Blindness can be caused by a host of factors including retinal infections, metabolic diseases such as diabetes, age-related conditions such as macular degeneration, or genetic disorders such as retinitis pigmentosa. 

More commonly, though, blindness is caused by glaucoma − a disease that affects millions of Americans and is the second leading cause of blindness worldwide, per the U.S. Centers for Disease Control and Prevention. 

Despite being so common, "about half the people who have glaucoma don't know they have it," says Dr. Jeffrey Schultz, director of the glaucoma division of the Montefiore Medical Center in the Bronx, New York.

What is glaucoma?

Glaucoma is a group of eye diseases that cause slow, progressive damage to the optic nerve in the back of the eye, says Schultz. This growing damage is due to unsafe fluid buildup that causes pressure inside of the eye, explains Dr. Tyler Barney, a Doctor of Optometry at Eagle Vision in Utah. 

In most cases, the increased pressure is not painful or even noticeable, he explains, but it nonetheless "slowly damages the optic nerve that connects the eye to the brain." 

This damage cannot be repaired once it occurs and eventually leads to diminished vision and sometimes even total blindness. While there are many types of glaucoma such as angle-closure glaucoma and congenital glaucoma, the most common one in the United States is called open-angle glaucoma. 

Doctors test for multiple types of glaucoma by checking eye pressure using a piece of equipment called a tonometer. "The doctor will also perform a test called a visual field examination to determine if blind spots are beginning to appear in the patient's vision," explains Dr. Mark Richey, an ophthalmologist for Revere Health. 

What causes glaucoma?

Beyond being a condition that is easy to miss, the exact cause of glaucoma is also not known or fully understood, explains Barney. At the same time, "there are several factors that may increase someone's risk of developing it," he says. These include a family history of glaucoma, one's ethnicity (research shows that African Americans and Hispanics are at higher risk of glaucoma), the presence of other medical conditions such as myopia or diabetes, and one's age as people over 40 are more likely to have glaucoma than younger individuals. 

Schultz adds that environmental factors may also contribute to the condition. Some such factors include air pollution, smoking and alcohol consumption, excessive dietary fat intake, climatic factors such as more sun exposure and higher temperatures, and even sleep apnea. 

Richey says that eye trauma can also lead to glaucoma, manifesting either immediately after an injury or sometimes even years later. 

Is glaucoma treatable? 

The good news is that, while there's no cure for glaucoma, early treatment can often stop or slow the damage from progressing, per the National Eye Institute. "The pressure in the eye can often be controlled by using daily eye drops prescribed by your eye care professional," says Barney. These drops work by improving how fluid drains from the eye or by reducing the amount of pressure-causing fluid the eye produces. They have been shown to be effective when taken regularly. 

Sometimes laser treatments or surgery are also recommended to slow the disease's progression, says Schultz. "In extreme cases, stents may be placed in the eye to act as a drain for the excess fluid that is putting pressure on the optic nerve," adds Richey. 

But the severity of treatment recommended is usually determined by how early the condition is caught and how effective initial interventions are. Because of this, and because the disease can be so easy to miss, Barney says "it's imperative that everyone has annual eye exams with an optometrist or ophthalmologist to look for early indications that they may have glaucoma."