The most common causes of blindness worldwide are cataracts, uncorrected refractive errors, glaucoma, age-related macular degeneration, and diabetic retinopathy. Together, these five conditions account for the vast majority of vision loss globally. Of the estimated 2.2 billion people living with some form of vision impairment, at least 1 billion have a condition that could have been prevented or still can be treated. More than 90% of all vision impairment has a preventable or treatable cause using interventions that already exist.
Cataracts
Cataracts are the single largest cause of blindness in the world, responsible for an estimated 94 million cases of distance vision impairment or blindness. The lens of your eye is made of tightly organized proteins that keep it clear. Over time, those proteins break down and clump together, scattering light instead of letting it pass through cleanly. The result is a gradual clouding of vision, like looking through a foggy window.
Most cataracts are age-related and develop slowly over years. Colors may seem faded, night driving becomes harder, and eventually the clouding can become dense enough to cause functional blindness. The good news is that cataract surgery, which replaces the clouded lens with an artificial one, is one of the most commonly performed and effective surgeries in medicine. In many low-income regions, however, access to that surgery remains limited, which is why cataracts still top the list.
Glaucoma
Glaucoma affects roughly 7.7 million people severely enough to cause significant vision loss or blindness. It damages the optic nerve, the cable that carries visual information from your eye to your brain. In most cases, the damage comes from elevated pressure inside the eye. That pressure creates mechanical strain where the nerve fibers exit the back of the eye, gradually killing the cells responsible for transmitting your visual signals.
The process involves multiple overlapping problems: inflammation around nerve cells, disrupted blood supply, and direct physical stress on the nerve fibers. Pressure-sensitive channels in nerve cells allow too much calcium to flood in, which triggers a cascade of damage. What makes glaucoma especially dangerous is that it typically destroys peripheral vision first, so you may not notice anything wrong until a significant amount of nerve tissue is already gone. By the time central vision is affected, the loss is permanent. Regular eye exams that measure eye pressure and examine the optic nerve are the only reliable way to catch it early.
Age-Related Macular Degeneration
Age-related macular degeneration (AMD) attacks the macula, the small central area of the retina responsible for sharp, detailed vision. About 8 million people worldwide have significant vision loss from it. AMD comes in two forms, and they behave very differently.
Dry AMD is far more common and progresses slowly. The layer of cells that nourishes your retina gradually breaks down, and small yellow deposits called drusen accumulate underneath. Over time, patches of retinal tissue waste away entirely. When this degeneration reaches the center of the macula, central vision drops significantly. You might struggle to read, recognize faces, or see fine detail, even though your side vision stays intact.
Wet AMD is less common but more aggressive. Weakened tissue beneath the retina allows abnormal blood vessels to grow up through it. These new vessels are fragile and leak fluid or blood into the retina, distorting and destroying central vision quickly. People with wet AMD often notice that straight lines appear wavy or that a dark spot develops in the center of their vision. Treatments that block the growth signal driving those abnormal vessels can slow or sometimes reverse the damage if caught early, but lost vision from scarring is permanent.
Diabetic Retinopathy
Diabetes damages the tiny blood vessels throughout your body, and the retina is especially vulnerable. About 3.9 million people have significant vision loss from diabetic retinopathy. The condition progresses through two main stages.
In the earlier stage, weakened blood vessels leak fluid and blood into the retina. Your body tries to repair the damage by closing off those vessels, but this cuts off blood supply to parts of the retina that need it. In the later stage, the retina responds to oxygen deprivation by growing new blood vessels, but these new vessels are abnormal and fragile. They bleed easily, can pull on the retina and cause detachment, and ultimately destroy enough light-detecting cells to cause permanent blindness.
Higher blood sugar levels are directly tied to faster progression. Keeping blood sugar lower and more consistent is the most effective way to slow the disease. This is one reason why regular dilated eye exams matter so much for anyone with diabetes, since the earliest signs of damage are visible to an eye doctor long before you notice any symptoms yourself.
Infections: Trachoma and Others
Trachoma is the leading infectious cause of blindness worldwide. It’s caused by a bacterium that spreads through direct contact with eye or nose secretions, contaminated hands, shared bedding, or certain species of flies. The infection is most common in crowded living conditions with limited access to clean water.
A single infection isn’t blinding. The damage comes from repeated infections over many years, typically starting in childhood. Each round of infection adds scar tissue to the inside of the eyelid. Eventually, the scarring becomes severe enough to pull the eyelid inward, forcing the eyelashes to scrape against the surface of the eye with every blink. This constant abrasion causes pain, light sensitivity, and progressive scarring of the cornea. Left untreated, the corneal scarring becomes dense and irreversible, leading to blindness.
Vitamin A Deficiency
Vitamin A deficiency is the world’s leading preventable cause of childhood blindness. An estimated 250,000 to 500,000 children who lack adequate vitamin A go blind each year, and half of them die within 12 months of losing their sight.
Vitamin A is essential for maintaining the surface of the eye and for the chemical reactions that allow you to see in low light. Night blindness is one of the earliest signs of deficiency. As the deficiency worsens, the cornea becomes extremely dry and can develop ulcers. In the most severe stage, the cornea softens and breaks down entirely. This progression from dryness to ulceration to corneal destruction can happen surprisingly fast in a malnourished child, making supplementation programs in affected regions one of the most cost-effective public health interventions available.
Retinal Detachment
Unlike most causes of blindness, retinal detachment happens suddenly. The retina peels away from the back of the eye, cutting off its blood supply and oxygen. Symptoms come on fast: a burst of new floaters (small dark spots or squiggly lines drifting across your vision), flashes of light, and a shadow or curtain creeping over part of your visual field.
The most common triggers are aging and eye injuries, though severe nearsightedness and previous eye surgery also raise risk. Retinal detachment is a medical emergency. The longer the retina stays detached, the more photoreceptors die and the greater the chance of permanent vision loss. Prompt treatment can often reattach the retina and preserve most of your vision, but delays of even a day or two can make a meaningful difference in outcomes.
Inherited Conditions
Some forms of blindness are genetic. Retinitis pigmentosa (RP) is one of the most well-known inherited eye diseases, affecting the retina’s light-sensitive cells. It typically begins with difficulty seeing in dim light during adolescence or early adulthood, then slowly narrows the visual field over decades, sometimes progressing to complete blindness.
RP can be inherited in several patterns. About 20 to 25% of cases follow a dominant pattern, meaning a single copy of the mutated gene from one parent is enough to cause the disease. Another 15 to 20% are recessive, requiring copies from both parents. Around 10 to 15% are linked to the X chromosome, which is why they affect males more severely. In about 30% of cases, there’s no family history at all. Over 50 different genes have been linked to RP, with mutations in the rhodopsin gene alone accounting for 30 to 40% of dominant cases. This genetic complexity is one reason why RP has been difficult to treat, though gene therapies targeting specific mutations are beginning to change that picture for some patients.
Why So Much Blindness Is Preventable
The striking thing about global blindness is how much of it doesn’t have to happen. Cataracts can be surgically removed. Refractive errors can be corrected with glasses. Glaucoma can be slowed with pressure-lowering drops if caught early. Diabetic retinopathy progression can be dramatically reduced with blood sugar management. Trachoma can be eliminated with antibiotics, clean water, and basic sanitation. Vitamin A deficiency can be addressed with supplements costing pennies per dose.
The barriers are not mainly scientific. They’re economic and geographic. Most preventable blindness is concentrated in low- and middle-income countries where eye care infrastructure is thin, trained specialists are scarce, and the cost of even simple interventions like eyeglasses remains out of reach for many families. For the billion people currently living with addressable vision impairment, the treatments already exist. The gap is in delivering them.