August 2010
Volume 51, Issue 8
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Glaucoma  |   August 2010
Effect of Myopia on the Thickness of the Retinal Nerve Fiber Layer Measured by Cirrus HD Optical Coherence Tomography
Author Affiliations & Notes
  • Shin Hee Kang
    From the Department of Ophthalmology, Armed Forces Capital Hospital of Korea, Seongnam, Korea;
    the Department of Ophthalmology, College of Medicine, Hallym University, Gangwon-do, Korea;
  • Seung Woo Hong
    From the Department of Ophthalmology, Armed Forces Capital Hospital of Korea, Seongnam, Korea;
    the Department of Ophthalmology and Visual Science, College of Medicine, The Catholic University of Korea, Seoul, Korea;
  • Seong Kyu Im
    From the Department of Ophthalmology, Armed Forces Capital Hospital of Korea, Seongnam, Korea;
    the Department of Ophthalmology, Chonnam National University Medical School and Hospital, Gwangju, Korea; and
  • Sang Hyup Lee
    From the Department of Ophthalmology, Armed Forces Capital Hospital of Korea, Seongnam, Korea;
    the Department of Ophthalmology, Yonsei University College of Medicine, Seoul, Korea.
  • Myung Douk Ahn
    the Department of Ophthalmology and Visual Science, College of Medicine, The Catholic University of Korea, Seoul, Korea;
  • Corresponding author: Seung Woo Hong, Department of Ophthalmology, Seoul St. Mary's Hospital, Banpo-dong 505, Seocho-gu, Seoul, Korea 137-701; anne-h@hanmail.net
Investigative Ophthalmology & Visual Science August 2010, Vol.51, 4075-4083. doi:10.1167/iovs.09-4737
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      Shin Hee Kang, Seung Woo Hong, Seong Kyu Im, Sang Hyup Lee, Myung Douk Ahn; Effect of Myopia on the Thickness of the Retinal Nerve Fiber Layer Measured by Cirrus HD Optical Coherence Tomography. Invest. Ophthalmol. Vis. Sci. 2010;51(8):4075-4083. doi: 10.1167/iovs.09-4737.

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      © 2017 Association for Research in Vision and Ophthalmology.

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Abstract

Purpose.: To evaluate the effect of myopia on the peripapillary retinal nerve fiber layer (RNFL) thickness measured by Cirrus HD optical coherence tomography (OCT).

Methods.: Comprehensive ophthalmic examinations were performed, including measurement of visual acuity, refraction, and axial length on 269 subjects (age, 19–26 years) with no ophthalmic abnormality. Further, 200 × 200-cube optic disc scans of the subjects' eyes were obtained with Cirrus HD OCT. The RNFL thickness at 256 points of the RNFL thickness profile and the average RNFL thickness were recorded. The correlations between these values and the axial length and spherical equivalent (SE) of refractive errors were then analyzed by simple linear regression, before and after adjustment of the ocular magnification.

Results.: Before ocular magnification adjustment, the uncorrected average RNFL thickness decreased as the axial length increased and as the SE decreased. However, after the adjustment, the corrected average RNFL thickness exhibited no correlation with the spherical equivalent and a weak positive correlation with the axial length. Myopia also affected the RNFL thickness distribution. As the axial length increased and the spherical equivalent decreased, the thickness of the temporal peripapillary RNFL increased and that of the superior, superior nasal, inferior, and inferior nasal peripapillary RNFL decreased.

Conclusions.: The axial length affected the average RNFL thickness, and myopia affected the RNFL thickness distribution. High myopes are likely to exhibit different RNFL distribution patterns. Since ocular magnification significantly affects the RNFL measurement in such patients, it should be considered in diagnosing glaucoma.

Myopia is one of the most common ocular abnormalities reported worldwide, and its association with glaucoma is well recognized. The prevalence of myopia is high in patients with ocular hypertension, primary open-angle glaucoma, and normal-tension glaucoma. 14 The risk of developing glaucoma is two to three times higher in myopic individuals than in nonmyopic individuals, and this risk is independent of other risk factors for glaucoma. 4  
Currently, glaucoma is diagnosed by considering the appearance of the optic disc and retinal nerve fiber layer (RNFL) and by standard achromatic perimetry. 5 However, myopic individuals often have enlarged optic discs with a more oval configuration and larger areas of peripapillary atrophy. 6,7 Because of these features, glaucomatous changes cannot be easily interpreted in myopic discs, possibly leading to a misdiagnosis of glaucoma. 
In early glaucoma, structural change is known to precede functional damage. 8,9 The RNFL is a sensitive indicator for predicting early glaucomatous changes, 911 and the extent of RNFL damage correlates with the severity of functional deficit in the visual field (VF). 1214 Thus, RNFL assessment may be more valuable than optic disc assessment in the case of myopic subjects. The RNFL can now be quantitatively assessed by means of scanning laser polarimetry or optical coherence tomography (OCT). OCT is a modern imaging technique based on coherence interferometry, and newer versions of OCT based on spectral domain technology have been developed. These novel techniques offer higher axial resolution and scanning speed than do conventional time-domain techniques. Compared with Stratus OCT, Cirrus high-definition (HD) OCT (Carl Zeiss Meditec Inc., Dublin, CA), which is one of the newest versions of OCT, offers reduced test variability and provides detailed information. 15  
The relationship between the RNFL thickness and myopia has been extensively investigated. 1622 However, whether the RNFL thickness could vary with the refractive status of the eye remains unclear. It is therefore important to investigate whether there is any correlation between RNFL measurements and the axial length/refractive error in myopic patients, considering that the risk of developing glaucoma increases with the severity of myopia. 4,23 The purpose of this study was to determine the relationship between myopia and the RNFL thickness measured by Cirrus HD OCT. 
Materials and Methods
Subjects
From December 2008 to April 2009, soldiers stationed in the Gyeonggi province were invited to participate in the study as volunteer subjects. All subjects provided their informed consent and were given a copy of the consent form. The procedures used adhered to the tenets of the Declaration of Helsinki, and the protocol was approved by the Institutional Review Board of Armed Forces Capital Hospital. 
The following individuals were excluded: those whose best-corrected visual acuity (VA) was <20/20; those with a history of severe ocular trauma, intraocular or refractive surgery, or any ocular or neurologic disease that could have affected the optic nerve head or RNFL; those with any pathologic ocular condition that could cause a visual disturbance; those who exhibited a closed or occludable angle in a gonioscopic examination; those with glaucoma or an intraocular pressure (IOP) >21 mm Hg in either eye; those showing evidence of a reproducible VF defect (with significant SD at the <5% level or abnormal results in the glaucoma hemifield test) in either eye, as detected using the Humphrey VF analyzer (HFA II 750-4.1 2005, Carl Zeiss Meditec Inc., Dublin, CA); those whose VF test results were unreliable (>15% false positives or false negatives or >20% fixation losses); those who had definite incyclotorsion or excyclotorsion of the eye on fundus photographs; and those whose RNFL and disc photographs indicated an RNFL defect or a disc anomaly. 
Each subject underwent a comprehensive ophthalmic evaluation, including VA measurement, slit-lamp examination, IOP measurement with Goldmann applanation tonometry, gonioscopic examination with a Goldmann three-mirror lens, and dilated fundus examination. SITA 24-2 standard tests were performed for all subjects by using the Humphrey Visual Field Analyzer. Manifest refractions were recorded with an automatic refractometer (model R-F10; Canon Inc., Tokyo, Japan). The automatic refractometer was set to measure the refractive error in increments of 0.125 D, and the value was recorded after confirmation by a designated optometrist or ophthalmologist. Axial lengths were measured using an ocular biometer (IOL Master; Carl Zeiss Meditec Inc.). Photographs of the optic disc and RNFL were obtained using digital fundus cameras (CF-60UD; Canon, Inc.). The images were digitally recorded and then analyzed by a single author (SWH). 
OCT Technique
The eyes of the subjects who satisfied the study criteria after pharmacologic dilation of the pupil were scanned with the Cirrus HD OCT system (ver. 3.0.0.64). After the subject was seated and aligned properly, three 200 × 200-cube optic disc scans were obtained per eye. In cases in which an involuntary saccade within a 1.73-mm radius occurred during the scan or the signal strength was below 7, the scan was discarded and a new one was obtained. The scan that had the highest signal strength and was obtained with the least eye movement was selected. The RNFL thicknesses at the 256 points (0–255) of the RNFL thickness profile, the mean RNFL thicknesses in each clock hour, and the average RNFL thickness were recorded. Clock hour RNFL thickness was recorded based on right eye orientation. The superior clock hour was 12 o'clock, and the others were assigned accordingly in a clockwise manner in the right eye and counterclockwise in the left. 
Adjustment for Ocular Magnification
The relationship between the measurement obtained from the Cirrus HD OCT image and the actual size of fundus dimension can be expressed as t = p · q · s, 24 where t is the actual fundus dimension, s is the measurement obtained using OCT, p is the magnification factor for the camera of the imaging system, and q is the magnification factor for the eye. The ocular magnification factor q of the eye can be determined with the formula q = 0.01306 · (axial length − 1.82). 25 Further, p is a constant in a telecentric system, and the Cirrus HD OCT system has the same magnification factor as the Stratus OCT system. The p of the latter system is known to be 3.382. 26 Therefore, the actual size of the 1.73-mm scan circle in the optic disc cube scan can be calculated using this formula. According to the formula, we arrive at:    
Now, the actual RNFL thickness at a 1.73-mm radius can be estimated from the RNFL thickness observed at a 1.73-mm radius before magnification correction as follows: All the retinal nerve fibers emerging from the retinal ganglion cells exit the eyeball through the optic nerve head. Thus, if we draw a circle on the fundus containing the optic nerve head, all the retinal nerve fibers emerging outside the circle would cross it. If the same number of retinal nerve fibers cross the different-sized circles with the same center, the larger scan circle would have a lower average RNFL thickness. Because the same number of retinal nerve fibers would be dispersed over the longer circumference, however, the total cross-sectional areas of the retinal nerve fibers under the different circles would be equal, because the same number of retinal nerve fibers cross the circles. From this, presuming that the same number of retinal nerve fibers cross the 1.73-mm radius scan circle and magnified scan circle, we can calculate the actual average RNFL thickness under 1.73-mm radius scan circle by multiplying the observed average RNFL thickness on the optic disc cube by 3.3382 · 0.1306 · (axial length − 1.82):    By substituting   and   in the second formula, we get    
Statistical Methods
One eye of each subject was randomly selected for the analysis. Linear regression analysis was performed independently for the axial length and spherical equivalent (SE), considering the average RNFL thickness and every RNFL thickness at the 256 points of the RNFL profile as the dependent variables (SPSS ver. 13.0; SPSS Corp., Chicago, IL). P < 0.05 at the 5% significance level was used in all tests. 
Results
The study involved 269 eyes of 269 normal subjects (258 men and 11 women); 135 of these were right eyes. Every subject enrolled reported being ethnically pure Korean. The mean subject age was 21.30 ± 1.70 years (range, 19–26 years). The mean SE of refractive errors (SE) was −2.521 ± 2.297 D (range, +1.5 to −10.125 D), and the mean axial length was 24.801 ± 1.170 mm (range, 22.25–28.13 mm). Among the enrolled subjects, none had a large peripapillary chorioretinal atrophy crossing the scanning circle. Figure 1 shows the mean RNFL thickness profile for the entire study population. The 95% CIs for distributions of RNFL thicknesses and mean RNFL thickness profiles for each subgroup of patients, classified on the basis of the SE and axial length, are shown in Figure 2. Figure 3 shows the results of comparisons between RNFL thicknesses in subgroups at individual points. Except the nasal area (around point 128), the RNFL thicknesses of the subgroups showed significant differences in many areas. Notably, low to moderate and high myopia groups had significantly higher RNFL thickness around the temporal area than the hyperopia and emmetropia groups did, and the high myopia group had significantly lower peak RNLF thicknesses in both superior and inferior areas than the low to moderate myopia group and hyperopia and emmetropia group did (Fig. 3). Analyses on clock-hour bases are shown in Tables 1 and 2
Figure 1.
 
The mean RNFL thickness profile of the entire study population, measured by Cirrus HD OCT before ocular magnification adjustment (circle with a 1.73-mm radius centered on the optic disc) (TEMP, temporal; SUP, superior; NAS, nasal; INF, inferior; 95% CI of mean).
Figure 1.
 
The mean RNFL thickness profile of the entire study population, measured by Cirrus HD OCT before ocular magnification adjustment (circle with a 1.73-mm radius centered on the optic disc) (TEMP, temporal; SUP, superior; NAS, nasal; INF, inferior; 95% CI of mean).
Figure 2.
 
The 95% CIs for distributions of uncorrected RNFL thicknesses and mean RNFL thickness profiles in the subgroups of participants, classified on the basis of axial length and SE (gray area, 95% CI; solid line, mean; Confidence limits of the 95% distributions were calculated with mean ± 1.96 SD; INF, inferior; NAS, nasal; SUP, superior; TEMP, temporal). (A) SE more than −0.5 (n = 36). (B) SE between −6.0 and −0.5 D (n = 209). (C) SE ≤ −6.0 D (n = 24). (D) Axial length < 24.0 mm (n = 75). (E) Axial length between 24.0 and 26.0 mm (n = 152). (F) Axial length ≥ 26.0 mm (n = 42).
Figure 2.
 
The 95% CIs for distributions of uncorrected RNFL thicknesses and mean RNFL thickness profiles in the subgroups of participants, classified on the basis of axial length and SE (gray area, 95% CI; solid line, mean; Confidence limits of the 95% distributions were calculated with mean ± 1.96 SD; INF, inferior; NAS, nasal; SUP, superior; TEMP, temporal). (A) SE more than −0.5 (n = 36). (B) SE between −6.0 and −0.5 D (n = 209). (C) SE ≤ −6.0 D (n = 24). (D) Axial length < 24.0 mm (n = 75). (E) Axial length between 24.0 and 26.0 mm (n = 152). (F) Axial length ≥ 26.0 mm (n = 42).
Figure 3.
 
Comparison of the RNFL thicknesses of subgroups classified on the axial length and SE with each other before ocular magnification adjustment. (95% CI of mean RNFL thickness at corresponding point. The shaded area implies no statistical significance; P > 0.05, Student's t-test.) (A) SE > −0.5 (n = 36) and −6.0 D < SE ≤ −0.5 D (n = 209). (B) −6.0 D < SE ≤ −0.5 D (n = 209) and SE ≤ −6.0 D (n = 24). (C) SE > −0.5 (n = 36) and SE ≤ −6.0 D (n = 24). (D) Axial length < 24.0 mm (n = 75) and 24.0 ≤ axial length < 26.0 mm (n = 152). (E) 24.0 ≤ axial length < 26.0 mm (n = 152) and axial length ≥ 26.0 mm (n = 42). (F) Axial length < 24.0 mm (n = 75) and axial length ≥ 26.0 mm (n = 42).
Figure 3.
 
Comparison of the RNFL thicknesses of subgroups classified on the axial length and SE with each other before ocular magnification adjustment. (95% CI of mean RNFL thickness at corresponding point. The shaded area implies no statistical significance; P > 0.05, Student's t-test.) (A) SE > −0.5 (n = 36) and −6.0 D < SE ≤ −0.5 D (n = 209). (B) −6.0 D < SE ≤ −0.5 D (n = 209) and SE ≤ −6.0 D (n = 24). (C) SE > −0.5 (n = 36) and SE ≤ −6.0 D (n = 24). (D) Axial length < 24.0 mm (n = 75) and 24.0 ≤ axial length < 26.0 mm (n = 152). (E) 24.0 ≤ axial length < 26.0 mm (n = 152) and axial length ≥ 26.0 mm (n = 42). (F) Axial length < 24.0 mm (n = 75) and axial length ≥ 26.0 mm (n = 42).
Table 1.
 
Clock Hour RNFL Thickness Measurements of Subgroups Classified on SE
Table 1.
 
Clock Hour RNFL Thickness Measurements of Subgroups Classified on SE
Clock Hour SE > −0.5 D (Group 1, n = 36) −6 D < SE ≤ −0.5 D (Group 2, n = 209) SE ≤ −6 D (Group 3, n = 24) Groups 1 and 2 P Groups 2 and 3 P Groups 1 and 3 P
12 140.98 ± 23.61 (137.61 ± 23.89) 130.17 ± 24.76 (131.38 ± 24.05) 114.11 ± 20.11 (125.66 ± 19.97) 0.015 (0.164) 0.001 (0.203) 0.000 (0.043)
1 125.38 ± 18.23 (122.21 ± 18.34) 118.30 ± 22.59 (119.44 ± 22.15) 106.60 ± 16.33 (117.85 ± 19.11) 0.043 (0.421) 0.003 (0.707) 0.000 (0.383)
2 91.94 ± 17.92 (89.41 ± 16.60) 83.16 ± 15.82 (83.95 ± 15.38) 68.17 ± 9.51 (75.29 ± 10.70) 0.008 (0.072) 0.000 (0.001) 0.000 (0.000)
3 58.19 ± 17.92 (56.66 ± 9.93) 58.51 ± 10.17 (59.19 ± 10.61) 56.22 ± 11.61 (62.10 ± 13.21) 0.867 (0.168) 0.363 (0.307) 0.507 (0.093)
4 65.02 ± 12.31 (63.31 ± 11.94) 62.26 ± 11.68 (62.94 ± 11.84) 60.28 ± 8.83 (66.64 ± 10.53) 0.218 (0.860) 0.324 (0.118) 0.088 (0.262)
5 108.43 ± 23.83 (105.42 ± 22.04) 98.71 ± 19.30 (99.56 ± 18.38) 82.012 ± 16.27 (90.53 ± 18.30) 0.025 (0.139) 0.000 (0.030) 0.000 (0.006)
6 144.43 ± 26.97 (140.46 ± 24.62) 135.02 ± 25.05 (136.19 ± 23.78) 110.42 ± 23.62 (121.62 ± 25.07) 0.057 (0.339) 0.000 (0.011) 0.000 (0.006)
7 141.08 ± 20.05 (137.76 ± 21.81) 144.94 ± 21.67 (146.66 ± 22.94) 142.58 ± 25.91 (157.25 ± 28.01) 0.296 (0.029) 0.670 (0.086) 0.812 (0.006)
8 70.99 ± 12.02 (69.40 ± 13.38) 75.91 ± 16.60 (76.94 ± 17.89) 85.15 ± 18.35 (94.17 ± 21.34) 0.037 (0.004) 0.026 (0.001) 0.002 (0.000)
9 53.90 ± 7.55 (52.56 ± 7.58) 57.50 ± 8.24 (58.22 ± 9.19) 63.32 ± 9.80 (69.94 ± 11.03) 0.012 (0.000) 0.009 (0.000) 0.000 (0.000)
10 78.30 ± 11.57 (76.41 ± 12.09) 83.96 ± 14.89 (85.06 ± 16.33) 92.87 ± 19.94 (102.62 ± 22.60) 0.012 (0.000) 0.044 (0.001) 0.003 (0.000)
11 129.59 ± 21.73 (126.45 ± 22.37) 133.36 ± 22.57 (135.00 ± 23.85) 135.48 ± 18.29 (149.62 ± 20.84) 0.343 (0.041) 0.604 (0.003) 0.262 (0.000)
Table 2.
 
Clock Hour RNFL Thickness Measurements of Subgroups Classified on Axial Length
Table 2.
 
Clock Hour RNFL Thickness Measurements of Subgroups Classified on Axial Length
Clock Hour AxL < 24 mm (Group 1, n = 75) 24 ≤ AxL < 26 mm (Group 2, n = 152) AxL ≥ 26 mm (Group 3, n = 42) Group 1 and 2 P Group 2 and 3 P Group 1 and 3 P
12 139.29 ± 23.40 (133.61 ± 22.57) 130.37 ± 24.15 (132.58 ± 24.30) 113.26 ± 22.00 (125.01 ± 23.38) 0.008 (0.753) 0.000 (0.070) 0.000 (0.057)
1 126.73 ± 20.74 (121.60 ± 20.17) 115.95 ± 21.65 (117.88 ± 21.59) 111.13 ± 21.00 (122.69 ± 22.57) 0.000 (0.204) 0.195 (0.222) 0.000 (0.794)
2 90.43 ± 17.02 (86.68 ± 16.07) 82.20 ± 15.80 (83.56 ± 15.70) 72.61 ± 11.70 (80.22 ± 12.97) 0.001 (0.167) 0.000 (0.164) 0.000 (0.020)
3 58.56 ± 9.68 (56.15 ± 9.20) 58.54 ± 10.51 (59.58 ± 10.80) 56.71 ± 10.85 (62.70 ± 12.32) 0.990 (0.014) 0.332 (0.142) 0.360 (0.004)
4 63.80 ± 10.81 (61.18 ± 10.26) 62.07 ± 12.32 (63.11 ± 12.31) 61.43 ± 9.89 (67.91 ± 11.10) 0.280 (0.216) 0.729 (0.018) 0.233 (0.002)
5 110.17 ± 20.36 (105.67 ± 19.51) 96.29 ± 18.59 (97.85 ± 18.21) 85.78 ± 17.78 (94.74 ± 19.56) 0.000 (0.004) 0.001 (0.359) 0.000 (0.005)
6 148.49 ± 24.47 (142.46 ± 23.66) 132.88 ± 23.88 (135.05 ± 23.53) 112.71 ± 22.58 (124.45 ± 24.74) 0.000 (0.028) 0.000 (0.016) 0.000 (0.000)
7 140.98 ± 19.74 (135.22 ± 18.87) 146.24 ± 22.52 (148.84 ± 23.30) 142.67 ± 22.51 (157.63 ± 25.07) 0.073 (0.000) 0.365 (0.045) 0.685 (0.000)
8 70.14 ± 13.20 (67.26 ± 12.56) 77.36 ± 16.94 (78.82 ± 17.94) 82.04 ± 17.43 (90.79 ± 20.10) 0.001 (0.000) 0.126 (0.001) 0.000 (0.000)
9 55.21 ± 6.70 (52.97 ± 6.50) 58.00 ± 9.11 (59.07 ± 9.71) 59.98 ± 8.68 (66.39 ± 10.44) 0.010 (0.000) 0.200 (0.000) 0.003 (0.000)
10 79.09 ± 10.82 (75.88 ± 10.56) 85.02 ± 15.82 (86.58 ± 16.67) 89.07 ± 18.11 (98.56 ± 20.90) 0.001 (0.000) 0.192 (0.001) 0.002 (0.000)
11 128.13 ± 22.09 (123.01 ± 21.75) 135.75 ± 23.26 (138.17 ± 24.02) 132.01 ± 15.68 (145.98 ± 18.54) 0.017 (0.000) 0.225 (0.027) 0.273 (0.000)
Before adjustment for ocular magnification, the mean of average peripapillary RNFL thickness of the subjects was 98.247 ± 8.586 μm. Linear regression analysis revealed that the average peripapillary RNFL thickness correlated significantly with the SE and the axial length (P < 0.001). The regression coefficients determined by linear regression analysis for the SE and axial length were 1.310 μm/D and −2.205 μm/mm, respectively (R 2 = 0.123 and 0.089; P < 0.01, Fig. 4). 
Figure 4.
 
(A) Scatterplot showing the uncorrected RNFL thickness against the SE (P < 0.01, simple linear regression analysis). (B) Scatterplot showing the uncorrected average RNFL thickness against the axial length (P < 0.01, simple linear regression analysis).
Figure 4.
 
(A) Scatterplot showing the uncorrected RNFL thickness against the SE (P < 0.01, simple linear regression analysis). (B) Scatterplot showing the uncorrected average RNFL thickness against the axial length (P < 0.01, simple linear regression analysis).
Linear regression analysis revealed that the correlations between the RNFL thickness and the SE varied at the 256 points of the peripapillary RNFL profile. Around the 12- to 2- and 5- to 6-o'clock positions (on the right eye orientation, at points 48–73, 78–121, and 152–209), the RNFL thickness increased significantly and proportionately with the SE, and around the 8- to 10-o'clock position (on the right eye orientation, at points 0–42 and 216–255), the RNFL thickness decreased significantly and proportionately with an increase in the SE (P < 0.05). Around the 3- to 4-o'clock position (on the right eye orientation, at points 43–47, 74−77, 122–151, and 210–215), no statistically significant correlation was observed between the RNFL thickness and SE (P > 0.05). Figure 5 shows the results of the linear regression analysis for the correlation between the SE and the RNFL thickness at the 256 points in the RNFL profile. 
Figure 5.
 
(A, C) The correlation between the SE and the uncorrected RNFL thickness at each of 256 points of the peripapillary RNFL profile. (B, D) The correlation between the axial length and the uncorrected RNFL thickness at each of 256 points of the peripapillary RNFL profile. (A, B) Regression coefficient for each of 256 points of the peripapillary RNFL profile. (C, D) R 2 obtained with the regression formula. Shaded areas imply no statistical significance (P > 0.05, simple linear regression test). INF, inferior; NAS, nasal; SUP, superior; TEMP, temporal.
Figure 5.
 
(A, C) The correlation between the SE and the uncorrected RNFL thickness at each of 256 points of the peripapillary RNFL profile. (B, D) The correlation between the axial length and the uncorrected RNFL thickness at each of 256 points of the peripapillary RNFL profile. (A, B) Regression coefficient for each of 256 points of the peripapillary RNFL profile. (C, D) R 2 obtained with the regression formula. Shaded areas imply no statistical significance (P > 0.05, simple linear regression test). INF, inferior; NAS, nasal; SUP, superior; TEMP, temporal.
In contrast to the SE, the axial length correlated inversely with the RNFL thickness at the 256 points of the RNFL thickness profile. Around the 8- to 10-o'clock position (on the right eye orientation, at points 0–43 and 215–255), the RNFL thickness increased significantly and proportionately with the axial length, while around the 12- to 2- and 5- to 6-o'clock positions (on the right eye orientation, at points 49–73, 80–122, and 152–209), the RNFL thickness decreased significantly and proportionately with an increase in the axial length (P < 0.05). There was no statistically significant correlation between the RNFL thickness and axial length around the 3- to 4-o'clock position (on the right eye orientation, at points 44–48, 74–79, 123–151, and 210–214, P > 0.05). Figure 5 shows the results of the linear regression analysis for the correlation between the axial length and the RNFL thickness at the 256 points of the RNFL profile. 
After adjustment for ocular magnification, the corrected average RNFL thickness was 99.544 ± 8.686 μm. The mean ocular magnification factor (q) and overall magnification factor (p · q) were 0.3001 ± 0.0153 (range, 0.2668–0.3410) and 1.015 ± 0.05176 (range, 0.9024–1.1533), respectively. Table 3 shows the refractive error and ocular magnification factors recorded in the subjects. Linear regression analysis revealed no significant correlation between the corrected average RNFL thickness and the SE (P = 0.152), but a correlation (regression coefficient, +1.996 μm/mm) between the corrected average RNFL thickness and the axial length (P < 0.001, R 2 = 0.074, Fig. 6). 
Table 3.
 
Summary of Recorded Refractive Errors and Magnification Factors
Table 3.
 
Summary of Recorded Refractive Errors and Magnification Factors
Spherical Equivalent (D) Subjects (n) Axial Length Ocular Magnification Factor (q) Scan Circle Magnification in Cirrus HD OCT (p · q) (p = 3.382)
+1.5 to −0.49 36 23.894 ± 0.774 0.2883 ± 0.0101 0.9750 ± 0.0342
−0.5 to −1.99 105 24.165 ± 0.685 0.2918 ± 0.0089 0.9869 ± 0.0303
−2.0 to −3.99 68 25.040 ± 0.805 0.3033 ± 0.0105 1.0256 ± 0.0356
−4.0 to −5.99 36 25.764 ± 0.921 0.3127 ± 0.0120 1.0575 ± 0.0406
< −6.0 24 26.823 ± 0.852 0.3265 ± 0.0111 1.1043 ± 0.0376
Total 269
Figure 6.
 
(A) The corrected RNFL thickness against the SE (P > 0.05, multiple linear regression analysis). (B) The corrected average RNFL thickness against the axial length (P < 0.01, simple linear regression analysis).
Figure 6.
 
(A) The corrected RNFL thickness against the SE (P > 0.05, multiple linear regression analysis). (B) The corrected average RNFL thickness against the axial length (P < 0.01, simple linear regression analysis).
Discussion
The effect of myopia on the average RNFL thickness is debatable. Hoh et al. 18 reported no correlation between these parameters, whereas many other researchers reported that the average RNFL thickness decreased with myopia and with an increase in the axial length. In this study, before adjusting the ocular magnification, we found that the average RNFL thickness decreased with SE and an increase in the axial length, and the corresponding regression coefficients were 1.310 μm/D and −2.205 μm/mm; this finding was consistent with those of previous studies. 16,17,19,20 In contrast, after adjusting the ocular magnification, we found that the average RNFL thickness increased with the axial length (regression coefficient, +1.996 μm/mm; R 2 = 0.074; P < 0.001) and exhibited no correlation with the SE. The main difference between the methods used in the study by Hoh et al. 18 and those used in other studies was the adjustment for ocular magnification. Hoh et al. used OCT 1, which employs a Littman-based formula to avoid errors in ocular magnification. However, the other researchers used Stratus OCT without the adjustments that would have avoided these errors. In the present study, we found that the correlation between myopia and the average RNFL thickness varied after adjustment for ocular magnification. 
A limitation of the method used for ocular magnification adjustment in this study was that the total cross-sectional area of the RNFL under the circle in the magnified scan was not exactly equal to that of the RNFL under the actual circle with a radius of 1.73 mm. The total cross-sectional area of the RNFL under the outer circle was slightly smaller than that of the RNFL under the inner circle, because the axons of ganglion cells located between the two circles did not cross the outer circle. By referring to a previous study on the density topography of ganglion cells, 27 we calculated the number of ganglion cells by multiplying the cell density by the area between the two circles. From this, we could calculate the difference between the cross-sectional areas under the outer and inner circles. The difference was then converted into a percentage, and the mean percentage was found to be 0.3360% ± 0.3359% (range, 0.0014%–1.13%). This value represented the percentage of ganglion cell axons that did not cross the outer circle. Only a very small percentage of axons did not cross the outer circle; thus, the difference between the corrected and actual average RNFL thickness seemed to be minimal, consistent with the results of a previous study on the scanning radius and RNFL thickness. 28  
Previous studies have reported that with myopia and an increase in the axial length, the thickness of the superior and inferior peripapillary RNFLs decreases. 17,20,29 Choi et al. 20 and Leung et al. 17 reported a slight but statistically insignificant increase in the thickness of the temporal peripapillary RNFL in myopia. In our study, we found that the thickness of the temporal peripapillary RNFL increased with myopia and with an increase in the axial length. Previous studies did not include emmetropic and hyperopic eyes. Further, most previous studies used Stratus OCT, in which the measurement reproducibility for the superotemporal and inferotemporal areas is lower than in Cirrus HD OCT. 15 These factors may explain why results similar to ours were not obtained in previous studies. On comparing the mean RNFL profile between the subjects with high myopia and the other subjects, we found that the peak elevation points in the RNFL thickness profiles of the former subjects were more temporally deviated than those in the profiles of the latter subjects (Fig. 3). Recently, Lee and Shields 30 reported a high percentage of peak RNFL thicknesses with temporal deviation. Myopia, especially high myopia, may be associated with temporal deviations in the peak RNFL thickness, and this may cause temporal RNFL thickening in myopia. 
In this study, we found that the average RNFL thickness was significantly affected by the ocular magnification. Therefore, to accurately determine the correlation between the RNFL thickness distribution and myopia, we attempted to eliminate the errors caused by ocular magnification. Presuming that (1) the retinal nerve fibers were radially distributed in a completely uniform manner between the circle on the magnified scan and the actual circle with a 1.73-mm radius and (2) that the total cross-sectional area of the RNFL under the magnified arc equaled that of the RNFL under the arc of the actual circle, we could calculate the corrected RNFL thickness at each of the 256 points of the peripapillary RNFL thickness profile by magnifying the uncorrected RNFL thickness at these points by 3.3382 · 0.01306(axial length − 1.82). Figure 7 shows the correlation between myopia and the corrected RNFL thickness at the 256 points of the RNFL profile. The corrected RNFL thicknesses on a clock-hour basis are shown in parenthesis in Tables 1 and 2. A comparison of Figures 5 and 7 reveals that after ocular magnification adjustment, the thickness of the temporal and nasal peripapillary RNFLs increased with myopia, whereas that of the superior and inferior peripapillary RNFLs did not exhibit a strong correlation with myopia, as was observed before the adjustment. These findings suggest that myopic eyes have a thicker temporal RNFL rather than a thinner superior and inferior RNFL. However, as mentioned, the ganglion cell axons between the two circles did not cross the outer circle, and more important, the retinal nerve fibers were not distributed radially in a completely uniform manner. 31,32 Thus, these data can only be interpreted in the context of attempts to eliminate artifacts caused by ocular manifestations. 
Figure 7.
 
(A, C) The correlation between the SE and the corrected RNFL thickness at each of 256 points of the peripapillary RNFL profile. (B, D) The correlation between the axial length and the corrected RNFL thickness at each of 256 points of the peripapillary RNFL profile. (A, B) Regression coefficient for each of 256 points of the peripapillary RNFL profile. (C, D) R 2 obtained using the regression formula. Shaded areas show statistical significance (P > 0.05, simple linear regression analysis). INF, inferior; NAS, nasal; SUP, superior; TEMP, temporal.
Figure 7.
 
(A, C) The correlation between the SE and the corrected RNFL thickness at each of 256 points of the peripapillary RNFL profile. (B, D) The correlation between the axial length and the corrected RNFL thickness at each of 256 points of the peripapillary RNFL profile. (A, B) Regression coefficient for each of 256 points of the peripapillary RNFL profile. (C, D) R 2 obtained using the regression formula. Shaded areas show statistical significance (P > 0.05, simple linear regression analysis). INF, inferior; NAS, nasal; SUP, superior; TEMP, temporal.
Various methods have been introduced to estimate the ocular magnification factor on the basis of the degree of ametropia, keratometry, and/or the axial length. 24,25,33,34 We selected the modified axial length method derived from Bennet et al., 25 because this method is considered more accurate than other methods. 35 In this study, the corrected average RNFL thickness exhibited different correlations with the axial length and SE. A different conclusion would have been arrived at had the Cirrus OCT measurements not been corrected for ocular magnification (Figs. 4, 6). This finding can explain the previously reported relationship between myopia and the average RNFL thickness. 
In this study, the corrected average RNFL thickness showed weak positive correlation with axial length and no correlation with SE. This result suggests that in young individuals, myopic eyes do not have fewer ganglion cells than nonmyopic eyes. In many studies, investigators reported that myopic eyes had lower RNFL thickness than nonmyopic eyes, and these findings were thought to account for the glaucoma susceptibility of myopic eyes, whereas the findings of the present study suggest that the glaucoma susceptibility of myopic eyes has another explanation and needs further investigations. 
Because 200 × 200-cube optic disc scan of the Cirrus HD OCT is a volume scan designed to measure RNFL thickness over an area of 6 × 6 mm2 in the optic disc region, the actual RNFL thickness at 1.73 mm (magnification-corrected radius) can be directly calculated from the volume scan. However, the current versions of Cirrus HD OCT software do not include the scan circle adjustment function for ocular magnification. Thus, to obtain accurate average RNFL thickness within a circle of radius 1.73 mm, we calculated the corrected average RNFL thickness through the ocular magnification adjustment. This author (SWH) thinks that the above-mentioned method is useful, because it enables correct quantitative comparison of RNFL thickness with normative RNFL thickness and can also be applied to other OCT systems. However, it is not necessary to adjust ocular magnification for every patient. According to the results of this study, the mean scan circle magnification for the myopic eye of −4 D or less was >5% (Table 3). Hence, we recommend considering the effect of ocular magnification when myopia exceeds −4 D. Further, the RNFL thickness values shown in Tables 1 and 2 and Figure 2 can be used as normal values for myopic patients. The young nature of our subjects provides enhanced sensitivity. 
The myopic eyes showed thicker temporal RNFL and thinner superior and inferior RNFL. In addition, the RNFL thickness profile of highly myopic eyes showed temporal deviations in peak RNFL thickness. Many factors like misalignment of the scan circle can affect the RNFL thickness profile. 36 However, the Cirrus HD OCT used in this study has a short scan time and a line scanning ophthalmoscope that can detect eye movement during imaging. Owing to these properties, the Cirrus HD OCT is believed to be free of this artifact. Another possible source of the artifacts in myopic eyes is peripapillary chorioretinal atrophy. However, the subjects enrolled in this study did not have peripapillary chorioretinal atrophy beyond the scanning circle. Therefore, we believe that the findings of this study are free of these artifacts and speculate that the temporal deviation of the peak RNFL thickness and thicker temporal RNFL in the myopic eyes were caused by uneven expansion of the macular and nonmacular areas during development or growth of the eye. 
The present study is one of the most large-scale studies conducted to evaluate the correlation between myopia and the peripapillary RNFL thickness. It involved a group of young, healthy subjects of uniform age and ethnicity and was thus free of confounding factors. However, these features may limit the application of these data to subjects of other age groups or ethnicities. 
Among the myopic subjects in this study, measurement of the RNFL thickness was significantly affected by ocular magnification, and temporal deviation of the RNFL thickness profile was observed (especially in cases of high myopia). Thus, in myopic subjects, the measured RNFL thickness may be less than the actual thickness, and owing to the temporal shift in the contour of the RNFL thickness profile, the superior and inferior RNFLs may be deemed abnormal in comparison with the normative reference provided by Cirrus OCT. To avoid such misdiagnosis, a new normative RNFL profile for myopic patients is needed, to improve the sensitivity and specificity of glaucoma detection, and the properties of the RNFL thickness profile in cases of high myopia should be considered. 
Footnotes
 Supported by Grant 09002 from the Committee of Armed Forces Capital Hospital. Ophthalmology Alumni, Seoul. The sponsor or funding organization had no role in the design or conduct of this research.
Footnotes
 Disclosure: S.H. Kang, None; S.W. Hong, None; S.K. Im, None; S.H. Lee, None; M.D. Ahn, None
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Figure 1.
 
The mean RNFL thickness profile of the entire study population, measured by Cirrus HD OCT before ocular magnification adjustment (circle with a 1.73-mm radius centered on the optic disc) (TEMP, temporal; SUP, superior; NAS, nasal; INF, inferior; 95% CI of mean).
Figure 1.
 
The mean RNFL thickness profile of the entire study population, measured by Cirrus HD OCT before ocular magnification adjustment (circle with a 1.73-mm radius centered on the optic disc) (TEMP, temporal; SUP, superior; NAS, nasal; INF, inferior; 95% CI of mean).
Figure 2.
 
The 95% CIs for distributions of uncorrected RNFL thicknesses and mean RNFL thickness profiles in the subgroups of participants, classified on the basis of axial length and SE (gray area, 95% CI; solid line, mean; Confidence limits of the 95% distributions were calculated with mean ± 1.96 SD; INF, inferior; NAS, nasal; SUP, superior; TEMP, temporal). (A) SE more than −0.5 (n = 36). (B) SE between −6.0 and −0.5 D (n = 209). (C) SE ≤ −6.0 D (n = 24). (D) Axial length < 24.0 mm (n = 75). (E) Axial length between 24.0 and 26.0 mm (n = 152). (F) Axial length ≥ 26.0 mm (n = 42).
Figure 2.
 
The 95% CIs for distributions of uncorrected RNFL thicknesses and mean RNFL thickness profiles in the subgroups of participants, classified on the basis of axial length and SE (gray area, 95% CI; solid line, mean; Confidence limits of the 95% distributions were calculated with mean ± 1.96 SD; INF, inferior; NAS, nasal; SUP, superior; TEMP, temporal). (A) SE more than −0.5 (n = 36). (B) SE between −6.0 and −0.5 D (n = 209). (C) SE ≤ −6.0 D (n = 24). (D) Axial length < 24.0 mm (n = 75). (E) Axial length between 24.0 and 26.0 mm (n = 152). (F) Axial length ≥ 26.0 mm (n = 42).
Figure 3.
 
Comparison of the RNFL thicknesses of subgroups classified on the axial length and SE with each other before ocular magnification adjustment. (95% CI of mean RNFL thickness at corresponding point. The shaded area implies no statistical significance; P > 0.05, Student's t-test.) (A) SE > −0.5 (n = 36) and −6.0 D < SE ≤ −0.5 D (n = 209). (B) −6.0 D < SE ≤ −0.5 D (n = 209) and SE ≤ −6.0 D (n = 24). (C) SE > −0.5 (n = 36) and SE ≤ −6.0 D (n = 24). (D) Axial length < 24.0 mm (n = 75) and 24.0 ≤ axial length < 26.0 mm (n = 152). (E) 24.0 ≤ axial length < 26.0 mm (n = 152) and axial length ≥ 26.0 mm (n = 42). (F) Axial length < 24.0 mm (n = 75) and axial length ≥ 26.0 mm (n = 42).
Figure 3.
 
Comparison of the RNFL thicknesses of subgroups classified on the axial length and SE with each other before ocular magnification adjustment. (95% CI of mean RNFL thickness at corresponding point. The shaded area implies no statistical significance; P > 0.05, Student's t-test.) (A) SE > −0.5 (n = 36) and −6.0 D < SE ≤ −0.5 D (n = 209). (B) −6.0 D < SE ≤ −0.5 D (n = 209) and SE ≤ −6.0 D (n = 24). (C) SE > −0.5 (n = 36) and SE ≤ −6.0 D (n = 24). (D) Axial length < 24.0 mm (n = 75) and 24.0 ≤ axial length < 26.0 mm (n = 152). (E) 24.0 ≤ axial length < 26.0 mm (n = 152) and axial length ≥ 26.0 mm (n = 42). (F) Axial length < 24.0 mm (n = 75) and axial length ≥ 26.0 mm (n = 42).
Figure 4.
 
(A) Scatterplot showing the uncorrected RNFL thickness against the SE (P < 0.01, simple linear regression analysis). (B) Scatterplot showing the uncorrected average RNFL thickness against the axial length (P < 0.01, simple linear regression analysis).
Figure 4.
 
(A) Scatterplot showing the uncorrected RNFL thickness against the SE (P < 0.01, simple linear regression analysis). (B) Scatterplot showing the uncorrected average RNFL thickness against the axial length (P < 0.01, simple linear regression analysis).
Figure 5.
 
(A, C) The correlation between the SE and the uncorrected RNFL thickness at each of 256 points of the peripapillary RNFL profile. (B, D) The correlation between the axial length and the uncorrected RNFL thickness at each of 256 points of the peripapillary RNFL profile. (A, B) Regression coefficient for each of 256 points of the peripapillary RNFL profile. (C, D) R 2 obtained with the regression formula. Shaded areas imply no statistical significance (P > 0.05, simple linear regression test). INF, inferior; NAS, nasal; SUP, superior; TEMP, temporal.
Figure 5.
 
(A, C) The correlation between the SE and the uncorrected RNFL thickness at each of 256 points of the peripapillary RNFL profile. (B, D) The correlation between the axial length and the uncorrected RNFL thickness at each of 256 points of the peripapillary RNFL profile. (A, B) Regression coefficient for each of 256 points of the peripapillary RNFL profile. (C, D) R 2 obtained with the regression formula. Shaded areas imply no statistical significance (P > 0.05, simple linear regression test). INF, inferior; NAS, nasal; SUP, superior; TEMP, temporal.
Figure 6.
 
(A) The corrected RNFL thickness against the SE (P > 0.05, multiple linear regression analysis). (B) The corrected average RNFL thickness against the axial length (P < 0.01, simple linear regression analysis).
Figure 6.
 
(A) The corrected RNFL thickness against the SE (P > 0.05, multiple linear regression analysis). (B) The corrected average RNFL thickness against the axial length (P < 0.01, simple linear regression analysis).
Figure 7.
 
(A, C) The correlation between the SE and the corrected RNFL thickness at each of 256 points of the peripapillary RNFL profile. (B, D) The correlation between the axial length and the corrected RNFL thickness at each of 256 points of the peripapillary RNFL profile. (A, B) Regression coefficient for each of 256 points of the peripapillary RNFL profile. (C, D) R 2 obtained using the regression formula. Shaded areas show statistical significance (P > 0.05, simple linear regression analysis). INF, inferior; NAS, nasal; SUP, superior; TEMP, temporal.
Figure 7.
 
(A, C) The correlation between the SE and the corrected RNFL thickness at each of 256 points of the peripapillary RNFL profile. (B, D) The correlation between the axial length and the corrected RNFL thickness at each of 256 points of the peripapillary RNFL profile. (A, B) Regression coefficient for each of 256 points of the peripapillary RNFL profile. (C, D) R 2 obtained using the regression formula. Shaded areas show statistical significance (P > 0.05, simple linear regression analysis). INF, inferior; NAS, nasal; SUP, superior; TEMP, temporal.
Table 1.
 
Clock Hour RNFL Thickness Measurements of Subgroups Classified on SE
Table 1.
 
Clock Hour RNFL Thickness Measurements of Subgroups Classified on SE
Clock Hour SE > −0.5 D (Group 1, n = 36) −6 D < SE ≤ −0.5 D (Group 2, n = 209) SE ≤ −6 D (Group 3, n = 24) Groups 1 and 2 P Groups 2 and 3 P Groups 1 and 3 P
12 140.98 ± 23.61 (137.61 ± 23.89) 130.17 ± 24.76 (131.38 ± 24.05) 114.11 ± 20.11 (125.66 ± 19.97) 0.015 (0.164) 0.001 (0.203) 0.000 (0.043)
1 125.38 ± 18.23 (122.21 ± 18.34) 118.30 ± 22.59 (119.44 ± 22.15) 106.60 ± 16.33 (117.85 ± 19.11) 0.043 (0.421) 0.003 (0.707) 0.000 (0.383)
2 91.94 ± 17.92 (89.41 ± 16.60) 83.16 ± 15.82 (83.95 ± 15.38) 68.17 ± 9.51 (75.29 ± 10.70) 0.008 (0.072) 0.000 (0.001) 0.000 (0.000)
3 58.19 ± 17.92 (56.66 ± 9.93) 58.51 ± 10.17 (59.19 ± 10.61) 56.22 ± 11.61 (62.10 ± 13.21) 0.867 (0.168) 0.363 (0.307) 0.507 (0.093)
4 65.02 ± 12.31 (63.31 ± 11.94) 62.26 ± 11.68 (62.94 ± 11.84) 60.28 ± 8.83 (66.64 ± 10.53) 0.218 (0.860) 0.324 (0.118) 0.088 (0.262)
5 108.43 ± 23.83 (105.42 ± 22.04) 98.71 ± 19.30 (99.56 ± 18.38) 82.012 ± 16.27 (90.53 ± 18.30) 0.025 (0.139) 0.000 (0.030) 0.000 (0.006)
6 144.43 ± 26.97 (140.46 ± 24.62) 135.02 ± 25.05 (136.19 ± 23.78) 110.42 ± 23.62 (121.62 ± 25.07) 0.057 (0.339) 0.000 (0.011) 0.000 (0.006)
7 141.08 ± 20.05 (137.76 ± 21.81) 144.94 ± 21.67 (146.66 ± 22.94) 142.58 ± 25.91 (157.25 ± 28.01) 0.296 (0.029) 0.670 (0.086) 0.812 (0.006)
8 70.99 ± 12.02 (69.40 ± 13.38) 75.91 ± 16.60 (76.94 ± 17.89) 85.15 ± 18.35 (94.17 ± 21.34) 0.037 (0.004) 0.026 (0.001) 0.002 (0.000)
9 53.90 ± 7.55 (52.56 ± 7.58) 57.50 ± 8.24 (58.22 ± 9.19) 63.32 ± 9.80 (69.94 ± 11.03) 0.012 (0.000) 0.009 (0.000) 0.000 (0.000)
10 78.30 ± 11.57 (76.41 ± 12.09) 83.96 ± 14.89 (85.06 ± 16.33) 92.87 ± 19.94 (102.62 ± 22.60) 0.012 (0.000) 0.044 (0.001) 0.003 (0.000)
11 129.59 ± 21.73 (126.45 ± 22.37) 133.36 ± 22.57 (135.00 ± 23.85) 135.48 ± 18.29 (149.62 ± 20.84) 0.343 (0.041) 0.604 (0.003) 0.262 (0.000)
Table 2.
 
Clock Hour RNFL Thickness Measurements of Subgroups Classified on Axial Length
Table 2.
 
Clock Hour RNFL Thickness Measurements of Subgroups Classified on Axial Length
Clock Hour AxL < 24 mm (Group 1, n = 75) 24 ≤ AxL < 26 mm (Group 2, n = 152) AxL ≥ 26 mm (Group 3, n = 42) Group 1 and 2 P Group 2 and 3 P Group 1 and 3 P
12 139.29 ± 23.40 (133.61 ± 22.57) 130.37 ± 24.15 (132.58 ± 24.30) 113.26 ± 22.00 (125.01 ± 23.38) 0.008 (0.753) 0.000 (0.070) 0.000 (0.057)
1 126.73 ± 20.74 (121.60 ± 20.17) 115.95 ± 21.65 (117.88 ± 21.59) 111.13 ± 21.00 (122.69 ± 22.57) 0.000 (0.204) 0.195 (0.222) 0.000 (0.794)
2 90.43 ± 17.02 (86.68 ± 16.07) 82.20 ± 15.80 (83.56 ± 15.70) 72.61 ± 11.70 (80.22 ± 12.97) 0.001 (0.167) 0.000 (0.164) 0.000 (0.020)
3 58.56 ± 9.68 (56.15 ± 9.20) 58.54 ± 10.51 (59.58 ± 10.80) 56.71 ± 10.85 (62.70 ± 12.32) 0.990 (0.014) 0.332 (0.142) 0.360 (0.004)
4 63.80 ± 10.81 (61.18 ± 10.26) 62.07 ± 12.32 (63.11 ± 12.31) 61.43 ± 9.89 (67.91 ± 11.10) 0.280 (0.216) 0.729 (0.018) 0.233 (0.002)
5 110.17 ± 20.36 (105.67 ± 19.51) 96.29 ± 18.59 (97.85 ± 18.21) 85.78 ± 17.78 (94.74 ± 19.56) 0.000 (0.004) 0.001 (0.359) 0.000 (0.005)
6 148.49 ± 24.47 (142.46 ± 23.66) 132.88 ± 23.88 (135.05 ± 23.53) 112.71 ± 22.58 (124.45 ± 24.74) 0.000 (0.028) 0.000 (0.016) 0.000 (0.000)
7 140.98 ± 19.74 (135.22 ± 18.87) 146.24 ± 22.52 (148.84 ± 23.30) 142.67 ± 22.51 (157.63 ± 25.07) 0.073 (0.000) 0.365 (0.045) 0.685 (0.000)
8 70.14 ± 13.20 (67.26 ± 12.56) 77.36 ± 16.94 (78.82 ± 17.94) 82.04 ± 17.43 (90.79 ± 20.10) 0.001 (0.000) 0.126 (0.001) 0.000 (0.000)
9 55.21 ± 6.70 (52.97 ± 6.50) 58.00 ± 9.11 (59.07 ± 9.71) 59.98 ± 8.68 (66.39 ± 10.44) 0.010 (0.000) 0.200 (0.000) 0.003 (0.000)
10 79.09 ± 10.82 (75.88 ± 10.56) 85.02 ± 15.82 (86.58 ± 16.67) 89.07 ± 18.11 (98.56 ± 20.90) 0.001 (0.000) 0.192 (0.001) 0.002 (0.000)
11 128.13 ± 22.09 (123.01 ± 21.75) 135.75 ± 23.26 (138.17 ± 24.02) 132.01 ± 15.68 (145.98 ± 18.54) 0.017 (0.000) 0.225 (0.027) 0.273 (0.000)
Table 3.
 
Summary of Recorded Refractive Errors and Magnification Factors
Table 3.
 
Summary of Recorded Refractive Errors and Magnification Factors
Spherical Equivalent (D) Subjects (n) Axial Length Ocular Magnification Factor (q) Scan Circle Magnification in Cirrus HD OCT (p · q) (p = 3.382)
+1.5 to −0.49 36 23.894 ± 0.774 0.2883 ± 0.0101 0.9750 ± 0.0342
−0.5 to −1.99 105 24.165 ± 0.685 0.2918 ± 0.0089 0.9869 ± 0.0303
−2.0 to −3.99 68 25.040 ± 0.805 0.3033 ± 0.0105 1.0256 ± 0.0356
−4.0 to −5.99 36 25.764 ± 0.921 0.3127 ± 0.0120 1.0575 ± 0.0406
< −6.0 24 26.823 ± 0.852 0.3265 ± 0.0111 1.1043 ± 0.0376
Total 269
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