How common are refractive errors?
What harm do refractive errors cause?
What can be done about poor vision due to uncorrected refractive error?
What is a refractive error?
A refractive error is an optical defect of the eye that results in light
not being focused clearly on the retina. If an image comes into focus
in front of the retina, this is called myopia, and the person is nearsighted
- able to see more clearly at near distance than at far distance. If
the image comes into focus behind the retina, a condition called hyperopia,
then the person is farsighted and able to see better at far distance
than at near distance. Other types of refractive errors include astigmatism
and presbyopia. Astigmatism represents "uneven" focusing in different
planes of the eye. Presbyopia refers to the difficulty focusing up close
and occurs with aging, resulting in the need for reading glasses in
many people over the age of 40. The cause of refractive errors is unknown.
For more-detailed information about refractive conditions and vision correction, see the webpage "Vision Correction" (under "Prevention and Care") and the pdf document "Do You or Your Child Need Glasses"; both are on this website. The American Optometric Association also provides helpful information about these eye conditions.
How common are refractive errors?
Refractive errors, and the need to correct them, are extremely common all over the world. For example, refractive errors affect approximately one-third of persons 40 years or older in the United States and Western Europe. About 150 million Americans wear spectacles or contact lenses to correct refractive errors. Myopia is more common than hyperopia; among Americans over the age of 40, about 30 million have myopia and at least 12 million have hyperopia. While the prevalence of myopia decreases with age, that of hyperopia increases. The prevalence of myopia is greater in women than men up to the age of about 60 years.
What harm do refractive errors cause?
To permit clear vision, myopia, hyperopia, and astigmatism must be corrected, usually with eyeglasses. Uncorrected refractive error can result in visual impairment ranging from mild blurring to legal blindness, depending on the magnitude of the refractive error. If the refractive error of the two eyes is very different and this is not corrected in childhood, then permanent loss of vision, called amblyopia, can result.
It has been estimated that more than a half of vision impairment, and about a quarter of legal blindness, is due to refractive error correctable with glasses. The number of people with poor vision due to uncorrected (or undercorrected) refractive error increases greatly in older age groups. In Australia this affects more than a third of the elderly folks who undergo an aged-care assessment. Particularly in older people, poor eyesight can lead to accidents, social withdrawal, and loss of independence.
What can be done about poor vision due to undercorrected refractive error?
In industrialized countries like the U.S., people need to be aware that they should visit an eye-care professional (optometrist or ophthalmologist) if they notice any change in their vision, such as fuzziness of a movie or slowed reading speed. The trouble is that such changes often take place over a long time, and may not be noticed. Therefore, it is recommended that everyone over age 50 see an eye specialist regularly. See "Checkups" under "Prevention and Care" on this website for a schedule. The National Institutes of Health is aware that education is needed; one of the goals in its "Healthy People 2010" campaign is to reduce uncorrected visual impairment due to refractive errors (NEI Press Release June 1, 2000)
In the developing world, the problem is often lack of resources - money, trained medical personnel, and transportation to the clinic. Furthermore, there is often social pressure against women seeking medical care and, in some cases, against their wearing glasses. A partnership among the World Health Organization, the US National Eye Institute, and local implementing partners has made correction of refractive error throughout the world one of its chief goals. See: Refractive Errors and Vision 2020 for more on this topic.
References
Ellwein, LB. Case finding for Refractive errors: Assessment of refractive error and visual impairment in children. Commun Eye Health. 2002; 15:37-38.
The Eye Diseases Prevalence Group. Causes and prevalence of visual impairment among adults in the United States. Arch Ophthalmol. 2004; 122:477-485.
The Eye Diseases Prevalence Group. The prevalence of refractive errors among adults in the United States, Western Europe, and Australia. Arch Ophthalmol. 2004; 122:495-505.
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