These quick and non-invasive tests help your doctor determine if you have glaucoma, and how best to treat it.

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Glaucoma is known as the "thief of sight," because if it goes undiagnosed and untreated, it can cause vision loss. Add to that the fact that the disease typically has zero noticeable symptoms until damage to the optic nerve has already been done, and it makes perfect sense that eye doctors want you to go for a yearly eye exam and get screened. Even if you have perfect vision, your annual eye exam is a chance for the doctor to thoroughly explore your eyes and make sure there's nothing silent and pervasive (like glaucoma) going on.

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While eye doctors screen everyone for glaucoma, actually making a glaucoma diagnosis takes time, Sriranjani Padmanabhan, MD, ophthalmologist and glaucoma specialist at UCSF Health tells Health. "Just having high eye pressure is not usually enough to make a diagnosis—it's just a risk factor," she says. Some people can have high eye pressure, but it never progresses to glaucoma (meaning it doesn't have any negative impact on the optic nerve or other structures in the eye).

To figure out if high pressure is cause for concern, ophthalmologists often need to look deeper at additional factors—including corneal thickness, family history, and the status of the optic nerve. By putting all these puzzle pieces together, they can accurately assess your risk and figure out the best treatment, says Dr. Padmanabhan.

To do that, an eye doctor will perform a series of tests. They may also choose to repeat these tests over a few months to keep tabs on whether or not things change. Some are standard for every single eye exam, while others may get added on if your pressure test shows potentially concerning results. If it's your first time ever seeing an eye doctor, they may want to do all the tests, just to get a baseline assessment to compare your future test results to. They can all be done with tools found in most ophthalmologists' offices.

Here, we dig into all the tests that can help diagnose glaucoma, and what to expect from each, according to experts.

Tonometry

There are a few different forms of tonometry, which is a test to measure eye pressure. In any regular old eye exam, your doctor will test eye pressure and also do a test to look at the optic nerve (more on that later), Christopher Starr, MD, ophthalmologist at Weill Cornell Medicine and NewYork-Presbyterian tells Health. That's because elevated eye pressure is what usually tips eye doctors off to potential glaucoma. 

The most accurate way to measure eye pressure: After receiving numbing eye drops, your doctor will have you place your head on the slit lamp (the big microscope eye doctors use to look inside your eyes) and look straight ahead while the doctor moves the tip of the device to touch your cornea. The device will measure how much force is required to temporarily flatten the cornea, and from that, produces a pressure reading.

The second method is very similar to the first, but uses a handheld pencil-shaped device to measure the pressure instead of the attachment on the slit lamp. The third method, known as the air puff, uses a beam of light and an actual puff of air to gauge how much air pressure is required to flatten the cornea. The air puff is the most uncomfortable of the three, since the eye is usually not numbed. If your eye pressure is high from any of these tests, your doctor will then do more specific tests (below) to diagnose glaucoma.

Ophthalmoscopy

During a routine eye exam, you'll likely also have an ophthalmoscopy. This exam lets your doctor see the back of your eye and make sure your retina, blood vessels, and the optic nerve all look normal. There are a few types of ophthalmoscopy exams, some of which require eye drops to dilate your eyes—which just means your pupil expands so that your doctor can get a better look inside your eye. Then, depending on the method, your doctor will use some sort of light beam and hold a lens up to your eye to get a look. You may need to rest your head against a slit lamp, or your doctor may wear a light on their head (like a headlamp you'd wear for a night run or a camping trip) and hold the lens up with their hands.

The exam usually takes 5-10 minutes, and shouldn't hurt, though the light shining in your eye may be a little uncomfortable. 

Gonioscopy

This test is used to check if the eye's drainage angle is open or closed, Courtney Ondeck, MD, ophthalmologist in the Glaucoma Service at Mass Eye and Ear tells Health. This close-up look at the drainage system in the eye can give your doctor insight into what type of glaucoma is at play (which will help them figure out how to treat it best) and if you're at risk for acute angle-closure glaucoma, a sudden and severe increase in eye pressure that warrants immediate medical attention. Gonioscopy also lets your doctor check for abnormal blood vessels, adhesions, damage from previous eye trauma, and any other intricacies or abnormalities in the drainage system, according to the Glaucoma Research Foundation.

Before a gonioscopy, your doctor will give you eye drops to numb the eye and then place a special contact lens right on the top of the eye. They will then position your head in the slit lamp so that a light beam will reach the special lens and make the angle visible to your doctor. This exam is quick and easy—shouldn't take more than a few minutes to do.

Pachymetry

Pachymetry measures the thickness of the cornea, which is the outermost part of the front of the eye that covers the pupil and iris. "There's some evidence that if the central corneal thickness is on the thinner side, it is a possible risk factor for glaucoma," Dr. Starr says. Corneal thickness can also impact eye pressure tests—if the cornea is really thick, it can give a false high pressure reading; if it's thin, you may get a false low pressure reading, Dr. Ondeck says. 

Pachymetry is done using a pachymeter, a small handheld probe. Your doctor will give you eye drops to numb the eye first, and then place the device on the front of the eye (the cornea) to measure its thickness. The test is very quick—it takes a minute or less to measure both eyes—and you shouldn't feel a thing.

Visual Field Testing

Also known as perimetry, a visual field test is done to assess whether you've lost any peripheral vision, Dr. Starr says. Peripheral vision is usually the first thing to deteriorate if you have glaucoma, he adds. During the test, you'll place your head in a dome-shaped device and look straight ahead at a target, while lights pop up in different areas throughout your field of vision. You'll click a button whenever you see a light, and this helps "map" your peripheral vision.

According to the Glaucoma Research Foundation, it's normal to experience a delay in seeing the light as it moves around your blind spot (we all have a blind spot where the optic nerve and retina connect). So, if you experience this during the test, don't freak out.

If you are diagnosed with glaucoma, you'll likely have to do a visual field test one to two times per year to check for vision changes.

Optic Nerve Scanning

There are a few different technologies used to scan the optic nerve and take complex pictures. But ophthalmologists talk about OCT, which stands for Optical Coherence Tomography, as the most advanced method we have right now. Dr. Starr describes an OCT scan as "a sophisticated picture test that can take a picture of the optic nerve, analyze its contour and shape, and quantify the number of fiber layers." It can give your eye doctor precise measurements of the nerve's shape and volume and reveal any areas that are thinning. "This scan can help us detect anatomic loss of the optic nerve, in many cases before vision is even lost," Dr. Starr adds. Finding even the tiniest amount of damage early on gives you a chance to treat the glaucoma before it progresses and results in any vision loss. Eye doctors praise this technology for making it possible to detect changes earlier than ever before.

If your optic nerve looks abnormal, your doctor may want to do regular nerve scans every six months or once a year to check if things are progressing or staying stable, Dr. Ondeck says. 

The test is quick and painless and only requires you to place your face against a machine so that it can take a picture of your eye. 

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