Myopia is an ocular condition closely associated with biomechanical and dimensional changes. In this study, the distribution of the posterior eye curvature was investigated using 400 kHz, 68×68 degrees SS-OCT, and the associations with AL, SE, age, and ChT were explored in school-aged children. The Gaussian curvature map revealed bilateral symmetry between the right and left eyes and flatter curvature in the macular region compared to the adjacent areas. In a previous report on high myopia, the posterior pole was considered to have greater curvature [21]. However, in the present study, on the contrary, the macula was observed as a flatter shape with less curvature than the surrounding regions. This difference is likely due to variations in the age and myopic condition of the subject population, suggesting it might reflect the process of ocular growth. Indeed, another study reported that 20% of school-aged children had a dome-shaped macula [26]. Longitudinal studies are needed to confirm these findings.
The mean curvature exhibited significant correlations with AL and SE across the superior, macular, and inferior regions. In these areas, a longer AL (indicative of higher myopia) corresponded to a steeper curvature, reflecting the elongation of the eyeball and its increased ellipticity. However, this relationship was notably absent in the temporal region, where it was entirely reversed. Also, in relation to ChT, significant negative correlations were observed in the superior, macular, and inferior regions, contrasting with the lack of significant correlation in the temporal region. This discrepancy led to an intriguing observation: the presence of a local maximum curvature point over the posterior pole located within the temporal region in most cases. Moreover, this maximum curvature point aligned closely with the extension of the line connecting the optic disc and fovea (Fig. 6e), and it appeared proximate to the entry site of the LPCA (Fig. 6f). While this is the first report of such findings, the exact origin of this characteristic shape remains speculative, with a potential link to LPCA suggested. Although the location does not precisely match the LPCA entry site, its proximity raises the possibility of mechanical pressure from the LPCA influencing structural features during ocular development. Additionally, variations in the rigidity of vascular walls and the sclera may have contributed to this phenomenon. Posterior ciliary arteries (PCA) circulation is the main source of blood supply in the eye [27]. Recently, topographic distribution of the short (S)-PCA entry sites into the choroid has been studied using SS-OCT but has not yet been fully understood in a large population [28]. Alternatively, other factors such as interactions between optic nerve sheaths, oblique muscles, and biomechanical scleral resistance, implicated in shaping the posterior globe in high myopia [5], could also influence the formation of the temporal local maximum curvature point. Further investigations, particularly into the topographic distribution of both SPCA and LPCA entry sites, are warranted to elucidate these findings in larger populations.
The strength of the methodology lies in the utilization of high-speed 400 kHz SS-OCT. This technology allowed for the acquisition of widefield, high-density point cloud data in just 1.2 seconds, facilitated by the high-penetration depth of SS-OCT, which enabled automated CSI segmentation. To validate the technique, we assessed the variability of results from multiple imaging sessions of the same eye. It was confirmed that the variation between measurements was less than 0.00016 mm-2, a sufficiently small margin relative to the measured values. Despite the inherent assumptions in OCT image distortion correction, including tissue refractive index parameters and estimation errors in curvature calculation, the minimal inter-measurement variability suggests the utility of Gaussian curvature as an ocular shape representation, complementing conventional AL measurements. Considering the posterior eye's curvature distribution, there's potential to optimize sampling density slightly and expand widefield imaging capabilities further. Widefield OCT not only allows for local ocular shape assessment, as demonstrated in this study, but also offers simultaneous evaluation of global ocular shape and size parameters. This broader scope opens avenues for assessing peripheral refraction [29] and investigating the relationship between global ocular shape and myopia treatment modalities such as outdoor activities, atropine eye drops, orthokeratology, and multifocal intraocular lenses [30].
In this study, we utilized the BM segmentation line to represent ocular shape. However, similar measurements can be applied to other retinochoroidal boundaries such as the inner limiting membrane, retinal pigment epithelium (RPE), and CSI, including their ratios. Hence, our technique can extend beyond myopia to assess idiopathic macular hole [31], irregular RPE curvature in age-related macular degeneration [32], and ONH shape, crucial in myopia and glaucoma [33], all central themes for future research.
In conclusion, we quantitatively analyzed posterior eye curvature using distortion-corrected widefield OCT, and characterized small, localized structural deformities with micrometer-level spatial resolution. Furthermore, we observed a common local maximum curvature point in the temporal region of young, healthy eyes. Investigating its origin, we measured the geometrical distance to the LPCA site into the choroid and discussed their relationship. Further longitudinal studies are warranted to elucidate their causal connection.