Normal vision depends on a multifaceted, complex process. Light enters the eye through the cornea, where some of it passes through the pupil, the black-appearing opening in the iris, which controls how much light enters the eye.
The light then passes through the lens, which works together with the cornea to focus it on the retina, a layer of tissues at the back of the eye that’s made up of cells called photoreceptors that transform the light into electrical signals. From here, those signals are transmitted to the brain through the optic nerve, turning them into the image you see in front of you.
When any of these eye parts are damaged, either through illness or injury, blindness can occur. Examples of damage to the eye include:
- The lens may cloud, obscuring the light entering the eye.
- The eye’s shape can change, altering the image projected onto the retina.
- The retina can degrade and deteriorate, affecting the perception of images.
- The optic nerve can become damaged, interrupting the flow of visual information to the brain.
With early detection, diagnosis, and intervention, however, blindness can often be prevented, and vision maintained.
Blindness vs. ‘Legal’ Blindness
Blindness is generally defined as either an inability to see or a lack of vision. It can affect one or both eyes and doesn’t necessarily cause total darkness. Some people’s blindness is so severe they can’t even detect light, while others still have low vision.
According to the Centers for Disease Control and Prevention, one million people in the United States are blind, while three million have vision impairment even after correction.
“Legal blindness” refers to the definition established by the U.S. Social Security Administration to determine who is eligible for disability benefits or supplemental income because of low vision. The SSA defines someone as legally blind when the person’s corrected central vision is no better than 20/200, or the person has lost peripheral vision so that he or she sees 20 degrees or less outside of central vision.
Normal visual acuity is 20/20, meaning that at 20 feet from an eye chart, a person can see what normally should be seen at that distance, according to the American Optometric Association. In addition, people can usually see up to 90 degrees with their peripheral vision.
Leading Causes of Blindness
Nearly all cases of blindness around the world are caused by eye diseases, with cataracts being the number-one cause for adults age 50 and older, followed by glaucoma, uncorrected refractive error, age-related macular degeneration (AMD), and diabetic retinopathy, according to an analysis published in the Lancet Global Health in February 2021.
These eye diseases are common causes of blindness, but you should not assume you are going blind if you have any of these conditions. There are treatments available for each — though some are more treatable than others.
Cataracts occur when the normally crystal-clear lens of the eye becomes cloudy. This causes blurry vision, faded colors, light sensitivity, and problems seeing through the glare.
The risk of cataracts increases with age, with more than half of Americans age 80 and up either having them or having had surgery to remove them, per the National Eye Institute (NEI).
Other risks for cataracts include a high exposure to sunlight, smoking, diabetes, too much alcohol, and steroids.
Blindness from cataracts can be prevented with surgery to remove the damaged lens from the eye and, in most cases, insert an artificial one.
Because cataracts typically develop slowly, surgery generally isn’t recommended until they interfere with a person’s ability to see and function in a meaningful way. In the meantime, it may be possible to deal with cataracts through the use of antiglare glasses, magnifying lenses, and brighter lighting.
Uncorrected Refractive Error
Refractive errors happen when the shape of your eye prevents light from focusing correctly on your retina, which results in a blurred image.
Common kinds of refractive errors include myopia (nearsightedness), hyperopia (farsightedness), astigmatism, and presbyopia, all of which are correctable.
But since so many cases are left uncorrected, these errors are a top cause of vision impairment and blindness worldwide, per the World Health Organization (WHO).
Treatment depends on the kind of refractive error you have and how far your condition has progressed. Some common methods used include customized glasses, contact lenses, and refractive laser surgery.
In glaucoma, the optic nerve is damaged, although it’s not always clear how or why this occurs.
High intraocular pressure, or internal eye pressure — something an eye doctor can test for — raises the risk of glaucoma, but not everyone with high eye pressure develops glaucoma, and some people with normal eye pressure have it.
Typically, glaucoma causes a gradual decrease in peripheral vision, according to the NEI, but because it happens so slowly, many people don’t realize their vision is changing at first.
There is one exception, however: A type of glaucoma called angle-closure glaucoma causes sudden, severe eye pain, nausea, blurry vision, and red eyes. This is a medical emergency that requires immediate treatment.
Most vision loss due to glaucoma cannot be reversed, although prescription eye drops, laser treatments, and certain kinds of surgery can prevent your eyesight from deteriorating. Two types of surgery that lower intraocular pressure, tube shunt surgery and augmented trabeculectomy, can lead to improvements in peripheral vision for some people.
To prevent blindness from glaucoma, it’s important to have regular eye exams to catch glaucoma early, because treatment can save your vision.
Age-Related Macular Degeneration
Age-related macular degeneration (AMD) involves the gradual deterioration of the macula, the portion of the retina that provides sharp central vision. People with AMD retain their peripheral vision but develop blurring and blind spots in their central vision, and without central vision, it can be hard or impossible to read, recognize faces, or drive.
There is no cure for macular degeneration, but treatments are available to slow its progress. These include multivitamin supplements for intermediate-stage AMD, and medications that are injected into the eye for later-stage disease.
Again, regular eye exams are the key to catching AMD early so it can be monitored and treated.
Diabetic retinopathy occurs when high blood glucose caused by diabetes begins to affect the small blood vessels of the retina. The blood vessels can bleed, swell, and leak fluid into the retina.
In its early stages, diabetic retinopathy may cause no symptoms. Over time, it can cause blurry vision, color blindness, poor night vision, and eye floaters or streaks in your vision. And eventually it can lead to blank spots or dark areas in your vision.
Diabetic retinopathy places a person at higher risk of glaucoma, macular edema, and retinal detachment, all of which can lead to vision loss and blindness.
According to the CDC, 90 percent of blindness caused by diabetes is preventable.
Prevention includes keeping blood sugar and blood pressure levels in a healthy range, having comprehensive eye exams at least annually if there are any signs of retinopathy, and — at more advanced stages — anti-VEGF injections or laser treatments, as recommended by your eye doctor, to stop your vision from getting worse.