Glaucoma is tested through a combination of eye exams that measure pressure inside your eye, examine the optic nerve, check your peripheral vision, and assess the eye’s internal drainage system. No single test can confirm or rule out glaucoma on its own. A full glaucoma workup typically takes 1.5 to 2 hours and involves five or six different assessments, each targeting a different piece of the puzzle.
Most glaucoma develops slowly and without symptoms, which is why testing matters long before you notice any vision changes. By the time peripheral vision loss becomes noticeable, at least 30% of the nerve fibers connecting the eye to the brain have already been destroyed.
Eye Pressure Check (Tonometry)
Measuring the pressure inside your eye is usually the first step. Normal eye pressure falls between 10 and 21 mmHg. Readings above 21 mmHg raise suspicion, but high pressure alone doesn’t mean you have glaucoma. Some people develop glaucoma at pressures well within the normal range (called normal-tension glaucoma), while others tolerate elevated pressure without any nerve damage.
There are several ways to measure eye pressure. The most common method in a screening setting is the “air puff” test, where a quick burst of air flattens the surface of your cornea and the machine calculates pressure based on how much force was needed. It requires no contact with the eye, though it can feel startling. In a full eye exam, your doctor will likely use a more precise method called applanation tonometry. After numbing your eye with drops, a small sensor gently touches the cornea and measures the force required to flatten a tiny area. This is considered the gold standard for accuracy.
A newer option, rebound tonometry, uses a lightweight probe that briefly taps the cornea and bounces back. The speed of the rebound corresponds to the pressure inside. This method is quick, doesn’t require numbing drops, and is sometimes used for children or people who have difficulty sitting still for other methods.
Optic Nerve Examination
Your optic nerve is the cable that carries visual information from the eye to the brain, and it’s the structure glaucoma damages. To get a clear view, a technician will put dilating drops in your eyes. These take about 20 to 30 minutes to fully open your pupils, and they’ll leave your vision blurry and light-sensitive for a few hours afterward. Bring sunglasses, and plan to have someone drive you home.
Once your pupils are dilated, the doctor examines the optic nerve head, which looks like a small disc at the back of the eye. The center of this disc has a natural depression called the “cup.” In a healthy eye, the cup takes up a relatively small portion of the overall disc. Glaucoma gradually enlarges this cup as nerve fibers die off. Doctors express this as a cup-to-disc ratio: a ratio of 0.4 or less is generally considered normal, while ratios above 0.6 start to raise concern. The doctor also looks for other warning signs like tiny hemorrhages near the nerve, thinning of the tissue rim around the cup, and changes in the nerve fiber layer surrounding the disc.
Structural changes to the optic nerve often show up before any vision loss is detectable on other tests, which makes this exam one of the most important parts of a glaucoma evaluation.
Visual Field Testing (Perimetry)
This test maps your peripheral vision to find blind spots or areas of reduced sensitivity. You sit in front of a bowl-shaped instrument and focus on a central point while small lights flash in different locations around your field of view. Each time you see a flash, you press a button. The machine builds a detailed map showing which areas of your vision are intact and which are weakened.
Each eye is tested separately. This is important because glaucoma often affects the two eyes unevenly. A blind spot in the upper visual field of one eye might be compensated by the other eye when both are open, masking the damage entirely.
Glaucoma produces characteristic patterns on these maps. Early damage often appears as a “nasal step,” a difference in sensitivity between the upper and lower halves of the visual field on the side closest to your nose. As the disease progresses, comma-shaped defects called arcuate scotomas can develop, curving outward from the natural blind spot. In advanced cases, vision narrows to a small central tunnel or is lost almost entirely. Your doctor will repeat visual field tests over time to track whether these defects are stable or getting worse, which is one of the main ways to judge whether treatment is working.
Drainage Angle Assessment (Gonioscopy)
Fluid constantly flows in and out of your eye to maintain its shape and nourish internal structures. That fluid drains through a tiny channel where the iris meets the cornea, called the drainage angle. If this angle is too narrow or blocked, fluid backs up and pressure rises.
Gonioscopy lets your doctor see this angle directly. After numbing your eye, a special lens with a built-in mirror is placed on the surface of your cornea. The mirror reflects light into the angle so the doctor can judge how open or closed it is. Doctors grade the angle on a scale. An angle between 20 and 45 degrees is considered wide open with no risk of closure. Angles below 20 degrees are potentially capable of closing, and an angle that’s effectively shut signals angle-closure glaucoma, which is a different condition from the more common open-angle type and may require different treatment.
This distinction matters because open-angle and angle-closure glaucoma behave differently, progress at different rates, and are managed with different strategies. Gonioscopy is the only way to tell them apart definitively.
Corneal Thickness Measurement (Pachymetry)
Your cornea’s thickness affects how accurately eye pressure can be measured. A thicker cornea can make pressure readings appear higher than they truly are, while a thinner cornea can make them appear falsely low. The measurement is quick and painless: after numbing drops, an ultrasonic probe or optical scanner measures the cornea in seconds.
Beyond its effect on pressure readings, corneal thickness appears to be an independent risk factor for glaucoma. Research from large population studies found that people with thinner corneas have a higher prevalence of glaucoma even after adjusting for pressure measurement errors. Scientists believe this is because corneal thickness may serve as a marker for structural characteristics elsewhere in the eye, particularly in the tissues that support the optic nerve. A thin cornea doesn’t cause glaucoma, but it does signal that your doctor should monitor you more closely.
Optical Coherence Tomography (OCT)
OCT is an imaging technology that creates high-resolution cross-sectional scans of the structures at the back of your eye. It works somewhat like an ultrasound but uses light instead of sound, producing images detailed enough to measure the thickness of individual tissue layers. The scan is quick, non-invasive, and doesn’t require any drops.
For glaucoma, OCT focuses on two key structures. The first is the retinal nerve fiber layer, the sheet of nerve fibers that converge to form the optic nerve. Thinning of this layer is one of the earliest detectable signs of glaucoma damage. The second is the ganglion cell layer in the macula (the central part of the retina), where changes can sometimes appear even before nerve fiber thinning becomes obvious.
OCT produces color-coded maps that flag areas falling outside normal ranges. However, these summary maps can occasionally miss localized defects like small wedge-shaped areas of thinning. For this reason, doctors get the most accurate picture by looking at the detailed deviation maps and correlating them with visual field results rather than relying on the color-coded summaries alone.
When and How Often to Get Tested
The American Academy of Ophthalmology recommends a baseline comprehensive eye exam at age 40 for everyone. After that, the schedule depends on your age and risk profile. For people without risk factors, exams every 2 to 4 years are recommended from ages 40 to 54, every 1 to 3 years from 55 to 64, and every 1 to 2 years after age 65.
You may need earlier or more frequent testing if you have risk factors: a family history of glaucoma, African or Hispanic ancestry, high nearsightedness, diabetes, a history of eye injury, or long-term corticosteroid use. If you fall into any of these categories, discuss a personalized screening timeline with your eye care provider rather than relying on the general schedule.