MF was first reported by Takano and Kishi in 1999 [8], and it affects 8–34% of highly myopic eyes [9]. The pathogenesis of MF is still unclear. Most studies have focused on the macular area and few have evaluated the changes in the optic disc or peripapillary region [1]. In our clinical observation, the optic disc is more oval in eyes with MF than in eyes without MF. This finding prompted discussions on why the optic disc is more oval in MF, and whether there is a relationship between an oval disc and the occurrence of MF.
The optic disc was markedly stretched in highly myopic eyes but its morphology is difficult to evaluate in a standardized manner [10]. The OI, defined as the ratio between the minimal and maximal optic disc diameters [3, 10, 15,16,17], is an objective and easily calculated parameter that can be measured on fundus examination without requiring additional imaging devices [10, 27]. In this study, the OI was significantly smaller in the eyes with MF than in the contralateral eyes. The optic disc tilt angle was significantly greater in the eyes with MF, but the optic disc torsion angle was not significantly different between the eyes with MF and the contralateral eyes. Simple linear regression analysis revealed that the optic disc tilt degree was significantly correlated with the OI. These results suggest that the ovality of the optic disc in eyes with MF results from the optic disc tilt around the vertical disc axis [10], in which the temporal disc edge is angled towards the rear of the eye and the nasal edge is angled towards the front of the eye [12].
We investigated the relationship between the optic disc tilt and the occurrence of MF further. Previous studies reported that optic disc tilt may be caused by the increased axial length and the step configuration of the scleral bed [6, 10, 28, 29]. The biometric features of the optic disc, PPA-β/γ zone, peripapillary scleral bending, and posterior staphyloma were determined. GEE model (multiple linear regression) revealed that spherical equivalent refraction, narrow macular staphyloma, and the occurrence of MF were the independent factors associated with the OI. Several other studies have reported similar findings. How et al. [28] and Samarawickrama et al. [6] reported that myopia spherical equivalent was a significant risk factor for the optic disc tilt. However, Tay et al. reported that greater optic disc ovality correlated with longer axial length [17]. Asai et al. reported that the OI showed significant associations with age, macular choroidal thickness, and the depth of staphyloma [10]. In the present study, neither axial length nor depth of staphyloma were associated with the OI, unlike in previous studies [10, 17, 28]. Our results indicate that optic disc tilt is affected by local protrusion in the macular area rather than general global enlargement. In parallel, we investigated the factors associated with the occurrence of MF. The OI was smaller, and the PPA-β/γ zone was wider, and the choroid in the central foveal region was thinner in the eyes with MF than in the contralateral eyes. Narrow macular staphyloma was more frequent and the staphyloma was deeper in the eyes with MF compared with the contralateral eyes. After adjusting for confounding variables, GEE model (multivariate logistic regression) revealed that narrow macular staphyloma was the only independent factor associated with the occurrence of MF. Overall, these results indicate that the optic disc tilt is likely to be due to local staphyloma protrusion in the macular area, which shows a significant association with the occurrence of MF.
Posterior staphyloma, a circumscribed outpouching of the posterior pole, was classified into six types according to a study by Ohno-Matsui and Jonas [24]. Wide macular, narrow macular, and peripapillary staphyloma were found in our study. Narrow macular staphyloma was more frequent in the eyes with MF than in the contralateral eyes. Narrow macular staphyloma, characterized by a protrusion in the macular area, has a much smaller curvature radius in the fovea than in the adjacent eye wall [24]. A distinct and abrupt change in the scleral curvature was visible at the staphyloma edge. We speculate that during the development of staphyloma, the posterior pole expands in an asymmetrical temporal manner [29], and the optic nerve head is mechanically pulled toward the nasal direction (Fig. 4) [6, 23, 30]. Therefore, the optic disc may tilt over the vertical disc axis. The enlarged PPA-β/γ zone and the shortened distance from the foveal center to the temporal PPA-β/γ margin in the eyes with MF suggest that the PPA-β/γ zone margin has shifted temporally [3, 30], supporting our proposed mechanism. Additionally, progressive mechanical stretching of the posterior pole in the macular area resulting from the staphyloma protrusion may increase the outward posterior traction on the neurosensory retina [9], thereby increasing the susceptibility of highly myopic eyes to experience foveoschisis [8]. However, chorioretinal atrophy was more frequent than foveoschisis in eyes with wide macular staphyloma [1, 24, 29]. We speculate that the sclera of the posterior pole protrudes relatively inwardly in this flat-based staphyloma (i.e., from the nasal optic nerve head to the temporal macula) [24]. This protrusion may weaken the outward posterior traction of the staphyloma and the inward tangential traction from the posterior vitreous cortex or the epiretinal membrane [8], and hence reduce the incidence of retinal splitting and protect against the development of MF. In addition, the optic disc may be flatter in eyes with a wide macular staphyloma because the traction force is more isotropic under the optic nerve head in these eyes.
In this study, high-resolution images of the macula and optic disc morphological features were obtained using SS-OCT. The high penetration of the SS-OCT light source and the low signal decay along the optic axis improved the visualization of deep ocular structures [7, 10, 14]. Our study demonstrates that SS-OCT is a useful imaging tool for highly myopic eyes. Using this imaging technique, we also measured the optic disc tilt angle, torsion angle, and other peripapillary features that were not based on the traditional horizontal axis but instead on the fovea–BMO axis. Several recent studies have suggested that it is better to acquire images along the fovea–BMO axis because the horizontal axis may be affected by the disc–fovea angle, and may introduce potential bias [13, 16, 31].
There are several limitations that should be acknowledged. First, this is a retrospective cross-sectional study. The sequential SS-OCT images should be obtained to observe the changes of optic disc tilt and MF over time. The longitudinal analysis, which may better determine the relationship between the OI and the occurrence of MF, should be performed in future studies. Second, we did not perform three-dimensional magnetic resonance imaging to determine the posterior staphyloma location [5]. Conventional fundus photographs and SS-OCT images may not be wide enough to visualize the entire length of the staphylomas, so some staphylomas, mainly peripapillary and nasal staphylomas [24], might not be classified accurately. Third, all of the parameters were measured on two-dimensional images rather than over the real curvature of the eyes [3]. Fourth, the current study comprised a small number of subjects, so the results may not represent the general population of individuals with MF. Therefore, further studies with a larger sample size are necessary.