Refractive errors of the eye

Principles of refraction

Before analysing the most common types of refractive errors, it is important to discuss the fundamental principles of refraction. Refraction is the bending of light rays when they pass through different mediums, in this case the components of the human eye. The ability of the eye to converge light into the retina is called refractive power, measured in diopters. Most of the ocular refractive power comes from the cornea, followed by the lens providing additional refractive power when needed (accommodation), to focus on different objects at various distances. Under normal conditions light rays are precisely converged to the retina providing clear, detailed images. A state of normal vision is called emmetropia. However, abnormalities in the mediums involved in refraction cause pathologies.

Myopia

Myopia, commonly referred to as nearsightedness, is characterized by the inability of the eye to focus on distant objects, while focusing on nearby objects with no limitation. This phenomenon occurs when the refractive power of the eye is too high in relation to the length of the eyeball. Various pathologies can lead to myopia, the most common being the increase of the axial length of the eye’s globe, where the condition is called axial myopia. Meridional myopia, often a manifestation of astigmatism, occurs when the curvature of the cornea and the lens is irregulated. Lenticular myopia is caused by the increase of the refractive power of the lens. As a result of all these conditions, images of distant objects form in front of the retina, thus appearing blurred at the fovea. However, objects close to the viewer are focused properly on the retina with no need for accommodation. Due to the problems widespread nature, there have been developed many effective treatments. The most common treatment of myopia are concave lenses in the form of eyeglasses and contact lenses. These lenses displace the focus of the image posteriorly from the vitreous body to the retina, resulting in clear image perception. Another option for individuals seeking permanent treatment is refractive surgery, where the cornea is surgically forged to properly focus light.

Hyperopia

Hyperopia, also called farsightedness, is a refractive error that involves difficulty in seeing nearby objects while distant objects can be seen relatively clearly. In contrast with myopia, here the refractive power of the eye is too small, in relation to its length, causing parallel light rays coming from infinity to focus behind the retina, with accommodation at rest. Younger individuals can compensate for this abnormality by accommodation, where an alteration in the lens’ curvature adjusts the total refractive power of the eye to focus light rays directly on the retina. To bring nearby objects to focus much more accommodation is needed therefore increasing the risk of asthenopic symptoms (eye pain, redness, headaches etc.). It is important to note that with high hyperopia both near and distant visions can be defective. Additionally, as accommodation diminishes with age, hyperopia might increase. The most prevalent cause of hyperopia is the axial shortening of the eye where the condition is called axial hyperopia. Other causes include the flattening of the cornea and the lens (curvature hyperopia) and abnormalities of the refractive indexes of the ocular media (index hyperopia). The correctional procedures of hyperopia target to calibrate the eyes abnormal focus. The usual treatment for hyperopia is convex lenses in eyeglasses and contact lenses that positively affect the refractive power of the eye. Thus, the picture of distant objects previously shaped behind the retina is displaced again on the retina, providing clear pictures. Refractive surgery is another option for the treatment of hyperopia where the normal refractive properties of the eye are restored.

Presbyopia

Presbyopia, commonly referred to as age-related farsightedness, is a naturally progressive refractive error of the eye where the ability to focus on nearby objects is restricted. It usually develops in the middle of the fifth decade of life. Presbyopia is characterized by loss of the lens’ accommodative ability. As a result, individuals with this refractive error have clear distant vision but defective nearby vision, as the portrait of those objects forms behind the retina due to the lack of accommodative refractive power. The diminishment of the eye’s accommodative ability most commonly occurs due to stiffening of the ocular lens and/or changes in the area of the zonular fibers and the ciliary body. The typical symptom is ‘’short arms’’, where the presbyopic patient finds that they can’t hold a text far enough to read clearly. Additional symptoms include eye strain, fatigue and/or headaches. The usual treatment of presbyopia involves reading glasses, which can be single vision addressing only near vision refractive complications, while bifocal lenses for treating both distant and near vision errors are also available. Surgical treatment by means of monovision is the most common operative method to treat presbyopia. In this procedure one eye is corrected for distance and the other for near vision. This procedure, though, often comes at the cost of some vision quality.

Astigmatism

Astigmatism is a refractive error in which visual distortions occur at any distance. This phenomenon occurs due to rotational asymmetry in the eye’s refractive power. As a result, parallel light rays do not converge on a focal point on the retina but form separate focal lines. This refractive error can be caused by corneal, lenticular or retinal abnormalities.

Astigmatism can be divided into categories based on the axis and the focus of the principal meridian. Based on the orientation of the axis there are two types of astigmatism, regular and irregular. In regular astigmatism meridians are perpendicular to each other while in irregular astigmatism meridians are not perpendicular. Regular astigmatism is further divided into three subcategories, with-the-rule, against-the-rule, and oblique astigmatism. In with-the-rule astigmatism the vertical meridian is the steepest, whereas in against-the-rule astigmatism the horizontal meridian is the steepest. In oblique astigmatism, the steepest curve lies between 120-150 and 30-60 degrees.

Based on the focus of the principal meridian astigmatism is divided into simple, compound and mixed. Simple astigmatism involves the focus of the light rays in one meridian in front of the retina where it is called simple myopic astigmatism, while when focused behind the retina it is called simple hypermetropic astigmatism. If the light rays in both meridians are focused in front of the retina or behind the retina, these are labeled compound myopic and compound hypermetropic astigmatism, respectively. In mixed astigmatism light rays in one meridian are focused in front of the retina while light rays from the other meridian behind the retina, thus in this condition one meridian is myopic and the other hypermetropic. As with other refractive errors, treatment for astigmatism includes eyeglasses, contact lenses and refractive surgery options.

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