This post is part of our “Simply Explained” series, where we take a common eye health topic and break it down into plain, everyday language — no medical degree required. Look out for the “Simply Explained” badge on future posts covering other eye conditions and treatments.
Glaucoma is one of the most common reasons people are referred to an eye specialist, but it’s also one of the most misunderstood conditions. A lot of patients ask us: “How do you actually know if I have it?” Here’s a simple answer.
What is glaucoma, in plain terms?
Glaucoma is damage to the nerve at the back of your eye — the optic nerve — which is like the cable that carries what you see to your brain. This damage usually happens slowly, over years, and it’s often linked to pressure inside the eye being too high for that particular nerve.
The tricky part is that glaucoma usually has no symptoms in its early stages. You won’t feel pain, and your central vision stays sharp for a long time. That’s why it’s sometimes called the “silent thief of sight” — by the time people notice a problem themselves, some damage has often already occurred. This is also why diagnosis relies on specific tests rather than how your eyes feel.
The tests we use to diagnose glaucoma
No single test can diagnose glaucoma on its own. Instead, we build a picture using a combination of simple, painless checks.
1. Eye pressure check (tonometry) A quick test that measures the pressure inside your eye, usually with a puff of air or a gentle instrument that lightly touches the surface of the eye (numbing drops are used first, so you won’t feel it). High pressure doesn’t always mean glaucoma, and normal pressure doesn’t always rule it out — but it’s an important piece of the puzzle.
2. Looking at the optic nerve Using a special lens and a bright light, we examine the optic nerve directly to check its shape, colour, and how it compares to what’s expected for a healthy nerve. Glaucoma changes the way the nerve looks, and an experienced eye specialist can pick up on subtle warning signs.
3. A photo or scan of the nerve (OCT) OCT stands for Optical Coherence Tomography — think of it like an ultra-detailed photograph that measures the thickness of the nerve fibre layer. It’s quick, painless, and doesn’t touch your eye. This scan can pick up very early changes, sometimes before they’d be visible to the naked eye, and lets us track changes over time by comparing scans year to year.
4. Visual field test This test maps out your peripheral (side) vision. You’ll look into a machine and press a button each time you see a small flash of light appear. Glaucoma tends to affect side vision before central vision, so this test can pick up gaps you wouldn’t otherwise notice yourself.
5. Corneal thickness measurement (pachymetry) A quick, painless measurement of how thick the clear front window of your eye (the cornea) is. This matters because corneal thickness can affect how accurate your eye pressure reading is, so it helps us interpret the results correctly.
Why so many tests?
Because glaucoma is a “connect the dots” diagnosis rather than a single test result. Your eye pressure, the appearance of your nerve, the OCT scan, and your visual field results all need to line up. We’re also looking for change over time — a single visit gives us a baseline, but repeat visits let us see whether things are stable or progressing.
What happens after testing?
If the results suggest glaucoma, or if you’re at higher risk of developing it (for example, due to family history, age, or other risk factors), we’ll talk through what that means for you specifically, and what monitoring or treatment options are appropriate. Many people with glaucoma keep good vision for life with the right monitoring and treatment — the key is catching it early and staying on top of it.
If your results are normal, that’s reassurance — but for some patients, especially those with risk factors, we may still recommend periodic check-ups, since glaucoma can develop later.
The bottom line
Diagnosing glaucoma isn’t about one scary test — it’s a series of quick, painless checks that together build a clear picture of your eye health. If you haven’t had a glaucoma check recently, or if you have a family history of glaucoma, it’s worth booking a comprehensive eye examination.
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