Remodeling Mechanisms of Eye in Myopia

Introduction

Sclera is the white part of the eye in the front part that is continuous with the cornea of the eye. Sclera has special remodeling mechanisms, where it changes its shape and properties to adapt to external stimuli, and a clear vision is formed. In myopic patients, the focus of light rays is changed and a clear image is not formed. In such situations, the remodeling of the sclera occurs and helps to form a clear image.

What Is Myopia?

Myopia, or nearsightedness, is a common vision problem where near objects appear clear and it is difficult to see objects far away. Due to changes in the eyeball's shape, the light rays focus at different points forward or backward on the retina. Primary myopia is corrected by using a concave or diverging lens. The use of a concave lens helps to focus the light rays correctly on the Retina.

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What Is the Remodeling Mechanism of the Human Eye?

The complex and dynamic mechanism of the eye constantly adapts to changes in light and visual stimuli. The fundamental mechanism of the eye that helps in proper functioning is remodeling. Remodeling allows the eye to adjust its shape and size based on visual demands..

• Accommodation - Accommodation is the process by which the eye changes its focus to see objects at various distances. This is achieved by the ciliary muscle, which controls the shape of the lens. The ciliary muscle relaxes when looking at faraway objects, causing the lens to flatten. This reduces its refractive power, enabling it to see far-away objects. Similarly while looking at closer objects, the ciliary muscle contracts, causing the lens to become more rounded. This increases its refractive power and helps to see close objects.

• Axial Length - The axial length refers to the distance between the front and back of the eye. This distance plays an important role in determining the eye's refractive power. In people with myopia (nearsightedness), the axial length is longer than normal. This causes light to focus in front of the retina, resulting in blurry vision. In people with hyperopia (farsightedness), the axial length of the eye is shorter than normal. This causes light to focus behind the retina, resulting in blurry vision.

• Curvature of Cornea - The cornea is the clear outer layer of the eye that covers the iris and pupil. It is responsible for refracting most of the light that enters the eye. The cornea's curvature is also an important factor in determining the eye's refractive power. In people with astigmatism, the cornea is irregularly shaped, causing light to focus unevenly on the retina. This results in distorted or blurry vision.

What Is Connective Tissue Remodeling in the Myopic Eye?

In myopia, the eye lens does not have a long enough focal length to focus on faraway objects. In the face of eye development, the feedback mechanism modulates the scleral remodeling and changes the focal length to help visualize faraway objects. The eye length is adjusted to maintain clear vision called homeostatic control, where the properties are changed but affects the function of the retina and is able to sharply focusing. The retina of the eye sends signals to the fibroblasts of the sclera and alters its composition through mechanical properties, remodeling rate, and finally the axial length of the eye.

Other factors that cause axial length elongation are light-intensity contrast and visual indicators. Some changes increase the scleral modeling, while other visual changes slow down the remodeling process. The visual indicators allow a group of photoreceptor cells to determine whether the organ has to elongate faster or slower. These processes occur in a coordinated way without a connection to the brain, and they occur only at the cellular level of the retina. And finally, the organ level size changes occur for the eye to obtain clear vision.

What Is the Remodeling Mechanism in Myopia?

The homeostatic control mechanism explains the scleral remodeling and progression of myopia in individuals with common lens correction using a single-vision lens. In myopia, the eyes have a defocus on central and peripheral vision. Using a single-vision lens only corrects the central vision, and the hyperopic shift in the periphery remains the same.

The peripheral photoreceptor cells receive visual indicators that suggest the eye is too short, which accelerates the remodeling process of the sclera. In myopic eyes, the axial elongation causes increased connective tissue remodeling. The tissue growth in the posterior side of the sclera has not increased. During the development of myopia, the microstructural and material properties of the sclera keep changing as long as the remodeling is accelerated. The increase in properties returns to its normal level after stabilizing the remodeling process.

What Is an Emmetropization Mechanism?

Emmetropization is a mechanism of the eyes where the refractive components and axial length of the eye come into balance during post-nature development to reduce refractive error. This process occurs in early childhood and is complete by the age of six. Emmetropization occurs when the length of the eye and the optical power come into perfect balance.

Emmetropia refers to a condition where there is no refractive error. Infants are commonly born with hyperopia or farsightedness. If infants are born with myopia, the myopia is reduced and reaches a state of emmetropia by the age of 2-3 years.

How Remodeling Mechanism Utilized in the Treatment of Myopia?

In the highly myopic eye, there is significant thinning of the sclera. The thickness of the sclera comes to about half when compared with an emmetropic eye. The collagen fiber bundles are also thinner in myopic eyes. The remodeling mechanism can be utilized to increase the thickness of the sclera, which can help reduce the progression of myopia. Alterations in the biomechanical properties also helped in increasing scleral thickness, and the myopia is corrected.

• Posterior Scleral Reinforcement (PSR) - A surgical procedure performed for scleral strengthening, by governing the scleral biomechanical properties. PSR surgery is typically done in patients with the highest levels of pathological myopia. PSR surgery is done by placing donor tissue or synthetic materials that help in increasing the thickness of the sclera.

• Collagen Cross Linking - It is performed to increase the thickness and strengthen the sclera by scleral collagen crosslinking.

• Sub-Tenon’s Injections - These are scleral thickening injections that increase the collagen cross-linking and thickness of the sclera.

Conclusion

The sclera is a dynamic tissue of the eye that is capable of responding to alterations in the visual environment and resulting in changes in ocular size and refraction. The changes in shape and properties of the sclera are important in minimizing refractive error. Understanding the mechanisms of remodeling is important in the treatment of myopia for each patient, as the conditions differ in each individual.