This study demonstrated that the foveal position relative to the optic disc was a significant determinant of the RNFL distribution in healthy myopic subjects (
Table 3 and
Fig. 5). Whereas AL particularly affected the temporal-side RNFL, the relative foveal position was associated with the vertical asymmetry of RNFL distribution. The disc–foveal angle was also affected by the distance between the fovea and the disc center. There is a wide range of normal RNFL thickness variation, which is affected by age, ethnicity, AL, and optic disc area.
3,5,25 To the best of our knowledge, this is the first study to characterize the relative foveal position as a significant determinant of the normal RNFL distribution in relation to other known determinant factors.
To measure the position of the fovea relative to the optic disc, we used the disc–foveal angle determined by fundus photography, which is the standard method for quantifying cyclotropia in strabismus clinics.
10,26 The effect of static ocular counter-roll, a compensatory torsional eye movement during head tilt, is reported to remain within 1°, provided the head tilt was <5°.
26 The disc–foveal angle is not affected by laterality, sex, or age.
11 However, small amounts of changes in physiological ocular rotation exist among each test session, which may affect the intertest measurement reproducibility of peripapillary RNFL thicknesses.
27 Therefore, in this study, we compensated for the potential effect of the ocular rotation on the disc–foveal angle by using fundus photographs overlaid on spectral-domain (SD)-OCT images (
Fig. 1).
19 Similar to results in previous studies,
8,11,20 the mean disc–foveal angle of our study participants was 6.08° ± 3.48°. As shown in
Figure 2, there was considerable variation in the normal disc–foveal angle, and most values were positive, which indicates that the fovea is positioned below the optic disc.
The development and progression patterns in the superior and inferior retina differ in glaucomatous damage.
28–30 The neuroretinal rim in normal eyes is broadest on the inferior side.
31,32 In addition, the RNFL thickness on the inferior side tends to be slightly thicker than on the superior side in normal subjects.
4,33 However, the superior VF corresponding to the inferior retina is involved more frequently in the early stages of glaucoma, with faster progression than the inferior VF.
34,35 In addition, the inferotemporal meridian was found to be the most frequent location where RNFL progression was detected.
36
The regional susceptibility of the inferior temporal optic disc to glaucomatous damage appears to be associated with the inferior foveal position relative to the optic disc. Hood et al.
47 suggested that, because of the characteristic foveal position, the inferior temporal side of the optic disc is more crowded, with a higher density of retinal ganglion cell axons than other disc regions, rendering the region more vulnerable to glaucomatous damage. In this study, as the fovea became more parallel with the optic disc, the superior RNFL increased significantly (
P = 0.003), whereas the inferior RNFL and I-S differences decreased (
P = 0.010 and
P < 0.001, respectively) (
Fig. 3). In addition, eyes with a more inferior foveal position relative to the optic disc (greater disc–foveal angle) had a thicker inferior RNFL and a greater I-S difference than eyes with the fovea positioned parallel to the optic disc (lesser disc–foveal angle) (
Table 1). Our results support the hypothesis that the relative foveal position is one of the key factors determining the RNFL distribution.
Previous studies have found that myopia redistributes the RNFL with axial elongation.
37 With increased AL, there is temporalization of the retinal vessels and thickening of the temporal RNFL.
14,25 In agreement with this finding, we observed thickening of the average RNFL and on the temporal side with increased AL (
P = 0.001 and
P < 0.001, respectively) (
Fig. 4). Consistent with the previous studies where the Littmann formula was used to eliminate the effect of ocular magnification related to the AL,
25,38 we found that the average RNFL thickness increased with AL. The thicker average RNFL thickness in myopic eyes compared to nonmyopic eyes may be related to the overshooting effect related to the adjustment for ocular magnification. In addition, eyes with a larger retinal surface area (i.e., eyes with long AL) may retain more RNFL than normal eyes, considering that optic disc size is positively correlated with AL
39 and that the larger optic disc is associated with more retinal nerve fiber axons, as shown in the previous histomorphometric study.
40
Intriguingly, we also observed that the position of the fovea becomes parallel to the optic disc as the distance between the fovea and disc center increases (
P = 0.033), which may suggest that there is asymmetrical enlargement in the posterior sclera between the superior and inferior regions in general axial myopia. The posterior sclera is more immature, produces more collagen, and is more extensible than the anterior and equatorial scleral regions, which makes it particularly susceptible to myopic changes.
41 The posterior sclera is the outer shell of the posterior pole and changes in the posterior sclera are reflected in the posterior pole, as shown in studies that analyzed outer deformities of the posterior sclera in cross-sectional images of the macula and optic disc.
15,16 Further studies using other imaging devices, such as swept-source OCT and magnetic resonance imaging (MRI) are needed to elucidate the changes in the posterior sclera with axial elongation in general myopia.
One of the limitations of this study was that it was clinically based and not population-based screening. The participants were all Koreans of similar age. In this study, participants with concurrent ophthalmic abnormalities or with a best-corrected VA of <20/20 were excluded. Furthermore, by virtue of our study design, the information regarding the shifts of disc–foveal angle with progressive vision loss cannot be addressed. Future studies of ocular rotational orientation in imaging eyes with vision loss or progressive vision loss are necessary. In addition, most participants were myopic, and we adjusted the measurement of RNFL thickness by using the Littmann formula
21 to remove the potential bias of AL-related ocular magnification. However, a population-based study including individuals with various refractive errors, as well as different ethnicities and ages, is needed to confirm our findings.
In summary, the foveal position relative to the optic disc was a significant determinant of the normal RNFL thickness profile. The relative position of the fovea was affected by the distance between the fovea and disc center. Knowledge of the normal anatomical variation associated with the relative foveal position will help to identify and follow glaucoma patients.