Anything that blocks the passage of light to the back of the eye or disrupts the transmission of optic nerve impulses to the brain will interfere with vision. A loss or lack of vision that can’t be corrected with eyeglasses is known as blindness.

A person who is partially blind may see shapes and/or colors but can’t focus properly or see details, while someone with total blindness is clinically referred to as having NLP, or “no light perception.” The definition of legally blind in the United States and most of Europe is vision that’s worse than 20/200 in the better eye; this means a legally blind person can’t see with the same degree of clarity from 20 feet away what a normal person can see from 200 feet.

Worldwide, leading causes of blindness include cataracts, leprosy, trachoma, vitamin A deficiency, and infections such as onchocerciasis (or water blindness), a parasitic disease spread by flies. In developed countries where cataract surgery is common and quality of life is better, leading causes are diabetes, glaucoma, macular degeneration, and injuries usually involving detachment of the retina. Anyone experiencing sudden vision loss should be examined by a doctor immediately, because many forms of blindness must be treated quickly to ensure recovery.

The World Health Organization estimates that about 2.6 percent of the world’s population is visually impaired or blind. Blind people can learn to function independently. They can read by touch with Braille, an international alphabet consisting of raised bumps. Some use white canes with red tips—the international symbol of blindness—or trained guide dogs to help them navigate safely around their neighborhoods.

In 2008, scientists attached a normal version of the gene RPE65 to a common cold virus (a safe and common delivery method in this type of research) and introduced it into the eyes of three young adults with a rare form of inherited blindness caused by an abnormal gene. The patients reported improved vision following the procedure, and researchers hope this type of gene therapy can be used in the future. Other ongoing studies include helping blind people “see” by way of sensors on their tongues that send pixilated images directly to the parts of the brain that are involved in vision.

ADDITIONAL FACTS

  1. Some blind people experience insomnia or have trouble sleeping at conventional times because they lack the daily rhythmic cues of light and darkness.
  2. Training a guide dog for the blind may take up to 18 months. Guide miniature horses and helper monkeys are preferred by some because of their much longer life spans.
  3. Strategies that allow blind people to handle normal daily activities include folding banknotes in different ways to indicate their denominations, labeling personal items and home appliances, and placing different types of food at specific positions on dinner plates.