What is Low Vision?
Normal vision is the ability to see comfortably what is around us, whether far away or near, with or without glasses. This is vision between 20/20 and 20/30. Almost everyone can continue to manage their activities when changes in vision are small, which is vision between 20/30 and 20/60.
Normal vision is known as "20/20". This simply means that the eye being tested is able to see an object at 20 feet as well as any eye with very good vision. If you have 20/60 vision, this means you can see at 20 feet what a person with good vision can see at 60 feet. As changes in vision become larger, most people experience more and more difficulty in their efforts to continue their usual visual activities, even with the best possible glasses or contact lenses. If this is a change in vision between 20/60 and 20/190 it is called being partially sighted or having low vision. If the change in vision is to 20/200 or worse, the person will still keep some vision but will be classified as "blind" (some people may be classified as blind if their field of vision, or the area that they can see, is less than 20° across--even if their vision is better than 20/200).
Low vision can occur at any age, but by far the greatest number of people who are partially sighted are the elderly. Low vision is most often due to a change in central vision. Occasionally it is associated with loss of side (peripheral) vision when it is close to centre. In a few cases it is associated with loss of colour vision or difficulty adapting to changes in brightness within the field of vision.
What Causes Low Vision?
A variety of disorders that affect the eye and the visual system may cause low vision. Birth defects, injuries, certain diseases of the body and aging all may lead to loss of sight. Most commonly, it is due to scarring because of deterioration of the central part of the retina (the light-sensitive tissue that lines the back of the eye). Loss of sight may also result from other conditions, such as cataract and glaucoma, or from damage to the optic nerve, which carries visual images to the brain.
Who Can Diagnose the Cause of Low Vision?
Since there are many diseases and conditions that can result in low vision, it is important to have a comprehensive eye examination by an ophthalmologist, a medically trained eye doctor. Once the cause of low vision is identified, your ophthalmologist may refer you to other low-vision and rehabilitation specialists or suggest low-vision aids such as magnifying glasses or special, strong reading glasses. There are also many other helpful devices that are not optical devices including large-print books, magazines, newspapers, playing cards and telephone dials, and large-character calculators. Increased lighting that is properly positioned is essential. Tinted lenses are sometimes used to reduce glare from bright sunlight. Electronic aids such as closed-circuit television systems with built-in magnification and computerized reading devices are also useful in some circumstances.
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