Glaucoma OCT Scans Explained: What the Images and Numbers Mean

Author: Dr Val Phua
Estimated reading time: 13–15 minutes

What Is an OCT Scan?

Optical coherence tomography, usually shortened to OCT, is a non-invasive imaging test that produces detailed cross-sectional pictures of the retina and optic nerve.

It uses reflected light rather than X-rays or sound waves.

An OCT scan can measure structures only a few microns thick and allows the ophthalmologist to examine individual retinal layers that cannot be measured accurately through ordinary visual inspection alone.

In glaucoma care, OCT is commonly used to examine:

• The retinal nerve-fibre layer around the optic nerve
• The ganglion-cell layers in the macula
• The optic-nerve head
• The neuroretinal rim
• Structural differences between the two eyes
• Structural change over time

The American Academy of Ophthalmology describes OCT as a test that allows the distinctive retinal layers and optic-nerve fibre layer to be viewed and measured. Modern glaucoma guidelines consider optic-nerve and retinal nerve-fibre imaging an important part of diagnosis and monitoring.

Is an OCT Scan the Same as a CT Scan?

No.

Despite the similar abbreviations, OCT and CT are completely different tests.

OCT

• Uses light waves
• Produces highly detailed images of the retina and optic nerve
• Is non-contact or near-contact
• Does not use ionising radiation
• Is usually completed within a few minutes

CT Scan

• Uses X-rays
• Produces images of the head, orbit or other body structures
• Exposes the patient to ionising radiation
• Is usually performed in a radiology department

An OCT scan does not expose the eye to the radiation used in a CT scan.

Is OCT Painful?

No.

During the test, the patient normally:

1. Places the chin and forehead against the machine.
2. Looks at a fixation target.
3. Keeps the eyes open for several seconds.
4. Allows the machine to capture the scan.

Nothing usually touches the eye.

The patient may notice:

• A bright fixation light
• Brief flashes
• The machine moving internally
• A need to blink between scans

The test is quick, painless and does not require an injection.

Do the Pupils Need to Be Dilated?

Not always.

Many modern OCT machines can obtain good-quality images without dilation.

Dilation may be helpful when:

• The pupil is small
• Cataract reduces image quality
• The scan is repeatedly weak
• A complete retinal examination is also required
• The optic nerve needs clearer photographic documentation

The decision depends on the patient’s eyes and the purpose of the visit.

What Does OCT Measure in Glaucoma?

Glaucoma damages retinal ganglion cells and the nerve fibres that connect them to the brain.

OCT attempts to measure this structural loss.

The principal glaucoma measurements include:

• Circumpapillary retinal nerve-fibre layer thickness
• Macular ganglion-cell measurements
• Optic-nerve head measurements
• Neuroretinal rim measurements
• Asymmetry and deviation maps
• Serial progression analysis

Different OCT manufacturers may use different names and measurement algorithms.

What Is the Retinal Nerve-Fibre Layer?

The retinal nerve-fibre layer, or RNFL, is formed mainly by the long nerve fibres, or axons, of retinal ganglion cells.

These fibres travel across the retina and converge at the optic nerve.

Glaucoma causes the nerve fibres to become progressively thinner as ganglion cells are damaged.

A glaucoma OCT normally measures the RNFL along a circular path around the optic nerve.

The machine may report:

• Average RNFL thickness
• Superior RNFL thickness
• Inferior RNFL thickness
• Nasal and temporal thickness
• Clock-hour sectors
• Thickness at numerous individual points

The current American Academy of Ophthalmology guideline identifies circumpapillary RNFL thinning as one of the characteristic structural features of primary open-angle glaucoma.

Why Is the RNFL Normally Thicker Above and Below the Optic Nerve?

In a healthy eye, the RNFL is generally thickest in the superior and inferior regions.

It is usually thinner nasally and temporally.

When plotted around the optic nerve, the normal thickness profile often forms two peaks, sometimes described as a double-hump pattern.

Glaucoma commonly damages the superior and inferior nerve-fibre bundles, which may produce:

• Localised thinning
• A missing or reduced peak
• A wedge-shaped defect
• Asymmetry between corresponding sectors
• A matching visual-field defect

However, normal anatomical variation can alter the expected pattern.

What Is the TSNIT Graph?

Some OCT reports show the RNFL thickness around the optic nerve as a circular graph.

The letters TSNIT represent the direction in which the measurement is displayed:

• Temporal
• Superior
• Nasal
• Inferior
• Temporal

The patient’s line is compared with a reference range.

The clinician looks for:

• Reduction in the superior or inferior peaks
• Focal thinning
• Differences between the eyes
• Change compared with earlier scans
• Whether the scan circle is centred correctly

A single unusual curve does not automatically confirm glaucoma.

What Are Retinal Ganglion Cells?

Retinal ganglion cells are nerve cells in the retina.

Their cell bodies are concentrated within the inner retina, particularly around the macula. Their axons form the RNFL and then the optic nerve.

Glaucoma may therefore be assessed at two related levels:

• Ganglion-cell bodies in the macula
• Ganglion-cell axons around the optic nerve

What Is a Ganglion-Cell Scan?

A ganglion-cell scan measures one or more of the inner retinal layers in the macula.

Depending on the OCT device, the report may use terms such as:

• Ganglion cell–inner plexiform layer, or GCIPL
• Ganglion cell complex, or GCC
• Ganglion cell layer, or GCL

These measurements are not identical, but they all aim to assess structures containing retinal ganglion cells or their associated layers.

Macular ganglion-cell analysis is particularly useful because a large proportion of retinal ganglion cells are concentrated near the macula.

Why Is the Ganglion-Cell Map Important?

Ganglion-cell analysis can help:

• Detect early central glaucomatous damage
• Assess defects close to fixation
• Confirm suspicious RNFL findings
• Monitor moderate or advanced disease
• Evaluate some highly myopic eyes
• Compare the superior and inferior macula

Studies support the usefulness of both RNFL and ganglion-cell measurements for detecting structural progression, including in eyes with central or more advanced damage.

A ganglion-cell scan should not be interpreted independently of the macular anatomy because retinal disease can also alter these layers.

What Is the Optic-Nerve Head Analysis?

The optic-nerve head is the visible front portion of the optic nerve.

An OCT may measure features such as:

• Disc area
• Cup area
• Cup-to-disc ratio
• Cup volume
• Rim area
• Rim thickness
• Bruch’s membrane opening
• Minimum rim width

Some machines use a measurement called Bruch’s membrane opening–minimum rim width, or BMO-MRW.

This estimates the shortest distance between the opening in Bruch’s membrane and the internal limiting membrane, providing an anatomical measure of the neuroretinal rim.

These measurements can support diagnosis, but none should be used as a standalone test.

What Is the Neuroretinal Rim?

The neuroretinal rim is the remaining nerve tissue surrounding the optic cup.

Glaucoma may cause:

• Generalised rim thinning
• Focal notching
• Progressive enlargement of the cup
• Asymmetry between the eyes
• Loss corresponding to RNFL and visual-field defects

A large cup does not automatically mean glaucoma.

People with naturally large optic nerves may have large cups while retaining healthy rim tissue.

Conversely, a small optic nerve may develop glaucoma without appearing dramatically cupped.

What Do the Green, Yellow and Red Colours Mean?

Most OCT machines compare the patient’s measurements with an internal normative database.

The exact colour thresholds vary by manufacturer, but reports commonly use:

• Green: measurement falls within the machine’s expected reference range
• Yellow: borderline or near the lower end of the reference range
• Red: measurement falls below most eyes in the reference database

These colours are statistical classifications.

They are not direct diagnoses.

Does Red Mean I Definitely Have Glaucoma?

No.

A red area means that the measurement differs statistically from the machine’s reference population.

It may reflect true glaucoma, but it can also be caused by:

• High myopia
• A tilted optic nerve
• A naturally thin RNFL
• Incorrect scan positioning
• Segmentation error
• Poor scan quality
• Peripapillary atrophy
• Retinal disease
• Unusual but healthy anatomy
• A reference database that does not match the patient well

This type of false-positive interpretation is sometimes called red disease.

Studies have shown that OCT artefacts are particularly common in eyes with both high myopia and glaucoma.

Does Green Mean the Optic Nerve Is Definitely Healthy?

No.

A measurement may appear green even when glaucoma is present.

Possible explanations include:

• The patient began with unusually thick nerve layers.
• Localised damage is hidden within a normal global average.
• The defect falls between analysed sectors.
• The normative database has a broad range.
• Early change has occurred but remains statistically “normal.”
• Both eyes are abnormal in a similar way.
• The disease has progressed from the patient’s own baseline but not yet crossed the red threshold.

False reassurance from normal colour coding is sometimes called green disease.

The clinician must inspect the actual images, thickness maps and progression from baseline—not simply the colour label.

Why Is Comparing with the Patient’s Own Baseline So Important?

A normative database compares the patient with other people.

Progression analysis compares the patient with their own previous scans.

A patient may lose a clinically important amount of tissue while remaining within the green population range.

Conversely, a patient may remain in the red range for years because of naturally unusual anatomy without showing any true progression.

For long-term glaucoma care, the most important questions are often:

• Is the scan technically reliable?
• Is the same region becoming thinner?
• Is the rate of thinning faster than normal ageing?
• Does the change match the optic nerve or visual field?
• Is the treatment sufficient to prevent further loss?

Can Normal Ageing Cause OCT Thinning?

Yes.

Healthy retinal nerve and ganglion-cell layers gradually become thinner with age.

Longitudinal studies have measured slow age-related loss in RNFL and ganglion-cell thickness even in people without glaucoma.

For this reason, progression software should distinguish between:

• Expected slow physiological ageing
• Faster or localised glaucomatous loss
• Measurement variation
• Scan artefact

Not every reduction of one or two microns represents glaucoma progression.

What Is a Micron?

OCT thickness is commonly reported in micrometres, also called microns and written as µm.

One micron is one-thousandth of a millimetre.

The RNFL may be only several dozen to slightly over one hundred microns thick, depending on:

• Age
• Eye size
• Measurement location
• Optic-disc anatomy
• OCT device
• Individual variation

Because the layers are extremely thin, small errors in scan placement or segmentation can materially change the reported measurement.

Is There One Normal RNFL Thickness for Everyone?

No.

Normal thickness varies with:

• Age
• Ethnicity
• Axial length
• Degree of myopia
• Optic-disc size
• Blood-vessel position
• Scan device
• Scan-circle diameter
• Individual anatomy

The average RNFL number should never be interpreted without reviewing the sectoral pattern and scan image.

A global average may hide a focal defect.

Why Can Two OCT Machines Produce Different Numbers?

Different manufacturers may use different:

• Light wavelengths
• Scan patterns
• Segmentation boundaries
• Reference databases
• Magnification corrections
• Optic-disc definitions
• Ganglion-cell layer combinations
• Progression software

A thickness value from one OCT device should not automatically be compared directly with a value from another device.

For reliable progression monitoring, it is generally preferable to use:

• The same device
• The same scan protocol
• The same anatomical registration
• Similar image quality

Switching devices may require establishing a new baseline.

What Is Scan Quality or Signal Strength?

The OCT report usually contains an indicator of image quality.

Poor quality may be caused by:

• Blinking
• Eye movement
• Dry eye
• Cataract
• Corneal opacity
• Small pupils
• Vitreous floaters
• Incorrect focus
• Poor fixation
• Eyelash or eyelid obstruction

A low-quality scan may produce inaccurate thickness measurements.

However, a high signal-strength number alone does not prove that the scan is free from errors.

The actual image and segmentation lines must also be inspected.

What Is Segmentation?

The OCT software automatically identifies the boundaries between retinal layers.

This process is called segmentation.

The machine then calculates the thickness between selected boundaries.

For example, RNFL analysis requires the software to identify:

• The inner retinal surface
• The lower boundary of the nerve-fibre layer

If the computer places either line incorrectly, the reported thickness may be falsely high or low.

What Is a Segmentation Error?

A segmentation error occurs when the software misidentifies one or more retinal boundaries.

It may be caused by:

• Poor scan quality
• High myopia
• Very thin tissue
• Epiretinal membrane
• Macular oedema
• Vitreoretinal traction
• Optic-disc abnormalities
• Retinal scarring
• Eye movement
• Blood-vessel shadows

Automated segmentation errors can cause false-positive or false-negative glaucoma classifications and may persist across repeat scans if not recognised.

The clinician should inspect the raw cross-sectional scan rather than accepting the automated number without review.

What Is Scan Decentration?

An optic-nerve RNFL scan is normally positioned around the optic disc.

If the measurement circle is shifted:

• One sector may appear thinner.
• Another may appear thicker.
• The TSNIT curve may change.
• Apparent progression may be created.
• Comparison with previous scans may become unreliable.

Modern machines attempt to register repeat scans to the same location, but errors remain possible.

How Do Blood Vessels Affect RNFL Measurements?

Retinal blood vessels travel within or close to the RNFL and contribute to the measured thickness.

Differences in vessel position can alter the normal thickness peaks.

A sector containing a large blood vessel may appear thicker than a neighbouring sector.

Local glaucoma damage may therefore need to be interpreted using:

• The RNFL map
• The raw B-scan
• Blood-vessel locations
• Optic-disc photographs
• Macular ganglion-cell analysis
• Visual fields

How Does High Myopia Affect OCT?

High myopia can make glaucoma OCT interpretation considerably more difficult.

Myopic eyes may have:

• Longer axial length
• Tilted optic nerves
• Rotated optic discs
• Peripapillary atrophy
• Stretched retinal tissue
• Temporally shifted RNFL bundles
• Posterior staphyloma
• Irregular scan geometry

These features may cause normal myopic anatomy to resemble glaucoma.

High myopia also increases the risk of:

• Scan-circle magnification error
• Segmentation error
• Normative-database mismatch
• False red sectors
• False-negative interpretation of real disease

Research shows that OCT artefacts are most frequent in eyes with combined high myopia and glaucoma, particularly in eyes with longer axial length, thinner RNFL and more advanced field loss.

Does a Red OCT in a Highly Myopic Eye Mean Glaucoma?

Not necessarily.

The clinician may need to consider:

• Optic-disc photographs
• Macular ganglion-cell analysis
• Visual-field testing
• Axial length
• Corneal thickness
• Pressure history
• Serial progression
• Comparison between the eyes
• Whether the structural pattern is anatomically glaucomatous

Macular ganglion-cell measurements can sometimes be useful in high myopia, although they can also be affected by myopic macular changes or segmentation error.

The diagnosis may depend more on change over time than on one colour-coded scan.

Can Cataract Affect Glaucoma OCT?

Yes.

Cataract can reduce the strength and clarity of the OCT signal.

This may lead to:

• Poorer image quality
• Artificially low thickness measurements
• Greater variability
• Segmentation problems

After cataract surgery, the OCT signal may improve and the measured RNFL may appear thicker even though nerve fibres have not regenerated.

A new post-cataract baseline may sometimes be helpful.

Can Dry Eye Affect the Scan?

Yes.

An irregular tear film can reduce image quality.

The scan may improve after:

• Blinking
• Preservative-free lubrication
• Repeating the image
• Adjusting focus
• Briefly resting the eye

A thick gel or ointment immediately before the scan may temporarily blur the optical surface, so the technician may advise when to use it.

Can Retinal Disease Affect the Ganglion-Cell Scan?

Yes.

Macular ganglion-cell analysis may be distorted by:

• Epiretinal membrane
• Macular oedema
• Diabetic macular disease
• Macular degeneration
• Retinal vein occlusion
• Vitreomacular traction
• Previous retinal surgery
• Myopic macular degeneration
• Retinal scars

A thickened or irregular macula may conceal ganglion-cell loss or cause segmentation errors.

The clinician should inspect the macular OCT anatomy before relying on the automated ganglion-cell map.

Can Other Optic-Nerve Diseases Cause OCT Thinning?

Yes.

RNFL and ganglion-cell thinning are not specific to glaucoma.

Other causes include:

• Optic neuritis
• Ischaemic optic neuropathy
• Compressive optic neuropathy
• Hereditary optic neuropathy
• Toxic or nutritional optic neuropathy
• Previous papilloedema
• Neurological disease
• Trauma

The pattern, symptoms, colour vision, pupil reactions and visual field help distinguish these conditions.

Atypical findings may require neurological imaging or neuro-ophthalmic assessment.

Can OCT Diagnose Glaucoma by Itself?

No.

OCT is an important part of glaucoma diagnosis, but it should be interpreted together with:

• Eye pressure
• Gonioscopy
• Optic-nerve examination
• Optic-disc photographs
• Visual-field testing
• Corneal thickness
• Medical and family history
• Change over time

The NICE glaucoma guideline incorporates optic-nerve assessment and appropriate imaging within a broader diagnostic and monitoring process rather than treating OCT as an independent diagnosis.

Can OCT Detect Glaucoma Before the Visual Field Becomes Abnormal?

Sometimes.

Structural thinning may be detected before a reproducible defect appears on standard automated perimetry.

This is often called pre-perimetric glaucoma.

OCT may be particularly sensitive for early structural change, while visual-field testing becomes increasingly important as functional damage develops.

Studies comparing OCT and visual-field progression have found that OCT may detect progression earlier in glaucoma suspects and early glaucoma.

However, early OCT abnormalities must be distinguished from normal anatomy and artefact.

Can the Visual Field Become Abnormal Before OCT?

Yes.

Possible reasons include:

• The OCT measurement is already near its lower floor.
• Damage occurs in an area not sampled effectively.
• High myopia reduces structural reliability.
• The visual-field defect is caused by another condition.
• The OCT scan contains segmentation or registration error.
• Functional change is easier to detect at that stage.

Structural and functional tests do not always change at the same time.

Why Are Both OCT and Visual Fields Needed?

OCT measures structure.

The visual-field test measures function.

OCT shows whether nerve tissue appears thin or is becoming thinner.

The visual field shows whether the patient has lost sensitivity in corresponding areas of vision.

Together, they help answer:

• Is glaucoma present?
• How advanced is it?
• Is the disease worsening?
• Does structural change have a functional effect?
• Is the pressure target low enough?
• Is treatment preventing further damage?

What Is OCT Progression Analysis?

Progression analysis compares a series of scans over time.

The software may evaluate:

• Global RNFL thickness
• Sectoral RNFL thickness
• Ganglion-cell thickness
• Optic-nerve rim measurements
• Change maps
• Rate of loss per year

The purpose is to determine whether the observed thinning exceeds normal measurement variability and expected ageing.

What Is Event-Based Analysis?

Event-based analysis asks whether a follow-up measurement has changed beyond a predefined amount compared with baseline scans.

It may label results using terms such as:

• Possible loss
• Likely loss
• Change from baseline

The exact terminology varies between devices.

A recent study found that event-based OCT analysis identified structural progression in more glaucoma patients and at an earlier time than trend-based analysis in the population studied.

A flagged event still requires confirmation that the scan is technically valid.

What Is Trend-Based Analysis?

Trend-based analysis estimates the rate at which a measurement changes over time.

It may report values such as:

• Microns of RNFL loss per year
• Rate of GCIPL thinning
• Statistical significance of the trend

Trend analysis requires several good-quality scans over an adequate period.

A slope based on only a small number of scans may be unstable.

The clinician should consider:

• Number of scans
• Length of follow-up
• Scan quality
• Normal ageing
• Treatment changes
• Whether the loss is localised or global

How Many OCT Scans Are Needed to Detect Progression?

There is no single number suitable for every patient.

Reliable progression analysis generally requires:

• At least two good baseline scans
• Several follow-up scans
• Consistent scan protocol
• Adequate time between measurements
• Sufficient testing frequency to detect rapid change

More frequent scans may be appropriate when:

• Glaucoma is newly diagnosed
• Progression is suspected
• Pressure remains above target
• Disease is advanced
• Treatment has recently changed
• The patient is young
• Damage threatens central vision

One isolated thinner measurement should not automatically lead to major treatment escalation without checking for artefact or confirmation.

Why Did My OCT Improve?

True glaucomatous nerve loss does not normally regenerate.

An apparently thicker or “better” OCT may result from:

• Improved signal strength
• Better scan centration
• Less dry eye
• Removal of cataract
• Different segmentation
• Normal measurement variability
• Swelling or retinal disease
• A different machine

Improvement in the colour map does not necessarily mean that glaucoma damage has reversed.

Why Did the OCT Suddenly Become Thinner?

Possible reasons include:

• True glaucoma progression
• Poor signal
• Cataract
• Dry eye
• Incorrect scan placement
• Segmentation error
• Eye movement
• Change of OCT device
• Retinal pathology
• Posterior vitreous changes
• Normal test variability

The raw scan and previous images should be reviewed before interpreting the new value.

What Is the OCT Floor Effect?

As glaucoma becomes advanced, the RNFL reaches a residual thickness containing structures that do not disappear completely, such as:

• Blood vessels
• Glial tissue
• Supporting tissue
• Remaining non-neural elements

Once the RNFL approaches this measurement floor, further ganglion-cell loss may occur without a large additional reduction in the reported RNFL thickness.

Research has estimated structural measurement floors and confirmed that RNFL monitoring becomes less sensitive in advanced disease.

Does OCT Become Useless in Advanced Glaucoma?

No, but its limitations become more important.

In advanced disease, clinicians may rely more heavily on:

• Macular ganglion-cell measurements
• Central 10-2 visual fields
• Other visual-field strategies
• Optic-disc photographs
• Clinical examination
• Functional change
• Remaining regions that have not yet reached the floor

Ganglion-cell measurements may retain useful measurable tissue after some RNFL sectors have reached their floor, although they also eventually become limited.

Can OCT Show Whether Treatment Is Working?

OCT cannot directly show whether a drop, laser or operation is biologically active.

It can help show whether structural damage is continuing despite treatment.

A successful treatment plan may be associated with:

• Stable RNFL
• Stable ganglion-cell measurements
• Stable visual fields
• Pressure at or below target
• No new optic-disc changes

If confirmed progression continues, the ophthalmologist may consider:

• Lowering the target pressure
• Improving drop adherence
• Changing medication
• Performing laser treatment
• Recommending surgery
• Investigating another cause of optic-nerve loss

Does a Stable OCT Mean the Glaucoma Is Cured?

No.

Stable OCT means no clinically significant structural progression has been detected during the observed period.

The patient still requires follow-up because:

• Pressure may change.
• Progression may resume.
• Visual fields may change.
• Measurement floors may conceal further loss.
• Treatment may become less effective.
• Adherence may change.
• New ocular conditions may develop.

Glaucoma is generally controlled rather than cured.

Can OCT Measure Eye Pressure?

No.

OCT does not replace tonometry.

The scan measures retinal and optic-nerve structure, not the fluid pressure inside the eye.

A glaucoma assessment may still require:

• Goldmann or another form of tonometry
• Corneal-thickness measurement
• Gonioscopy
• Visual fields
• Clinical optic-nerve examination

Can OCT Show Whether the Drainage Angle Is Narrow?

A standard posterior-segment glaucoma OCT does not fully replace gonioscopy.

Anterior-segment OCT can image the angle and anterior chamber, but it may not show every clinically important feature.

Gonioscopy remains necessary for assessing:

• Whether the angle is truly open or closed
• Peripheral anterior synechiae
• Abnormal pigment
• Angle recession
• Abnormal blood vessels
• Dynamic angle behaviour

What Is OCT Angiography?

OCT angiography, or OCTA, uses repeated OCT scans to map movement of blood cells within retinal and optic-nerve vessels.

It can measure vessel density without injecting fluorescent dye.

Glaucoma may be associated with reduced vessel density around the optic nerve and macula.

However, OCTA is affected by:

• Eye movement
• Segmentation errors
• Signal strength
• High myopia
• Blood pressure and vascular factors
• Device-specific algorithms

OCTA may provide useful supplementary information, but it has not replaced standard RNFL, ganglion-cell and visual-field monitoring. The AAO has reviewed evidence that OCTA can detect reduced peripapillary and macular vessel density in glaucoma.

Can Artificial Intelligence Diagnose Glaucoma from OCT?

Artificial intelligence systems are increasingly being developed to:

• Identify poor-quality scans
• Detect RNFL thinning
• Classify glaucoma risk
• Predict visual-field damage
• Estimate progression

These tools may improve screening and efficiency.

They remain dependent on:

• Training data
• Image quality
• Population diversity
• Device compatibility
• Accurate segmentation
• Clinical validation

An AI label should not replace examination by a clinician who can review the patient’s pressure, angle anatomy, medical history, visual fields and actual scan images.

Should I Ask for a Copy of My OCT?

It can be useful to retain copies or ensure the clinic stores serial scans.

Long-term comparison is particularly important when:

• Changing clinics
• Moving overseas
• Undergoing refractive or cataract surgery
• Glaucoma is suspected but not yet confirmed
• Progression is slow
• High myopia makes interpretation difficult

The most valuable record includes:

• Actual scan images
• Thickness maps
• Device name
• Scan date
• Signal quality
• Progression report

A photograph of only the colour-coded summary may not contain enough information.

What Questions Should I Ask About My OCT?

Useful questions include:

• Is the scan quality reliable?
• Is there any segmentation error?
• Does the pattern look like glaucoma?
• Does it match my visual field?
• Is the abnormality explained by myopia or another retinal condition?
• Has the scan changed from my previous baseline?
• What is the estimated rate of thinning?
• Has any region reached the measurement floor?
• Does my target pressure need to change?
• When should the OCT be repeated?

Frequently Asked Questions

Does an abnormal OCT mean I have glaucoma?

Not necessarily.

The result must be compared with the optic nerve, pressure, visual field, angle anatomy and previous scans.

Can I have glaucoma with a normal OCT?

Yes.

Disease may be very early, anatomically unusual, inadequately sampled or functionally detectable before the structural report becomes abnormal.

Can OCT detect glaucoma before symptoms?

Yes, structural damage may be detectable before the patient notices symptoms.

Can OCT detect glaucoma before a visual-field defect?

Sometimes.

This is often described as pre-perimetric glaucoma.

Does red always mean nerve damage?

No.

Red means the measurement lies outside the machine’s statistical reference range.

Does green mean I do not have glaucoma?

No.

Early or progressive loss can remain within the green range.

Why is one eye thinner than the other?

Possible reasons include:

• Normal asymmetry
• Glaucoma
• High myopia
• Different optic-disc sizes
• Previous optic-nerve disease
• Retinal disease
• Scan error

Is an average RNFL of 80 microns normal?

The number cannot be interpreted accurately without considering age, device, axial length, sectoral pattern, image quality and previous measurements.

Why is my average RNFL thin but my visual field normal?

Possible explanations include:

• Early structural glaucoma
• High myopia
• Naturally thin RNFL
• Scan artefact
• Optic-nerve disease
• A field defect not yet detectable

Why is my visual field abnormal but the OCT normal?

Possible explanations include:

• Unreliable visual-field testing
• Neurological or retinal disease
• Early functional damage
• OCT measurement limitations
• Advanced focal disease not reflected in the global average

Can cataract make the OCT look worse?

Yes.

Cataract may reduce signal and artificially lower measurements.

Can cataract surgery improve the OCT?

It may improve scan quality and measured thickness, but it does not restore glaucomatous nerve tissue.

Can LASIK affect glaucoma OCT?

LASIK mainly affects pressure measurement rather than the retinal OCT itself.

However, patients who had LASIK often had myopia, which can complicate optic-nerve and OCT interpretation.

Can contact lenses be worn during OCT?

Usually yes, but they may be removed if they reduce image quality, are uncomfortable or interfere with fixation.

Can OCT damage the retina?

Routine diagnostic OCT uses low-power light and is regarded as non-invasive and safe.

How often should glaucoma OCT be repeated?

The interval depends on diagnosis, severity, progression risk and scan reliability.

It may range from several months to a year or longer in stable low-risk cases.

Why did my doctor repeat the scan during the same visit?

The first scan may have had:

• Low signal
• Movement
• Blinking
• Decentration
• Segmentation error
• Poor focus

Repeating improves reliability.

Does optic-nerve cupping always appear on OCT?

The OCT may quantify cup and rim anatomy, but cupping is also assessed clinically and through photographs.

Can an OCT tell whether I will go blind?

It can help estimate disease severity and rate of structural loss, but it cannot predict the future with certainty.

Risk also depends on:

• Age
• Visual fields
• Eye pressure
• Treatment response
• Rate of progression
• Other eye disease
• Adherence

Can nerve tissue grow back after treatment?

Current glaucoma treatment does not normally regenerate lost retinal ganglion cells or optic-nerve fibres.

Treatment aims to protect the tissue that remains.

A Practical Approach to Reading a Glaucoma OCT

Step 1: Confirm the Correct Patient and Eye

Check:

• Name
• Date
• Right or left eye
• Scan type
• Device

Step 2: Assess Scan Quality

Look for:

• Adequate signal
• Good focus
• Minimal movement
• Clear retinal boundaries
• Absence of blink artefact

Step 3: Inspect the Raw Images

Do not rely only on the summary colours.

Check whether the segmentation lines follow the correct retinal layers.

Step 4: Check Scan Centration

Confirm that:

• The optic-disc scan is centred.
• The scan circle matches previous visits.
• The macular scan is correctly positioned.

Step 5: Assess the RNFL Pattern

Review:

• Global thickness
• Superior and inferior bundles
• Focal defects
• TSNIT profile
• Sectoral asymmetry

Step 6: Assess the Ganglion-Cell Map

Look for:

• Superior–inferior asymmetry
• Arcuate or wedge-shaped thinning
• Central involvement
• Macular pathology causing artefact

Step 7: Consider Anatomy and Risk Factors

Account for:

• High myopia
• Tilted discs
• Large or small optic nerves
• Peripapillary atrophy
• Retinal disease
• Previous optic neuropathy

Step 8: Compare with the Visual Field

Ask whether structural loss matches the functional defect.

Step 9: Compare with Previous Scans

Look for:

• Reproducible change
• Rate of thinning
• Event-based alerts
• Trend-based progression
• Change exceeding normal ageing

Step 10: Interpret Within the Full Clinical Picture

Include:

• Eye pressure
• Target pressure
• Corneal thickness
• Gonioscopy
• Disc examination
• Treatment adherence

Common Misunderstandings About Glaucoma OCT

“Red means glaucoma.”

False.

Red means statistically outside the reference range.

“Green means healthy.”

False.

Progression may occur while values remain green.

“The lowest number is always the worst.”

False.

The pattern, device, anatomy and change over time matter.

“A normal OCT means no glaucoma.”

False.

Very early or functionally evident disease may still be present.

“One thinner scan proves progression.”

False.

Artefact and measurement variability must first be excluded.

“OCT replaces the visual field.”

False.

OCT measures structure, while visual fields measure function.

“The OCT can measure eye pressure.”

False.

Tonometry is still required.

“Nerve fibres grow back when the pressure falls.”

False.

Pressure lowering protects remaining nerve tissue but does not usually restore tissue already lost.

The Bottom Line

A glaucoma OCT scan uses light to measure microscopic structures within the retina and optic nerve.

The main measurements include:

• Retinal nerve-fibre layer thickness
• Macular ganglion-cell thickness
• Optic-nerve head and neuroretinal rim measurements
• Structural change over time

OCT is valuable because it can:

• Detect early structural damage
• Support glaucoma diagnosis
• Monitor progression
• Identify central ganglion-cell loss
• Complement visual-field testing

However, OCT is not a standalone diagnosis.

Colour coding can be misleading:

• Red does not always mean glaucoma.
• Green does not always mean healthy.
• High myopia frequently produces false abnormalities.
• Poor quality and segmentation errors can change the numbers.
• Retinal and neurological disease can also cause thinning.

The most reliable interpretation combines:

• Raw scan inspection
• Scan quality
• RNFL and ganglion-cell patterns
• Serial progression
• Optic-nerve examination
• Eye pressure
• Gonioscopy
• Visual-field testing

The most important OCT result is not whether today’s scan is red or green. It is whether a reliable, anatomically consistent pattern of nerve loss is present—and whether that pattern is progressing over time.

References

1. American Academy of Ophthalmology. What Is Optical Coherence Tomography? Updated 2024.
2. Gedde SJ, et al. Primary Open-Angle Glaucoma Preferred Practice Pattern. Ophthalmology. 2026.
3. National Institute for Health and Care Excellence. Glaucoma: Diagnosis and Management. Guideline NG81. Reviewed March 2025.
4. Palmer LD, et al. Diagnosing Glaucoma Progression With Optical Coherence Tomography. 2025.
5. Poon LYC, et al. The Prevalence of Optical Coherence Tomography Artifacts in High Myopia and Its Influence on Glaucoma Diagnosis. Journal of Glaucoma. 2023.
6. Chen JJ, Kardon RH. Avoiding Clinical Misinterpretation and Artifacts of Optical Coherence Tomography Analysis of the Optic Nerve, Retinal Nerve Fiber Layer and Ganglion Cell Layer. Journal of Neuro-Ophthalmology. 2016.
7. Nagarkatti-Gude N, et al. Optical Coherence Tomography Segmentation Errors of the Retinal Nerve Fibre Layer Persist Over Time. 2019.
8. Mansberger SL, et al. Automated Segmentation Errors When Using Optical Coherence Tomography to Measure Retinal Nerve Fiber Layer Thickness in Glaucoma. American Journal of Ophthalmology. 2017.
9. Bowd C, et al. Estimating Optical Coherence Tomography Structural Measurement Floors to Improve Detection of Progression in Advanced Glaucoma. American Journal of Ophthalmology. 2017.
10. Zhang X, et al. Comparison of Glaucoma Progression Detection by Optical Coherence Tomography and Visual Field. American Journal of Ophthalmology. 2017.
11. Scott TD, et al. Comparison of Event-Based Analysis Versus Trend-Based Analysis for Detecting Optical Coherence Tomography Progression. 2025.
12. Wang WW, et al. Diagnostic Ability of Ganglion Cell Complex Thickness to Detect Glaucoma in High Myopia. 2018.
13. Zhang X, et al. Longitudinal and Cross-Sectional Analyses of Age Effects on Retinal Nerve Fiber Layer and Ganglion Cell Complex Thickness. 2016.
14. Munuera I, et al. Study of the Macular Ganglion Cell Complex by Optical Coherence Tomography in the Diagnosis of Glaucoma Progression. 2024.