High eye pressure creeps up on you, which is why you need to catch it before the damage is done.

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There's a reason glaucoma screening is always done in your yearly eye exam: Typically it is asymptomatic until it's already done a good amount of damage to your optic nerve. Since the optic nerve can't be repaired or restored, catching glaucoma early—before you notice any changes in your vision—is the best way to prevent it from messing with your eyesight.

What actually causes glaucoma to develop in the first place isn't always clear, but in most types of glaucoma (yes, there are multiple types) something goes awry and causes the pressure in the eye—called intraocular pressure or IOP—to get too high. But eye pressure doesn't always need to be high for glaucoma to occur (more on that later).

Before we get into the different causes of each type of glaucoma, here's a quick refresher on eye anatomy, so that you know what the heck we're talking about.

Cropped Image Of Woman Eye

How High Pressure Causes Glaucoma

OK, picture an eyeball. The outermost layer is the cornea. It's a protective film that covers the more delicate parts below—namely, the iris, the colored part of the eye that contracts and expands to accommodate changes in light; and the pupil, the black hole right smack in the middle of the iris, where light actually enters. Behind the iris is the lens, which focuses light onto the back of the eye, called the retina. The retina then turns that light into electrical signals and sends them off to the brain via the optic nerve.

The front of the eye is filled with fluid, called aqueous humor. Aqueous humor keeps the eye plump and also brings nutrients to the lens and cornea (since they have no direct blood supply). The eyes constantly make new fluid and drain the old fluid at a rate that maintains a healthy pressure in the eye. A normal eye drains via an area of spongy tissue between the cornea and iris, called the trabecular meshwork. If the eye makes new fluid too quickly or doesn't drain fluid quickly enough, the imbalance can cause high pressure.

"Most people don't have an issue making fluid," says Sriranjani Padmanabhan, MD, ophthalmologist and glaucoma specialist at UCSF Health. "We suspect the issue is that the drainage apparatus of the eye doesn't work any longer or work adequately enough." Over time, high pressure essentially presses on the delicate optic nerve, says Christopher Starr, MD, ophthalmologist at Weill Cornell Medicine and NewYork-Presbyterian. "The pressure causes the fibers [of the optic nerve] to slowly die," he explains. When optic nerve fibers die, signals can't be sent from the eye to the brain, and you essentially lose the ability to see. With glaucoma, both the damage and vision loss happen slowly over time, which is why it can be hard to notice until it's really drastic and a lot of damage has already been done.

Glaucoma is categorized into different types depending on the actual cause of high eye pressure.

Causes of Primary Glaucoma

Open-angle glaucoma cause

The most common form of glaucoma, open-angle glaucoma happens when the fluid drains too slowly through the meshwork. Dr. Padmadhaban explains that usually the drain looks normal to the naked eye, but on a microscopic level there is something abnormal about the covering on the drainage area (which she describes as looking like a series of colanders) that causes increased resistance. The fluid can't drain as quickly as it needs to, resulting in a build-up of pressure. Glaucoma experts are still trying to pinpoint what causes this to happen, but genetics and age are both big risk factors.

Angle-closure glaucoma cause

Angle-closure glaucoma happens when the iris blocks the area where the iris and cornea meet, preventing fluid from draining properly. This can happen slowly over time, similarly to open-angle glaucoma, according to Merck Manual. But it can also happen suddenly, causing a very quick and dramatic increase in eye pressure and noticeable symptoms like redness, blurred vision, halos around lights, and severe eye and head pain. When this happens, it's a medical emergency.

The reason this happens isn't always clear—sometimes genetics and your personal eye anatomy just make you more prone to it. The most identifiable cause is the enlarging lens inside the eye, says Daniel Laroche, MD, director of glaucoma services and president of Advanced Eyecare of New York and assistant professor of ophthalmology at the Mount Sinai School of Medicine. The lens slowly gets larger as we age, and eventually, it can get wide enough that it blocks or closes off the drainage angle, he explains.

Normal-tension glaucoma cause

A form of open-angle glaucoma, normal-tension glaucoma is a bit puzzling. It's a type of glaucoma that happens when the eye pressure level is totally normal, according to the National Eye Institute (NEI). "Pressure is normal, it looks like the drain is working, but for whatever reason, the pressure is still not low enough," Dr. Padmadhaban says.

Experts don't really know what causes normal-tension glaucoma. The theory is that some people's optic nerves are just more sensitive and prone to damage than others, so even "normal" pressure levels can cause issues, according to NEI.

Since the pressure is technically normal, this type of glaucoma is usually only detected if optic nerve damage is seen during an eye exam, Dr. Starr says. It still needs to be treated to avoid any further damage to the optic nerve. Luckily, most treatments that balance out the fluid in the eye to fix other types of glaucoma also effectively control normal-tension glaucoma.

Congenital or childhood glaucoma cause

When a baby is born with a defect in the eye that prevents fluid from draining properly, it's called congenital glaucoma. The cause? For some reason, the drainage area of the eye just didn't develop properly, Dr. Laroche says. Experts don't know why it happens or how to prevent it, though sometimes, there is a genetic component. According to the American Association for Pediatric Ophthalmology and Strabismus, about 10% of primary congenital/infantile glaucoma cases are inherited, and a few specific gene mutations have been linked to the disease. The good news is that symptoms are usually noticeable after the baby is born, and timely surgery can fix the problem so that the child can go on to develop good vision.

Childhood or juvenile glaucoma—glaucoma that develops after age 3—is pretty rare in the US, Dr. Padmanabhan says. Many cases have no identifiable cause, while others are associated with a separate condition or disease. 

Other causes of glaucoma

When glaucoma is connected to or caused by another condition that raises eye pressure, it's called secondary glaucoma. In some cases, health conditions can cause a direct increase in pressure; other times, they can cause damage that ultimately results in blocking the drainage area and causing the pressure to rise (angle-closure glaucoma). For example, neovascular glaucoma is a secondary glaucoma that happens when extra blood vessels form in the eye as a result of a medical condition and cover the drainage areas.

Per NEI, some of the more common causes of various types of secondary glaucoma include:

  • Diabetes or high blood pressure.
  • Cataracts and cataract surgery.
  • Tumor in the eye.
  • Inflammation of the eye.
  • Eye trauma/injury.
  • Scarring from an injury or past eye surgery.
  • Chronic use of steroid eye drops.

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