Risk factors for eye disease and injury: literature review

2.4 Eye disorders

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      2.4.1 Refractive errors (myopia, hyperopia, presbyopia, astigmatism)

      The term ‘refractive error’ encompasses myopia (near-sightedness or short-sightedness), hyperopia (far-sightedness or long-sightedness), presbyopia and astigmatism. Most people have one or more of these disorders, which are routinely corrected with glasses, contact lenses or surgery. The cornea, the lens and the axial length (the length of the eye) all contribute to the eye’s refractive power. When each eye has a different refractive error, this is known as anisometropia.

      Myopia

      Myopia occurs when images are formed in front of the retina because the eye is relatively too long or the refractive powers of the cornea and lens of the eye are relatively too strong. The result is a blurred image.

      Very high levels of myopia are associated other eye conditions such as cataracts, glaucoma and retinal detachment, which can lead to low vision and even legal blindness.

      Hyperopia

      Hyperopia occurs when images are formed behind the retina because the eye is relatively too short or the refractive powers of the cornea and lens of the eye are relatively too weak. The result is a blurred image.

      Astigmatism

      Astigmatism results from the cornea (or the lens) not being ‘perfectly’ round. A cornea or lens with such smooth surface curvature refracts all incoming light the same way and makes a sharply focused image on the retina. However, if your cornea or lens is not evenly and smoothly curved this causes a refractive error, resulting in a blurred image. Astigmatism of some degree is present in approximately 30–40% of people who wear glasses or contact lenses.

      Most investigations look at refractive error in the central part of the retina; however, there have been some studies that have shown that peripheral refractive errors can exist in eyes that have little central astigmatism.

      Presbyopia

      As we age, the lens gradually hardens and becomes less pliable, making it increasingly difficult to focus on near objects. By about 45 years of age, most people require reading correction. Many people assume that this correction is for hyperopia; however, the condition is called presbyopia.

      2.4.2 Amblyopia

      Amblyopia, also known as ‘lazy eye’, is the term used when vision in one eye is reduced because the pathways from the eye to the visual cortex of the brain do not develop or mature properly. The eye itself looks normal, but is not used normally because the brain favours the other eye. Amblyopia may be caused when the position of the two eyes is not balanced, or when one eye is more near-sighted, far-sighted or astigmatic than the other eye.
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