Effect of refractive status and axial length on retinal nerve fiber layer thickness using OCT

Aim of work To evaluate the effect of refractive status and axial length of the eye on retinal nerve fiber layer (RNFL) thickness. Method A t tal of 116 eyes 62 subjects were enroll d in the study. RNFL thickness was measured using TOPCON 3D OCT 2000. Results Hyperopic group, as compared to normal, did not show any significant thickening for average, superior, inferior, nasal, and temporal quadrants (p = 0.760 , p = 0.160, p = 0.139, p = 0.345, p = 0.762, respectively). Also myopic group, as compared to normal, did not sh w any significant thinning for average, superior, inferior, nasal, and temporal quadran (p = 0.991, p = 0.746, p = 0.848 p = 0.382, p = 0.894, respectively). The RNFL thickness decreased with increasi g axial length and this was statist cally signific t in average, superior, inferior, and nasal quadrants (p = 0.0009***, p = 0.0003***, p = 0.0031**, p = 0.0129**, respectively), except for temporal quadrant (p = 0.8118). Conclusion Neither hyperopia nor myopia affect retinal nerve fiber layer thickness significantly. The RNFL thickness decreased with increasing axial length and this was statistically significant in average, superior, inferior, and nasal quadrants except for temporal quadrant.


Introduction
Previous studies using time domain (TD) OCT have shown that RNFL thickness measurements could be influenced by refractive status and the axial length of the eye . These variables should be taken into account to make more clinically useful normative databases of peripapillary RNFL thickness.
Optical coherence tomography (OCT) has been shown to be valuable in the diagnosis and monitoring of retinal diseases and glaucoma (Wollstein et

Subjects and Methods
A total of 116 eyes of 62 female students and employee from King Saud University Female Campus, comprising 40 emmetropic eyes of 20 subjects (+0.5 to -0.5 DS), 36 hyperopic eyes of 22 subjects (+0.75 and above), 40 myopic eyes of 20 subjects (-0.75 and above) in the age group of 20 to 46 years were enrolled in this cross sectional study. Patients with astigmatism higher than

The retinal nerve fiber layer thickness results
There was no significant differences between the groups concerning age. In the hyperopic group, one patient had a visual acuity of 20/30 and the rest of patients in hyperopic group and all of the subjects in the myopic and emmetropic group had visual acuities of 20/20. The mean SE value in emmetropic, hyperopic, and myopic group were -0.30 ± 0.33, + 1.27 ± 1.02, and -3.05 ± 1.25 respectively. In all groups, mean astigmatism was less than 1.00 D. The Average RNFL thickness in normal group was 111.73±10.31microns. This was the reference thickness. In increasing myopia the RNFL thickness was seen to decrease and in increasing hypermetropia the RNFL thickness was increased ( Table 2).  Figure 1).
1.00 D, amblyopia, strabismus, glaucoma, retinal and optic disc anomalies or any systemic diseases with ocular complications were excluded.

All subjects underwent detailed clinical examination including the following:
1. Measurement of refractive error using Auto Refractometer.
7. Retinal nerve fiber layer analysis of both eyes (without dilatation) using 3D OCT-2000 Spectral Domain OCT.

Statistical analysis
Statistical analysis was performed using a commercially available statistical software package (SPSS for windows, version 22). Both eyes were taken for the statistical analysis. Analysis of variance (ANOVA) has been used to find the significance of study parameters between three or more groups of patients. Student t-test was used for normally distributed variables. Pearson correlation analysis according to the distribution type of the variables was used to analyze relationships between variables. P-value less than or equal to 0.05 was considered statistically significant.

Results
In this study 116 eyes of 62 individuals (40 emmetropic eyes of 20 subjects, 36 hyperopic eyes of 22 subjects, and 40 myopic eyes of 20 subjects) underwent evaluation of retinal nerve fiber layer thickness in the age group of 20-46 years using 3D Spectral Domain OCT. Demographic details of which are given in Table 1. Group II, as compared to normal, shows increase in average and all quadrants RNFL thickness, but this was insignificant for average, superior, inferior, nasal, and temporal quadrants (p = 0.760 , p = 0.160, p = 0.139, p = 0.345, p = 0.762, respectively).

Myopic group
there is no significant differences in the superior and nasal quadrant of myopic group. He also documented a significant increase in the RNFL thickness in the inferior and temporal quadrant, but there is no significant differences in the superior or nasal RNFL thickness of hyperopic group.
Veysi Oner et al. (2013), using stratus OCT documented a significant reduction in the RNFL thickness in the superior, inferior, temporal, and nasal quadrant of myopic group. He also documented a significant increase in the RNFL thickness in the nasal quadrant and no significant differences in the superior, inferior, or temporal RNFL thickness of hyperopic group.

Conclusion
Neither hyperopia nor myopia affect retinal nerve fiber layer thickness significantly. In the hyperopic group, mean RNFL thickness and RNFL thickness in superior, inferior, and nasal quadrants had weak positive correlations with spherical equivalent, but temporal quadrant did not show any significant correlation with spherical equivalent. In the myopic group, mean RNFL thickness and RNFL thickness in superior, inferior, and nasal quadrants did not show any significant correlation with spherical equivalent, but temporal quadrant had a weak negative correlation with spherical equivalent.
The RNFL thickness decreased with increasing axial length and this was statistically significant in average, superior, inferior, and nasal quadrants except for temporal quadrant. The mean RNFL thickness and RNFL thickness in superior, inferior, and nasal quadrants had weak negative correlations with axial length, except for the temporal quadrant was positive and insignificant.

Compliance with Ethical Standards
Conflict of Interest: I declare no potential conflicts of interest with respect to the authorship, and/or publication of this article.
Ethical approval: All procedures performed were in accordance with the ethical standards of the institutional research committee and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards.
Informed consent: Informed consent was obtained from all individual participants included in the study.