Glaucoma Screening: What Happens During the Test

Glaucoma is the second leading cause of blindness worldwide and the leading cause of irreversible blindness — and roughly half the people who have it do not know yet. The screening tests that catch it are quick, painless, and built into every comprehensive eye exam. This guide walks through exactly what each test does, what it feels like, what the results mean, and what happens if anything is flagged.

Why glaucoma screening matters

Glaucoma damages the optic nerve, almost always silently. There are no early symptoms. Vision loss starts in the far peripheral field where you do not normally look, and the brain fills in the missing area so seamlessly that most people do not notice anything is wrong until central vision begins to be affected. By that point, a substantial amount of the optic nerve has already been permanently damaged.

The good news is that glaucoma, when detected early, is highly treatable. Eye-pressure-lowering drops, laser procedures, and (in advanced cases) surgery can slow or halt progression for the rest of a patient's life. The bad news is that no treatment can restore vision that has already been lost. Everything depends on catching it early — which means everything depends on screening.

A brief primer on glaucoma

The eye continuously produces a clear fluid (aqueous humor) that nourishes the front of the eye and drains out through a small mesh-like channel called the trabecular meshwork. When that drain does not work as efficiently as it should, fluid backs up and pressure inside the eye rises. Over time, that elevated pressure damages the optic nerve fibers at the back of the eye.

That is the most common form — primary open-angle glaucoma. There are other forms too: angle-closure glaucoma (a sudden, painful blockage of the drain), normal-tension glaucoma (optic nerve damage despite normal eye pressures), and secondary glaucomas caused by trauma, steroids, or other eye conditions. A proper screening tests for all of them, not just the pressure component.

Who needs to be screened — and how often

The American Optometric Association and American Academy of Ophthalmology both recommend glaucoma screening as part of routine comprehensive eye exams for adults. Risk factors that raise priority include:

• Age 40 or older (risk rises with each decade after 40, and steeply after 60)
• African American, Hispanic, or Asian heritage
• Family history of glaucoma in a parent or sibling
• Diabetes or high blood pressure
• Severe nearsightedness (high myopia) or farsightedness
• Long-term steroid use (oral, topical, or inhaled)
• History of significant eye injury
• Thin corneas (a finding that is itself detected by screening)

A reasonable rule of thumb: every adult should have glaucoma screening as part of a comprehensive eye exam at least every two years before age 40, every year from 40 onward, and every six to twelve months if any high-risk factors are present or a previous screening flagged borderline findings. For patients diagnosed with glaucoma or with documented suspicious findings, monitoring intervals are set by the optometrist and may be more frequent.

The actual tests in a glaucoma screening

A modern glaucoma screening is not a single test — it is a layered set of measurements that together build a complete picture of optic nerve health. No single test can rule glaucoma in or out on its own. Here is what each one does.

1. Tonometry — measuring eye pressure

Tonometry measures intraocular pressure (IOP), reported in millimeters of mercury (mmHg). Normal range is roughly 10–21 mmHg, with the average around 15. Pressures above 21 are flagged as ocular hypertension and increase glaucoma risk; pressures within range do not rule out glaucoma (about a third of glaucoma patients have normal pressures).

You may have experienced the classic "air puff" test (non-contact tonometry) at past exams. Modern clinics often use a rebound tonometer instead — a quick, painless tap from a tiny disposable probe that takes a fraction of a second and does not require numbing drops or air. Both produce reliable measurements. The whole test takes seconds.

2. Optic nerve evaluation and retinal imaging

A high-resolution digital retinal photograph captures the optic nerve head, the retina, and surrounding structures in a single image. The optometrist examines the size, shape, and color of the optic nerve — particularly the cup-to-disc ratio, the relative size of the central depression in the nerve. A larger or asymmetric cup, or a cup that has grown over time, is one of the earliest visible signs of glaucoma damage.

Storing the image year-over-year is a major part of why retinal imaging is valuable. Subtle changes that would be hard to catch by examining the eye directly become obvious when this year's image is overlaid on last year's.

3. Optical coherence tomography (OCT)

OCT is a non-contact imaging test — think of it as ultrasound for the eye, but using light instead of sound — that produces a cross-sectional image of the retina layer by layer at micron-level resolution. For glaucoma screening, OCT measures the thickness of the retinal nerve fiber layer (RNFL) around the optic nerve, where the first cells to die in glaucoma live.

OCT can detect glaucoma damage years before it shows up in a visual field test. In current clinical practice it is one of the most sensitive screening tools available. The test takes a couple of minutes per eye, no contact, nothing in or near your eye.

4. Visual field test (perimetry)

A visual field test maps your peripheral vision. You sit at a machine, focus on a central target, and click a button each time you see a flash of light appear in your side vision. The machine records which spots you saw and which you missed, building a map of your functional field.

Glaucoma damages peripheral vision in characteristic patterns long before central vision is affected. Visual field testing detects that damage. The full test takes around 5–10 minutes per eye and can feel a little tedious — staying focused on the center while flashes appear all around you takes concentration. Visual field tests are typically performed when other findings warrant follow-up rather than at every routine screening.

5. Pachymetry — corneal thickness

Corneal thickness affects how accurately tonometry measures eye pressure: thin corneas can read artificially low, thick corneas artificially high. More importantly, thin corneas are an independent risk factor for glaucoma development. Pachymetry — a quick ultrasound or optical measurement of corneal thickness — adjusts the interpretation of pressure readings and identifies risk.

6. Gonioscopy — examining the drainage angle (when indicated)

Gonioscopy uses a special contact lens to look at the drainage angle of the eye — the structure that determines whether glaucoma, if present, is "open angle" (most common) or "closed angle" (less common but more dangerous). It is performed when other findings suggest examining the angle is necessary; it is not part of every routine screening.

What it actually feels like

For the typical patient at a routine comprehensive exam, the entire glaucoma screening component adds maybe 5–10 minutes to the visit and consists almost entirely of looking into machines while they take measurements. The most "intrusive" part is rebound tonometry — a barely perceptible tap. Nothing else touches your eye. There are no drops required for OCT, retinal imaging, or tonometry. Visual field testing, when performed, takes longer but is just clicking a button.

Patients are sometimes surprised at how brief and uneventful glaucoma screening actually is, given how important it is. Modern technology has made what used to require dilation and a manual exam something you can complete in minutes.

What the results mean

The optometrist reviews the data from all the screening tests together rather than treating any single number as a diagnosis. Common categories of result include:

• Normal. Pressures in range, optic nerves healthy, RNFL thickness normal, no risk factors elevated. Continue routine annual or biannual screening.
• Glaucoma suspect. One or more findings warrant attention — perhaps a slightly elevated pressure, an asymmetric optic nerve, borderline RNFL thinning, or a strong family history. The optometrist will recommend more frequent monitoring (typically every 6–12 months) and may add a baseline visual field test.
• Ocular hypertension. Eye pressure is above 21 mmHg but optic nerves and visual fields look healthy. Not glaucoma yet, but elevated risk; close monitoring or pre-emptive treatment may be discussed depending on overall risk profile.
• Glaucoma diagnosed. Multiple findings together (optic nerve damage, RNFL loss, visual field changes, with or without elevated pressure) confirm the disease. Treatment usually starts with daily prescription eye drops to lower pressure; laser or surgical options exist for cases where drops are not enough.

The optometrist explains your specific results, what they mean, and what the recommended next step is at the end of your exam. You leave with a clear understanding, not a question mark.

If glaucoma is suspected

A "suspect" or borderline finding is not a diagnosis. It is a flag that something is worth watching. The most common next steps are some combination of:

• A baseline visual field test, if not already done
• Repeat OCT in 3–6 months to look for progression
• Repeat eye-pressure measurement at a different time of day (IOP fluctuates)
• Review of relevant family or medical history that might raise or lower risk

For a confirmed diagnosis or complex case, the optometrist coordinates with a glaucoma specialist (a fellowship-trained ophthalmologist) for treatment planning. Many patients are managed entirely in primary eye care for years; only a minority ultimately require surgical intervention.

Cost and insurance

Glaucoma screening is a built-in part of a comprehensive eye exam, not a separate add-on charge. Vision insurance plans (VSP, EyeMed, Davis Vision, Spectera, and most others) cover one comprehensive exam per year that includes screening. Medical insurance — including Medicare — covers glaucoma evaluation when there are risk factors or symptoms; the visit is billed as a medical exam rather than a vision exam.

For self-pay patients, our pricing for a comprehensive exam (which includes glaucoma screening) is published transparently. There is no scenario where you save money by skipping screening — it is part of the same visit.

Schedule a glaucoma screening

Glaucoma is the canonical example of a sight-threatening condition you cannot feel coming. The single most important thing any adult — particularly anyone over 40 or with risk factors — can do for long-term vision is to keep up with comprehensive eye exams that include glaucoma screening. Every Summit Eye Care exam includes the screening as standard.

You can find your nearest Summit Eye Care location and call to book. If you would like more clinical detail on the procedure side, our glaucoma screening service page covers it. For a closer look at the disease itself — what it does, who is at risk, and how it is treated — see our glaucoma condition guide.