Highly myopic eyes differ in morphology from emmetropic eyes, and the correct estimation of the vitreous volume is difficult [17]. This study aims to explore an effective method to estimate ocular volume using refractive factors in children. Using high-end CT 3D reconstruction data, the results indicate that in children, the ocular volume is negatively and linearly correlated with the diopter and curvature, and positively and linearly correlated with the axial length.
Highly myopic eyeballs have asymmetric growth, as characterized by the elongation of the ocular equator, expansion of the posterior pole, and multi-directional and spherical ocular expansion [19]. Atchison et al. [21] showed that compared with emmetropic eyes, myopic eyes are elongated, more in length than in height, and even less in width. Bron et al. [22] suggest that the bone wall around the eyeball is closer to the eyeball than that behind it in terms of the anatomic relationship between the eyeball and the orbital bone wall; therefore, the anterior and posterior diameters of the eyeball change significantly more than the vertical and horizontal diameters. The reasons for this might be that the osseous structure of the orbit limits the expansion direction of the eyeball, and some ocular factors might cause different changes in the diameters of highly myopic eyeballs. Assuming that defocusing leads to myopia and that the ocular development may be affected by retinal development, the reason for greater anterior and posterior diameters than vertical and horizontal diameters in highly myopic eyeballs may be that the equator has milder defocusing than the posterior pole [22]. The uneven distribution of retinal growth factors might also affect ocular development [23, 24], but it still lacks strong evidence to support it. Cheng et al. [25] measured the thickness of scleral and choroidal tissues of eight subjects with mild-to-moderate hyperopia, six with emmetropia, and seven with mild-to-moderate myopia, and found that subjects with myopia had thinner sclera and choroid than the subjects of the other two groups, and such extension of sclera and choroid caused an expanded vitreous body.
This study shows that the ocular volume is correlated with diopter, total curvature, and axial length. Linear regression equations were also determined. This is supported by Nagra et al. [26], who revealed that the ocular volume could be estimated using the axial length. The determination of the ocular volume of a patient with high myopia will enable more efficient planning before surgery, such as customizing an artificial vitreous body that fits the size of the patient's eyeball and determining the needed amount of silicon oil or expansive gas during surgery. This would be beneficial to the patient's postoperative recovery and might reduce postoperative complications (e.g., anterior segment ischemia syndrome, postoperative ocular hypertension, postoperative ocular hypotension, and retinal re-detachment). Future studies will have to examine the applicability of those equations in real-world clinical practice. Validation in large cohorts is necessary.
The estimation of the eye volume by CT has been shown to be feasible [27], but it is not practical and should not be done in all patients. Indeed, CT examination and 3D reconstruction require time and human resources, and consuming such resources might be unnecessary if simple and reliable equations can be designed. In addition, radiation exposure remains an issue, particularly in children.
This is an exploratory study, but as high-end CT technology has yet to be widely used in ophthalmic examinations, this study was affected by a small sample size and selection bias. Future studies should include more high myopia patients, including adult patients, to verify and improve the ocular volume estimation model, and promote the application of high-end CT in ophthalmology in selected cases. Currently, studies on ocular morphology require the manual collection of images, which is time and labor cost consuming. We expect big data collection and analysis by artificial intelligence for a way to determine the ocular volume by refractive factors and hope to design patient-friendly surgery for severe eye disease before surgery.