Blindness is the absence of all or most vision.
Corrine tried to imagine how she would describe a bird to her sister Amy, who might never be able to see one. She could say that it is small, that it has feathers, and that it sings. At least, those are the characteristics her sister can feel and hear. But how could she describe the red of a male cardinal and distinguish it from the reddish-brown of a female cardinal, when Amy might never see anything at all? And what about all the other things Corrine's sister might not see: the television shows and the movies, the picture books Corrine saved to give her, the blue sky, the faces of their mom and dad?
Corrine's sister Amy was born prematurely * . The doctors told Amy's family that she had retinopathy (ret-i-NAH-puh-thee) of prematurity. This condition occurs when the blood vessels in the eyes of a premature baby grow abnormally and cause bleeding and scarring. Retinopathy can result in total or partial blindness.
Blindness is complete loss of vision, as well as seriously compromised vision, as described in the section on diagnosis. A variety of diseases and injuries can cause blindness. Some people can take advantage of treatments to restore partial or complete eyesight, but many others who are blind will remain so for the rest of their lives.
Blindness can result from several breakdowns in the visual system, which includes the eyes, the brain, and the nerve pathway that connects them. The first steps in vision occur when light enters the eye's pupil and passes through a transparent lens, which focuses it on the retina * . The retina contains millions of receptors called rods and cones. Rods help individuals see light and are especially useful in low-light conditions.
Cones help people see light and colors and provide detail to the images they see. When light strikes the rods and cones, it activates chemicals that create electrical impulses. An optic nerve * tract transmits the impulses to a portion of the brain known as the visual cortex * . The visual cortex translates the information from the nerves into the image that individuals ultimately see.
Many conditions can cause blindness. The most common cause of vision loss in infants and young children is amblyopia (am-blee-O-pee-uh).
Amblyopia, sometimes called lazy eye, is reduced vision in one eye. Although it can result from several conditions, it most frequently arises as a consequence of constant strabismus * , in which one eye consistently turns away from an object under view and may give a person a cross-eyed appearance. About 3 to 5 percent of children have either constant strabismus or intermittent strabismus (in which the eye's deviation occurs only occasionally). Of these, an estimated 30 to 50 percent develop amblyopia. In most cases, strabismus has no known cause. Early detection of amblyopia can often lead to successful treatment. Treatment may include an eye patch placed over the stronger eye to help strengthen the affected eye, or in some cases, surgery is performed to align the affected eye correctly.
Some children are born with cataracts, which cloud the lens in the eye and prevent images from being seen clearly or at all. People also develop cataracts as they age, which makes it one of the most common causes of reduced vision. More than 400,000 new cases of cataracts develop each year in the United States. Surgery to remove cataracts is common for adults and for those few children with them. This procedure restores vision in more than 95 percent of cases.
The best treatment for diabetic retinopathy is prevention, which means managing diabetes (and high blood pressure, if present) with proper nutrition, exercise, and medications. In some cases, laser treatment may avert worsening of diabetic retinopathy. People with diabetes also are at higher risk for cataracts and glaucoma.
Glaucoma (glaw-CO-muh) is a disorder that causes fluid pressure to build up inside the eye and may cause optic nerve damage. The disorder can go undetected for many years before its effects on vision are noticed. Glaucoma is one of the leading causes of blindness in the United States, with elderly people and people of African ancestry at increased risk.
Macular degeneration is the leading cause of blindness in adults age 60 and older in the United States. Some people have dry macular degeneration, which is caused by a breakdown in the macula, the part of the eye that helps people see details. With wet macular degeneration, abnormal blood vessels forming behind the retina burst and leak, displacing the macula. The wet form is more severe but easier to manage with lasers or injections of special drugs.
Changes to vision from macular degeneration occur gradually. The first signs of macular degeneration include shapes of objects appearing wrong or a dark area appearing in the center of vision. If the vision loss cannot be treated, those with macular degeneration can compensate by using magnifying devices. According to the American Macular Degeneration Foundation, the disease affects more than 10 million Americans.
Ocular (OCK-yoo-lar) herpes may cause vision loss as a result of the herpes simplex virus that causes cold sores or the herpes zoster virus that causes chickenpox and shingles. The National Eye Institute estimates that ocular herpes affects approximately 400,000 people in the United States, with close to 50,000 new and recurring cases occurring each year.
Infection of the eyes by the Chlamydia trachomatis organism is a leading cause of blindness in developing nations. The eyes of newborn babies may become infected during childbirth if the mother has chlamydia.
Blindness is diagnosed through a standard eye test, called the Snellen test, which requires a patient to read from a test chart containing lines of increasingly smaller type. The notation for normal eyesight is 20/20 (or 6/6 in countries where metric measurements are used). Expressed as a fraction, this notation includes a numerator (the first number) that refers to the distance between the patient and the test chart, which is usually 20 feet (6 meters). The denominator (the second number) denotes the distance at which a normal-sighted person could read the line with the smallest letters that the patient is able to decipher. For example, if the patient's visual acuity is 20/100, the patient can correctly read at 20 feet a line that a person with normal vision can read at 100 feet. People are considered to have visual impairment if their vision cannot be corrected with eyeglasses or contact lenses to better than 20/40. Blindness is defined as 20/200 vision or worse when corrected by eyeglasses or contact lenses.
Medical professionals check for loss of side vision or peripheral vision. In a typical peripheral-vision test, a doctor asks a patient to look straight ahead and to announce when an object that is moving along one side or the other disappears from view. A person is defined as blind if the individual has a visual field of less than 20 degrees. Older adults often have their pupils dilated as part of the vision examination. The doctor places special drops in the eyes that make the pupils larger so that the doctor can see inside the eye with the help of a magnifying lens. A slit lamp examination uses a special light source and microscope to help the eye doctor better see the cornea and other structures of the eye.
Other individuals whose conditions may not be reversed or improved through available treatment options may benefit from special devices such as voice-recognition software for computers and from programs that train guide dogs to assist with daily tasks such as walking, going to school, and working. A large percentage of people who are legally blind or visually impaired still have sufficient visual capabilities to do many of the same activities that people with normal sight perform, including reading books and newspapers, participating in some sports, and navigating independently through their daily routines.
Millions of people who have visual impairments, including more than 1 million people who are legally or totally blind, can engage in many of the same activities that people with normal vision perform. People with partial sight can use powerful eyeglasses and magnifying devices to improve their ability to read and see objects. People with blindness can do the following:
Individuals who are blind are at risk for developing non-24 sleep-wake disorder. This is a disorder of the circadian rhythm that regulates when to sleep and when to be awake. The average person's circadian rhythm is a bit longer than 24 hours. Over time, the additional minutes and hours add up. This means that the body can change the times it wants to sleep or be awake.
Non-24 is actually a serious, chronic circadian rhythm disorder that is common in people who are totally blind. This disorder causes the following symptoms:
These changes are caused by the timing of the release of the hormones melatonin and cortisol. Melatonin controls sleep and cortisol controls when to wake up and when to eat. For sighted people, environmental light cues signal the brain to reset their body clock every day to 24 hours. For people who are totally blind, the master body clock lacks light as a visual cue and runs its natural course. This means that over time the blind person's times to be awake or sleep can switch.
See also Cataracts • Diabetes • Eye Disorders: Overview • Glaucoma • Herpes Simplex Virus Infections • Ophthalmoplegic Syndromes
Bowling, Brad. Kanski's Clinical Ophthalmology: A Systematic Approach. 8th ed. Philadelphia: Saunders, 2015.
World Health Organization. “Water-Related Diseases.” WHO.int. http://www.who.int/water_sanitation_health/diseases/trachoma/en (accessed March 14, 2016).
American Council of the Blind. 2200 Wilson Blvd., Suite 650, Arlington, VA 22201-3354. Telephone: 202-467-5081. Toll-free: 800-424-8666. Website: http://www.acb.org (accessed March 14, 2016).
American Foundation for the Blind. 2 Penn Plaza, Suite 1102, New York, NY 10121. Telephone: 212-502-7600. Website: http://www.afb.org (accessed March 14, 2016).
American Macular Degeneration Foundation. PO Box 515, Northampton, MA 01061-0515. Toll-free: 888-622-8527. Website: https://www.macular.org (accessed March 14, 2016).
Lighthouse Guild. 15 W 56th St., New York, NY 10022. Toll-free: 800-284-4422. Website: http://www.lighthouse.org (accessed March 14, 2016).
National Eye Institute. 31 Center Dr., MSC 2510, Bethesda, MD 20892-2510. Telephone: 301-496-5248. Website: http://www.nei.nih.gov (accessed March 14, 2016).
National Federation of the Blind. 200 E Wells St. at Jernigan Pl., Baltimore, MD 21230. Telephone: 410-659-9314. Website: http://www.nfb.org (accessed March 14, 2016).
Research to Prevent Blindness. 645 Madison Ave., Fl. 21, New York, NY 10022-1010. Toll-free: 800-621-0026. Website: http://www.rpbusa.org (accessed March 14, 2016).
Seeing Eye Inc. PO Box 375, Morristown, NJ 07963-0375. Telephone: 973-539-4425. Website: http://www.seeingeye.org (accessed March 14, 2016).
* premature birth (pre-ma-CHUB) means born too early. In humans, it means being born after a pregnancy term lasting less than 37 weeks.
* retina (REH-tuh-na) is the tissue that forms the inner surface of the back of the eyeballs. It receives the light that enters the eye and transmits it through the optic nerve to the brain to produce visual images.
* optic nerve is the nerve that sends messages, or conducts impulses, from the eyes to the brain, making it possible to see. The optic nerve is also referred to as the second cranial nerve.
* visual cortex is a portion of the brain's cerebrum that processes visual information transmitted from the eyes.
* strabismus (struh-BIZ-mus), is a condition that causes the eyes to cross or not work together correctly, which may lead to permanent loss of vision in one eye.