Eye Refractive Errors
- Curvature of the Cornea: If the cornea is not perfectly spherical, then the image is refracted or focused irregularly to create a condition called astigmatism. A person can be nearsighted or farsighted with or without astigmatism.
- Curvature of the Lens: If the lens is too steeply curved in relation to the length of the eye and the curvature of the cornea, this causes nearsightedness. If the lens is too flat, the result is farsightedness.
- Farsightedness (hyperopia): A common type of eye refractive error error where distant objects may be seen more clearly than objects that are near. However, people experience farsightedness differently. Some people may not notice any problems with their vision, especially when they are young. For people with significant farsightedness, vision can be blurry for objects at any distance, near or far. Parallel rays converge at a focal point posterior to the retina. This is caused by an eye that is too short, whose cornea is not curved enough, or whose lens sits farther back in the eye than normal. Farsightedness is usually inherited. A person with one or more parents who are farsighted is likely to be farsighted also.
- Astigmatism: A condition in which the eye does not focus light evenly onto the retina, the light-sensitive tissue at the back of the eye. This can cause images to appear blurry and stretched out. Others will develop an astigmatism resulting from an injury to the eye or cornea. Astigmatism is usually it is caused by an irregularly shaped cornea. Usually astigmatism is hereditary. Still others may develop astigmatism resulting from surgery to the eye or from a disease resulting in thinning of the cornea (called keratoconus).
- Presbyopia: An age-related condition in which the ability to focus up close becomes more difficult. As the eye ages, the lens can no longer change shape enough to allow the eye to focus close objects clearly. However several factors are thought to increase the likelihood of developing presbyopia, including long-sightedness, an occupation which has near vision demand, ocular disease or trauma which causes damage to lens or its surrounding muscles, conditions such as diabetes and multiple sclerosis, and use of drugs such as alcohol, antidepressants and antihistamines. Greater exposure to ultraviolet radiation and higher temperature climate may also increase the risk of the condition.
- Contact lenses: Higher quality of optical image and less influence on the size of retinal image than spectacle lenses.
- Indication: Cosmetic, athletic activities, occupational, irregular corneal astigmatism, high anisometropia, corneal disease.
- Complication: Infectious keratitis, giant papillary conjunctivitis, corneal vascularization, severe chronic conjunctivitis.
- Give best optical correction for aphakia, avoid significant magnification and distortion caused by spectacle lenses.
- photorefractive keratectomy (PRK)
- LASIK (laser in situ keratomileusis)
- ICR (intracranial reinforcement)
- Phakic IOL
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