Blindness

Health effects

Visual impairments vary widely in form and degree, and visual acuity alone does not always predict an individual's functional ability. Someone with relatively good acuity, such as 20/40, may still struggle with daily tasks, while a person with worse acuity, like 20/200, might function reasonably well if their visual demands are low. Best-corrected visual acuity differs from presenting visual acuity; a person with normal best-corrected acuity can have poor presenting acuity due to uncorrected refractive error.

The American Medical Association estimates that loss of one eye equals 25% impairment of the visual system and 24% impairment of the whole person, while total vision loss in both eyes is considered 100% visual impairment and 85% impairment of the whole person. Some individuals with significant vision loss can use their residual vision to complete daily tasks without alternative methods. Low vision specialists aim to maximize functional vision through optical or non-optical means, such as magnification. Older adults with visual impairment face increased risks of physical inactivity, slower gait speeds, and fear of falls.

Cause

The most common causes of visual impairment globally in 2010 were refractive error (42%), cataract (33%), glaucoma (2%), age-related macular degeneration (1%), corneal opacification (1%), diabetic retinopathy (1%), childhood blindness, trachoma (1%), and undetermined causes (18%). The most common causes of blindness worldwide in 2010 were cataracts (51%), glaucoma (8%), age-related macular degeneration (5%), corneal opacification (4%), childhood blindness (4%), refractive errors (3%), trachoma (3%), diabetic retinopathy (1%), and undetermined causes (21%).

About 90% of visually impaired people live in the developing world. In developed countries, age-related macular degeneration, glaucoma, and diabetic retinopathy are the leading causes of blindness. Among working-age adults newly blind in England and Wales in 2010, the most common causes included hereditary retinal disorders (20.2%), diabetic retinopathy (14.4%), optic atrophy (14.1%), glaucoma (5.9%), and congenital abnormalities (5.1%). Cataracts, the greying or opacity of the crystalline lens, double in prevalence every ten years after age 40 and are the leading cause of child and adult blindness.

Management

Many people with serious visual impairments can travel independently using a range of tools and techniques. Orientation and mobility specialists are trained to teach visually impaired individuals how to travel safely and independently in the home and community, including practicing specific routes. The white cane with a red tip is the international symbol of blindness and is used to extend the user's range of touch sensation, typically swung in a low sweeping motion to detect obstacles.

Some visually impaired persons use a shorter identification cane or a support cane, depending on their vision and motivation. A small number of people employ guide dogs, which are trained to navigate around obstacles and indicate steps; the handler directs the dog based on prior mobility training. GPS devices can assist with orientation and navigation but are not a replacement for traditional mobility tools. Some blind people are skilled at echolocating silent objects by producing mouth clicks and listening to echoes, using what is normally the visual part of the brain to process the sounds.

Society and culture

Various governments define legal blindness to determine eligibility for special assistance. In North America and most of Europe, legal blindness is defined as visual acuity of 20/200 or less in the better eye with best correction, or a visual field of 20 degrees or less. Approximately 15% of those deemed legally blind have no light or form perception; the rest have some vision, from light perception to relatively good acuity. Low vision is sometimes used to describe visual acuities from 20/70 to 20/200.

The Moche people of ancient Peru depicted the blind in their ceramics. In Greek myth, Tiresias was a blind prophet famous for his clairvoyance; one myth says he was blinded by the gods for revealing secrets, another for seeing Athena naked. In the Odyssey, the Cyclops Polyphemus is blinded by Odysseus. In Norse mythology, the blind god Höðr is tricked into killing his brother Baldr.

The New Testament describes Jesus performing miracles to heal the blind. The parable of the blind men and an elephant appears in Jain, Buddhist, Sufi, and Hindu traditions, where each blind man feels a different part of an elephant and they disagree on what it is like.

Did You Know?

• The Centers for Disease Control and Prevention estimates that half of blindness in the United States is preventable.
• Near vision impairment is defined as near visual acuity worse than N6 or M 0.8 at 40 cm.