Glaucoma is diagnosed through a combination of tests measuring eye pressure (10–21 mmHg is average), checking for optic nerve damage, and mapping peripheral vision loss. Key parameters include intraocular pressure (IOP), optic nerve structure (OCT), corneal thickness (pachymetry), and visual field (perimeter) tests. A diagnosis indicates irreversible damage to the optic nerve.
+5
Key Parameters and Tests
Tonometry (Eye Pressure): Measures the pressure inside the eye, typically using a Goldmann applanation tonometer or air-puff test. While
–
mm Hg is normal, some individuals develop glaucoma at "normal" pressures.
Optic Nerve Imaging/Fundus Photography: A dilated eye exam allows doctors to look at the optic nerve at the back of the eye, looking for "cupping" or thinning, which is a key indicator of damage.
Optical Coherence Tomography (OCT): A non-invasive imaging test that takes cross-sectional pictures of the retina and optic nerve to check for nerve fiber layer thinning.
Visual Field Test (Perimetry): Maps your peripheral and central vision to detect blind spots, a common sign of damage, particularly with side vision loss.
Pachymetry (Corneal Thickness): Measures the thickness of the cornea, as thinner corneas can lead to an underestimation of eye pressure.
Gonioscopy: Uses a special lens to examine the eye's drainage angle to determine if it is open or closed.
+8
How to Understand if You Have Glaucoma
A diagnosis is generally made when multiple tests reveal:
Elevated Intraocular Pressure (IOP): Consistently high pressure, often >22 mmHg.
Optic Nerve Damage: Visible structural changes in the optic nerve head (cupping).
Visual Field Defects: Loss of peripheral vision.
Barnet Dulaney Perkins Eye Center
+3
Important Notes:
No Symptoms Initially: Glaucoma is often asymptomatic until significant vision loss occurs.
Normal Tension Glaucoma: You can have glaucoma even with normal pressure.
Emergency Symptoms: Sudden eye pain, severe headaches, halos around lights, and blurry vision indicate acute angle-closure glaucoma.
Glaucoma Research Foundation
Glaucoma Research Foundation
+2
Regular comprehensive eye exams with dilation are the only way to detect glaucoma early.
Learn
While intraocular pressure (IOP) is the most common metric, doctors use several other specific "reading numbers" from structural and functional tests to diagnose and monitor glaucoma.
1. Optic Nerve: Cup-to-Disc Ratio (CDR)
The Cup-to-Disc Ratio (CDR) measures the size of the "cup" (the white center) relative to the entire "disc" (the optic nerve head).
Normal: Typically around 0.3.
Suspicious: Ratios above 0.5 or 0.6 often raise suspicion.
Glaucomatous: Ratios of 0.8 or higher are strongly indicative of glaucoma.
Asymmetry: A difference of >0.2 between your two eyes is found in less than 1% of healthy individuals and is a major warning sign.
JAMA
+5
2. Retinal Nerve Fiber Layer (RNFL) Thickness
Using OCT (Optical Coherence Tomography), doctors measure the thickness of the nerve fiber layer.
Normal: Average thickness is typically >90 µm (micrometres).
Glaucoma: Thinning often leads to readings below 75–80 µm.
Sectoral loss: Thinning specifically in the inferior (bottom) or superior (top) quadrants is more characteristic of glaucoma than general global thinning.
JAMA
+3
3. Visual Field: Mean Deviation (MD)
This number from a Perimetry test tells the doctor how much your vision deviates from a healthy person of your same age.
Normal: Values near 0 dB (decibels).
Mild Glaucoma: MD up to -6 dB.
Moderate Glaucoma: MD between -6 dB and -12 dB.
Severe Glaucoma: MD worse than -12 dB.
National Institutes of Health (.gov)
National Institutes of Health (.gov)
+1
4. Ganglion Cell Complex (GCC)
This newer OCT measurement looks at the nerve cell bodies in the macula (central vision area).
Normal: Average thickness is often around 85–95 µm.
Glaucoma: Readings frequently drop to ~70 µm or lower.
ScienceDirect.com
ScienceDirect.com
+2
5. Central Corneal Thickness (CCT)
This number is used to "correct" your IOP reading.
Average: Roughly 540–550 µm.
Impact: If your cornea is very thin (<500 µm), your IOP reading might be falsely low, meaning your actual eye pressure is higher than it appears on the test.
+2
Do you have your test results and want help understanding a specific number like the CDR or RNFL thickness?
This is for informational purposes only. For medical advice or diagnosis, consult a professional. AI responses may include mistakes. Learn more
The Retinal Nerve Fiber Layer Thickness in Ocular ... - JAMA
JAMA
Can ganglion cell complex assessment on cirrus HD ...
Macular and Retinal Nerve Fiber Layer Thickness - IOVS
