Diagnostic Tests & Monitoring

Diagnosing and monitoring open-angle glaucoma (OAG) is a complex process that goes far beyond a simple eye pressure check. Because glaucoma is a “multimodal” disease—meaning it affects the structure of the eye and the function of your vision in different ways—your doctor must use a suite of tests to get a complete picture ^[1][2].

The Limitations of Eye Pressure

While measuring intraocular pressure (IOP) using a method called tonometry is a standard part of every exam, it is not enough to diagnose glaucoma on its own ^[3]. Some people have consistently high pressure but never develop nerve damage (ocular hypertension), while others have “normal” pressure but lose vision (normal-tension glaucoma) ^[1]. Furthermore, your eye pressure changes throughout the day, much like blood pressure, so a single reading is only a snapshot in time ^[4].

Essential Diagnostic Tests

To confirm a diagnosis and track changes, your care team will perform three primary types of assessments.

1. Gonioscopy: Looking at the Drain

Gonioscopy is the only way to confirm you have “open-angle” glaucoma rather than “angle-closure.” Your doctor uses a special mirrored lens to look directly at the eye’s drainage angle ^[2].

• Why it matters: It ensures the drain isn’t physically blocked by the iris. It also allows the doctor to see if “clogs” like pigment or flaky material (pseudoexfoliation) are present ^[5][6].
• What to expect: The doctor will numb your eye with drops and briefly place the lens against your eye.

2. Optical Coherence Tomography (OCT): Mapping the Structure

OCT is a non-invasive imaging test that creates a highly detailed, 3D map of your optic nerve and retina ^[1].

• RNFL (Retinal Nerve Fiber Layer): This measures the thickness of the nerve fibers. Thinning in this layer can be an early sign of glaucoma ^[7][8].
• Macular GCIPL (Ganglion Cell-Inner Plexiform Layer): This looks at the specialized cells in the center of your retina. It is especially helpful for detecting early damage and monitoring progression ^[9][10].
• Important Caveats: A single “abnormal” OCT scan does not automatically mean you have glaucoma. OCT measurements can be skewed by high myopia (nearsightedness), normal anatomical variations, or imaging artifacts. Your doctor looks for trends over multiple tests rather than relying on one scan.

3. Visual Field Testing: Measuring the Function

While OCT shows what the eye looks like structurally, a visual field test (perimetry) shows how it works functionally ^[11].

• The Test: You look into a bowl-shaped instrument and press a button every time you see a flash of light. This maps your peripheral and central vision.
• Structure vs. Function: It is common for an OCT to show structural damage while the visual field remains “normal.” This just means the damage hasn’t reached the point where it’s affecting your ability to see the lights yet. If your test results are unreliable because you were tired or distracted, your doctor will likely ask you to repeat it ^[12][8].

Understanding Your Anatomy

Two other measurements help your doctor assess your overall risk:

• Central Corneal Thickness (Pachymetry): The thickness of your cornea (the clear front window of the eye) can skew pressure readings ^[13]. A thin cornea might make your pressure seem lower than it actually is, while a thick cornea can make it seem higher ^[14]. However, there is no universally reliable mathematical formula to perfectly “correct” your pressure reading based on thickness. A thin cornea is also considered an independent risk marker for glaucoma progression ^[15].
• Corneal Hysteresis (CH): This measures the biomechanical properties of your cornea—essentially, how well it absorbs and dissipates energy. Low hysteresis is an adjunctive risk marker for progression, though this test is not available at every clinic ^[16][17].

The Optic Nerve Exam

During a dilated exam, your doctor will look directly at your optic nerve for specific warning signs:

• Cupping: The “cup” is the small indentation in the center of the nerve. Doctors measure the cup-to-disc ratio. While a large ratio can be a sign of glaucoma, it can also be a perfectly normal, physiological trait you were born with. The doctor looks for thinning or “notching” of the nerve tissue over time ^[1].
• Disc Hemorrhages: These are tiny “splinter” bleeds on the edge of the nerve. While they are a risk marker associated with glaucoma progression, they are not definitive proof of “active” disease by themselves ^[18].

Your Completeness Checklist

A comprehensive glaucoma evaluation is a combination of findings over time. Ensure your baseline exam and routine follow-ups include the following:

Baseline & Occasional Tests:

• [ ] Gonioscopy: To see if your drainage angle is open.
• [ ] Pachymetry: Measurement of your corneal thickness.

Routine Monitoring (Repeated Regularly):

• [ ] Tonometry: Accurate measurement of eye pressure.
• [ ] Dilated Nerve Exam: A physical look at your optic nerve.
• [ ] OCT Imaging: Structural scans to establish reliable trends.
• [ ] Visual Field Test: Functional mapping of your sight to detect subtle changes.
• [ ] Target IOP Discussion: Reviewing whether your specific eye pressure goal needs adjustment.