A recent prospective study analysed the relationship between corneo-scleral geometry and the axial length of the eye to develop preliminary prediction models of the axial length of the eye based on clinical data.
Myopia is the most common refractive defect of the eye, affecting substantial proportions of adult populations; in one UK study, it was found to affect 49% of adults.1 The exponential increase in the prevalence of the condition is sparking increased interest in the geometric properties of the conjunctival-scleral tissue.
Generally referred to as short sightedness, myopia is an ametropia characterised by an eye that is too long for its optical system (cornea and lens). In recent years, there has been growing concern about the sharp rise in the prevalence of myopia in the juvenile population, and this has increased the need for research and development of new techniques to control axial elongation of the eye.
Because in vitro and post-mortem studies in humans have shown that the sclera is more extensible in high myopes than emmetropes, and that weakening of the scleral structure is related to the elongation of the eye, in vivo examination of the geometric properties of the anterior sclera in eyes with different levels of myopia may improve our understanding of the variations in the anterior eye properties associated with increased ocular length.2-4
The risk of suffering from diseases such as glaucoma or maculopathies increases very significantly as myopia increases. Therefore, it is important to understand the weakening procedure that takes place in the sclera, leading to progressive eye elongation, and to see whether this process also generates changes in the anterior portion of the structure (covered by the conjunctiva), which can be measured clinically. In this way, geometric factors at the level of the sclera might predict which cases are more susceptible to scleral changes.
Considering the existing technology for the clinical evaluation of the corneo-scleral geometry,5 as well as the comparison and validation of the main technologies applied in the clinic,6,7 Fourier domain profilometry (Eye Surface Profiler [ESP], Eaglet-Eye) stands out as being a reasonable way of studying the relationship between myopia and corneo-scleral geometry. Measurement with this device is minimally invasive and quickly captures all the data necessary to characterise such geometry with high levels of precision.7
We recently conducted a prospective study to analyse the relationship between corneo-scleral geometry, measured with this type of technology, and the axial length of the eye, defining the differential aspects existing in myopic eyes and developing prediction models of the axial length of the eye based on clinical data.8
In total, 64 eyes of 32 healthy participants (mean age 33 years) were evaluated in the Optometry Clinic of the University of Alicante, Spain, including complete analysis of the anterior segment, analysis of visual function and evaluation of the corneo-scleral topographic profile with the ESP system. The study methods adhered to the tenets of the Declaration of Helsinki and were approved by the ethics committee of the University of Alicante (Exp UA-2019-08-28).
A statistically significant and moderate-to-strong negative association was found between the axial length and the difference between temporal and nasal sagittal heights for different chord lengths (from r = −0.701; P < 0.001 for 11-mm chord length to r = −0.502; P < 0.001 for 15-mm chord length). This suggests that nasotemporal sagittal height asymmetry has potential as a biomarker of myopic changes.
It is possible that the decrease in nasotemporal sagittal height asymmetry in myopic eyes is due to different insertion of the medial rectus muscle and the lateral rectus muscle9 combined with the noticeably lower rigidity of the sclera in such eyes.2,10,11 Specifically, there is a rearward displacement of the insertion site of the medial and lateral rectus muscles in myopic eyes,12 with rotation of the eyeball around the same nasal and anterior point to its geometric centre regardless of the axial length of the eye.13
A multiple linear regression analysis showed that the axial length could be predicted with acceptable levels of precision by means of a linear equation relating refractive, corneal and corneo-scleral variables, although the variables involved for the prediction seem to differ in right and left eyes. Specifically, the following linear equations for predicting the axial length (AL) were obtained:
where SE is spherical equivalent; CD, corneal diameter; HOA RMS, high order aberration root mean square; MinSH14, minimum sagittal height for a chord length of 14 mm; and Km, mean keratometry.
The slight difference between the linear models used to predict axial length from the anterior segment parameters for the right and left eyes may be related to differences in the ocular globe between eyes: the asymmetry of the axial length between them has been shown to increase with increasing axial length.14
These models should be considered as a preliminary approach that provides information on the potential value of the geometry of the anterior sclera for the prediction of axial length. Future studies should validate and refine these models in large samples including subjects with different ethnicities.
The analysis of the relationship between the corneo-scleral geometry measured with Fourier domain profilometry and the axial length of the eye has shown that nasotemporal sagittal height asymmetry may be a good biomarker of myopic changes. More studies are needed to clarify this issue as well as determine how these corneo-scleral parameters change during myopia progression.