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Vincent, Stephen, Collins, Michael, Read, Scott, Carney, Leo, & Yap, Mau-rice(2011)Interocular symmetry in myopic anisometropia.Optometry and Vision Science, 88(12), pp. 1454-1462.
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https://doi.org/10.1097/OPX.0b013e318233ee5f
1
Interocular Symmetry in Myopic Anisometropia 1
Stephen J Vincent BAppSc(Optom)(Hons)1 2
Michael J Collins PhD FAAO1 3
Scott A Read PhD1 4
Leo G Carney DSc PhD FAAO1 5
Maurice KH Yap PhD FAAO2 6
1 Contact Lens and Visual Optics Laboratory, School of Optometry, Queensland University of 7
Technology, Brisbane, Queensland, Australia 8
2 Centre for Myopia Research, School of Optometry, The Hong Kong Polytechnic University, Kowloon, 9
Hong Kong, PR China 10
11
Corresponding author: 12
Stephen Vincent 13
Contact Lens and Visual Optics Laboratory 14
School of Optometry 15
Queensland University of Technology 16
Room B556, O Block, Victoria Park Road, Kelvin Grove 4059 17
Brisbane, Queensland, Australia 18
Phone: 617 3138 5732, Fax: 617 3138 5665 19
Email: sj.vincent@qut.edu.au 20
21
Number of Tables: 6 22
Number of Figures: 2 23
Word count: 5038 (excluding abstract, figures and references) 24
Date submitted: May 12, 2011 25
26
2
Abstract 27
Purpose: To investigate the interocular symmetry of optical, biometric and biomechanical 28
characteristics between the fellow eyes of myopic anisometropes. 29
Methods: Thirty-four young, healthy myopic anisometropic adults (≥ 1 D spherical equivalent 30
difference between eyes) without amblyopia or strabismus were recruited. A range of biometric and 31
optical parameters were measured in both eyes of each subject including; axial length, ocular 32
aberrations, intraocular pressure (IOP), corneal topography and biomechanics. Ocular sighting 33
dominance was also measured. 34
Results: Mean absolute spherical equivalent anisometropia was 1.70 ± 0.74 D and there was a strong 35
correlation between the degree of anisometropia and the interocular difference in axial length (r = 36
0.81, p < 0.001). The more and less myopic eyes displayed a high degree of interocular symmetry for 37
the majority of biometric, biomechanical and optical parameters measured. When the level of 38
anisometropia exceeded 1.75 D, the more myopic eye was more likely to be the dominant sighting 39
eye than for lower levels of anisometropia (p=0.002). Subjects with greater levels of anisometropia 40
(> 1.75 D) also showed high levels of correlation between the dominant and non-dominant eyes in 41
their biometric, biomechanical and optical characteristics. 42
Conclusions: Although significantly different in axial length, anisometropic eyes display a high degree 43
of interocular symmetry for a range of anterior eye biometrics and optical parameters. For higher 44
levels of anisometropia, the more myopic eye tends to be the dominant sighting eye. 45
Key words: myopia, anisometropia, aberrations, biomechanics, dominance 46
47
3
Introduction 48
While there is evidence to suggest that both genetics and the environment may influence the 49
development of myopia, there is no theory which adequately explains the mechanisms underlying all 50
aspects refractive error development, including the refractive condition of anisometropia. Two 51
commonly proposed hypotheses include those where optical factors (such as ocular aberrations) or 52
mechanical factors (such as intraocular pressure (IOP) or forces generated during convergence) 53
promote excessive axial eye growth. 54
Anisometropia is a condition characterised by a difference in refractive error between fellow eyes 55
typically due to an interocular difference in axial lengths, in particular the depth of the vitreous 56
chamber.1 Hyperopic anisometropia that persists during early childhood is often associated with 57
amblyopia and strabismus due to the disruption of normal visual development.2 However, in myopic 58
anisometropia, in which the more myopic eye may still receive a clear image during close viewing, 59
amblyopia and strabismus are less likely to develop.3 Anisometropia may be used as an 60
experimental paradigm in refractive error research. Comparing the more and less ametropic eyes of 61
the same anisometropic subject allows for greater control of potential confounding inter-subject 62
variables (e.g. age, gender and ethnicity) compared to traditional cohort studies (i.e. myopes 63
compared to emmetropes). 64
If mechanical factors are primarily involved in anisometropic eye growth, then we would expect 65
differences in the biomechanical properties between the fellow eyes (such as corneal thickness, 66
corneal hysteresis, or IOP). Previous studies have confirmed that corneal thickness is similar 67
between the fellow eyes of anisometropes4, however recent studies have shown that the more 68
myopic eyes of anisometropes have slightly lower values of corneal hysteresis, suggesting a 69
reduction in mechanical strength of the cornea.4,5 70
4
Cross-sectional studies of IOP in anisometropic subjects using both contact and non-contact 71
applanation techniques have not consistently shown significant differences between the more and 72
less ametropic eyes.6-9 These studies suggest that axial elongation due to a simple IOP induced 73
expansion of the globe is unlikely to be involved in the development of myopia or axial 74
anisometropia. If a relationship does exist between IOP and axial elongation, we might expect that 75
IOP would be higher in the more myopic eye of anisometropes, at least during myopia development 76
or progression. 77
If optical factors contribute to asymmetric eye growth, we might anticipate differences in the optical 78
properties of the two eyes. However, previous studies have found only small or non-significant 79
differences between the fellow eyes of anisometropes for corneal10-12 or crystalline lens power.1 80
Tian et al13 investigated the interocular symmetry of ocular aberrations in myopic anisometropes (> 81
1.00 D spherical equivalent {SEq}) and found no significant interocular differences in individual 82
Zernike terms, 3rd, 4th and 5th order aberrations or total higher order aberrations. Kwan et al11 also 83
examined the interocular symmetry and magnitude of total ocular aberrations in myopic 84
anisometropia (> 2.00 D SEq) and observed a strong correlation between fellow eyes for a range of 85
Zernike terms but significantly higher levels of aberrations in the less myopic eyes (total, 3rd order 86
and 4th order root mean square (RMS) values and spherical aberration). 87
Previous biometric studies have examined the interocular symmetry of moderate to high degrees of 88
anisometropia including subjects with amblyopia, structural ocular abnormalities or pathology. In 89
this study we have examined the interocular differences in the fellow eyes of axial myopic 90
anisometropes without amblyopia, strabismus or ocular disease for a comprehensive range of 91
biometric, biomechanical and optical parameters. While we cannot rule out the influence of genetic 92
or environmental factors in the development of myopic anisometropia, we have compared the two 93
eyes within the one subject in an attempt to minimise the inter-subject variations in such variables. 94
5
Our hypothesis was that the more and less myopic eyes may exhibit biometric or optical differences 95
which may provide insight into the mechanism underlying asymmetric refractive error development. 96
Methods 97
Subjects and screening 98
Thirty-four young, healthy adult subjects aged between 18 and 34 years (mean age 24 ± 4 years) 99
with a minimum of 1.00 D of spherical-equivalent myopic anisometropia were recruited for the 100
study (mean anisometropia 1.70 ± 0.74 D). Subjects were recruited from the staff and students of 101
QUT (Queensland University of Technology, Brisbane, Australia) (n = 10) and HKPU (Hong Kong 102
Polytechnic University, Hong Kong, PR China) (n = 24). Twenty-two of the 34 subjects were female 103
and 31 of the subjects were of East Asian descent, with the remaining three subjects of Caucasian 104
ethnicity. 105
Before testing, subjects underwent a screening examination to determine subjective refraction, 106
binocular vision and ocular health status. Ocular sighting dominance was assessed using a forced 107
choice method (a modification of the hole-in-the-card test).14 Stereopsis was assessed using a 108
random dot stereo test (TNO test). All subjects were free of ocular or systemic disease and had no 109
history of ocular surgery or trauma. Subjects with best corrected visual acuity worse than 0.10 110
logMAR, strabismus, unequal visual acuities (interocular difference of greater than 0.10 logMAR) or a 111
history of rigid contact lens wear were excluded from the study. Fourteen soft contact lens wearers 112
were included in the study, but ceased contact lens wear for 36 hours prior to participation. 113
Approval from both the QUT and HKPU human research ethics committees was obtained before 114
commencement of the study and subjects gave written informed consent to participate. All subjects 115
were treated in accordance with the tenets of the Declaration of Helsinki. 116
Data collection procedures 117
A range of biometric and optical measurements were collected from the more and less myopic eye 118
of each subject including axial length, corneal topography, corneal biomechanics, intraocular 119
6
pressure and ocular aberrations. Measurements were taken using the same instruments 120
(manufacturer and model) at both sites and by the same author (SJV) to minimise the potential for 121
inter-operator variability. In order to standardise measurement conditions at both sites, the lighting 122
conditions were kept at mesopic levels for all ocular measures. 123
Axial length 124
Axial length (defined as the distance from the anterior corneal surface to the retinal pigment 125
epithelium) was measured using the IOLMaster (Carl Zeiss Meditec, Inc., Jena; Germany). The 126
IOLMaster is a non-contact instrument based on the principle of partial coherence laser 127
interferometry and has been found to provide precise, repeatable measurements of axial length in 128
children15 and adults.16,17 Five measures of axial length with a signal-to-noise ratio of greater than 129
2.0 were taken and averaged for each eye. 130
Corneal topography 131
Corneal topography was measured using the E300 videokeratoscope (Medmont Pty. Ltd., Victoria, 132
Australia) based on the Placido disc principle. Four measurements, captured according to the 133
manufacturers recommendations, were performed on each eye. 134
Ocular biomechanics 135
Corneal biomechanics and intraocular pressure were measured using the Ocular Response Analyzer 136
(ORA; Reichert Ophthalmic Instruments, Buffalo, New York, USA). The ORA is a non-contact 137
tonometer that uses an air impulse to take two pressure measurements; one while the cornea is 138
moving inward, and the other as the cornea returns. The average of these two pressure values 139
provides a Goldmann-correlated IOP measurement (IOPg). The difference between these two 140
pressure values is corneal hysteresis (CH), or the viscoelasticity of the corneal tissue.18 The CH 141
measurement also allows the calculation of the corneal-compensated intraocular pressure (IOPcc), 142
which is less affected by corneal properties than other methods of applanation tonometry.19 The 143
7
ORA also provides a measure of the corneal resistance to deformation, the corneal resistance factor 144
(CRF). Four measurements were performed and the mean for each parameter was calculated for 145
each eye. 146
Ocular aberrations 147
Total ocular monochromatic aberrations of each eye were measured using a Complete Ophthalmic 148
Analysis System (COAS) wavefront aberrometer (Wavefront Sciences, New Mexico, USA). The 149
system was modified to allow fixation of an illuminated external target at 6 metres via a beam 150
splitter between the eye and the wavefront sensor. The subject’s distance prescription was inserted 151
into a lens holder outside of the path of the COAS beam (after taking into account the change in 152
vertex distance) to allow a clear view of the fixation target. The eye not being measured was 153
occluded. Subjects had natural pupil sizes without pharmacological dilation during COAS 154
measurements. Room illumination was kept in the mesopic range to maximize the pupil size and to 155
optimise the visibility of the externally illuminated distance target during measurements. One 156
hundred wavefront measurements (4 x 25 frames) were taken for each eye and later averaged. 157
Data analysis 158
Corneal topography 159
Following data collection, corneal refractive power and height data were exported from the 160
videokeratoscope. Topography maps that displayed poor focus or local irregularities such as tear 161
film instability were excluded from analysis. Topography data were analysed using customised 162
software. Refractive power maps and corneal height data were averaged using an established 163
technique20 assuming a corneal refractive index of 1.376. This analysis was conducted for right and 164
left eye data, taking into account midline symmetry (enantiomorphism). 165
A best-fit sphero-cylinder was calculated from each subject’s mean refractive power maps.21 The 166
sphero-cylindrical analysis was calculated around the line of sight. Corneal height data were used to 167
8
calculate the corneal wavefront error using a ray tracing procedure described by Buehren et al22. 168
Zernike wavefront polynomials were fitted to the wavefront error (up to and including the eighth 169
radial order) and expressed using the double index notation (Optical Society of America [OSA] 170
convention).23 The image plane was at the circle of least confusion and the wavelength used was 171
555 nm. The wavefront was centred on the line of sight by using the pupil offset value from the 172
pupil detection function in the Medmont videokeratoscope as the reference axis for the wavefront. 173
This procedure was conducted for 4 measurements per eye and the mean and standard deviations 174
were calculated. Corneal diameters of 4 and 6 mm were chosen for analysis purposes to 175
approximate mean pupil sizes in photopic and mesopic conditions respectively. Simulated 176
keratometry readings and corneal asphericity values (Q) were recorded for the principal corneal 177
meridians. The Medmont E300 software calculates the best fit ellipse at a specified chord (6 mm 178
diameter was chosen). 179
Ocular aberrations 180
Wavefront data from the COAS was fitted with an 8th order Zernike expansion and exported for 181
further analysis. Using customised software, the 100 wavefront measurements were rescaled to set 182
pupil diameters of 4 and 6 mm using the method of Schwiegerling24 and then the coefficients of the 183
Zernike polynomials were averaged. Pupil size scaling was only conducted to convert from a larger 184
natural pupil size to a smaller pupil which has been shown to be associated with only very small 185
errors that are not expected to be optically significant.24 This analysis was conducted for right and 186
left eye data, taking into account enantiomorphism. Although corneal and ocular wavefronts were 187
fit with 8th order Zernike expansions, given that the predominant higher order aberrations are 3rd 188
and 4th order terms25,26 we limited our analysis up to and including the 4th order. 189
190
9
Statistical analysis 191
Two tailed paired t-tests were used to assess the statistical significance of the mean interocular 192
difference between the more and less myopic eyes of the anisometropic subjects. Pearson’s 193
correlation coefficient was used to quantify the degree and statistical significance of the interocular 194
symmetry between the more and less myopic eyes. Pearson’s correlation coefficient was also used 195
to examine the relationship between the magnitude of refractive anisometropia and the interocular 196
difference of a range of parameters. Interocular differences were calculated by subtracting the less 197
myopic eye from the more myopic eye. Chi-square tests of independence were used to examine the 198
distribution of proportions. 199
Results 200
The mean components of subjective refraction, best corrected visual acuity and axial length of the 201
anisometropic subjects are presented in Table 1. The subjects’ mean spherical equivalent refraction 202
was -5.35 2.74 D for the more myopic eye and -3.64 ± 2.61 D for the less myopic eye. There were 203
statistically significant differences between the more and less myopic eyes for the spherical 204
component and spherical equivalent of the refractive error, however, the magnitude of refractive 205
astigmatism (cylinder) was similar between the two eyes. Mean best corrected visual acuity was not 206
significantly different between fellow eyes. The magnitude of spherical equivalent anisometropia 207
was significantly correlated with the interocular difference in axial length between fellow eyes (r = -208
0.81, p < 0.001) (Figure 1). However, given the r-value of -0.81, it appeared that for some subjects 209
there were various factors contributing to the magnitude of anisometropia other than an interocular 210
difference in axial length (e.g. interocular asymmetry in corneal or lenticular power, crystalline lens 211
position or retinal thickness). 212
213
10
Ocular biomechanics 214
Valid measurements were obtained for 31 subjects with the ORA. Measurements for 3 subjects 215
were excluded due to poor fixation or eyelash interference during data collection. A high degree of 216
symmetry was observed between the fellow eyes for all measures of intraocular pressure and 217
corneal biomechanics (Table 2). There were no statistically significant correlations between the 218
degree of anisometropia and the interocular difference in IOPg (r = 0.12), IOPcc (r = 0.19), CRF (r = -219
0.16) and CH (r = -0.16) (p > 0.05 for all parameters). 220
Corneal topography 221
Various measures of corneal shape were captured using the Medmont E300 videokeratoscope. One 222
subject was excluded from the Medmont data analysis due to substantial missing data due to 223
eyelash interference and reduced palpebral aperture size. The group mean and standard deviations 224
for the more and less myopic eyes are displayed in Table 3. 225
There was a high degree of interocular symmetry for all the corneal parameters that were measured 226
(p < 0.0001). The magnitude of refractive astigmatism was not statistically different between the 227
more and less myopic eyes (Table 1). However, examination of the principal corneal meridians 228
revealed that on average, the more myopic eyes were slightly more powerful in both the flattest and 229
steepest corneal meridians compared to fellow eyes. The average interocular difference between 230
the more and less myopic eyes was 0.19 ± 0.57 D for the steepest meridian (p = 0.06), 0.15 ± 0.37 D 231
for the flattest meridian (p = 0.03) and 0.17 ± 0.32 D for the average of the two principal meridians 232
(p < 0.01). 233
Average corneal asphericity (Q) values were slightly more prolate (greater peripheral flattening) in 234
the more myopic eyes in both the steepest and flattest meridians. This interocular difference was 235
statistically significant for the mean Q value (average of the steepest and flattest meridians) and for 236
the steepest corneal meridian, with Q values of -0.14 ± 0.09 in the less myopic eyes and -0.19 ± 0.12 237
in the more myopic eyes (p = 0.001). 238
11
The group mean and standard deviations for corneal sphero-cylinder and refractive power vectors M 239
(spherical corneal power), J0 (90/180 astigmatic power) and J45 (45/135 oblique astigmatic power) 240
in the more and less myopic eyes are displayed in Table 4. The more myopic eyes had a significantly 241
higher M for both 4 and 6 mm corneal diameters, however, the mean astigmatic vectors and 242
spherocylinder were not significantly different between fellow eyes. 243
Corneal aberrations 244
There was a high degree of interocular symmetry for corneal higher order aberrations up to the 245
fourth order, in particular within the 6 mm analysis diameter. On average, there were no 246
statistically significant differences between the more and less myopic eyes for third, fourth and 247
higher order RMS values (Table 5). There were few significant correlations between the interocular 248
difference in corneal aberrations for individual Zernike coefficients up to the fourth order and the 249
degree of spherical equivalent anisometropia. The strongest correlations for the interocular 250
difference in Zernike coefficients and the magnitude of anisometropia were observed for fourth 251
order terms Z (4,-2) secondary astigmatism (r = -0.35, p = 0.06) and Z (4,-4) tetrafoil along 22.5˚ (r = 252
0.41, p = 0.03) over a 4 mm corneal diameter. These correlations were relatively weak for the 4 mm 253
diameter and were weaker for the 6 mm analysis diameter. Correlation analysis for spherical 254
aberration revealed no significant relationship between the interocular difference in Zernike 255
coefficient Z (4,0) and the degree of anisometropia. 256
Total ocular monochromatic aberrations 257
Due to inter-subject variation in natural pupil sizes during data collection, some subjects were 258
excluded from analysis when examining aberrations over larger pupil diameters. Here we present 259
data for 31 subjects over a 4 mm pupil diameter and 19 subjects for a 6 mm diameter. A high degree 260
of interocular symmetry of Zernike coefficients was observed between the more and less myopic 261
eyes over both pupil sizes. There were no statistically significant differences between mean 262
individual Zernike coefficients for the more and less myopic eyes. The less myopic eyes had slightly 263
12
greater mean RMS values of 3rd, 4th and total higher order aberrations compared to more myopic 264
eyes. However, these interocular differences were small in magnitude and did not reach statistical 265
significance (Table 5). There were no significant correlations between the interocular difference in 266
individual Zernike coefficients up to the 4th order and the magnitude of anisometropia. 267
Sighting ocular dominance 268
The more myopic eye was the sighting dominant eye in 22 subjects (65%). However, when the level 269
of anisometropia exceeded 1.75 D (the approximate mean level of anisometropia in the subject 270
group), the more myopic eye was the dominant eye in 90% of subjects (Table 6, Figure 2). Although 271
two thirds of the subjects had anisometropia ≤ 1.75 D, there was a statistically significant difference 272
in the proportion of more myopic dominant eyes between the “low” (≤ 1.75 D) and “high” (> 1.75 D) 273
anisometropia groups (p = 0.002, Table 6). The more myopic eye was always the dominant sighting 274
eye when the level of anisometropia exceeded 2.25 D. 275
Stereoacuity was not significantly different between subjects with less myopic dominant 276
eyes (55 ± 64 seconds of arc) or more myopic dominant eyes (57 ± 31 seconds of arc) (p > 277
0.05, unpaired t-test). Although the “high anisometropia” group displayed a slightly 278
reduced level of mean stereopsis (72 ± 67 seconds of arc) compared to the “low 279
anisometropia” group (49 ± 28 seconds of arc), this difference did not reach statistical 280
significance (p > 0.05, unpaired t-test). 281
Given the significantly higher proportion of more myopic dominant sighting eyes in the subjects with 282
greater levels of anisometropia, we examined the interocular symmetry between the dominant and 283
non-dominant eyes of both the low and high anisometropia groups. On average, dominant eyes 284
were more myopic with greater axial lengths (-4.86 ± 2.80 D, 25.42 ± 1.01 mm) compared with non-285
dominant eyes (-4.13 ± 2.77 D, 25.15 ± 0.90 mm) (all p < 0.05). This was most evident for the high 286
13
anisometropia group (dominant: -6.70 ± 2.77 D and 25.69 ± 0.57 mm, non-dominant: -4.56 ± 3.51 D 287
and 24.93 ± 0.71 mm) (p < 0.0001). 288
There was no difference in best corrected visual acuity between the dominant and non-dominant 289
eyes. There were also no statistically significant differences between the dominant and non-290
dominant eyes for all measures of IOP, corneal biomechanics and corneal refractive power vectors 291
M, J0 and J45. Non-dominant eyes had larger RMS values for third, fourth and higher order corneal 292
and total aberrations compared to the dominant eyes in each anisometropic cohort examined, 293
however, these interocular differences were not statistically significant. 294
Discussion 295
This study provides a comprehensive examination of the optical and biomechanical properties of 296
anisometropic eyes not associated with pathology, amblyopia or strabismus. Since this was a cross 297
sectional study and not longitudinal, we cannot be certain if differences between the eyes represent 298
a possible cause or consequence of myopic eye growth. We observed a high degree of interocular 299
symmetry in myopic anisometropia. Aside from the interocular difference in axial length, there were 300
few significant differences between the more and less ametropic fellow eyes for a range of ocular 301
parameters. But interestingly, for higher levels of anisometropia (> 1.75 D), the more myopic eye 302
was typically the ocular sighting dominant eye. 303
A high degree of symmetry exists between fellow eyes for corneal power in both isometropic eyes 304
measured with slit scanning topography27 and anisometropic eyes measured with 305
keratometry.28,11,12,10 Although there is significant variability in corneal power in emmetropia and 306
myopia29, several studies have shown greater corneal power30-32 and a less prolate corneal shape33 307
(less peripheral flattening) in myopes compared to emmetropes. In our population of 308
anisometropes, our corneal measures with videokeratoscopy revealed, small interocular differences 309
between the flat and steep corneal meridians of fellow eyes. Also, the mean refractive corneal 310
14
power (average of the steep and flat corneal meridians) was significantly greater (steeper) in the 311
more myopic eyes. To our knowledge, this has not been observed in previous biometric studies of 312
anisometropic subjects. The more myopic eyes also exhibited more prolate corneas (flattening more 313
rapidly in the periphery), which is in contrast to previous studies which have shown that corneas 314
tend to become less prolate with increasing levels of myopia.34,35 315
We observed a high degree of interocular symmetry for measures of corneal resistance and 316
hysteresis. Hysteresis is positively correlated with central corneal thickness and is reduced in 317
conditions associated with corneal thinning such as advanced keratoconus, Fuch’s endothelial 318
dystrophy and the post LASIK cornea.18 Xu et al4 observed a small but statistically significant 319
reduction in corneal hysteresis in the more myopic eye compared to the fellow eye in a study of high 320
myopic anisometropia. A stretched or weakened sclera may be related to these lower values of 321
corneal hysteresis in high myopia. We found no such relationship in our cohort of anisometropes, 322
possibly due to the difference in the magnitude of anisometropia in our subjects (mean 1.70 D) 323
compared to those of Xu et al4 (mean 10.82 D). 324
The measurement of intraocular pressure may be influenced by variables such as age, blood 325
pressure, gender, corneal thickness and curvature and diurnal variation. We minimised the potential 326
confounding effect of such variables by comparing the fellow eyes in anisometropia and measured 327
IOP using a technique less influenced by corneal characteristics in comparison to applanation 328
tonometry. Our findings were similar to those of previous studies examining IOP in 329
anisometropia.6,36,8,9 We found no significant differences in IOP between the more and less 330
ametropic eyes and no correlation between the interocular difference in IOP and the magnitude of 331
anisometropia. Our results do not support a simple mechanical model of increased IOP leading to 332
axial elongation and myopia. However, it is possible that anisometropia may develop through an 333
IOP dependant mechanical mechanism with symmetrical IOPs, if there are interocular differences in 334
scleral biomechanics as suggested by Lee and Edwards8. The findings from our study and previous 335
15
studies of IOP in anisometropia are cross sectional in nature, which leaves open the possibility that 336
short term (e.g. diurnal variations) or longer term fluctuations in IOP may vary with anisometropia. 337
Plech et al37 observed that corneal higher order aberrations were similar between the fellow eyes of 338
anisometropes with amblyopia, however, to our knowledge our study is the first to report the 339
interocular symmetry of corneal aberrations in anisometropic eyes without amblyopia or strabismus. 340
We observed a high degree of interocular symmetry for corneal aberrations, which suggests that the 341
optical quality of the cornea is similar between the two eyes of myopic anisometropes. These 342
findings are in agreement with previous studies of between eye symmetry of corneal aberrations in 343
isometropic populations.38,26 344
A high degree of interocular symmetry also exists for total ocular monochromatic aberrations in 345
various isometropic populations.39-41,25 We observed a high level of symmetry between the fellow 346
eyes of anisometropes for Zernike coefficients up to the fourth order and higher levels of 3rd, 4th and 347
total RMS errors in the less myopic eyes. Although these differences were small and statistically 348
insignificant, our findings are in agreement with Kwan et al11 who also noted significant interocular 349
symmetry of higher order aberrations in anisometropes (> 2.00 D SEq) and significantly higher levels 350
of third order and total higher order aberrations in less myopic eyes. Tian et al13 also investigated 351
the interocular symmetry of ocular aberrations in ten myopic anisometropes (> 1.00 D SEq) similar to 352
the cohort in our study and found no significant interocular differences in individual Zernike terms, 353
3rd order, 4th order and 5th order aberrations or total higher order aberrations. 354
Since biometric or optical parameters may vary with ethnicity, we conducted an additional analysis 355
after removal of the three Caucasian subjects. Analysis of the 31 Asian subjects alone revealed 356
similar trends and p-values for the analysis of axial length, visual acuity, corneal topography, corneal 357
biomechanics, IOP and corneal and total ocular aberrations. 358
Our findings suggest that a high degree of symmetry exists between the fellow eyes of myopic 359
anisometropes for both corneal and total ocular higher order aberrations. Given the cross sectional 360
16
nature of our study we are unable to conclude whether higher order aberrations contribute to the 361
development of myopic anisometropia (i.e. an interocular difference in higher order aberrations 362
which diminishes over time may influence asymmetric axial elongation during refractive 363
development). Additionally, we cannot rule out the possibility that higher order aberrations during 364
accommodation (due to an unequal accommodative response between the eyes), following near 365
work (due to asymmetric palpebral aperture morphology and unequal eyelid pressure during 366
downward gaze) or that the sign of the aberrations may play a role in asymmetric refractive 367
development. 368
Along with the extensive range of optical and anterior biometric properties that we 369
examined in this study, there are many other factors that may be associated with the 370
development of anisometropia that we did not measure. Animal models have shown that 371
peripheral optics may play a role in the regulation of eye growth and refractive error 372
development.42,43 Asymmetries in retinal contour have also been reported between the two 373
eyes of myopic anisometropes.12 There is increasing evidence that orthokeratology, which 374
focuses light centrally at the fovea but induces peripheral myopic blur, slows the rate of axial 375
elongation during myopia development.44-46 Our current study limited its measurements to on 376
axis refraction, aberrations, biomechanics and biometrics, however given the potential role 377
of peripheral optics in refractive error development, investigations of peripheral refraction, 378
aberrations and biometrics in anisometropia are areas worthy of future study. 379
Developmental abnormalities of the optic nerve may also be associated with anisometropia. 380
Lempert47 reported that in hyperopic anisometropes, with or without amblyopia, the optic disc of 381
the more hyperopic eye is significantly smaller or underdeveloped in comparison to the fellow eye. 382
In myopes, asymmetric optic nerve head morphology due to unilateral tilted disc syndrome may 383
potentially result in differences in astigmatism between the fellow eyes and could initiate axial 384
17
anisometropic development. Examination of optic disc parameters in myopic anisometropes may 385
therefore provide more information regarding asymmetric axial elongation. 386
We observed that as the degree of anisometropia increased, the sighting dominant eye was more 387
often the more myopic of the two eyes. When anisometropia exceeded 1.75 D (n = 10), the more 388
myopic eye was the dominant eye in 90% of subjects. When greater than 2.25 D, the more myopic 389
eye was always the dominant eye. Our findings are in agreement with those of Cheng et al48 who 390
examined ocular dominance in 55 adults with spherical equivalent anisometropia ranging from 0.5 - 391
5.5 D and reported a threshold level of anisometropia (1.75 D), beyond which the more myopic eye 392
was always the dominant sighting eye. The authors hypothesised that during or following sustained 393
near work, the dominant eye may have a greater lag of accommodation in comparison to the non-394
dominant eye, resulting in greater axial elongation in the dominant eye. 395
Ibi49 examined the accommodative response in the dominant and non-dominant eyes of young 396
isometropic subjects and observed that the dominant eye showed a slight myopic shift at both 397
distance and near fixation following accommodation. The author speculated that the static tonus of 398
the ciliary muscle is increased in the dominant eye, which may explain why the dominant eye is 399
often the more myopic eye in non-amblyopic anisometropia. However, if the dominant eye shows a 400
slight lead of accommodation following near work, this myopic defocus would slow eye growth, 401
based on the theory of retinal image mediated eye growth. 402
In anisometropic amblyopia, the dominant sighting eye is typically the eye with better visual acuity, 403
although there may be exceptions in some cases with intermittent strabismus.50 We found no 404
significant difference in best corrected visual acuity between dominant and non-dominant eyes 405
(mean inter-eye difference ≤ 0.01 logMAR), or when dividing our subjects into low and high 406
anisometropia cohorts. We also examined the interocular difference between dominant and non-407
dominant eyes for a range of optical and biometric parameters including corneal power vectors M, 408
J0 and J45 and corneal and total higher order aberration RMS values, in order to provide insights 409
18
into the potential role of optical and mechanical factors in the interaction between ocular 410
dominance and anisometropia. Whilst some trends were observed for differences in ocular optics 411
(e.g. higher levels of 3rd, 4th and higher order corneal RMS values in non-dominant eyes) between the 412
dominant and non-dominant eyes, limited differences of statistical significance were found. These 413
data do not point to an obvious underlying optical or biomechanical reason for the more myopic eye 414
typically being the dominant eye for higher levels of anisometropia, however we cannot rule out the 415
possible role of optical or mechanical differences that are present at the time when ocular 416
dominance develops. The role of ocular dominance in refractive error development (or the role of 417
refractive error in relation to the determination of ocular dominance) remains unclear. Although 418
Yang et al51 reported no significant effect of ocular dominance on myopia development, given the 419
findings of this study and those of Cheng et al48, the association between myopic anisometropia and 420
ocular dominance requires further investigation. A more precise technique of measuring sensory 421
ocular dominance, described by Li et al52 may provide a clearer insight into this association. 422
Aside from an interocular difference in axial length, due to asymmetry in the posterior vitreous 423
chamber, anisometropic eyes display a high degree of interocular symmetry for a range of biometric 424
and optical characteristics. While we did not observe significant interocular differences for several 425
of the parameters investigated in this population of anisometropes (mean SEq anisometropia 1.70 426
D), this does not rule out the possibility that interocular differences may exist in more severe cases 427
of anisometropia (with or without amblyopia). 428
Unlike previous anisometropia studies, we observed that the more ametropic eye had a significantly 429
steeper mean corneal power in comparison to the fellow eye. There is a threshold level of 430
anisometropia, above which the more myopic eye is typically the dominant sighting eye. The 431
findings from our study do not support a single mechanical (IOP expansion) or retinal image 432
mediated (corneal or total monochromatic aberrations) mechanism in the unaccommodated eye in 433
the development of myopic anisometropia, however, longitudinal studies of myopes during (not 434
19
after) the development of anisometropia may shed more light on the possible role of mechanical 435
and optical factors. 436
Acknowledgements 437
The authors thank Po Wah Ng for his assistance with subject recruitment. Some aspects of this 438
study were presented at the American Academy of Optometry meeting, San Francisco, November 439
17-20, 2010. 440
441
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References 442
1. Sorsby A, Leary G, Richards M. The optical components in anisometropia. Vision Res 1962; 2(1-443 4):43-51. 444 445 2. Abrahamsson M, Sjostrand J. Natural history of infantile anisometropia. Br J Ophthalmol 1996; 446 80(10):860-3. 447 448 3. Tanlamai T, Goss DA. Prevalence of monocular amblyopia among anisometropes. Am J Optom 449 Physiol Opt 1979; 56(11):704-15. 450 451 4. Xu S, Xu A, Tao A, Wang J, Fan F, Lu F. Corneal biomechanical properties and intraocular pressure 452 in high myopic anisometropia. Eye Contact Lens 2010; 36(4):204-9. 453 454 5. Chang P, Chang S, Wang J. Assessment of corneal biomechanical properties and intraocular 455 pressure with Ocular Response Analyzer in childhood myopia. Br J Ophthalmol 2009; 94:877-81. 456 457 6. Bonomi L, Mecca E, Massa F. Intraocular pressure in myopic anisometropia. Int Ophthalmol 1982; 458 5(3):145-8. 459 460 7. Lam AK, Chan ST, Chan B, Chan H. The effect of axial length on ocular blood flow assessment in 461 anisometropes. Ophthalmic Physiol Optics 2003; 23(4):315-20. 462 463 8. Lee SM, Edwards MH. Intraocular pressure in anisometropic children. Optom Vis Sci 2000; 464 77(12):675-9. 465 466 9. Tomlinson A, Phillips CI. Unequal axial length of eyeball and ocular tension. Acta Ophthalmol 467 (Copenh) 1972; 50(6):872-6. 468 469 10. Weiss AH. Unilateral high myopia: optical components, associated factors, and visual outcomes. 470 Br J Ophthalmol 2003; 87(8):1025-31. 471 472 11. Kwan WCK, Yip SP, Yap MK. Monochromatic aberrations of the human eye and myopia. Clin Exp 473 Optom 2009; 92(3):304-12. 474 475 12. Logan NS, Gilmartin B, Wildsoet CF, Dunne MC. Posterior retinal contour in adult human 476 anisomyopia. Invest Ophthalmol Vis Sci 2004; 45(7):2152-62. 477 478 13. Tian Y, Tarrant J, Wildsoet C. Optical and biometric bases of anisomyopia. Invest Ophthalmol Vis 479 Sci 2006; 47:E-Abstract 3677. 480 481 14. Miles WR. Ocular dominance demonstrated by unconscious sighting. J Exp Psychol 1929; 12:113-482 26. 483 484 15. Carkeet A, Saw SM, Gazzard G, Tang W, Tan DT. Repeatability of IOLMaster biometry in children. 485 Optom Vis Sci 2004; 81(11):829-34. 486 487 16. Lam AK, Chan R, Pang PC. The repeatability and accuracy of axial length and anterior chamber 488 depth measurements from the IOLMaster. Ophthalmic Physiol Opt 2001; 21(6):477-83. 489 490
21
17. Sheng H, Bottjer CA, Bullimore MA. Ocular component measurement using the Zeiss IOLMaster. 491 Optom Vis Sci 2004; 81(1):27-34. 492 493 18. Luce DA. Determining in vivo biomechanical properties of the cornea with an ocular response 494 analyzer. J Cataract Refract Surg 2005; 31(1):156-62. 495 496 19. Lam A, Chen D, Chiu R, Chui WS. Comparison of IOP measurements between ORA and GAT in 497 normal Chinese. Optom Vis Sci 2007; 84(9):909-14. 498 499 20. Buehren T, Collins MJ, Iskander DR, Davis B, Lingelbach B. The stability of corneal topography in 500 the post-blink interval. Cornea 2001; 20(8):826-33. 501 502 21. Maloney RK, Bogan SJ, Waring GO, 3rd. Determination of corneal image-forming properties from 503 corneal topography. Am J Ophthalmol 1993; 115(1):31-41. 504 505 22. Buehren T, Collins MJ, Carney L. Corneal aberrations and reading. Optom Vis Sci 2003; 80(2):159-506 66. 507 508 23. Thibos LN, Applegate RA, Schwiegerling JT, Webb R. Report from the VSIA taskforce on standards 509 for reporting optical aberrations of the eye. J Refract Surg 2000; 16(5):S654-5. 510 511 24. Schwiegerling J. Scaling Zernike expansion coefficients to different pupil sizes. J Opt Soc Am A 512 Opt Image Sci Vis 2002; 19(10):1937-45. 513 514 25. Porter J, Guirao A, Cox IG, Williams DR. Monochromatic aberrations of the human eye in a large 515 population. J Opt Soc Am A Opt Image Sci Vis 2001; 18(8):1793-803. 516 517 26. Wang L, Dai E, Koch DD, Nathoo A. Optical aberrations of the human anterior cornea. J Cataract 518 Refract Surg 2003; 29(8):1514-21. 519 520 27. Myrowitz EH, Kouzis AC, O'Brien TP. High interocular corneal symmetry in average simulated 521 keratometry, central corneal thickness, and posterior elevation. Optom Vis Sci 2005; 82(5):428-31. 522 523 28. Holden BA, Sweeney DF, Vannas A, Nilsson KT, Efron N. Effects of long-term extended contact 524 lens wear on the human cornea. Invest Ophthalmol Vis Sci 1985; 26(11):1489-501. 525 526 29. Sorsby A, Leary G, Richards M. Correlation ametropia and component ametropia. Vision Res 527 1962; 2(1-4):309-13. 528 529 30. Grosvenor T, Scott R. Comparison of refractive components in youth-onset and early adult-onset 530 myopia. Optom Vis Sci 1991; 68(3):204-9. 531 532 31. Scott R, Grosvenor T. Structural model for emmetropic and myopic eyes. Ophthalmic Physiol Opt 533 1993; 13(1):41-7. 534 535 32. Goss DA, Van Veen HG, Rainey BB, Feng B. Ocular components measured by keratometry, 536 phakometry, and ultrasonography in emmetropic and myopic optometry students. Optom Vis Sci 537 1997; 74(7):489-95. 538 539
22
33. Davis WR, Raasch TW, Mitchell GL, Mutti DO, Zadnik K. Corneal asphericity and apical curvature 540 in children: a cross-sectional and longitudinal evaluation. Invest Ophthalmol Vis Sci 2005; 541 46(6):1899-906. 542 543 34. Carney LG, Mainstone JC, Henderson BA. Corneal topography and myopia. A cross-sectional 544 study. Invest Ophthalmol Vis Sci 1997; 38(2):311-20. 545 546 35. Horner DG, Soni PS, Vyas N, Himebaugh NL. Longitudinal changes in corneal asphericity in 547 myopia. Optom Vis Sci 2000; 77(4):198-203. 548 549 36. Lam AK, Wong S, Lam CS, To CH. The effect of myopic axial elongation and posture on the 550 pulsatile ocular blood flow in young normal subjects. Optom Vis Sci 2002; 79(5):300-5. 551 552 37. Plech AR, Pinero DP, Laria C, Aleson A, Alio JL. Corneal higher-order aberrations in amblyopia. Eur 553 J Ophthalmol 2010; 20(1):12-20. 554 555 38. Lombardo M, Lombardo G, Serrao S. Interocular high-order corneal wavefront aberration 556 symmetry. J Opt Soc Am A Opt Image Sci Vis 2006; 23(4):777-87. 557 558 39. Castejon-Mochon JF, Lopez-Gil N, Benito A, Artal P. Ocular wave-front aberration statistics in a 559 normal young population. Vision Res 2002; 42(13):1611-7. 560 561 40. Liang J, Williams DR. Aberrations and retinal image quality of the normal human eye. J Opt Soc 562 Am A Opt Image Sci Vis 1997; 14(11):2873-83. 563 564 41. Marcos S, Burns SA. On the symmetry between eyes of wavefront aberration and cone 565 directionality. Vision Res 2000; 40(18):2437-47. 566 567 42. Smith EL, 3rd, Hung LF, Huang J. Relative peripheral hyperopic defocus alters central refractive 568 development in infant monkeys. Vision Res 2009; 49(19):2386-92. 569 570 43. Smith EL, 3rd, Kee CS, Ramamirtham R, Qiao-Grider Y, Hung LF. Peripheral vision can influence 571 eye growth and refractive development in infant monkeys. Invest Ophthalmol Vis Sci 2005; 572 46(11):3965-72. 573 574 44. Cheung SW, Cho P, Fan D. Asymmetrical increase in axial length in the two eyes of a monocular 575 orthokeratology patient. Optom Vis Sci 2004; 81(9):653-6. 576 577 45. Cho P, Cheung SW, Edwards M. The longitudinal orthokeratology research in children (LORIC) in 578 Hong Kong: a pilot study on refractive changes and myopic control. Curr Eye Res 2005; 30(1):71-80. 579 580 46. Walline JJ, Jones LA, Sinnott LT. Corneal reshaping and myopia progression. Br J Ophthalmol 581 2009; 93(9):1181-5. 582 583 47. Lempert P. The axial length/disc area ratio in anisometropic hyperopic amblyopia: a hypothesis 584 for decreased unilateral vision associated with hyperopic anisometropia. Ophthalmology 2004; 585 111(2):304-8. 586 587 48. Cheng CY, Yen MY, Lin HY, Hsia WW, Hsu WM. Association of ocular dominance and 588 anisometropic myopia. Invest Ophthalmol Vis Sci 2004; 45(8):2856-60. 589 590
23
49. Ibi K. Characteristics of dynamic accommodation responses: comparison between the dominant 591 and non-dominant eyes. Ophthalmic Physiol Opt 1997; 17(1):44-54. 592 593 50. Rutstein RP, Swanson MW. Atypical fixation preference with anisometropia. Optom Vis Sci 2007; 594 84(9):848-51. 595 596 51. Yang Z, Lan W, Liu W, Chen X, Nie H, Yu M, Ge J. Association of ocular dominance and myopia 597 development: a 2-year longitudinal study. Invest Ophthalmol Vis Sci 2008; 49(11):4779-83. 598 599 52. Li J, Lam CS, Yu M, Hess RF, Chan LY, Maehara G, Woo GC, Thompson B. Quantifying sensory eye 600 dominance in the normal visual system: a new technique and insights into variation across 601 traditional tests. Invest Ophthalmol Vis Sci 2010; 51(12):6875-81. 602
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Table 1. Overview of the subjective refraction, best corrected visual acuity and axial length for the
more myopic and less myopic eyes of the anisometropic population.
More myopic eyes Less myopic eyes Paired t-test
Parameter Mean ± SD Mean ± SD p
Subjective refraction sphere (D) -4.87 ± 2.59 -3.18 ± 2.49 < 0.0001
Subjective refraction cylinder (D) -0.95 ± 0.85 -0.96 ± 0.82 0.85
Spherical equivalent (D) -5.35 2.74 -3.64 ± 2.61 < 0.0001
Best corrected visual acuity (logMAR) -0.01 ± 0.04 0.00 ± 0.04 0.26
Axial length (mm) 25.57 ± 0.89 25.00 ± 0.95 < 0.0001
25
Table 2. Mean ocular biomechanical parameters of the more and less myopic eyes for the anisometropic
population.
Biomechanical parameter
More myopic eyes Less myopic eyes Paired t-test Interocular symmetry
Pearson’s correlation coefficient
(mmHg) Mean ± SD Mean ± SD p r
p
IOPg 15.60 ± 2.98 15.66 ± 2.86 0.83 0.87 < 0.0001
IOPcc 15.05 ± 2.20 15.15 ± 2.14 0.66 0.66 < 0.0001
CRF 11.25 ± 1.80 11.11 ± 1.60 0.52 0.76 < 0.0001
CH 11.35 ± 1.37 11.30 ± 1.41 0.68 0.68 < 0.0001
IOPg – Goldman-correlated IOP, IOPcc- Corneal compensated IOP, CRF – corneal resistance factor, CH –
corneal hysteresis.
26
Table 3. Mean corneal parameters of the more and less myopic eyes for the anisometropic population.
Corneal parameter
More myopic eyes Less myopic eyes Paired t-test Interocular symmetry
Pearson’s correlation coefficient
Mean ± SD Mean ± SD p r p
Flat K (D) 42.91 ± 1.30 42.77 ± 1.30 0.03 0.96 < 0.0001
Steep K (D) 44.52 ± 1.78 44.32 ± 1.69 0.06 0.95 < 0.0001
Mean K (D) 43.72 ± 1.51 43.55 ± 1.43 < 0.01 0.98 < 0.0001
Astigmatism (D) -1.61 ± 0.81 -1.55 ± 0.93 0.70 0.71 < 0.0001
Flat Q -0.46 ± 0.17 -0.44 ± 0.15 0.21 0.92 < 0.0001
Steep Q -0.19 ± 0.12 -0.14 ± 0.09 0.001 0.71 < 0.0001
Mean Q -0.32 ± 0.13 -0.29 ± 0.10 < 0.001 0.91 < 0.0001
K - Simulated keratometric corneal power, Q - corneal asphericity (for 6 mm chord).
27
Table 4. Mean corneal refractive power vectors and sphero-cylinder for the more and less myopic eyes
over 4 and 6 mm corneal diameters.
Corneal parameter (D) Corneal diameter (mm) More myopic eyes Less myopic eyes
M 4 49.21 ± 1.8 * 49.06 ± 1.78
6 49.60 ± 2.13 ** 49.43 ± 2.06
J0 4 0.87 ± 0.48 0.85 ± 0.53
6 0.94 ± 0.55 1.05 ± 0.56
J45 4 0.09 ± 0.24 0.06 ± 0.29
6 0.14 ± 0.14 0.12 ± 0.35
Sphero-cylinder 4 50.08/-1.75 x 3 49.91/-1.70 x 2
6 50.55/-1.90 x 4 50.49/-2.11 x 3
All values mean ± SD
Corneal sphero-cylinder data presented as (Sphere/Cylinder x Axis)
* p < 0.01, ** p < 0.001 Paired t test (more vs less myopic eyes)
28
Table 5. Mean 3rd, 4th and total higher order RMS values up to 8th order for corneal and total ocular monochromatic aberrations in the more and less myopic
eyes of the anisometropic population.
Corneal aberrations Total monochromatic aberrations
Analysis diameter
More myopic eyes Less myopic eyes Paired t-test More myopic eyes Less myopic eyes Paired t-test
RMS (mm) Mean ± SD Mean ± SD p Mean ± SD Mean ± SD p
3rd
order 4 0.106 ± 0.043 0.132 ± 0.109 0.14 0.045 ± 0.021 0.048 ± 0.028 0.56
6 0.379 ± 0.279 0.435 ± 0.586 0.24 0.105 ± 0.047 0.107 ± 0.055 0.93
4th
order 4 0.055 ± 0.016 0.076 ± 0.079 0.16 0.019 ± 0.009 0.020 ± 0.009 0.26
6 0.266 ± 0.164 0.359 ± 0.455 0.10 0.083 ± 0.038 0.087 ± 0.035 0.50
Total higher order
4 0.130 ± 0.040 0.171 ± 0.151 0.12 0.115 ± 0.048 0.121 ± 0.052 0.49
6 0.498 ± 0.370 0.636 ± 0.845 0.13 0.313 ± 0.085 0.328 ± 0.113 0.55
Corneal aberrations: 4 and 6 mm analysis n = 33.
Total monochromatic aberrations: 4 mm analysis n = 31, 6 mm analysis n = 19.
29
Table 6. Distribution of ocular sighting dominance in more and less myopic eyes in the anisometropic
population.
Spherical equivalent anisometropia
(Mean ± SD) (D)
Sighting dominant eye Χ2 test
More myopic Less myopic p
“Low” ≤ 1.75 (1.30 ± 0.26)
13 11
0.002 “High” > 1.75 (2.53 ± 0.74)
9 1
30
Figure Legends
Figure 1. Interocular axial length difference versus spherical equivalent anisometropia. *More
myopic eye minus the less myopic eye.
Figure 2. Distribution of sighting dominance in more and less myopic eyes as a function of spherical
equivalent anisometropia. When anisometropia exceeded 1.75 D (dashed line) the dominant eye
was the more myopic eye in 90% of subjects. When greater than 2.25 D anisometropia was present
(solid line), the dominant eye was always the more myopic eye.
31
Figure 1.
32
Figure 2.
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