been used including Hartmann-Shack aberrometry, ²⁵ off-axis retinoscopy, ²⁶ the double-pass technique, ^27,28 photorefraction, ²⁹ and open-field autorefraction. ^23,30,31 Most of these studies are in agreement that refractive errors in the periphery differ from those measured at the fovea, with the amount and type presenting considerable individual variability. Hyperopic eyes tend to exhibit relative myopic shifts in the periphery, whereas most Caucasian myopic eyes often display relative hyperopic peripheral refractions compared with central refraction along the horizontal meridian. ^{7,23,30,32 –34} Most studies agree on this point for the horizontal meridian, but there is controversy regarding the relative peripheral shifts in other meridians. ^29,30,35  

The relatively peripheral hyperopia in myopic individuals suggests that the myopic retina has a more prolate/less oblate shape than emmetropic and hyperopic eyes. ³⁶ An increase in the relative prolate shape of the globe (or decrease in relative oblate shape) as myopia increases has also been evident in magnetic resonance imaging data. ³⁷  

There is also agreement that the average degree of astigmatism increases with retinal eccentricity with evidence of nasal–temporal asymmetry. ^34,38,39 Most studies measured only peripheral refraction along the horizontal meridian, with a minority investigating along both the horizontal and vertical meridians in human adults ^23,30 or children ³⁵ and even fewer across other retinal locations. ^29,40 Table 1 summarizes the published studies in which peripheral refraction was measured in more than the horizontal meridian. 

Table 1.

 

Summary of Peripheral Refraction Studies along More Than One Meridian in Myopic Individuals

Source	Peripheral Angle	Method	n (Myopia)	Age (y)	Alignment	Results	Meridians Difference*	Asymmetry†
Seidemann et al. 29	Central 22° × 22°	Photorefraction	18	21–33	Pupil center (eye-turn)	RP myopia across the retina (except IR with RP hyperopia)	1.75 D (right eye)	M: SR>IR Ast: TR>NR
Schmid 35	V & H (15°) NR excluded from analysis	Autorefraction	17 (TR) 10 (vertical)	7–15	Not stated	H: RP myopia in TR V: RP hyperopia	0.7 D	M: SR>IR
Atchison et al. 30	V & H (Out to 35°)	Autorefraction	84 (horizontal)	18–35	Pupil center (eye-turn)	H: RP hyperopia	3.5 D	M: SR>IR
	(5° steps)		31 (vertical)			V: RP myopia		J180: TR >NR
Bakaraju et al. 41	V & H (Out to 30°) (10° steps)	Ray tracing	3 myopic models	Not applicable	Pupil center (assume)	H: RP hyperopia V: RP hyperopia	0.75 D	Not stated
Mathur et al. 40	Central 42° × 32°	Aberrometry	9	22–35	Pupil center (head-turn)	RP myopia across the retina	1 D	Not stated
Berntsen et al. 42	H (30°)	Aberrometry	85	6–11		H: RP hyperopia	1.09 D	Not stated
	V (SR: 30°, IR: 20°)					V: RP myopia
Chen et al. 23	H (0°, 22°, 32°, 40°)	Autorefraction	23 children	8–12	Pupil center (eye-turn)	H: RP hyperopia	MM: 2.5 D	M: NR >TR
	V (22°, 32°)		23 adults	18–25		V: RP myopia in LM	LM: 1.8 D	J45: SR >IR
						V: Flat profile in MM

 RP, relative peripheral refraction; LM, low myopia; MM, moderate myopia; H, horizontal meridian; V, vertical meridian; SR, superior retina; IR, inferior retina, NR, nasal retina; TR, temporal retina.

*  This value represents the dioptric difference from the most positive and least negative value along both horizontal and vertical meridians.

†  The region with relatively more minus value > the region with relatively less minus value.

Refractive error (astigmatism and spherical equivalent refraction) can differ significantly with retinal eccentricity, as the visual signals that control eye growth may vary in different parts of the retina. ^14,34 To obtain a more comprehensive understanding of the relationship between the on-axis refraction to refractive errors in peripheral areas of the retina, an experiment was devised to measure the peripheral refraction across ±30° of the horizontal, vertical, and the two oblique (superior nasal–inferior temporal [SN-IT] and superior temporal–inferior nasal [ST-IN]) meridians in a group of myopic and emmetropic adults. To our knowledge, we are the first to use autorefraction to investigate peripheral refraction in four meridians. 

Methods

Study Population

Thirty-one myopic (defined as a mean spherical equivalent refraction of −0.75 D or worse) and 18 emmetropic (defined as a mean spherical equivalent between −0.50 and +0.50 D) volunteers were recruited from the University of Bradford student population according to the following inclusion criteria: no history of corneal and ocular surgery; on-axis subjective astigmatism ≤ −0.75 D, and no ocular disease. Subjects participating in this study exhibited a range of different ethnic backgrounds including: Caucasian, British Asian, and East Asian. The complete profile of participants is shown in Table 2. 

Table 2.

 

Participant Profile

Refractive Correction Group (n = 49)	Age (y)	MSE (D)*	Astig (D)†	Distribution of Ethnicity
Emmetropia (n = 18)	22.28 ± 5.19	0.07 ± 0.34	−0.15 ± 0.20	9 Caucasian,
	18 to 39	+0.50 to −0.50	−0.50 to 0.00	7 British Asian, 2 East Asian
Myopia (n = 31)	23.06 ± 4.63	−5.73 ± 1.80	−0.32 ± 0.26	12 Caucasian,
	18 to 37	−2.00 to −9.62	−0.75 to 0.00	14 British Asian, 5 East Asian

 Data are expressed as the mean ± SD, with the range.

*  Mean spherical equivalent based on subjective refraction in diopters.

†  Cylindrical power in diopter.

To ensure that subjects met the inclusion criteria, their spherocylindrical refractive errors were measured by subjective refraction. The traditional endpoint of maximum plus/minimum minus with the optimum visual acuity was adopted. To determine astigmatism, a crossed cylinder was used to locate the axis and power of a suitable correcting lens. Only the dominant eye results were submitted for analysis. All participants were able to achieve a corrected visual acuity of at least 0.00 (6/6) or better with the dominant eye on a high-contrast Bailey-Lovie logMAR chart at a distance of 6 m. 

Ethical Considerations

Informed consent was obtained from each subject after the nature of the experimental procedures had been explained. The research followed the tenets of the Declaration of Helsinki and was approved by University of Bradford Research Ethics Committee. 

Central and Peripheral Refraction Instrumentation

Measurement of central and peripheral refraction was performed with an NVision-K 5001 autorefractor (Shin-Nippon, Tokyo, Japan). This instrument has been shown to provide accurate and reliable on-axis refraction compared with subjective refraction. ⁴³ This model is very similar technically to the SRW-5000 (Shin-Nippon), which has also been shown to produce a comparable level of accuracy. ⁴⁴ The NVision-K 5001 autorefractor also has been used in many published studies to measure peripheral refractive errors. ^22,23,45 The open-field view design of the autorefractor allows measurement of off-axis refraction to ∼30° eccentricity in the horizontal meridian and to ∼15° eccentricity in the vertical and oblique meridians, due to the limitations imposed by the frame of the viewing window. Therefore, to be able to obtain measurements of the peripheral refraction across a wide range of eccentricities in different meridians, modifications were applied to the system (Fig. 1). 

Figure 1.

 

Optical attachments to the autorefractometer (NVision K-5001; Shin-Nippon, Tokyo, Japan) for measurements of refraction in horizontal (a), vertical (b), and oblique (c, d) meridians (45–225° and 135–315°), for the right eye. (e) The optical attachment.

The system consisted of one 1-mm-thick beam splitter (25% reflectance, 75% transmittance), a high-positive-power lens (36 mm focal length; Edmund Optics, Barrington, NJ) to collimate the target to ensure minimal accommodation, a high-contrast Maltese-cross target at optical infinity illuminated by an LED and a goniometer to position the whole setup at different peripheral locations. The apparatus was attached to the top rail of the autorefractor's chinrest frame for horizontal peripheral locations measurements and to the right or left side of the instrument (dependent on the subject's dominant eye) for measurement along the vertical and two major oblique meridians. 

Initially, the subject was asked to observe a green LED mounted on the laboratory wall at a 2-m distance, and the cross target through the beam splitter in the primary position. Next, the subject was requested to follow the primary position of the fixation target superimposed on the cross target for each eccentric location along each meridian to obtain the complete set of readings. The attachment was positioned and rotated around the center of rotation of the eye, located approximately 14.8 mm behind the corneal apex. ⁴⁶ The subject's head was held stable, requiring the subject to turn his or her eyes to view the fixation target in each eccentric location. 

All measurements were made at a low photopic level to ensure that the pupil size was a minimum of 4 mm in diameter, which was achieved in all participants. Only the dominant eye was assessed, and the nondominant eye was occluded throughout the experimental procedure. No cycloplegic or mydriatic was used for central and peripheral refraction ³¹ and at least five repeat measurements of refraction were averaged from each location. If there was an obvious fixation loss, the reading was discarded and repeated. The axis of the autorefractor was aligned with the corneal reflection for all measurements consistent within the acceptable alignment range recommended by Ehsaei et al. ⁴⁷ Peripheral refractions were determined at 25 retinal locations up to 30° eccentricity in horizontal, vertical, and two oblique meridians (45–225° and 135–315°) in 10° steps. 

Before commencing the experiment, we confirmed that the instrument gave the correct central reading for a calibration model eye provided with the instrument. In addition, to assess the reliability of our setup, we repeated peripheral refraction measurements at a few retinal locations in the horizontal and vertical meridians. Five LED targets were arranged at central fixation, 10° superiorly and inferiorly, and 20° temporally and nasally, on a flat wall 2 m in front of the participant, to create a series of fixation angles along the horizontal and vertical meridians. Peripheral refractions were then measured with the instrument aligned with the corneal reflex and recorded to compare with peripheral refraction measured through the previously described collimated optical setup. 

Reliability of the Technique

To investigate the reliability of our optical attachment for peripheral refraction measurements, the method of Bland and Altman ⁴⁸ was performed to examine the agreement between our aforementioned collimated optical setup and peripheral refraction measurements using a natural open view of the instrument of fixations targets on a flat wall for the M (mean spherical equivalent) component of the whole sample. 

Figure 2 shows the difference in M component between the two measurement techniques, compared with the mean of the two techniques, for central, 20° nasal and temporal, and 10° superior and inferior peripheral refraction. Although our optical setup consistently tended to give slightly more minus mean spherical values compared with those generated by the wall setup, it was within the repeatability of the instrument shown by Davies et al. ⁴³ for central objective refraction. 

Figure 2.

 

Difference in the mean spherical equivalent component (M) between the wall and collimated setups compared with the mean of two techniques for central (a), 20° nasal (b), 20° temporal (c), 10° superior (d), and 10° inferior (e) retina measurements. Solid line: the mean bias; dotted lines: the 95% confidence limits.

Statistical Analysis

The refraction results were converted to vector format,⁴⁹ by representing the sphere (S), cylinder (C), and axis (θ) as the mean spherical equivalent (M), 180° to 90° astigmatism J₁₈₀ and 45° to 135° astigmatism J₄₅ components based on the following equations:  

 

 

 

 

 

In addition, overall power of refraction (P) of the eye was calculated:²⁵ 

 

Vector components (M, J₁₈₀, and J₄₅) were averaged and summarized using descriptive statistics. Relative peripheral refraction (RPR) was calculated by subtracting the spherical equivalent refraction (M) obtained at the foveal position from that at each eccentric location. A positive number indicates a relative hyperopic shift. 

The biases between the two techniques for measurement of central and peripheral refraction (the mean difference, standard deviation, and 95% CI) were calculated as suggested by Bland and Altman. ⁴⁸  

Repeated-measures ANOVA was used to determine whether there were differences in refractive error as a function of eccentricity. The level of significance was set at P < 0.05 (SPSS ver. 15; SPSS, Inc., Chicago, IL). 

Results

The central objective autorefraction obtained for the whole sample ranged between −8.79 and +0.75 D for the spherical value, with a maximum astigmatism of −0.93 D. The mean value of the spherical equivalent (M) based on central objective refraction was −5.76 ± 1.82 D for the myopic group and −0.32 ± 0.44 D for the emmetropic group. 

Table 3 presents descriptive statistics (mean ± SD) of the refractive and astigmatic components (M, J ₁₈₀, and J ₄₅) and RPR for all measured meridians in myopic and emmetropic eyes. Although standard deviations of the power vectors within a set of measurements were small, they increased gradually with increases in retinal eccentricity in both refractive groups, indicating an increase in between-subject variance. 

Table 3.

 

Descriptive Statistics for Refractive Components

A. Horizontal Meridian
	Temporal Retina			Central	Nasal Retina
	−30°	−20°	−10°	0	+10°	+20°	+30°
Emmetropia
M	−0.18 ± 1.34	−0.26 ± 0.95	−0.34 ± 0.62	−0.32 ± 0.44	−0.34 ± 0.63	0.09 ± 0.74	0.40 ± 1.12
J 180	−0.59 ± 0.79	−0.23 ± 0.38	−0.06 ± 0.23	0.06 ± 0.16	−0.03 ± 0.34	−0.06 ± 0.41	−0.09 ± 0.75
J 45	0.10 ± 0.65	−0.04 ± 0.52	0.08 ± 0.18	0.05 ± 0.13	0.05 ± 0.31	0.04 ± 0.31	−0.09 ± 0.46
RPR	0.13 ± 1.17	0.05 ± 0.80	−0.03 ± 0.50	0.00	−0.02 ± 0.56	0.40 ± 0.67	0.72 ± 0.98
Myopia
M	−3.71 ± 2.09	−4.97 ± 1.78	−5.62 ± 1.84	−5.76 ± 1.82	−5.54 ± 1.96	−4.96 ± 2.43	−3.69 ± 2.89
J 180	0.14 ± 0.94	−0.05 ± 0.39	0.07 ± 0.18	0.17 ± 0.16	0.23 ± 0.30	0.23 ± 0.33	0.47 ± 0.59
J 45	0.31 ± 0.64	0.13 ± 0.40	0.10 ± 0.26	0.04 ± 0.14	−0.01 ± 0.30	−0.02 ± 0.34	0.01 ± 0.65
RPR	2.04 ± 1.43	0.79 ± 0.84	0.10 ± 0.56	0.00	0.22 ± 0.64	0.80 ± 0.93	2.06 ± 1.63

B. Vertical Meridian
	Inferior Retina			Central	Superior Retina
	−30°	−20°	−10°	0	+10°	+20°	+30°
Emmetropia
M	0.63 ± 1.45	−0.40 ± 1.03	−0.54 ± 0.79	−0.32 ± 0.44	−0.49 ± 0.61	−0.58 ± 0.62	−0.99 ± 1.15
J 180	−0.02 ± 0.98	0.24 ± 0.48	0.30 ± 0.29	0.06 ± 0.16	0.15 ± 0.23	0.32 ± 0.40	0.97 ± 0.87
J 45	−0.10 ± 0.32	−0.03 ± 0.33	0.02 ± 0.21	0.05 ± 0.13	0.10 ± 0.24	0.19 ± 0.46	0.23 ± 0.54
RPR	0.95 ± 1.25	−0.09 ± 0.83	−0.22 ± 0.62	0.00	−0.17 ± 0.57	−0.26 ± 0.57	−0.67 ± 1.10
Myopia
M	−3.32 ± 3.10	−4.96 ± 2.32	−5.50 ± 1.83	−5.76 ± 1.82	−5.56 ± 1.91	−5.22 ± 2.12	−4.36 ± 2.68
J 180	−0.21 ± 1.29	0.16 ± 0.68	0.27 ± 0.45	0.17 ± 0.16	0.26 ± 0.24	0.40 ± 0.37	0.71 ± 0.82
J 45	−0.09 ± 0.67	−0.12 ± 0.35	0.01 ± 0.30	0.04 ± 0.14	0.09 ± 0.25	0.21 ± 0.33	0.45 ± 0.58
RPR	2.43 ± 2.21	0.80 ± 1.31	0.26 ± 0.67	0.00	0.20 ± 0.57	0.53 ± 0.96	1.40 ± 1.63

C. Oblique Meridian: Superior Temporal-Inferior Nasal
	Superior Temporal Retina			Central	Inferior Nasal Retina
	−30°	−20°	−10°	0	+10°	+20°	+30°
Emmetropia
M	−0.76 ± 1.45	−0.58 ± 0.90	−0.57 ± 0.68	−0.32 ± 0.44	−0.36 ± 0.87	−0.31 ± 0.77	0.13 ± 1.06
J 180	0.01 ± 0.66	0.04 ± 0.33	0.01 ± 0.22	0.06 ± 0.16	0.26 ± 0.37	0.34 ± 0.34	0.19 ± 0.62
J 45	0.44 ± 1.06	0.22 ± 0.50	0.11 ± 0.30	0.05 ± 0.13	0.04 ± 0.26	0.03 ± 0.49	−0.11 ± 0.61
RPR	−0.44 ± 1.40	−0.27 ± 0.89	−0.25 ± 0.72	0.00	−0.04 ± 0.78	0.01 ± 0.64	0.45 ± 0.87
Myopia
M	−4.47 ± 2.34	−5.11 ± 2.06	−5.63 ± 1.87	−5.76 ± 1.82	−5.71 ± 2.08	−5.18 ± 2.32	−3.85 ± 2.79
J 180	0.28 ± 0.72	0.21 ± 0.35	0.14 ± 0.23	0.17 ± 0.16	0.21 ± 0.32	0.26 ± 0.41	0.11 ± 0.59
J 45	0.51 ± 0.94	0.19 ± 0.46	0.18 ± 0.26	0.04 ± 0.14	0.09 ± 0.24	0.09 ± 0.41	−0.27 ± 0.70
RPR	1.28 ± 1.39	0.64 ± 0.85	0.12 ± 0.57	0.00	0.05 ± 0.74	0.58 ± 1.13	1.91 ± 1.56

D. Oblique Meridian: Superior Nasal-Inferior Temporal
	Superior Nasal Retina			Central	Inferior Temporal Retina
	−30°	−20°	−10°	0	+10°	+20°	+30°
Emmetropia
M	−0.10 ± 0.97	−0.18 ± 0.87	−0.39 ± 0.77	−0.32 ± 0.44	−0.46 ± 1.07	−0.50 ± 1.32	−0.09 ± 1.60
J 180	0.34 ± 0.40	0.21 ± 0.27	0.14 ± 0.23	0.06 ± 0.16	0.01 ± 0.24	0.03 ± 0.46	−0.17 ± 0.60
J 45	−0.26 ± 0.47	−0.06 ± 0.29	−0.01 ± 0.23	0.05 ± 0.13	0.00 ± 0.17	−0.05 ± 0.35	−0.11 ± 0.55
RPR	0.22 ± 0.93	0.14 ± 0.89	−0.07 ± 0.77	0.00	−0.14 ± 1.01	−0.19 ± 1.18	0.23 ± 1.34
Myopia
M	−3.95 ± 2.96	−4.89 ± 2.37	−5.40 ± 2.17	−5.76 ± 1.82	−5.81 ± 1.95	−5.03 ± 2.05	−3.94 ± 2.11
J 180	0.62 ± 0.51	0.43 ± 0.40	0.21 ± 0.29	0.17 ± 0.16	0.02 ± 0.28	−0.05 ± 0.51	−0.21 ± 0.69
J 45	−0.11 ± 0.69	0.00 ± 0.38	−0.03 ± 0.21	0.04 ± 0.14	0.02 ± 0.21	−0.01 ± 0.53	−0.14 ± 0.94
RPR	1.81 ± 1.78	0.87 ± 0.98	0.35 ± 0.73	0.00	−0.06 ± 0.73	0.72 ± 1.05	1.81 ± 1.24

 Data are results for mean sphere M, horizontal and oblique components of power vectors J ₀ and J ₄₅, respectively, and RPR (mean ± SD) at 25 retinal locations in all the horizontal, vertical, and oblique meridians for each of the two refractive groups: myopes (n = 31) and emmetropes (n = 18).

Figures 3 to 6 illustrate the refraction components M, J ₁₈₀, and J ₄₅ and the pure cylindrical power, respectively, in both refractive groups. Standard errors are illustrated by error bars; however, in most cases they are sufficiently small to be contained within the symbols of the vector components. Figure 7 exhibits the polar presentations of the overall power of refraction (P) by illustrating the values at 10°, 20° and 30° across the retina in eight locations in myopic individuals. 

Figure 3.

 

Variation in spherical equivalent (M) as a function of retinal eccentricity in the horizontal (a), vertical (b), and oblique (c, d) meridians in the myopic and emmetropic groups. Error bars, SE. Average M demonstrates a general trend toward a relative hyperopic shift, with increasing eccentricity in the myopic group, but a flat profile in the emmetropic group. Lines: second-order polynomial fitted to data. In some cases, error bars are contained within the symbols.

Figure 4.

 

Variation in astigmatic component J ₁₈₀ as a function of retinal eccentricity in the horizontal (a), vertical (b), and oblique (c, d) meridians for myopic and emmetropic groups. The error bars represent the SE of the data. In some cases, the error bars are contained within the symbols.

Figure 5.

 

Variation in astigmatic component J ₄₅ as a function of retinal eccentricity in the horizontal (a), vertical (b), and oblique (c, d) meridians in the myopic and emmetropic groups. Error bars, SE. In some cases, the error bars are contained within the symbols.

Figure 6.

 

Variation in cylindrical power as a function of retinal eccentricity in the horizontal (a), vertical (b), and oblique (c, d) meridians in the myopic and emmetropic groups. Error bars, SE. Lines: the second-order polynomial fitted to the data. In some cases, the error bars are contained within the symbols.

Figure 7.

 

A plot of the profile of overall power refractive error (P) in myopes across 10°, 20°, and 30° eccentricities in all eight measured regions of the retina: S, superior retina, SN, superior–nasal retina, N, nasal retina, IN, inferior–nasal retina; I, inferior retina; IT, inferior–temporal retina; T, temporal retina and ST, superior–temporal retina. Average of overall power refractive error (P) for central refraction, −5.76 D.

Mean Spherical Equivalent

Figure 3 shows the pattern of peripheral refraction for the four measured meridians in both refractive groups by plotting spherical equivalent refraction (M) as a function of eccentricity. 

In the myopic group, the M component (Fig. 3) on average, showed a relative hyperopic shift in all meridians. In the emmetropic group, the M component was relatively consistent across all meridians (Table 3). 

In the myopic group, a repeated-measures ANOVA revealed that the refractive error (M) varied as a function of eccentricity for all four measured meridians: horizontal (F = 30.75, P = 0.000), vertical (F = 19.37, P = 0.000), ST-IN (F = 23.18, P = 0.000), and SN-IT (F = 26.03, P = 0.000) with the superior–temporal portion of the retina as the most myopic and the inferior retina as the least myopic region of the retina. 

In the emmetropic group, one-way repeated-measures ANOVA showed that the range of peripheral refractive errors (M) was similar to that for the central retina, and there was relatively little change in refraction along the horizontal (F = 2.81, P = 0.058), ST-IN (F = 2.55, P = 0.071), and SN-IT (F = 0.69, P = 0.531) meridians (Fig. 3). The only exception was the vertical meridian, which demonstrated a significant difference in variation of peripheral refraction compared with the central refraction (F = 7.39, P = 0.000), with more myopia in the superior retina. This sharp elevation from superior to inferior retina at 30° eccentricity can be seen in Figure 3b. This group exhibited a steeper change along the vertical meridian than in other parts of the retina. 

Astigmatic Component J ₁₈₀

Figure 4 shows the variation in astigmatic component (J ₁₈₀) on the four measured meridians in both refractive groups. In the myopic group, one-way repeated-measures ANOVA showed that the degree of J ₁₈₀ varied significantly with eccentricity in the horizontal (F = 4.04, P = 0.017), vertical (F = 5.89, P = 0.003) and SN-IT (F = 15.84, P = 0.000) meridians. However, J ₁₈₀ did not change significantly with retinal eccentricity on the ST-IN meridian (F = 0.73 P = 0.543). 

For the emmetropes, one-way repeated-measures ANOVA revealed that the degree of J ₁₈₀ component varied significantly with retinal eccentricity on the horizontal (F = 3.64, P = 0.020), vertical (F = 6.48, P = 0.001) and SN-IT (F = 3.87, P = 0.018) meridians, but did not vary significantly on the ST-IN meridian (F = 1.75, P = 0.168). 

Astigmatic Component J ₄₅

Figure 5 illustrates the variation in astigmatic component (J ₄₅) on the four measured meridians in both refractive groups. In the myopic group, one-way repeated-measures ANOVA illustrated that the magnitude of J ₄₅ did not change significantly with eccentricity in the horizontal (F = 2.35, P = 0.090) and SN-IT (F = 0.62, P = 0.574) meridians. J ₄₅ however, was found to vary significantly with retinal eccentricity on both the vertical (F = 7.47, P = 0.000) and ST-IN (F = 6.46, P = 0.001) meridians. 

In the emmetropes, J ₄₅ did not change significantly with eccentricity on any of the four measured meridians (one-way repeated-measures ANOV): horizontal (F = 0.57, P = 0.578), vertical (F = 2.13, P = 0.136), SN-IT (F = 2.04, P = 0.118), and ST-IN (F = 2.1, P = 0.147). 

Astigmatism

The amount of conventional cylindrical power increased consistently with increasing retinal eccentricity on all meridians (Fig. 6). It should be borne in mind that the conventional cylinder is generally greater in magnitude than in either J ₁₈₀ or J ₄₅. For instance, the cylinder C is −1.00 D, with an axis at 180° corresponding to a J ₁₈₀ of +0.50 D. The presentation of the conventional cylindrical power was included because this parameter demonstrated the most rapid change in refraction. 

Discussion

Our study provided cross-sectional data on variations in refractive error measurements in young myopic and emmetropic adults in four meridians of the peripheral retina. Similar to previously published studies of the horizontal meridian, the myopic group illustrated a relative hyperopic shift (Fig. 8a) with increasing eccentricity (according to M component), confirming the prolate shape of the myopic eye. ^23,30,34 However, our results contradict those in some previous studies of the vertical meridian, which showed a relative myopic shift (Fig. 8b) with increasing eccentricity. ^23,30  

Figure 8.

 

Comparisons of the RPR in the horizontal (a) and vertical (b) meridians in myopes measured in this study with published data (sources are denoted in the legends). For comparison, all published data were converted into RPR. In particular, data were derived from the two refractive ranges from Atchison et al. ³⁰ that were closest to the range of myopia in those recruited in the present study.

The establishment of a conclusive shape profile of the vertical meridian from previous investigations (summarized in Table 1) is difficult. The number of participants used in these studies was either restricted (e.g., in Atchison et al., ³⁰ vertical profile was derived based on two subjects in the −5-D range and three subjects in the −6-D range) or, when the sample was large, the number of retinal eccentricities measured was limited (e.g., Berntsen et al. ⁴² and Schmid ³⁵ ). From Figure 8, it can be seen that our data revealed a general trend for greater relative hyperopia in the periphery, compared with results in other studies. The difference may be related to ethnicity, as our cohort comprised few East-Asian participants. In studies comprising a greater number of East-Asian participants, the global expansion observed in these eyes as myopia progresses ³⁶ may minimize relative hyperopic shifts in the peripheral refraction. 

Chen et al. ²³ investigated the profile of the peripheral refraction in horizontal (0°, 22°, 32°, and 40°) and vertical (22° and 32°) meridians, in a group of Chinese adults (n = 42) and children (n = 40). In the horizontal meridian, they found a relatively hyperopic peripheral refraction in myopes (low and moderate degrees) and a flat profile for emmetropes. However, in the vertical meridian, they demonstrated a myopic shift for emmetropes and low myopes and a flat profile for a range of moderate myopic eyes away from fixation. For the J ₁₈₀ component, increases in off-axis astigmatism (positive shift for vertical and negative shift in horizontal meridian) were reported in all groups. In addition, the J ₄₅ component was increased to a lesser degree in the periphery, with evidence of inferior–superior asymmetry. ²³  

Seidemann et al. ²⁹ measured peripheral refractive error out to 22° along several meridians of the retina with the photorefraction technique (a pupillometry-based instrument) in a group of 18 myopic adult eyes (average myopic refraction: −4.75 ± 1.90 D). Their results are different from those of other studies along the horizontal meridian, in that they found peripheral myopic shifts in all refractive groups. Although consistent with other studies, the reported shifts were less for the myopic group. In addition, they found relative peripheral myopia in the superior retina but relative peripheral hyperopia in the inferior retina. ²⁹  

Schmid ³⁵ measured refractions with the NVision K5001 autorefractor (Shin-Nippon) at fixation and up to 15° in the horizontal and vertical meridians. However, due to the large variability of data associated with the optic disc region, he did not include the nasal retina in his analysis. In children with low myopic (horizontal meridian: n = 17, vertical meridian: n = 10), he found a small myopic refractive shift in the temporal retina, but reported relative hyperopic shifts in vertical meridians. In contrast, emmetropic (n = 21) and hyperopic (n = 18) participants had relative myopic shifts along temporal, inferior and superior retina. 

In another study, Atchison et al. ³⁰ measured peripheral refraction out to 35° eccentricity in horizontal and vertical meridians of emmetropic and myopic subjects up to −12 D. Relative hyperopic and myopic shifts were reported in horizontal and vertical meridians, respectively, in the myopic group. Moreover, J ₁₈₀ was found to increase negatively in horizontal, and positively in vertical meridians, relative to the fovea. In addition, Atchison et al. showed that the differences in peripheral refraction between myopic and emmetropic eyes are small when measured along the vertical meridian out to 30° eccentricity compared with those measured along the horizontal meridian. Figure 8 illustrates the comparison of our results with those in previous peripheral refraction studies on the horizontal and vertical meridians. Our data illustrate a relative hyperopic shift which was similar for all measured meridians for the myopic group, and a relatively constant refractive profile for emmetropic eyes based on the mean spherical equivalents (M), despite the large individual variations in peripheral refraction (see Fig. 3, Table 3). Our results are in agreement with the relative hyperopic shift in the horizontal and vertical meridian, illustrated by theoretical modeling. ⁴¹ The results presented here also support the findings of magnetic resonance imaging studies, ^37,50 in that the shape of myopic eyes tends toward an ellipsoid, whereas the emmetropic eye tends toward a globe shape. Our finding of relative hyperopic shifts in the peripheral refraction in all meridians suggest the existence of a ocular growth cue across the entire peripheral retina in myopes (consistent with theories of hyperopic defocus driving myopia progression in primate experiments). ^2,51,52 In contrast, the disparity in ocular shape between horizontal and vertical meridians reported by a few studies would be expected to create mixed signals for ocular growth. 

This disparity for the vertical meridian compared to previous studies may relate to fixation target arrangement and autorefractor alignment. In terms of fixation target arrangement, we used a specially constructed target collimation system that is viewed via a beam splitter. The use of this device ensured a consistent visual stimulus arrangement, regardless of the meridian or field angle being examined. Other investigators (e.g., Atchison et al., ³⁰ ) have used fixation systems where the arrangements were different for the horizontal and vertical meridians (i.e., direct fixation on targets at 3.3 meters for the horizontal meridian, and indirect fixation of targets at 2 m via a beam splitter for the vertical meridian in the example cited). The difference in fixation arrangement has the potential to account for the disparity in vertical data presented in our study, perhaps mediated by different accommodation responses between horizontal and vertical fixation. Some studies incorporating peripheral refraction aimed for the pupil center as the reference point for measurements, ^30,31,53,54 but the majority fail to provide details of instrument alignment in relation to the optics of the eye. Alignment position is likely to be important for peripheral refraction measurements because the power of the refractive components of the eye varies with angle of incidence. ⁵⁵  

In our study the corneal reflex was the primary reference position for peripheral refraction measurements. We investigated the effect of instrument alignment on peripheral refraction and the results have been presented elsewhere. ⁴⁷ The optimum alignment position for peripheral refraction measurements was found to be half-way between the pupil center and corneal reflex. The corneal reflex fell well within the range of acceptable positions, but the pupil center lay close to the limits of acceptable alignment positions for larger eccentricities. Moreover, the position of the entrance pupil center can be less stable than that of the corneal reflex and may shift asymmetrically with pupil constriction after active accommodation or changes in light level. ^{56 –58} Although these changes are usually minor, they can be significant when measuring peripheral refraction. In addition, the pupil center seen on the instrument's monitor is a virtual image of the real pupil, as imaged by the cornea. 

In agreement with previous studies, we found that the average amount of astigmatism is not significantly different between groups. ³⁴ In addition, we demonstrated greater values of J ₁₈₀ and absolute astigmatism in the temporal retina compared to the nasal retina. ^14,29,30,59 Further, the variations between meridians have been observed, with relatively more myopia (M) in the superior compared with the inferior retina ^29,60 and the ST compared with the IN oblique meridians, particularly at higher eccentricities. Regional differences in scleral growth patterns have been suggested as a possible reason for the reported asymmetry in the shape of the myopic eye in some studies. ^29,36 In addition, the eye is not rotationally symmetric and the centers of curvature of the cornea and crystalline lens do not lie on a common axis. These decentrations and tilts are associated with angle α which is positioned approximately 5° horizontally and 2° vertically ⁶¹ and can contribute to asymmetries in peripheral refraction. ⁶² The dioptric variation in the J ₄₅ component across all measured meridians is small, with evidence of superior–inferior regional asymmetry (superior retina relatively less minus) consistent with the work of Chen et al. ²³ (range from −0.45 to 0.51 for myopes and −0.26 to 0.44 D for emmetropes). Differences in trends in J ₄₅ and J ₁₈₀ across the different regions of the retina may be due to high variability in peripheral refraction ³⁵ and ocular shape ³⁷ usually seen in human eyes. 

Common parameters to describe peripheral refraction are mean spherical equivalent (M) and astigmatic components (J ₁₈₀ and J ₄₅) as a function of retinal eccentricity. In this study we also calculated the fourth equation (P) discussed by Atchison et al., ²⁵ to evaluate the overall power of refraction. This value quantifies the total spherocylindrical image blur on the peripheral retina. A polar plot for the myopic group (Fig. 7) confirms that the overall power of refraction (and hence blur) decreases with increasing eccentricity. Converting our data to relative overall refraction shows an eccentricity-dependent profile consistent with that in the study by Shen et al., ⁶³ as illustrated in Figure 9. 

Figure 9.

 

Comparison with published data. ⁶³ of the overall power of refraction on the horizontal meridian measured in this study. For ease of comparison, overall powers of refraction (P) from the present study were converted into relative values.

We demonstrated a relative hyperopic shift in all measured meridians in our myopic group. Our findings are particularly relevant to the design of the ophthalmic lenses that manipulate peripheral refractive errors of human eyes with the goal of reducing myopia progression based on multiple axis analysis of peripheral refraction. ⁵⁴ An imprecise shift in peripheral refraction in myopic subjects may lead to inaccurate manipulation of the curvature of the image shell with these novel lenses. Further work is now needed to provide an evaluation of multiaxis globe shape variations between eyes of different ethnicities. ²² It may be the case that peripheral image shell modifications adopted by myopia control lenses need to be tailored to a given retinal surface profile. 

Ocular growth across the retina would be expected to depend on refractive variations across the entire retina, not just those across the horizontal and vertical meridians. Therefore, we propose that our findings may contribute to the understanding the development and progression of myopia. 

Footnotes

 Disclosure: A. Ehsaei, None; E.A.H. Mallen, None; C.M. Chisholm, None; I.E. Pacey, None