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A refractive disorder is an ocular condition caused by changes in the shape of the eye, which prevents light from being focused sharply on the retina, creating unclear images. The causes may range from congenital shortening or lengthening of the eyeball through variations in the shape of the cornea, to anomalies of the lens.
Refraction is the process of bending of light that occurs when it passes through the cornea and the lens. This helps direct light from the objects we view exactly on the macula, the part of the retina that has the greatest number of cone cells. Cone cells are the photoreceptors responsible for detailed and sharp vision.
Refractive disorders are of the following main types:
Myopia, or short-sightedness, occurs when the eye refracts light so much that the rays converge to a spot in front of the retina, leading to a blurred image when one looks at objects that are beyond a certain distance. However, objects that are close by can be seen clearly, as the light rays are divergent at their origin and so undergo the right amount of refraction. The eyeball in such people may be too long, or the cornea may bulge more than usual, leading to the focusing of light before it reaches the retina. The condition is usually diagnosed in childhood, between 8 and 12 years. It stabilizes in the years between 20 and 40 in most cases. A family history of myopia may predispose to the condition. It is correctable using prescription eyeglasses, contact lenses or refractive surgery such as keratoplasty. People with high myopia have a greater risk of future retinal detachment.
Hypermetropia is also called long-sightedness. It occurs when light rays are refracted too little, and so focus beyond the retina, leading to poor vision for near objects. However, light from more distant objects is parallel and so usually comes to a focus at the retina with this lower level of refraction. Some hyperopic people cannot see well regardless of the distance of the object. The eyeball may be shorter, or the cornea flatter, than usual, or the lens may have less refractive power than is normal. Like myopia, eyeglasses, contact lenses or surgery may be recommended for correction.
Presbyopia is a refractive error resulting from aging. As the lens inside the eye ages it loses its power to change shape with the pull of the ciliary muscles in response to the need for greater or less refractive capability. As a result, it becomes less able to accommodate itself so as to focus light from nearby objects clearly. Near vision becomes a problem, leading to the characteristic picture of people over the age of 35 or 40 holding books farther away than usual in order to see them clearly. Eyeglasses are the best way to correct presbyopia. Bifocal glasses have two different refractive powers, with the lower part being designed for reading and the upper part for distant vision.
Astigmatism is a condition in which the shape of the cornea is uneven. For instance, it may be curved more in one direction than the other, or show areas with different curvatures. As a result, light rays are scattered and come to focus at different spots on the retina rather than forming a single sharp image. This leads to blurry vision. Eyeglasses are usually recommended, but contact lenses to smooth the refractive surface, and refractive surgery to chisel the cornea to a smooth contour, may also work well.