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Visual Impairment

Visual Impairment


Total blindness is the inability to tell light from dark, or the total inability to see. Visual impairment or low vision is a severe reduction in vision that cannot be corrected with standard glasses or contact lenses and reduces a person's ability to function at certain or all tasks. Legal blindness (which is actually a severe visual impairment) refers to a best-corrected central vision of 20/200 or worse in the better eye or a visual acuity of better than 20/200 but with a visual field no greater than 20° (e.g., side vision that is so reduced that it appears as if the person is looking through a tunnel).


Vision is normally measured using a Snellen chart. A Snellen chart has letters of different sizes that are read, one eye at a time, from a distance of 20 ft. People with normal vision are able to read the 20 ft line at 20 ft-20/20 visionor the 40 ft line at 40 ft, the 100 ft line at 100 ft, and so forth. If at 20 ft the smallest readable letter is larger, vision is designated as the distance from the chart over the size of the smallest letter that can be read.

Eye care professionals measure vision in many ways. Clarity (sharpness) of vision indicates how well a person's central visual status is. The diopter is the unit of measure for refractive errors such as nearsightedness, farsightedness, and astigmatism and indicates the strength of corrective lenses needed. People do not just see straight ahead; the entire area of vision is called the visual field. Some people have good vision (e.g., see clearly) but have areas of reduced or no vision (blind spots) in parts of their visual field. Others have good vision in the center but poor vision around the edges (peripheral visual field). People with very poor vision may be able only to count fingers at a given distance from their eyes. This distance becomes the measure of their ability to see.

The World Health Organization (WHO) defines impaired vision in five categories:

  • Low vision 1 is a best corrected visual acuity of 20/70.
  • Low vision 2 starts at 20/200.
  • Blindness 3 is below 20/400.
  • Blindness 4 is worse than 5/300
  • Blindness 5 is no light perception at all.
  • A visual field between 5° and 10° (compared with a normal visual field of about 120°) goes into category 3; less than 5° into category 4, even if the tiny spot of central vision is perfect.

Color blindness is the reduced ability to perceive certain colors, usually red and green. It is a hereditary defect and affects very few tasks. Contrast sensitivity describes the ability to distinguish one object from another. A person with reduced contrast sensitivity may have problems seeing things in the fog because of the decrease in contrast between the object and the fog.

According to the WHO there are over forty million people worldwide whose vision is category 3 or worse, 80% of whom live in developing countries. Half of the blind population in the United States is over 65 years of age.

Causes and symptoms

The leading causes of blindness include:

  • macular degeneration
  • glaucoma
  • cataracts
  • diabetes mellitus

Other possible causes include infections, injury, or nutrition.


Most infectious eye diseases have been eliminated in the industrialized nations by sanitation, medication, and public health measures. Viral infections are the main exception to this statement. Some infections that may lead to visual impairment include:

  • Herpes simplex keratitis. A viral infection of the cornea. Repeated occurrences may lead to corneal scarring.
  • Trachoma. This disease is responsible for six to nine million cases of blindness around the world, of the third of a billion who have the disease. Trachoma is caused by an incomplete bacterium, Chlamydia trachomatis, that is easily treated with standard antibiotics. It is transmitted directly from eye to eye, mostly by flies. The chlamydia gradually destroy the cornea.
  • Leprosy (Hansen's disease). This is another bacterial disease that has a high affinity for the eyes. It, too, can be effectively treated with medicines.
  • River blindness. Much of the tropics of the Eastern Hemisphere are infested with Onchocerca volvulus, a worm that causes "river blindness." This worm is transmitted by fly bites and can be treated with a drug called ivermectin. Nevertheless, twenty-eight million people have the disease, and 40% of them are blind from it.

Other causes

Exposure of a pregnant woman to certain diseases (e.g., rubella or toxoplasmosis ) can cause congenital eye problems. Injuries to the eyes can result in blindness. Very little blindness is due to disease in the brain or the optic nerves. Multiple sclerosis and similar nervous system diseases, brain tumors, diseases of the eye sockets, and head injuries are rare causes of blindness.


Vitamin A deficiency is a widespread cause of corneal degeneration in children in developing nations. As many as five million children develop xerophthalmia from this deficiency each year. Five percent end up blind.


A low vision exam is slightly different from a general exam. While a case history, visual status, and eye health evaluation are common to both exams, some things do differ. Eye charts other than a Snellen eye chart will be used. Testing distance will vary. A trial frame worn by the patient is usually used instead of the instrument containing the lenses the patient sits behind (phoropter). Because the low vision exam is slightly more goal oriented than a general exam, for example, what specifically is the patient having trouble with (reading, seeing street signs, etc.) different optical and nonoptical aids will generally be tried. Eye health is the last thing to be checked so that the lights necessary to examine the eyes will not interfere with the rest of the testing.


There are many options for patients with visual impairment. There are optical and nonoptical aids. Optical aids include:

  • Telescopes. May be used to read street signs.
  • Hand magnifiers. May be used to read labels on things at the store.
  • Stand magnifiers. May be used to read.
  • Prisms. May be used to move the image onto a healthy part of the retina in some eye diseases.
  • Closed circuit television (CCTV). For large magnification (e.g., for reading).

Nonoptical aids can include large print books and magazines, check-writing guides, large print dials on the telephone, and more.

For those who are blind, there are enormous resources available to improve the quality of life. For the legally blind, financial assistance for help may be possible. Braille and audio books are increasingly available. Guide dogs provide well-trained eyes and independence. Orientation and mobility training is available. There are special schools for blind children and access to disability support through Social Security and private institutions.


The prognosis generally relates to the severity of the impairment and the ability of the aids to correct it. A good low vision exam is important to be aware of the latest low vision aids.


Regular eye exams are important to detect silent eye problems (e.g., glaucoma). Left untreated, glaucoma can result in blindness.

Corneal infections can be treated with effective antibiotics. When a cornea has become opaque beyond recovery it must be transplanted. Good hygiene (e.g., washing hands frequently) to prevent infection, proper use of contact lenses, and not sharing makeup are just some ways to guard against corneal infections.

Cataracts should be removed when they interfere with a person's quality of life.

Primary prevention addresses the causes before they ever begin. Fly control can be accomplished by simple sanitation methods. Public health measures can reduce the incidence of many infectious diseases. Vitamin A supplementation (when appropriate) will eliminate xerophthalmia completely. It is possible that protecting the eyes against ultraviolet (UV) light will reduce the incidence of cataracts, macular degeneration, and some other eye diseases. UV coatings can be placed on regular glasses, sunglasses, and ski goggles. Patients should ask their eye care professional about UV coatings. Protective goggles should also be worn in certain situations (e.g., certain jobs, sports, even mowing the lawn).

Secondary prevention addresses treating established diseases before they cause irreversible eye damage. Having general physical checkups can also detect systemic diseases such as diabetes or high blood pressure. Control of diabetes is very important in preserving sight.



American Academy of Ophthalmology. 655 Beach Street, P.O. Box 7424, San Francisco, CA 94120-7424.

American Foundation for the Blind. 11 Penn Plaza, Suite 300, New York, NY 10001. (800) 232-5463.

Guide Dogs for the Blind. P.O. Box 1200, San Rafael, CA 94915. (415) 499-4000.

International Eye Foundation. 7801 Norfolk Ave., Bethesda, MD 20814. (301) 986-1830.

Lighthouse National Center for Education. 111 E. 59th Street. New York, NY 10022. (800) 334-5497.

National Association for the Visually Handicapped. 22 West 21st St., New York, NY 10010. (212) 889-3141.

National Center for Sight. (800) 221-3004.

National Children's Eye Care Foundation. One Clinic Center, A3-108, Cleveland, OH 44195. (216) 444-0488.

National Eye Institute. 2020 Vision Place, Bethesda, MD 20892-3655. (301) 496-5248.

National Federation of the Blind. 1800 Johnson St., Baltimore, MD 21230. (301) 569-9314.

Prevent Blindness America. 500 East Remington Road, Schaumburg, IL 60173. (800) 331-2020.

Research to Prevent Blindness. 598 Madison Ave., New York, NY 10022. (212) 363-3911.


Cornea The clear dome-shaped structure that is part of the front of the eye. It lies in front of the colored part of the eye (iris).

Diabetic retinopathy Retinal disease caused by the damage diabetes does to small blood vessels.

Phoropter The instrument used to measure refractive status of the eyes. It contains many lenses which are then changed in front of the eyes while the patient is looking at an eye chart. This is when the doctor usually asks, "Which is better, one or two?"

Xerophthalmia A drying of the cornea and conjunctiva.

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Vision Disorders


Human vision is dependent on the successful interaction of optical structures in the eye. When these structures malfunction, vision disorders occur. The key to treatment and resolution of these disorders is early detection through regular eye exams and prompt consultation with an ophthalmologist when problems occur.

The best way to describe how vision works is to use the analogy of a camera. The pupil manages the incoming light rays, opening and closinglike a camera shutteraccording to the amount of light available. These light rays are progressively refracted and focused by three structures: the cornea, a transparent, convex cover over the iris and pupil in front of the eye; the lens, a spherical body behind the cornea, and the vitreous humor, a gelatinous substance that fills the back of the eyeball. It is important that the rays be in sharp focus when they reach the retina, a sensory membrane that lines the back of the eye and acts like film in a camera. The retina converts the light rays into electrical signals that are sent to the brain by way of the optic nerve. The brain then translates these electrical signals into what we know as sight.

Refractive Errors. The most common vision disorders are refractive errorsspecifically nearsightedness, farsightedness, and astigmatism. In each case, the eye does not refract the incoming light properly, so the image is blurred. While they are not diseases, refractive errors affect every age range and comprise the largest treatment effort of ophthalmologists. Refractive errors can be successfully corrected with eyeglasses, contact lenses, and laser refractive surgery.

Cataract. A cataract results when the normally transparent lens of the eye clouds, blurring vision. Most cataracts are age-related, advancing slowly and progressively until functional blindness occurs. Cataract cannot be prevented or cured with medication or optical devices, but it can be successfully treated through a surgical procedure that removes the damaged, natural eye lens and replaces it with a permanent, intraocular lens implant. The procedure has over a 90 percent success rate. After refractive errors, cataract is the most common vision disorder.

Macular Degeneration. Located in the retina, the macula is responsible for central vision. When people have macular degeneration, they can no longer bring the center of the picture they see into focus. The most common type of the disease is agerelated, and there are two forms: "wet" and "dry." Whereas the wet form comprises only about 10 percent of cases, it causes the greatest vision loss, striking quickly and without warning as a result of erupting blood vessels. The dry form is characterized by a slow, progressive loss of vision from the thinning and tearing of the macula. Although both forms are being extensively researched, definitive causes and treatments have not yet been identified. Age-related macular degeneration is the leading cause of blindness in most developed countries.

Glaucoma. Glaucoma is a disease of the optic nerve. If the aqueous humor (the clear fluid that fills the front of the eye) does not drain properly, intraocular pressure builds, damaging the optic nerve and causing blind spots to develop. When the entire nerve is destroyed, blindness results. If glaucoma is detected and treated in the early stages, loss of vision can be averted. However, the disease is chronic and cannot be cured or reversed. Unfortunately, the early stages are symptomless. Once symptoms occur, usually manifested by loss of peripheral or side vision, irreversible vision loss has already taken place. Treatment consists of medication and/or surgery, depending on the type of glaucoma, the patient's medical history, and the stage of the disease. Glaucoma is the leading cause of blindness worldwide and the second-leading cause in developed countries.

Diabetic Retinopathy. Retinopathy is a side effect of diabetes and occurs as a result of fluctuations in the body's blood sugar, a daily problem for diabetics. When blood sugar fluctuates over time, it affects the blood vessels in various parts of the body, including the retina of the eye, where the blood vessels can break and bleed, causing blurred vision. The longer a person has diabetes, the higher the risk of retinopathy; good diabetic control can forestall the disease, however. Signs of retinopathy often occur before symptoms appear. Treatment includes the use of laser photocoagulation to seal leaking blood vessels. Often undetected and untreated, diabetic retinopathy is the leading cause of visual disability among working-age people.

Retinal Degenerations. Retinal degeneration is an umbrella term for a number of hereditary and degenerative disorders that range from mild to profound vision loss and blindness. Retinitis pigmentosa is the most common type of retinal degeneration, affecting one in three thousand people. Its many forms have widely varied symptoms, and onset and progress of the disease can be slow or rapid. In general, symptoms occur in childhood or young adulthood. Patients complain of night blindness followed by loss of visual field. There is no treatment, though researchers are hopeful that genetic therapies may be possible in the future.

Strabismus. Unlike most other vision disorders, strabismus is a physical defect. One or both of the eyes are misaligned and point in different directions. One eye may look ahead while the other eye points up, down, in, or out. Strabismus is more common in children than in adults. In adults it can be a side effect of head trauma or brain disorder. Treatment may involve eyeglasses, an eye patch (in some cases), or surgery on the eye muscles.

Barbara L. Pawley

(see also: Diabetes Mellitus )


O'Toole, M. (1997). Miller-Keane Encyclopedia & Dictionary of Medicine, Nursing, & Allied Health, 6th edition. Philadelphia, PA: W.B. Saunders.

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Vision Disorders



Vision disorders are conditions caused by the eye's inability to properly focus light rays. Three major types of vision disorder are hyperopia (pronounced HY-puh-ROH-pee-uh), or farsightedness; myopia (pronounced my-OH-pee-a) or nearsightedness; and astigmatism, which causes the eye to form a blurred image of an object.


Light that strikes the eyeball first passes through the cornea. The cornea is the tough, transparent covering at the front of the eye. It is shaped like a dome, with the top of the dome facing outward. Light then passes through the lens. The lens is located just inside the eyeball, behind the cornea. The lens has a double-convex shape. That is, it bulges outward in the middle.

The cornea and lens bend light as light waves pass through them. This process is known as refraction. In a normal eye, the light waves are brought to focus on the retina. The retina is a thin membrane at the back of the eye. Light waves cause chemical changes in the retina. These chemical changes set off a signal that passes through the optic nerve to the brain. The brain reads that signal as a visual image.

Muscles in the front of the eye can change the shape of the lens. These muscles adjust the lens shape to see objects close at hand or far away. This process is known as accommodation.

Visual disorders develop when the cornea and/or the lens do not properly focus light waves. The light waves do not come to a focus (that is, they do not all reach the same point) on the retina. They may come to a focus in front of the retina or behind the retina.

When light waves come to a focus in front of the retina, the person has myopia, or nearsightedness. People who are nearsighted can see objects close at hand, but they cannot see objects far away clearly. When light waves come to a focus behind the retina, the person has hyperopia, or farsightedness. People who are farsighted (hyperopic) can see objects far away but cannot clearly see objects close at hand.

In healthy eyes the cornea has a smooth, regular shape. Some people have corneas with an ellipsoidal shape, like a football or a squashed baseball. Such corneas scatter light waves across the retina, causing objects to appear as blurred images. This condition is called astigmatism.

Vision Disorders: Words to Know

The ability of the lens to change its shape in order to focus light waves from distant or near objects.
A condition in which light from a single point fails to focus on a single point of the retina. The condition causes the patient to see a blurred image.
The clear, dome-shaped outer covering of the front of the eye.
The unit of measure used for the refractive (light bending) power of a lens.
Farsightedness. A condition in which vision is better for distant objects than for close ones.
In the eye, a transparent, elastic, curved structure that helps focus light on the retina.
Nearsightedness. A condition in which far away objects appear fuzzy, because light from a distance doesn't focus properly on the retina.
Optic nerve:
A bundle of nerve fibers that carries visual signals from the retina to the brain.
Radial keratotomy (RK):
A surgical procedure in which the shape of the cornea is changed in order to correct myopia.
The bending of light waves as they pass through a dense substance, such as water, glass, or plastic.
A membrane at the back of the eye that is sensitive to light and that converts light waves into signals sent to the brain by way of the optic nerve.


Most vision disorders are thought to be inherited. That is, the disorders are passed down from one generation to the next. Most babies are born slightly hyperopic. However, this pattern changes as they grow older. In most cases, children develop normal vision. They may, however, become more hyperopic, or they may develop myopia.

Some eyecare specialists argue that vision disorders may be caused by a number of factors. For example, they think that factors in the environment, such as work conditions, stress, and eye strain, can contribute to the development of vision disorders.

The development of astigmatism has been attributed to a number of factors. For example, people with allergies often rub their eyes repeatedly. This constant rubbing may cause damage to the cornea. Diabetes (see diabetes mellitus entry) is also known to change the shape of the cornea over time.


The primary symptom of any vision disorder is the inability to see objects clearly. Objects near by or far away will seem blurry to a person with farsightedness or nearsightedness. Objects at almost any distance will seem blurry to a person with astigmatism.

Blurry vision may lead to other problems. People who have trouble seeing clearly may begin to squint in order to see better. Constant squinting can lead to discomfort, headaches, and eyestrain.


Vision disorders are sometimes difficult to diagnose. Many people with vision disorders are not aware of their problem. They have often grown up seeing the world slightly out of focus and their view of objects may seem perfectly normal.

As a result, many vision problems are detected by others. For example, school teachers may realize that a pupil is unable to read the blackboard. Or a parent may notice that a child is constantly squinting while trying to read. In such cases, the individual is referred to an eye specialist for examination.

Many tests are available for detecting vision disorders. The most common tests involve the use of the familiar eye chart. The eye chart contains rows of letters of decreasing size going from top to bottom. The patient is asked to read each row on the chart. The last row the patient is able to read tells how accurate is his or her vision.

The results of an eye chart test are usually indicated by a pair of numbers, such as 20/50. These numbers show how well a person can see an object compared to a person with normal eyesight. A person with 20/50 vision can see at 20 feet from an eye chart what a person with normal vision can see at 50 feet from the chart.

Vision disorders are measured by having a patient look through various lenses at an eye chart. The lens are changed until the patient is able to see the letters on the chart clearly. The degree of error in the patient's eye is determined by the lens needed to produce perfect vision. This degree of error is measured in units called diopters (abbreviated: D). The prescription a doctor writes for glasses will indicate the shape of the lens (the diopter measurements) needed to provide perfect vision.

A simple test for astigmatism consists of a dial containing lines that radiate outward from the center. A person with astigmatism will be able to see some lines more clearly than other lines.

Doctors may also use direct tests on the eye. For example, a device known as a corneal topographer can be used to measure the shape of the cornea and determine whether the shape is correct or not. Doctors can also shine a light directly at the retina in a person's eye to look for any damage.


Traditionally, the usual treatment for vision disorders has been fitting the patient with eyeglasses. Eyeglasses consist of glass or plastic lenses set in a

frame. These corrective lenses compensate (make up) for any errors in the patient's cornea and lens. If the patient's lens is too flat, for example, the lenses in the eyeglasses will be more round. Hyperopia, myopia, and astigmatism can all be treated very effectively, easily, and inexpensively with eyeglasses.

Some patients prefer to use contact lenses instead of eyeglasses. The principle behind contact lenses is the same as that behind eyeglasses. In the case of contact lenses, however, the corrective lens is placed directly on the eye.

Many vision disorders can now be treated surgically. An example is radial keratotomy (RK). Radial keratotomy (pronounced RAY-dee-uhl KARE-uh-TOT-uh-mee) is now a common procedure for the treatment of myopia. In RK, a doctor makes very small incisions (cuts) in the cornea to change its shape. The goal is to produce a cornea that has exactly the right shape to produce normal vision.

Traditional RK surgery is done with a very small diamond-tipped blade. Another approach now being studied involves the use of a laser beam to make the necessary incision. By 1999 this procedure had not yet received government approval in the United States for general use.

A form of RK surgery with lasers has been approved for treatment of astigmatism. The procedure is still quite new, however, and a patient should consider the risk carefully before choosing RK surgery.


Most cases of vision disorder can be treated successfully with eyeglasses or contact lenses. Surgical procedures are often successful, as well. However,

many of surgical procedures are still in the early stages of development and carry considerably more risk than the use of eyeglasses or contact lenses.


All forms of vision disorder are regarded as genetic. To the extent that is true, there is nothing a person can do to avoid vision problems. However, some experts believe that environmental factors may determine whether or not vision disorders develop. They suggest that maintaining good nutrition, reading and working in good light, and wearing corrective lenses as prescribed can reduce the risk of eye problems.

Regular eye examinations are also important. Eye characteristics change over time, so new corrective lenses may be needed occasionally. Also, eye examinations detect other types of eye disorders, such as glaucoma (see glaucoma entry), that are more serious than vision disorders.

See also: Color blindness.



Rosanes-Berrett, Marilyn B. Do You Really Need Eyeglasses? Barrytown, NY: Station Hill Press, 1990.

Zinn, Walter J., and Herbert Solomon. Complete Guide to Eyecare, Eyeglasses, and Contact Lenses. Hollywood, FL: Lifetime, 1996.


"Insight on Eyesight: Seven Vision Myths: Blind Spots about Vision Can Cause Needless Worry, Wasted Effort, and Unnecessary Treatment." Consumer Reports on Health (April 1997): p. 42.


American Academy of Ophthalmology. PO Box 7424, San Francisco, CA 941207424. (800) 222EYES.

American Optometric Association. 243 North Lindbergh Boulevard, St. Louis, MO, 63141. (314) 9914100.

International Myopia Prevention Association. RD No. 5, Box 171, Ligonier, PA 15658. (412) 238-2101.

Myopia International Research Foundation. 1265 Broadway, Room 608, New York, NY 10001. (212) 684-2777.

National Eye Institute. 2020 Vision Place, Bethesda, MD 208923655. (301) 496-5248.

Web sites

The Refractive Surgery Patient Resource Center. [Online] (accessed on November 1, 1999).

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photophobia (foh-toh-foh-biă) n. an abnormal intolerance of light, in which exposure to light produces intense discomfort of the eyes with tight contraction of the eyelids. Photophobia may be associated with migraine, measles, German measles, meningitis, iritis, or corneal injury.

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