To investigate optic nerve head size and retinal nerve fiber layer (RNFL) thickness according to refractive status and axial length.

In a cross-sectional study, 252 eyes of 252 healthy volunteers underwent ocular biometry measurement as well as optic nerve head and RNFL imaging by spectral-domain optical coherence tomography. Correlation and linear regression analyses were performed for all subjects. The magnification effect was adjusted by the modified axial length method.

Disc area and spherical equivalent were positively correlated (r = 0.225, r^{2} = 0.051, ^{2} = 0.129, ^{2} = 0.069, ^{2} = 0.129, ^{2} = 0.069, ^{2} = 0.129, ^{2} = 0.069,

Optic disc radius and RNFL thickness decreased in more severely myopic eyes, but they increased after adjustment for magnification effect. The error due to the magnification effect and optic nerve head size difference might be factors that should be considered when interpreting optical coherence tomography results.

Morphologic evaluation of the optic nerve head (ONH) is an essential step in the proper diagnosis of optic nerve diseases, including glaucoma. For example, the ONH of primary open-angle glaucoma patients is characterized by a large and deep cup with a narrow neuroretinal rim, resulting from the loss of ganglion cell axons [

Notably, myopia has been widely reported to affect the size and shape of the optic disc and peripapillary retinal nerve fiber layer (RNFL) [

Soldiers stationed in Gyeonggi province were invited to participate in the study, which was conducted between December 2008 and April 2009. The study met the ethical standards of the Declaration of Helsinki and was approved by the Armed Forces Capital Hospital institutional review board. In addition, informed consent was obtained from each participant, and individuals with any abnormal ocular findings or history of certain diseases were excluded. The specific exclusion criteria were as follows: (1) ocular hypertension (IOP >21 mmHg) or glaucoma; (2) evidence of reproducible visual field abnormality (defined as pattern standard deviation significant at

All subjects underwent comprehensive ophthalmologic examinations on both eyes; these examinations included best-corrected visual acuity, intraocular pressure with Goldmann applanation tonometry, automated refraction (RK-F1; Topcon, Tokyo, Japan), axial length (IOLMaster; Carl Zeiss Meditec, Dublin, CA, USA), slit-lamp examination, red-free fundus photography (CF-60UVi; Canon, Tokyo, Japan) with mydriasis, standard automated perimetry (Swedish Interactive Threshold Algorithm standard C24-2 program, Humphrey Field Analyzer II 750; Carl Zeiss Meditec), ONH parameter measurement (rim area, disc area, average cup-to-disc [C/D] ratio, vertical C/D ratio, and cup volume), and peripapillary RNFL thickness measurement by spectral-domain OCT (Cirrus OCT, Carl Zeiss Meditec). The data for one randomly selected eye were selected for analysis.

The refractive error was measured five times by autorefractometry (R-F10, Canon) without cycloplegia, and the result was subsequently converted to spherical equivalent. The average of three median values, after discarding the upper and lower values, was used in the analysis. The axial length was measured five times by partial coherence interferometry (IOLMaster, Carl Zeiss Meditec), and the average was calculated using the same process as that used for refractive error.

The ONH parameters and peripapillary RNFL thicknesses were measured after the red-free fundus photography by spectral-domain OCT with the optic disc cube 200 × 200 scan protocol under pupil dilatation and dim illumination by two expert examiners. Scanned images of signal strength lower than 8 were discarded. Also excluded were individuals with an extent of peripapillary atrophy that expanded across the 3.46 mm scan circle centered on the optic disc. Clock-hour RNFL thickness was recorded based on the right-eye orientation. The optic disc margin measured by spectral-domain OCT was sometimes different from the actual disc margin because spectral-domain OCT determines disc margin based on the retinal pigment epithelium. However, it did not have a significant impact on the analysis, so we included all data if peripapillary atrophy did not expand past the scan circle radius.

The ONH average radius was calculated from the OCT-measured disc area by the following equations: ^{2}

With this method, the distance from the disc margin to the scan circle (distance = radius of scan circle - radius of disc), which is suspected to influence the OCT peripapillary RNFL thickness measurement, was determined.

Adjustment for the ocular magnification effect was performed in the same way as in our previous work with the modified axial length method [

Adjustment for average RNFL thickness was performed in the same way as in our previous work [

Therefore,

Statistical analysis was carried out using the SPSS ver. 12.0 (SPSS Inc., Chicago, IL, USA). The spectral-domain OCT-measured ONH parameters and the calculated mean radius of the disc were compared. Bivariate and partial correlation analyses were performed to investigate the relationship between axial length or spherical equivalent and ONH parameters or peripapillary RNFL thickness. In addition, a partial correlation analysis for the same variables, which controlled for spherical equivalent, was performed to investigate the shear influence of axial length on ONH parameters and peripapillary RNFL thickness, apart from the effect of refractive status. Correlation and linear regression analyses of the ONH parameters, peripapillary RNFL thickness, calculated disc radius, distance from disc margin to scan circle, axial length, and refractive error were performed for all subjects. The

A total of 258 subjects were enrolled in this study. Among them, six were excluded because of extended peripapillary atrophy across the 1.73 mm radius scan circle or unacceptable OCT scans, leaving 252 eyes of 252 subjects for further analysis. The mean age of the 252 subjects was 21.06 ± 1.64 years (19 to 26). The average axial length was 24.74 ± 1.25 mm (21.38 to 28.59), and the mean refractive error was -2.51 ± 2.37 (-11.0 to +4.13) diopters (D). The ONH parameters and peripapillary RNFL thicknesses measured by spectral-domain OCT are listed in ^{2}, the r im area was 1.308 ± 0.264 mm^{2}, and the average C/D ratio was 0.539 ± 0.149. A correlation analysis of the axial length with disc area, rim area, cup area, C/D ratio, and peripapillary RNFL thickness showed negative results. The distance from the disc margin to the scan circle (1.73-disc radius) showed positive correlation with axial length. A partial correlation analysis of the same variables, which controlled for the spherical equivalent, showed negative correlation with axial length, whereas the distance from the disc margin to the scan circle showed a positive correlation. A correlation analysis of the spherical equivalent with the disc area, rim area, cup area, C/D ratio, and RNFL thickness showed positive correlations, while that of the spherical equivalent with the distance from the disc margin to the scan circle showed a negative correlation. A partial correlation analysis that controlled for the axial length also revealed positive correlations of the spherical equivalent with the ONH parameters and peripapillary RNFL thickness and a negative correlation with the distance from the disc margin to the scan circle (

Many studies have suggested that optic disc size is influenced by axial length and refractive error [

Considering both the homogeneous characteristics of the subjects enrolled in this study and the fact that their respective findings did not significantly diverge from those of prior studies on subjects of varying age, sex, and ethnicity, it was concluded that ONH size, peripapillary RNFL thickness, and myopia might be independent of such factors. If this is true, an ophthalmologist, when interpreting data obtained with imaging devices such as OCT or fundus photography, could apply the same considerations with regard to disc size, the effect of ocular magnification, and the degree of myopia for any age, gender, or ethnicity. Nonetheless, in the clinical setting, whereas peripapillary RNFL thickness evaluation provides some of the most important information regarding glaucomatous optic nerve damage, it is impossible to correct for the ocular magnification effect in every OCT scan result. Therefore, the present study analyzed the influence of several parameters on scanned peripapillary RNFL thickness without correction for the ocular magnification effect. According to our study, the adjusted peripapillary RNFL thickness showed a negative correlation with spherical equivalent without statistical significance and a positive correlation with axial length, results similar to those of a previous study [

The study was conducted only in a limited sample of healthy young Korean males. Therefore, these features may limit the application of these data to subjects of other age groups or ethnicities.

In conclusion, the peripapillary RNFL thickness was most strongly influenced by the distance from the disc margin to the scan circle. Disc radius and RNFL thickness decreased in more severely myopic eyes, they increased after adjustment for the magnification effect. Based on this, the ONH size and RNFL measurements were influenced by the magnification effect. Although the error by the magnification effect and the ONH size difference were clinically negligible because of the low coefficient of determination and extremely small optic disc size change according to the degree of myopia, they might remain as factors that should be considered.

Values are presented as mean ± standard deviation; The RNFL thicknesses were converted according to the right-eye orientation. ONH = optic nerve head; RNFL = retinal nerve fiber layer; OCT = optical coherence tomography; C/D = cup-to-disc.

The figures in parentheses are

AL = axial length; SE = spherical equivalent; RNFL = retinal nerve fiber layer; D = diopters; C/D = cup-to-disc.

The figures in parentheses are the

AL = axial length; SE = spherical equivalent; RNFL = retinal nerve fiber layer; D = diopters.

RNFL = retinal nerve fiber layer; r = correlation coefficient; r^{2} = coefficient of determination; β = regression constant.