Myopia & Conjunctival-Scleral Geometry

Myopia & Conjunctival-Scleral Geometry

October 04, 2021

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                                                               Οφθαλμός       

Most cases of myopia are associated with excessive axial eye growth. Axial myopia is a condition that develops principally during childhood and early adulthood. Ophthalmic News

The excessive elongation of the eye results in blurred distance vision due to images of distant objects focusing in front of the retina. Myopia is defined by a spherical equivalent (SE) lower than −0.5 dioptres.

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Several experimental and clinical studies have clearly demonstrated that both nature (genetics and heredity) and nurture (environment and lifestyle) are important factors in the development of myopia.

In light of analyzing the geometry of conjunctiva and/or sclera and its relationship between geometric changes in the scleral-conjunctival structure and myopia, several studies have been performed with different diagnostic technologies. Ophthalmology News

These technologies included optical coherence tomography, profilometry and Scheimpflug imaging, focused on detecting geometric changes of the scleral-conjunctival tissue in different physiological conditions of the eye, after use of contact lenses and in different ocular pathologies.

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The conjunctiva is the transparent, thin membrane that covers a great portion of the anterior surface of the ocular globe and the internal surface of the eyelids.

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It has two segments: palpebral conjunctiva, which is the portion that covers the internal surface of both the lower and upper eyelids, and bulbar conjunctiva, which is the portion that covers the front part of the sclera, stopping at the junction between sclera and cornea.

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These two portions of conjunctiva are continuous and have several relevant functions: to maintain the front surface of the eye lubricated and moist as well as the internal surface of the eyelids. Ophthalmic News

Thus, they eyelids open and close without friction or inducing eye irritation, protecting the eye from dust, debris and infection-causing microorganisms. Ophthalmic News

Ophthalmology News - “The word ophthalmos, “οφθαλμος”  from which the term ophthalmology is derived, appears often in Homer’s epic poetry: “Athena aimed the arrow towards the nose, near the ophthalmos [eye] and pierced the white teeth” [Il 5.291].”

A great variety of small blood vessels are present in the conjunctiva that provide nutrients. Likewise, there are also special cells in this structure that secrete components of the tear film. Ophthalmology News

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In spite of the large quantity of studies that have been performed about anatomical and histological aspects of this structure, there was a scarce peer-reviewed literature about their geometry which has been suggested to be mostly influenced by scleral geometry. Ophthalmic News

More than 80% of the surface of the eyeball is covered by the sclera, which is below the bulbar conjunctiva, extending from the optic nerve to the corneal area. The junction area between cornea and sclera is called limbus. Ophthalmology News

The thickness of the sclera is very variable, ranging from about 0.3 to 1.0 mm. The structure of this fibrous layer consists of collagen fibrils that are small fibers with an arrangement in irregular and interlacing bundles. Ophthalmology News

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This interweaving of fibers with random arrangement is the main factor accounting for the mechanical strength of the eyeball. As the sclera has a limited blood supply, it is almost inactive metabolically and can be considered as an avascular structure.

In conjunction with the intraocular pressure (IOP), the scleral structure is the main responsible for the maintenance of the shape of the eyeball, protecting the eye from damage, such as that produced by laceration or rupture as a consequence of an external trauma. Ophthalmology News

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This structure is also a sturdy attachment for the extraocular muscles allowing the movement and alignment of both eyes. Ophthalmology News

 As happened with the conjunctive, few studies have been conducted to analyse the geometry of this structure mainly due to the complexity of obtaining an in-vivo measurement of such geometry.

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Myopia is the most common refractive defect of the eye, aff ecting substantial proportions of adult populations; in one UK study, it was found to aff ect 49% of adults. Ophthalmology News

The exponential increase in the prevalence of the condition is sparking increased interest in the geometric properties of the conjunctival-scleral tissue.  Ophthalmology

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Generally referred to as short sightedness, myopia is an ametropia characterized by an eye that is too long for its optical system (cornea and lens). Ophthalmology News

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 postmortem 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.           Οφθαλμός

The risk of suffering from diseases such as glaucoma or maculopathies increases very significantly as myopia increases. Ophthalmology News

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.

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In this way, geometric factors at the level of the sclera might predict which cases are more susceptible to scleral changes. Ophthalmology News

Considering the existing technology for the clinical evaluation of the corneo-scleral geometry, as well as the comparison and validation of the main technologies applied in the clinic, Fourier domain profi lometry (Eye Surface Profi ler [ESP], Eaglet-Eye) stands out as being a reasonable way of studying the relationship between myopia and corneo-scleral geometry.  Ophthalmology

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Measurement with this device is minimally invasive and quickly captures all the data necessary to characterize such geometry with high levels of precision. Ophthalmology News

We recently conducted a prospective study to analyze 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. Ophthalmic News

Our study 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. Ophthalmology News

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). Ophthalmology News

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).

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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 muscle combined with the noticeably lower rigidity of the sclera in such eyes. Ophthalmic News

Specifically, there is a rearward displacement of the insertion site of the medial and lateral rectus muscles in myopic eyes, with rotation of the eyeball around the same nasal and anterior point to its geometric center regardless of the axial length of the eye. Ophthalmology News

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.

                                               Οφθαλμός                               Οφθαλμολογικά Νέα

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Specifically, the following linear equations for predicting the axial length (AL) were obtained: Right eye (P < .001, adjusted R2: 0.843): AL=15.62-0.27 ×SE+0.99 ×CD+2.77 ×HOA RMS-1.77 ×MinSH14; and Left eye (P < 0.001, adjusted R2: 0.730): AL=37.20-0.40 ×SE-0.33 ×Km+0.086 ×CD3.96 ×HOA RMS 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. Ophthalmology News      Ophthalmic News

The asymmetry of the axial length between them has been shown to increase with increasing axial length. 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.

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Future studies should validate and refine these models in large samples including subjects with different ethnicities. Ophthalmology News

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. Ophthalmic News

More studies are needed to clarify this issue as well as determine how these corneo-scleral parameters change during myopia progression. Ophthalmic News

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