What Causes Glaucoma? Here’s How This Eye Disease Impacts Your Vision
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.
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.
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