Eye Refractive Errors

Eye Refractive Error: Description, Causes and Risk Factors:Eye Refractive ErrorIn order to see, we must have light. A light ray can be deflected, reflected, bent or absorbed, depending on the different substances it encounters. When light travels through water or a lens, for example, its path is bent or refracted. Eye refractive error structures also have refractive properties similar to water or lenses and can bend light rays into a precise point of focus essential for sharp vision.Refractive errors are one of the most common and correctable visual impairment in the United States. According to a recent study led by the NEI (National Eye Institute), approximately half of all American adults do not have the 20/20 vision.Eye refractive error: Refractive errors are based on three eye anatomy features:Eye Length: If the eye refractive error is too long, light is focused before it reaches the retina, causing nearsightedness. If the eye is too short, light is not focused by the time it reaches the retina. This causes farsightedness or hyperopia.
  • 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.
Types may include of:Nearsightedness (myopia): A condition where objects which are close appear clearly, while objects far away appear blurry. With nearsightedness, light comes to focus in front of the retina instead of on the retina. Parallel rays converge at a focal point anterior to the retina. Myopia increase rapidly each year and is associated with vitreous opacities, fluidity of vitreous and chorioretinal change. Rate of increase in amount of myopia generally tapers off at about 20 years of age. The causes of myopia are unknown. Excessive amounts of reading, poor metabolism, poor diet; poor light, poor posture and genetic factors have been suggested.
  • 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.
Symptoms:The primary symptom of refractive errors is blurred vision for distant objects, near objects, or both. Sometimes, the excessive ciliary muscle tone can cause headaches. Occasionally, excessive staring can lead to ocular surface desiccation, causing eye irritation, itching, visual fatigue, foreign body sensation, and redness. Frowning when reading and excessive blinking or rubbing of the eyes are symptoms in children.Diagnosis:Refraction should be checked every year or two. Screening children helps detect refractive errors before they interfere with learning. A comprehensive eye examination should accompany refraction, whether done by an ophthalmologist or an optometrist.Treatment:Types of optical correction may include:Spectacle Lenses:Monofocal Lenses: Spherical lenses , cylindrical lenses, and multifocal lenses
  • 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.
Contact Lenses:Disadvantages: Careful daily cleaning and disinfection, expensive.
  • Complication: Infectious keratitis, giant papillary conjunctivitis, corneal vascularization, severe chronic conjunctivitis.
Intraocular Lenses:Replacement of cataract crystalline lens.
  • Give best optical correction for aphakia, avoid significant magnification and distortion caused by spectacle lenses.
Surgical correction:Keratorefractive surgery:Radial keratotomy (RK).
  • photorefractive keratectomy (PRK)
  • LASIK (laser in situ keratomileusis)
  • ICR (intracranial reinforcement)
  • Thermokeratoplasty
Intraocular surgery:Clear lens extraction with or without IOL (intraocular lens).
  • Phakic IOL
Disclaimer: The above information is educational purpose. The information provided herein should not be used during any medical emergency or for the diagnosis or treatment of any medical condition.DISCLAIMER: This information should not substitute for seeking responsible, professional medical care.

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