Glaucoma is detected through a combination of tests that measure eye pressure, examine the optic nerve, map your peripheral vision, and image the nerve tissue at the back of your eye. No single test confirms or rules out glaucoma on its own. Because the disease produces no noticeable symptoms in its early stages, an estimated 50 to 80 percent of people with glaucoma don’t know they have it, making routine screening the only reliable way to catch it early.
Eye Pressure Measurement (Tonometry)
Tonometry measures the fluid pressure inside your eye, called intraocular pressure. A normal reading falls between 10 and 21 mmHg for most people. Elevated pressure is the most well-known risk factor for glaucoma, though some people develop the disease with pressure in the normal range, and others tolerate higher pressures without damage.
There are a few ways to measure it. In applanation tonometry, a small disc-shaped tip gently presses against the surface of your eye and measures how much force is needed to flatten it slightly. Your eye is numbed with drops first, so you won’t feel pain. Rebound tonometry uses a tiny plastic probe that briefly taps the eye surface and calculates pressure based on how the probe bounces back. This method is quick and painless, often used in screening settings. Some newer approaches use a sensor worn like a contact lens for continuous monitoring over time.
One important detail: the thickness of your cornea affects how accurately pressure readings reflect what’s actually happening inside your eye. The standard pressure-measuring instrument was designed assuming a corneal thickness of about 520 micrometers. If your cornea is thinner, your true pressure may be higher than what the device reads, and if it’s thicker, your reading may overestimate the real pressure. The error can be as large as 4 to 7 mmHg in people whose corneas are significantly thinner or thicker than average. That’s why many eye doctors also measure corneal thickness (pachymetry) to put your pressure reading in context.
Optic Nerve Examination
Looking directly at the optic nerve head is one of the most important parts of a glaucoma evaluation. Your doctor will dilate your pupils with drops and then use a magnifying instrument to examine the back of the eye, where the optic nerve connects to the retina.
The key feature they’re assessing is the “cup,” a natural depression in the center of the optic nerve head. The ratio of this cup to the overall disc is expressed as a number. An average cup-to-disc ratio is about 0.4. Ratios of 0.7 or greater occur in only about 2.5 percent of the population, so a cup that large raises suspicion that glaucoma may already be underway. A difference of more than 0.2 between your two eyes is also a red flag, even if neither number is particularly high on its own.
Beyond the overall size of the cup, your doctor looks for specific patterns of damage. Glaucoma destroys nerve fibers in bundles that travel together, so it often creates a characteristic notch, a localized area of missing tissue, usually at the top or bottom of the nerve head. Small splinter-like hemorrhages on the nerve head are another sign that glaucoma is actively progressing. The doctor also examines the layer of nerve fibers just outside the disc to see whether any areas appear thinned or absent.
Visual Field Testing (Perimetry)
A visual field test maps how well you see across your entire range of vision, including the periphery that glaucoma attacks first. During the most common version, you sit in front of a bowl-shaped machine and stare at a central point. Tiny spots of light flash in different locations, and you press a button each time you notice one. The machine varies the brightness and sometimes the color of the lights to measure sensitivity at each point.
Results are printed as a pattern of dots or numbers that creates a map of your vision. Certain patterns are well-established indicators of specific diseases. In glaucoma, the earliest losses tend to appear as small blind spots in the upper or lower peripheral field. These spots often go completely unnoticed in daily life, which is why the test matters. As the disease progresses, the areas of vision loss grow larger and can eventually affect central vision.
Visual field testing is also essential for tracking glaucoma over time. Repeated tests, usually done every six to twelve months, reveal whether treatment is holding the disease stable or whether damage is continuing to progress.
OCT Imaging
Optical coherence tomography, or OCT, uses light waves to create detailed cross-sectional images of the retina and optic nerve. It can measure the thickness of the retinal nerve fiber layer down to the micrometer, providing an objective, quantifiable picture of how much nerve tissue remains.
This technology is particularly valuable because it can detect structural damage before you notice any vision loss on a visual field test. Studies show that nerve fiber layer measurements have an area under the curve of about 0.9 for distinguishing glaucomatous eyes from healthy ones, meaning the test is highly accurate overall. It performs best at identifying moderate to advanced disease, with somewhat lower accuracy for the earliest stages. Among the various measurements OCT can take, nerve fiber layer thickness has proven significantly better than optic nerve head or macular measurements for catching glaucoma before it shows up on visual field testing.
During the scan, you rest your chin on a support and look at a target while the machine captures images in a few seconds. There’s no contact with your eye and no discomfort.
Drainage Angle Inspection (Gonioscopy)
Gonioscopy lets your doctor see the drainage angle where fluid exits the eye. This is the area where the iris meets the cornea. It’s not visible without a special lens placed on the eye surface. The exam helps distinguish between the two main types of glaucoma: open-angle, where the drainage system looks physically open but isn’t functioning efficiently, and angle-closure, where the iris physically blocks the drainage pathway.
The room is kept dim during the exam because bright light causes the pupil to constrict, which pulls the iris away from the drainage angle and can mask a problem. If the angle appears closed, the doctor may press gently on the cornea with the lens to push fluid toward the angle. This compression technique helps determine whether the closure is caused by the iris simply resting against the drainage area (which can be relieved) or by scar-like tissue that has permanently fused the iris in place.
The distinction matters because the two types of glaucoma are managed differently, and angle-closure glaucoma can escalate into an emergency with sudden, severe pressure spikes.
How Often You Should Be Screened
The American Academy of Ophthalmology recommends a baseline comprehensive eye exam at age 40 for adults with no risk factors. After that, the schedule depends on your age: every two to four years from ages 40 to 54, every one to three years from 55 to 64, and every one to two years from age 65 onward.
If you’re at higher risk, the timeline accelerates. African Americans, who have a significantly elevated risk of glaucoma, should consider exams every two to four years even before age 40, every one to three years between ages 40 and 54, and every one to two years between 55 and 64. A family history of glaucoma, very high or very low eye pressure, high myopia, or a history of eye injury also place you in the higher-risk category.
People with type 1 diabetes should have their first eye exam five years after diagnosis and then at least yearly. Those with type 2 diabetes should be examined at the time of diagnosis and yearly after that, since diabetic eye changes can overlap with and complicate glaucoma detection.