OCT Eye Scans: What They Show, How They Work and What the Results Mean

Author: Dr Val Phua
Estimated reading time: 23 minutes

Optical coherence tomography, commonly abbreviated as OCT, is a non-invasive imaging test that produces detailed cross-sectional images of structures inside the eye.

An OCT scan can examine:

The macula
The retina
The optic nerve
The retinal nerve fibre layer
The retinal ganglion-cell layers
The choroid
The cornea
The drainage angle
The anterior chamber
An implanted lens or other anterior-segment structures

OCT is sometimes described as an “optical biopsy” because it allows the ophthalmologist to examine microscopic tissue layers without cutting into or touching the eye.

It is widely used to diagnose and monitor conditions such as:

Age-related macular degeneration
Diabetic macular oedema
Epiretinal membrane
Macular hole
Vitreomacular traction
Retinal vein occlusion
Central serous chorioretinopathy
Glaucoma
Optic-nerve disease
Keratoconus
Corneal scarring
Narrow drainage angles
Implantable Collamer Lens vault
Healing after corneal or retinal surgery

The test is usually:

Quick
Painless
Non-contact
Repeatable
Safe to perform repeatedly
Completed without injections or radiation

However, OCT does not replace a complete eye examination.

An apparently abnormal OCT may be caused by:

Poor scan quality
Eye movement
Incorrect automated segmentation
Cataract
Dry eye
High myopia
An unusual but normal anatomical variation
A comparison database that does not accurately represent that patient

The scan must therefore be interpreted together with the patient’s symptoms, visual acuity, clinical examination and other investigations. OCT artefacts are common enough to cause both false diagnoses and missed progression if the underlying scan images are not reviewed carefully.

The Quick Answer

What Does OCT Stand For?

OCT stands for optical coherence tomography.

“Optical” means that the test uses light.

“Coherence” refers to the way reflected light waves are compared.

“Tomography” means that the device produces sectional images through tissue.

Is an OCT Scan Like an X-Ray?

No.

OCT uses reflected light rather than ionising radiation.

It is more similar in concept to ultrasound imaging, except that it uses light rather than sound.

Because light has a much shorter wavelength than sound, OCT can produce extremely detailed images of thin ocular tissue layers.

Does the Machine Touch the Eye?

Most routine retinal and optic-nerve OCT scans are non-contact.

The patient rests the chin and forehead against the machine while looking at a fixation target.

Some specialised intraoperative, handheld or contact-based systems may work differently, but routine clinic scans do not normally touch the eye.

Is an OCT Scan Painful?

No.

The test itself should not be painful.

The patient may notice:

A bright fixation light
Brief flashes
Mild dryness from keeping the eye open
Temporary blur if dilating drops are used
Neck or back discomfort from positioning

How Long Does an OCT Scan Take?

A basic scan may take only a few seconds to acquire.

The complete imaging session commonly takes approximately 5 to 15 minutes, depending on:

The number of scan patterns required
Whether both eyes are scanned
Whether the pupil needs dilation
The patient’s ability to fixate
Whether images need to be repeated
Whether OCT angiography or anterior-segment imaging is performed

Does OCT Show the Same Thing as a Retinal Photograph?

No.

A retinal photograph provides a surface view of the retina.

OCT shows cross-sectional tissue layers beneath the retinal surface.

The tests provide complementary information.

Can OCT Diagnose Every Eye Condition?

No.

OCT is extremely useful but cannot independently diagnose every disease.

A normal OCT does not exclude:

A peripheral retinal tear
Early retinal detachment outside the scan area
Some optic-nerve disorders
Early glaucoma
Intermittent angle closure
A neurological cause of visual symptoms
Disease hidden by poor image quality

How OCT Works

Light Is Directed into the Eye

The OCT machine sends a low-power beam of light into the eye.

Different tissue structures reflect or scatter different amounts of this light.

Reflected Light Is Compared

The device compares light returning from the eye with light travelling through a reference pathway.

The time delay and intensity of the returning signal allow the machine to calculate the depth and reflectivity of ocular structures.

An A-Scan Is Created

A single depth measurement is called an A-scan.

It represents the reflective profile along one line through the tissue.

Multiple A-Scans Form a B-Scan

Many adjacent A-scans are combined to create a cross-sectional B-scan.

This resembles a microscopic slice through the retina, optic nerve or cornea.

Multiple B-Scans Form a Volume

A dense group of B-scans can be combined into a three-dimensional volume.

The software may then generate:

Thickness maps
En face images
Layer-by-layer measurements
Progression analyses
Blood-flow maps
Three-dimensional reconstructions

OCT has transformed ophthalmic care because it provides rapid, high-resolution, cross-sectional imaging that can reveal tissue changes that are difficult or impossible to identify from surface examination alone.

What Do the Colours on an OCT Mean?

False-Colour Cross-Sectional Images

Some OCT displays assign colours according to reflectivity.

Commonly:

Highly reflective structures appear red or white.
Moderately reflective structures appear yellow or green.
Low-reflectivity areas appear blue or black.

These colours do not represent the tissue’s true colour.

Thickness Maps

Thickness maps often use colours to show relative thickness.

Depending on the software:

Warmer colours may represent thicker tissue.
Cooler colours may represent thinner tissue.

The colour scheme differs between devices.

Normative Colour Coding

Glaucoma reports commonly compare measurements with a reference database.

A typical display may use:

Green: within the expected reference range
Yellow: borderline
Red: outside the expected reference range

This colour code is not a diagnosis.

A red sector does not automatically mean glaucoma, and a green report does not guarantee that the eye is healthy.

What Is a Normative Database?

A normative database contains OCT measurements from a group of people considered to have healthy eyes.

The patient’s measurements are compared with people of a similar age.

This may help identify tissue that is statistically thinner or thicker than expected.

However, the database may not adequately represent every patient, particularly those with:

High myopia
Very long or short eyes
Tilted optic discs
Large or small optic nerves
Unusual retinal anatomy
Certain ethnic backgrounds
Previous retinal or optic-nerve disease

High-myopia-specific databases improve glaucoma diagnostic accuracy compared with conventional databases, demonstrating why standard colour coding must be interpreted cautiously in long myopic eyes.

The Main Types of OCT Eye Scan

Macular OCT

A macular OCT examines the central retina responsible for detailed vision.

It may assess:

Retinal thickness
Foveal contour
Intraretinal fluid
Subretinal fluid
Retinal pigment epithelium
Photoreceptor layers
Vitreomacular attachment
Epiretinal membrane
Macular holes
Choroidal abnormalities

Optic-Nerve OCT

An optic-nerve OCT assesses the optic-disc structure and tissue surrounding the nerve.

It may measure:

Optic-nerve-head dimensions
Neuroretinal rim
Cup-to-disc relationships
Bruch’s membrane opening
Peripapillary retinal nerve fibre layer

Retinal Nerve Fibre Layer OCT

The retinal nerve fibre layer, or RNFL, contains axons travelling from retinal ganglion cells towards the optic nerve.

Glaucoma commonly causes progressive RNFL thinning.

A circular scan is usually obtained around the optic nerve.

Ganglion-Cell Analysis

Ganglion-cell analysis measures inner retinal layers in the macula.

Depending on the machine, the report may measure:

Ganglion-cell layer
Inner plexiform layer
Ganglion-cell–inner plexiform layer
Ganglion-cell complex

Macular ganglion-cell analysis is particularly helpful when:

Early central glaucomatous damage is suspected
The optic disc is difficult to interpret
High myopia complicates RNFL analysis
The RNFL has reached an advanced measurement floor

Anterior-Segment OCT

Anterior-segment OCT examines structures at the front of the eye, including:

Cornea
Corneal epithelium
Conjunctiva
Sclera
Anterior chamber
Iris
Drainage angle
Intraocular lenses
Implantable Collamer Lenses

Modern anterior-segment OCT has clinical applications in corneal disease, ocular-surface lesions, glaucoma angle assessment and surgical planning.

OCT Angiography

OCT angiography, or OCTA, uses repeated OCT scans to detect movement of blood cells.

It can produce depth-resolved maps of:

Superficial retinal vessels
Deep retinal vessels
Choriocapillaris
Choroidal neovascular networks
Optic-nerve microvasculature

OCTA does not normally require an injected dye.

It is particularly useful for examining vascular changes in:

Age-related macular degeneration
Diabetic retinopathy
Retinal vein occlusion
Retinal artery occlusion
Macular telangiectasia
Glaucoma
Inherited retinal disease

OCTA is rapid and non-invasive, but it can be affected by motion, projection and low-flow artefacts. It also does not show vascular leakage in the same way as fluorescein angiography.

Swept-Source OCT

Swept-source OCT uses a longer-wavelength light source and rapid wavelength scanning.

Potential advantages include:

Faster image acquisition
Wider scans
Better penetration through some media opacities
Improved imaging of the choroid
Deeper anterior-segment imaging
Reduced signal loss beneath certain retinal structures

The actual advantage depends on the device, scan pattern and condition being examined.

Spectral-Domain OCT

Spectral-domain OCT is widely used in routine ophthalmology.

It provides:

High-resolution retinal imaging
Rapid scan acquisition
Reliable repeat measurements
Automated layer segmentation
Progression analysis

Different machines use different:

Wavelengths
Scan speeds
Algorithms
Normative databases
Definitions of tissue boundaries

Measurements from different OCT brands should not automatically be treated as interchangeable.

Enhanced-Depth Imaging OCT

Enhanced-depth imaging improves visualisation of deeper structures, particularly:

Choroid
Choroidal vessels
Scleral interface
Optic-nerve structures

It may be useful in:

Central serous chorioretinopathy
Pachychoroid disease
Inflammatory choroidal disease
Age-related macular degeneration
Choroidal tumours
High myopia

En Face OCT

A conventional B-scan shows a cross-sectional slice.

An en face image shows a horizontal tissue plane viewed from above.

The operator selects a depth or “slab”.

En face imaging may help evaluate:

Epiretinal membranes
Macular holes
Drusen
Geographic atrophy
Choroidal vessels
Nerve fibre defects
Corneal lesions

Incorrect slab placement can produce misleading images.

Widefield OCT

Widefield and ultra-widefield OCT systems image a larger retinal area than conventional macular scans.

They may help assess:

Peripheral retinal disease
Retinal detachments
Diabetic retinopathy
Retinal vascular disease
Posterior vitreous detachment
Choroidal abnormalities

A widefield OCT does not necessarily image the entire far peripheral retina, and indirect ophthalmoscopy may still be required.

Macular OCT

Why Is the Macula Important?

The macula is responsible for:

Reading
Recognising faces
Fine detail
Colour discrimination
Central vision
Driving vision

The fovea is the central specialised region of the macula.

What Does a Normal Macular OCT Show?

A normal macular OCT commonly shows:

A smooth foveal depression
Organised retinal layers
An intact external limiting membrane
A continuous ellipsoid zone
An intact retinal pigment epithelium
No intraretinal or subretinal fluid
No significant vitreomacular traction

Normal anatomy varies between individuals.

Intraretinal Fluid

Intraretinal fluid appears as dark spaces within the retinal tissue.

Possible causes include:

Diabetic macular oedema
Retinal vein occlusion
Age-related macular degeneration
Inflammation
Postoperative cystoid macular oedema
Retinal telangiectasia
Traction

Not every dark space is active fluid.

Some degenerative cavities and schisis spaces may resemble oedema but behave differently.

Subretinal Fluid

Subretinal fluid accumulates between the neurosensory retina and the retinal pigment epithelium.

Possible causes include:

Neovascular age-related macular degeneration
Central serous chorioretinopathy
Inflammatory disease
Polypoidal choroidal vasculopathy
Retinal detachment
Choroidal tumours

Pigment Epithelial Detachment

A pigment epithelial detachment occurs when the retinal pigment epithelium is elevated from the underlying tissue.

It may be:

Drusenoid
Serous
Fibrovascular
Haemorrhagic

The OCT appearance may help determine the likely cause but should be interpreted with the clinical examination and, when needed, angiography.

Age-Related Macular Degeneration

OCT may detect and monitor:

Drusen
Subretinal drusenoid deposits
Pigment epithelial detachments
Intraretinal fluid
Subretinal fluid
Hyperreflective material
Photoreceptor disruption
Geographic atrophy
Fibrosis
Macular neovascularisation

OCT is central to the diagnosis and follow-up of neovascular AMD and frequently guides anti-VEGF injection intervals. Numerous OCT biomarkers have been associated with disease activity and prognosis, although individual findings must be interpreted in context.

Diabetic Macular Oedema

OCT may show:

Increased retinal thickness
Intraretinal cysts
Subretinal fluid
Hyperreflective foci
Disorganisation of retinal inner layers
Disruption of the ellipsoid zone
Vitreomacular traction

OCT is used to:

Confirm macular oedema
Measure central retinal thickness
Assess treatment response
Guide anti-VEGF treatment intervals
Identify structural features associated with visual prognosis

Current clinical guidelines consistently recommend OCT for diagnosing and monitoring diabetic macular oedema, particularly during anti-VEGF treatment.

Retinal Vein Occlusion

OCT may identify:

Intraretinal fluid
Subretinal fluid
Macular thickening
Haemorrhage-related shadowing
Ischaemic structural damage
Epiretinal membrane
Chronic retinal atrophy

It helps determine whether macular oedema is present and whether treatment is working.

OCTA may provide additional information about:

Capillary non-perfusion
Foveal avascular-zone enlargement
Collateral vessels
Microvascular remodelling

Epiretinal Membrane

An epiretinal membrane is a thin sheet of tissue on the retinal surface.

OCT may show:

A reflective membrane over the macula
Retinal wrinkling
Thickening
Loss of the normal foveal depression
Distortion of inner retinal layers
Cystic change
Ectopic inner foveal layers

OCT assists with:

Confirming the diagnosis
Grading severity
Distinguishing an epiretinal membrane from a macular hole
Planning surgery
Estimating visual potential
Monitoring after vitrectomy

Vitreomacular Traction

The vitreous gel normally separates from the retina with age.

When it remains abnormally attached to the macula, it may pull on the fovea.

OCT may show:

Focal vitreous attachment
Distortion of the fovea
Retinal cysts
Subretinal fluid
Early macular-hole formation

OCT has transformed understanding and classification of vitreomacular-interface disorders.

Macular Hole

A macular hole is a defect in the central retina.

OCT can distinguish:

Full-thickness macular hole
Lamellar macular hole
Macular pseudohole
Epiretinal membrane with foveoschisis
Vitreomacular traction without a hole

It may measure:

Minimum hole diameter
Base diameter
Height
Retinal-fluid distribution
Outer retinal integrity

OCT is used before surgery, after surgery and, in selected cases, beneath an intraocular gas bubble to confirm closure. Modern OCT has fundamentally changed macular-hole classification and management.

Central Serous Chorioretinopathy

OCT may show:

Subretinal fluid
Pigment epithelial detachments
Thickened choroid
Elongated photoreceptor outer segments
Retinal pigment epithelial changes
Chronic outer retinal damage

OCTA may help identify secondary macular neovascularisation without requiring dye, although dye angiography may still be needed in complex cases.

Retinal Detachment

OCT may help assess:

Whether the macula is attached or detached
Foveal involvement
Retinal folds
Subretinal fluid
Outer retinal changes
Postoperative recovery

However, a standard macular OCT does not exclude a peripheral retinal tear or detachment.

Patients with flashes, new floaters or a curtain in the vision require a dilated retinal examination even when a central OCT appears normal.

Optic-Nerve and Glaucoma OCT

What Does Glaucoma Damage?

Glaucoma damages retinal ganglion cells and their axons.

These axons form the retinal nerve fibre layer and converge at the optic nerve.

OCT can therefore measure structural tissue that glaucoma progressively damages.

Retinal Nerve Fibre Layer Analysis

RNFL analysis usually measures tissue along a circular path around the optic nerve.

The report may show:

Average RNFL thickness
Superior and inferior quadrant thickness
Clock-hour sectors
Deviation maps
Comparison with age-matched reference values
Change over time

Glaucoma commonly produces localised or diffuse thinning, particularly in superior and inferior arcuate regions.

Ganglion-Cell Analysis

Macular ganglion-cell analysis may detect damage near central vision.

It can be particularly valuable for:

Early glaucoma
Normal-tension glaucoma
Paracentral visual-field defects
Advanced RNFL loss
Unusual optic-disc anatomy

Bruch’s Membrane Opening–Minimum Rim Width

Some OCT systems measure the minimum distance between the Bruch’s membrane opening and the internal limiting membrane.

This provides an anatomically based estimate of the neuroretinal rim.

It may be helpful when the clinical disc margin is difficult to define.

Can OCT Diagnose Glaucoma by Itself?

No.

Glaucoma diagnosis should consider:

Eye pressure
Gonioscopy
Optic-disc appearance
RNFL OCT
Ganglion-cell OCT
Visual-field testing
Corneal thickness
Family history
Previous progression
Other causes of optic-nerve damage

An abnormal OCT without corresponding clinical or functional evidence may reflect an artefact or anatomical variation.

OCT Progression Analysis

Progression software compares scans over time.

It may use:

Event analysis
Trend analysis
Thickness maps
Rate-of-change graphs
Probability maps

Event Analysis

Event analysis identifies whether a new scan differs significantly from baseline variability.

Trend Analysis

Trend analysis calculates the rate of tissue change over time.

For example, it may estimate annual RNFL thinning.

Why Are Several Scans Needed?

One abnormal measurement may represent:

Noise
Poor centring
Segmentation error
Cataract
Motion
Dry eye
Temporary image-quality reduction

Repeated high-quality scans help determine whether a change is reproducible.

The Glaucoma Floor Effect

In advanced glaucoma, RNFL thickness reaches a level below which further axonal loss may not produce substantial additional measured thinning.

This is called the floor effect.

When the RNFL has reached the floor, monitoring may rely more heavily on:

Macular ganglion-cell measurements
Visual fields
Optic-disc examination
Central visual-field testing
Clinical progression

Red Disease

“Red disease” occurs when a healthy eye is incorrectly flagged as abnormal by OCT colour coding.

Possible causes include:

High myopia
Tilted optic disc
Unusual nerve-fibre distribution
Incorrect segmentation
Small normative database
Scan decentration

Green Disease

“Green disease” occurs when a diseased eye is incorrectly displayed within the green reference range.

Possible causes include:

Early localised disease
Large natural baseline thickness
Incorrect segmentation
An insensitive summary parameter
Progression that remains numerically within the database range

The raw scan and longitudinal change matter more than the final colours alone.

OCT in High Myopia

High myopia can make OCT interpretation difficult because of:

Tilted optic discs
Peripapillary atrophy
Long axial length
Magnification error
Shifted nerve-fibre bundles
Posterior staphyloma
Retinal thinning
Segmentation error
Limited representation in standard normative databases

A red RNFL sector in a highly myopic eye may be anatomical rather than glaucomatous.

Diagnosis should emphasise:

Reproducible progression
Corresponding visual-field loss
Ganglion-cell patterns
Optic-disc examination
Myopia-adjusted databases when available

Myopic normative databases reduce false-positive classification and improve diagnostic accuracy in highly myopic eyes.

OCT in Optic-Nerve and Neurological Disease

OCT may also assist in assessing:

Optic neuritis
Multiple sclerosis
Ischaemic optic neuropathy
Compressive optic neuropathy
Toxic optic neuropathy
Hereditary optic neuropathy
Optic-disc swelling
Papilloedema
Optic-nerve drusen

Different diseases produce different patterns of:

RNFL thickening
RNFL thinning
Ganglion-cell loss
Optic-nerve-head elevation

OCT cannot determine every cause of optic-nerve damage by itself.

The patient may also require:

Colour-vision testing
Visual fields
Pupil examination
Neurological examination
Blood investigations
MRI or other imaging

Anterior-Segment OCT

Corneal OCT

Corneal OCT may assess:

Corneal thickness
Scar depth
Corneal dystrophy
Keratoconus
Corneal infection
Previous laser-treatment planes
LASIK flap thickness
Corneal graft attachment
Descemet membrane
Intrastromal implants

Epithelial Thickness Mapping

The corneal epithelium can become thicker or thinner in response to underlying stromal shape.

Epithelial mapping may help identify:

Early keratoconus
Contact-lens warpage
Epithelial compensation
Irregular corneal shape
Previous laser-ablation patterns
Recurrent refractive error

Epithelial mapping complements rather than replaces corneal topography and tomography.

Keratoconus

Anterior-segment OCT may help assess:

Corneal thickness
Epithelial distribution
Cone location
Corneal scarring
Intracorneal ring position
Cross-linking demarcation line
Postoperative healing

Corneal tomography remains essential because it analyses anterior and posterior corneal shape in ways that may not be fully duplicated by every OCT system.

Narrow Drainage Angles

Anterior-segment OCT may show:

Angle width
Iris configuration
Lens vault
Anterior-chamber depth
Angle opening after laser iridotomy
Persistent angle crowding
Plateau-iris-like configuration

It is non-contact and useful for documenting anatomy.

However, it does not replace gonioscopy because gonioscopy can assess:

Pigmentation
Peripheral anterior synechiae
Abnormal blood vessels
Dynamic angle opening
Structures hidden from some OCT scans

Implantable Collamer Lens Assessment

Anterior-segment OCT may measure:

ICL vault
Distance from the ICL to the natural lens
Angle width
ICL position
Central port
Lens tilt
Relationship with the iris

The measurement must be interpreted with:

Eye pressure
Natural-lens clarity
Endothelial health
Drainage-angle anatomy
Changes over time

After Cataract Surgery

Anterior-segment OCT may help evaluate:

Intraocular lens position
Corneal wounds
Descemet membrane detachment
Corneal swelling
Anterior-chamber anatomy
Selected lens complications

Posterior-segment OCT is also commonly used after cataract surgery to assess unexplained blurred vision or cystoid macular oedema.

OCT Angiography

How Is OCTA Different from Standard OCT?

Standard structural OCT analyses reflected light from tissue.

OCTA analyses changes between repeated scans caused by moving blood cells.

This creates a map of blood flow.

What Can OCTA Show?

OCTA may show:

Retinal capillary networks
Capillary dropout
Foveal avascular-zone changes
Choroidal neovascular membranes
Collateral vessels
Abnormal optic-disc vessels
Microvascular density

Advantages of OCTA

Potential advantages include:

No injected dye
No needle
Rapid acquisition
Layer-by-layer vascular analysis
Repeatability
Three-dimensional vascular imaging

Limitations of OCTA

OCTA does not directly show leakage.

It may miss:

Blood flow that is too slow
Blood flow that is too fast for the algorithm
Vessels hidden by haemorrhage
Lesions outside the scan area

It is particularly vulnerable to:

Eye-movement artefact
Projection artefact
Segmentation error
Blink artefact
Shadowing
Poor fixation

OCTA should not automatically replace fluorescein or indocyanine-green angiography when dye-based information is clinically important.

OCT Versus Other Eye Tests

OCT Versus Retinal Photography

Retinal Photography Shows

Surface appearance
Haemorrhages
Exudates
Pigmentation
Optic-disc colour
Retinal vessels
Peripheral retinal findings with widefield systems

OCT Shows

Cross-sectional layers
Fluid
Traction
Tissue thickness
Photoreceptor integrity
Retinal pigment epithelial elevation

Both may be required.

OCT Versus Fluorescein Angiography

Fluorescein angiography requires dye injection and photographs dye movement through retinal blood vessels.

It can show:

Leakage
Blockage
Staining
Pooling
Vascular timing

OCT shows:

Structural fluid
Retinal thickness
Tissue layers

OCTA shows flow but not leakage.

OCT Versus Indocyanine-Green Angiography

Indocyanine-green angiography provides deeper vascular imaging of the choroid.

It may be particularly useful for:

Polypoidal choroidal vasculopathy
Choroidal vascular lesions
Selected inflammatory conditions
Occult choroidal neovascularisation

OCT and OCTA provide complementary structural and flow information.

OCT Versus Ocular Ultrasound

Ultrasound uses sound waves.

It is particularly useful when light cannot pass through the eye because of:

Dense cataract
Vitreous haemorrhage
Severe corneal opacity
Dense inflammation

Ultrasound can assess deeper and wider structures but generally does not provide the same microscopic retinal-layer resolution as OCT.

OCT Versus Visual-Field Testing

OCT measures structure.

Visual-field testing measures visual function.

In glaucoma:

OCT may detect structural damage.
Visual fields show how the damage affects the patient’s vision.

Both are required because structural and functional progression do not always occur at the same time.

What Happens During an OCT Scan?

Step 1: Registration

The patient’s details and the eye to be scanned are confirmed.

Previous scans may be selected for comparison.

Step 2: Positioning

The patient places:

The chin on a chin rest
The forehead against a support

The operator adjusts the machine to the correct height.

Step 3: Fixation

The patient looks at a target inside the machine.

The target may appear as:

A light
A star
A cross
A moving pattern

Step 4: Image Acquisition

The device scans the eye while the patient keeps still and avoids blinking briefly.

The patient may see flashes or moving lines.

Step 5: Repeating Poor Images

A scan may be repeated when:

The eye moves
The patient blinks
The image is off-centre
Signal quality is poor
Segmentation is unreliable
The scan does not match the previous follow-up location

Step 6: Image Review

The operator or ophthalmologist checks:

Signal quality
Centring
Segmentation
Motion
Scan completeness
Whether the relevant lesion has been captured

A computer-generated report should not be accepted without checking the underlying B-scans.

Do the Pupils Need to Be Dilated?

Not always.

Many OCT scans can be performed through an undilated pupil.

Dilation may be recommended when:

The pupil is small
Cataract reduces the signal
A dense scan is required
The patient has difficulty fixating
A full retinal examination is also needed
The peripheral retina must be assessed

Dilating drops may cause:

Temporary blurred near vision
Light sensitivity
Reduced driving confidence
Rare angle-closure symptoms in susceptible eyes

Can Children Have OCT?

Yes.

OCT can be performed in many cooperative children.

It may be useful for:

Childhood glaucoma
Optic-nerve disease
Retinal dystrophy
Macular disease
Papilloedema
Myopia-related research or monitoring
Congenital retinal abnormalities

Handheld OCT may be used for infants or patients unable to sit at a conventional machine.

Can OCT Be Performed During Pregnancy?

Routine OCT is non-invasive and does not use ionising radiation.

It can generally be performed when clinically required during pregnancy.

Dilation or other associated tests should be considered individually.

Does OCT Harm the Retina?

Routine clinical OCT uses low-power light within device safety standards.

It is widely used repeatedly for chronic conditions.

The scan may be uncomfortable for patients with severe light sensitivity, but it should not injure a healthy retina.

Does the Bright Light Affect an Intraocular Lens or ICL?

No.

Routine OCT does not damage:

Cataract intraocular lenses
Implantable Collamer Lenses
Corneal implants
Glaucoma drainage devices

These structures may alter image quality or create reflections in selected scans.

Common OCT Artefacts

Motion Artefact

Movement may cause:

Broken vessels
Duplicated structures
Misaligned scan sections
False thickness changes
Distorted OCTA maps

Blink Artefact

Blinking interrupts the scan and may create:

Missing sections
Horizontal black bands
Incomplete data
Segmentation failure

Decentration

A scan taken away from the intended centre may produce misleading measurements.

For example, an optic-nerve circle scan placed too close to or too far from the disc can alter RNFL thickness.

Segmentation Error

The software attempts to identify the boundaries between retinal layers.

It may fail because of:

Epiretinal membrane
Macular oedema
Macular hole
High myopia
Retinal scarring
Poor signal
Vitreomacular traction
Drusen
Pigment epithelial detachment

Segmentation artefacts can create false thickening, false thinning, false progression or missed progression. In one longitudinal glaucoma study, artefacts were present in half of the examined eyes and sometimes masked real progression or created false progression.

Shadowing

A structure that blocks light may cast a shadow beneath it.

Possible causes include:

Retinal haemorrhage
Exudates
Dense vessels
Pigment
Vitreous opacity
Corneal opacity
Cataract

Projection Artefact

In OCTA, superficial vessels may appear falsely within deeper layers.

Software can reduce but not always eliminate this.

Mirror Artefact

Deep structures may be displayed in an inverted position when the scan crosses the device’s imaging boundary.

Clipping or Truncation

Part of the tissue may be cut off when the scan is positioned too high or too low.

Poor Signal

Poor signal may result from:

Cataract
Dry eye
Corneal opacity
Small pupil
Vitreous haemorrhage
Eye movement
Poor fixation
Defocus

Poorer signal can affect measured thickness and classification.

How Cataract Affects OCT

Cataract scatters and attenuates the light entering and leaving the eye.

This may:

Reduce signal strength
Make retinal layers less distinct
Produce falsely thin RNFL measurements
Increase segmentation error
Make serial scans appear to worsen

After cataract surgery, measured RNFL and ganglion-cell thickness may appear to increase because image quality has improved rather than because nerve tissue has regenerated.

A new OCT baseline may therefore be useful after cataract surgery.

How Dry Eye Affects OCT

An unstable tear film can reduce image quality.

The scan may improve after:

Blinking
Applying lubricating drops
Treating meibomian gland dysfunction
Repeating the scan after ocular-surface treatment

Dry eye has also been associated with a higher frequency of OCT artefacts in clinical studies.

How Vitreous Haemorrhage Affects OCT

Blood inside the vitreous may prevent sufficient light from reaching the retina.

A partial image may still be possible with some devices, but dense haemorrhage often requires ultrasound assessment.

Why Comparing Different Machines Can Be Difficult

Different OCT systems may use different:

Segmentation boundaries
Scan diameters
Wavelengths
Image averaging
Normative databases
Magnification corrections
Progression algorithms

A central retinal thickness of one value on one machine may not equal the same value on another.

Long-term monitoring is most reliable when:

The same device is used
The same scan protocol is selected
The follow-up function aligns scans to the same location
Image quality remains comparable

How Often Should OCT Be Repeated?

There is no universal schedule.

The frequency depends on:

Diagnosis
Disease severity
Treatment
Risk of progression
Symptoms
Previous rate of change
Image quality

Glaucoma

OCT may be repeated:

Several times during the first year to establish a baseline
Every 6 to 12 months in stable disease
More frequently when progression is suspected
After a major change in treatment

Visual fields remain necessary.

Age-Related Macular Degeneration

OCT may be performed:

At each anti-VEGF treatment visit
According to a treat-and-extend regimen
When new distortion or central blur develops
At planned monitoring intervals for non-neovascular AMD

Diabetic Macular Oedema

OCT frequency depends on:

Presence of centre-involving oedema
Visual acuity
Injection schedule
Treatment response
Systemic control
Previous recurrence

Epiretinal Membrane or Macular Hole

OCT may be repeated:

When symptoms change
Before surgery
After surgery
To monitor stable mild disease
To confirm macular-hole closure

Keratoconus or ICL

Anterior-segment OCT may be repeated to monitor:

Corneal changes
Cross-linking recovery
ICL vault
Drainage-angle width
Implant position

Can Home OCT Be Used?

Home OCT systems are being developed and introduced for selected retinal conditions.

Potential uses include:

Frequent monitoring of neovascular AMD
Detection of recurrent retinal fluid
Remote treatment guidance
Reducing unnecessary clinic visits

Recent studies show that selected patients can acquire interpretable home OCT images, but the technology requires:

Patient training
Reliable image transmission
Automated analysis
Clinician oversight
Clear pathways for responding to new fluid

Home OCT does not replace urgent examination when symptoms suddenly worsen.

Artificial Intelligence and OCT

Artificial intelligence may assist with:

Automated segmentation
Fluid detection
Glaucoma classification
Progression prediction
Macular-disease screening
Treatment-response assessment
Home-monitoring triage

Potential limitations include:

Biased training data
Poor generalisability
Failure in unusual anatomy
Dependence on image quality
Lack of clinical context
False reassurance
False alarms

AI output should support rather than replace clinical interpretation.

What an OCT Scan Cannot Tell You

An OCT scan cannot independently determine:

Whether every visual symptom is caused by the finding
Whether a peripheral retinal tear is absent
Whether glaucoma is definitely present
Whether an abnormal colour code represents true disease
Whether a stable thickness means the disease is inactive
Whether vision will definitely improve after treatment
Whether urgent examination is unnecessary

The image must be interpreted in context.

Common Myths

“OCT Uses Radiation”

False.

OCT uses light rather than ionising radiation.

“An OCT Scan Is the Same as a CT Scan”

False.

A CT scan uses X-rays to image the body.

OCT uses reflected light to image ocular tissue.

“A Red Result Means I Definitely Have Glaucoma”

False.

Red colour coding means that the measurement lies outside a statistical reference range.

It may be caused by disease, high myopia, anatomy or artefact.

“A Green OCT Means My Eyes Are Healthy”

False.

Early or localised disease may remain within the green range.

“The Computer Diagnoses the Condition”

False.

The software measures and compares structures.

The ophthalmologist must interpret the findings.

“A Thicker Retina Is Always Worse”

False.

Thickness may increase from fluid or traction, but some conditions cause thinning and tissue loss.

“A Thin Retina Always Means Permanent Damage”

False.

Normal retinal thickness varies, and segmentation or high myopia may create falsely thin measurements.

“A Normal OCT Excludes a Retinal Tear”

False.

Most standard OCT scans do not image the entire peripheral retina.

“OCT Replaces Visual-Field Testing”

False.

OCT measures structure; visual fields measure function.

“OCT Replaces Fluorescein Angiography”

False.

OCT does not directly show dye leakage.

“OCTA Shows Every Blood Vessel”

False.

Very slow or undetectable flow may not appear.

“The Same Thickness Number Means the Same Thing on Every Machine”

False.

Devices and algorithms differ.

“OCT Can Restore Vision”

False.

OCT is an imaging test, not a treatment.

Frequently Asked Questions

Why Has My Doctor Ordered an OCT?

Common reasons include:

Blurred central vision
Distorted vision
Glaucoma assessment
Diabetes
Macular degeneration
Flashes or floaters
Unexplained reduced vision
Monitoring after eye surgery
Monitoring treatment response

Can OCT Detect Glaucoma Before Symptoms Develop?

It may detect structural tissue loss before the patient notices visual symptoms.

However, early glaucoma diagnosis still requires correlation with optic-disc findings, eye pressure and visual fields.

Can OCT Detect Macular Degeneration?

Yes.

OCT can identify many features of dry and wet AMD.

Dye angiography or other imaging may still be required in selected cases.

Can OCT Detect Diabetic Retinopathy?

OCT is particularly useful for detecting diabetic macular oedema.

Retinal photography and dilated examination are generally better for grading the full extent of peripheral diabetic retinopathy.

Can OCT Detect a Retinal Detachment?

It can detect retinal detachment within the scanned area.

A normal macular OCT does not exclude a peripheral detachment.

Can OCT Detect a Brain Tumour?

No.

It may show optic-nerve or retinal changes that raise concern about neurological disease.

MRI or other neurological investigations are required to assess the brain and orbit.

Can OCT Detect Multiple Sclerosis?

OCT may show retinal nerve fibre and ganglion-cell loss associated with optic neuritis or multiple sclerosis.

It cannot independently diagnose multiple sclerosis.

Can OCT Detect Raised Brain Pressure?

OCT may document optic-disc swelling or changes in the peripapillary retina.

It cannot independently determine the cause or accurately replace neurological assessment, brain imaging or lumbar puncture when these are required.

Why Was My Scan Repeated?

Common reasons include:

Blinking
Eye movement
Poor signal
Incorrect centring
Segmentation error
Failure to capture the lesion
Need for a denser scan

Why Do I Need Another Scan When My Vision Is Unchanged?

Some diseases progress before the patient notices symptoms.

Serial OCT may identify structural change that helps guide treatment.

Why Does My OCT Look Worse After Cataract Developed?

The cataract may have reduced scan quality and made tissue appear artificially thinner.

The ophthalmologist should inspect the signal and raw images.

Why Did My RNFL Become Thicker After Cataract Surgery?

Improved light transmission may produce a stronger signal and a higher measured thickness.

This does not mean that glaucoma damage has reversed.

Why Is My OCT Red Even Though My Visual Field Is Normal?

Possible explanations include:

Early structural disease
High myopia
Tilted optic nerve
Normal variation
Segmentation error
Scan decentration

The result should be monitored and correlated with the full examination.

Why Is My Visual Field Abnormal When My OCT Is Normal?

Possible reasons include:

Very early functional change
Neurological disease
Visual-field artefact
OCT measurement limitations
Structural measurements still within the database range
Disease outside the measured tissue

Does OCT Require an Injection?

Routine structural OCT and OCT angiography do not require an injection.

Fluorescein or indocyanine-green angiography does.

Can I Drive After OCT?

Yes, when the pupils have not been dilated and vision is otherwise suitable.

After dilation, near vision and light sensitivity may be affected for several hours.

Can I Wear Contact Lenses During OCT?

Soft contact lenses may sometimes remain in place for a retinal OCT.

They may need to be removed when:

Corneal imaging is required
Lens deposits reduce image quality
Contact-lens warpage is being assessed
The ocular surface needs examination

Can OCT Be Performed After LASIK or SMILE?

Yes.

Retinal and optic-nerve OCT can be performed normally.

Anterior-segment OCT may also assess:

LASIK flap thickness
Corneal interfaces
Corneal epithelial mapping
Previous treatment zones

Can OCT Be Performed with an ICL?

Yes.

Anterior-segment OCT is commonly used to assess ICL vault and position.

Can OCT Be Performed with a Gas Bubble in the Eye?

Selected scans may be possible through or beneath an intraocular gas bubble, depending on:

Gas size
Patient position
Scan location
Device

The retinal surgeon will determine whether the scan is useful.

Does OCT Show Cancer?

Anterior-segment OCT may help assess selected conjunctival or corneal lesions.

Retinal or choroidal OCT may provide structural information about ocular tumours.

It does not replace clinical oncology assessment, ultrasound or biopsy when these are required.

Can OCT Results Change from Day to Day?

Small variations may occur because of:

Tear film
Hydration
Fixation
Signal strength
Segmentation
Time of day
Device alignment

A meaningful change should be confirmed and interpreted clinically.

When to Seek Urgent Eye Care

An OCT appointment should not delay urgent assessment for:

Sudden loss of vision
A curtain or shadow in the vision
New flashes of light
A sudden shower of floaters
New distortion of straight lines
Sudden central blur
Severe eye pain
A red eye with nausea or vomiting
Sudden double vision
New neurological weakness or speech difficulty

These symptoms may indicate:

Retinal tear or detachment
Macular haemorrhage
Retinal vascular occlusion
Acute angle closure
Optic-nerve disease
Stroke or another neurological emergency

An OCT Assessment Checklist

Symptoms to Report

Central blur
Distortion
Ghosting
Missing areas of vision
Reduced colour vision
Flashes
Floaters
A curtain or shadow
Poor night vision
Difficulty reading
Eye pain
Headache
Double vision

Medical Information to Report

Diabetes
High blood pressure
High cholesterol
Autoimmune disease
Neurological disease
Multiple sclerosis
Kidney disease
Pregnancy
Steroid use
Hydroxychloroquine use
Cancer treatment

Eye History to Report

Glaucoma
Macular degeneration
Diabetic retinopathy
Retinal vein occlusion
Retinal detachment
Epiretinal membrane
Macular hole
Keratoconus
LASIK, PRK or SMILE
Cataract surgery
ICL surgery
Retinal injections
Vitrectomy
Corneal transplant

Questions to Ask the Ophthalmologist

Which part of my eye was scanned?
Is the scan quality reliable?
Is there any segmentation error?
Is the finding new?
Has it changed from my previous scan?
Is the change likely to be true progression?
Could cataract, dry eye or high myopia affect the result?
Does the OCT finding explain my symptoms?
Do I need another test?
Do I need treatment?
How soon should the scan be repeated?
Which symptoms require urgent assessment?

The Bottom Line

Optical coherence tomography is a rapid, non-invasive imaging test that uses reflected light to produce detailed cross-sectional images of the eye.

It can assess:

Retina
Macula
Optic nerve
Retinal nerve fibre layer
Ganglion-cell layers
Choroid
Cornea
Drainage angle
Implantable lenses and other anterior-segment structures

OCT is especially useful for:

Age-related macular degeneration
Diabetic macular oedema
Retinal vascular disease
Epiretinal membrane
Vitreomacular traction
Macular hole
Glaucoma
Optic-nerve disease
Keratoconus
Narrow angles
ICL vault assessment
Monitoring before and after eye surgery

Its principal advantages are:

No ionising radiation
No routine injection
High tissue resolution
Rapid acquisition
Objective measurements
Repeatable monitoring
Ability to identify subtle structural change

Its limitations include:

A restricted scan area
Dependence on image quality
Segmentation errors
Motion and blink artefacts
Difficulty imaging through dense cataract or haemorrhage
False-positive findings in high myopia
Dependence on device-specific databases and algorithms
Inability to replace clinical examination

OCT angiography can map blood flow without injected dye, but it does not directly show vascular leakage and remains vulnerable to movement, projection and segmentation artefacts.

The most important message is:

An OCT scan is a highly detailed measurement of eye structure, but it is not a diagnosis by itself. Reliable interpretation requires assessment of the raw images, scan quality, anatomy, previous scans, clinical examination and other tests. The computer’s colour coding should guide attention—not replace clinical judgement.

References

Jaffe GJ, Caprioli J. Optical coherence tomography to detect and manage retinal disease and glaucoma. Am J Ophthalmol. 2004;137(1):156–169. PMID: 14700659.
Chong YJ, Azzopardi M, Hussain G, et al. Clinical applications of anterior segment optical coherence tomography: an updated review. Diagnostics. 2024;14(2):122. doi:10.3390/diagnostics14020122. PMID: 38248000.
Bayer A, Akman A. Artifacts and anatomic variations in optical coherence tomography. Turk J Ophthalmol. 2020;50(2):99–106. doi:10.4274/tjo.galenos.2019.78000. PMID: 32367701.
Liu Y, Simavli H, Que CJ, et al. Patient characteristics associated with artifacts in spectral-domain optical coherence tomography imaging of the retinal nerve fibre layer in glaucoma. Am J Ophthalmol. 2015.
Asrani S, Essaid L, Alder BD, Santiago-Turla C. Artifacts in spectral-domain optical coherence tomography measurements in glaucoma. JAMA Ophthalmol. 2014.
Leung CKS, et al. Impact of artifacts from optical coherence tomography retinal nerve fibre layer and macula scans on detection of glaucoma progression. Am J Ophthalmol. 2021;221:235–245. PMID: 32818450.
Vazquez LE, Bye A, Aref AA. Recent developments in the use of optical coherence tomography for glaucoma. Curr Opin Ophthalmol. 2021;32(2):98–104. PMID: 33332883.
Chen JJ, Kardon RH. Avoiding clinical misinterpretation and artifacts of optical coherence tomography analysis of the optic nerve, retinal nerve fibre layer and ganglion-cell layer. J Neuroophthalmol. 2016;36(4):417–438. PMID: 27636747.
Lee SY, Kwon HJ, Bae HW, et al. Frequency, type and cause of artifacts in swept-source and spectral-domain optical coherence tomography in glaucoma and suspected glaucoma. Curr Eye Res. 2016;41(7):957–964. PMID: 26431251.
Mwanza JC, Bhorade AM, Sekhon N, et al. Effect of cataract and its removal on signal strength and peripapillary retinal nerve fibre layer optical coherence tomography measurements. J Glaucoma. 2011;20(1):37–43. PMID: 20179622.
Choi YJ, Seol BR. Effects of cataract on retinal nerve fibre layer and ganglion-cell–inner plexiform layer thickness on swept-source optical coherence tomography. Korean J Ophthalmol. 2023;37(1):37–46. PMID: 36281576.
Biswas S, et al. High-myopia normative database of peripapillary retinal nerve fibre layer thickness to detect myopic glaucoma in a Chinese population. Ophthalmology. 2023. PMID: 37499953.
Chong GT, Lee RK. Glaucoma versus red disease: imaging and interpretation in highly myopic eyes. Review literature.
Koutsiaris AG, Batis V, Liakopoulou G, et al. Optical coherence tomography angiography of the eye: basic principles, advantages, disadvantages and device specifications. Clin Hemorheol Microcirc. 2023;83(3):247–271. PMID: 36502308.
Hormel TT, Huang D, Jia Y. Artifacts and artifact removal in optical coherence tomographic angiography. Quant Imaging Med Surg. 2021;11(3):1120–1133. PMID: 33654681.
Ong CJT, Wong MYZ, Cheong KX, et al. Optical coherence tomography angiography in retinal vascular disorders. Diagnostics. 2023;13(9):1620. PMID: 37175011.
Ashraf M, et al. Optical coherence tomography imaging biomarkers associated with neovascular age-related macular degeneration: a systematic review. Eye. 2023. PMID: 36526863.
Guymer RH, et al. OCT prognostic biomarkers for progression to late age-related macular degeneration: a systematic review and meta-analysis. Ophthalmol Retina. 2024. PMID: 38154619.
McAllister M, Ko S, Bawa K, et al. Clinical practice guidelines for the treatment and management of diabetic macular oedema: a systematic review. Eye. 2025. PMID: 41034347.
Nanji K, Grad J, Hatamnejad A, et al. Baseline OCT biomarkers associated with visual acuity in diabetic macular oedema: a systematic review and meta-analysis. Ophthalmology. 2026;133(1):75–90. PMID: 40803536.
Govetto A, Lalane RA, Sarraf D, Figueroa MS, Hubschman JP. Epiretinal membrane: optical coherence tomography-based diagnosis and classification. Am J Ophthalmol. 2017;175:99–113. PMID: 27099458.
Tsuboi K, Fukushima M, Akai R. How optical coherence tomography has changed the management of macular holes: a narrative review. Taiwan J Ophthalmol. 2025;15(3):344–353. PMID: 40995310.
Hubschman JP, Govetto A, Spaide RF, et al. Optical coherence tomography-based consensus definition for lamellar macular hole. Br J Ophthalmol. 2020;104(12):1741–1747. PMID: 32107208.
Steel DHW, Lotery AJ. Vitreomacular disorders: a review of classification, pathogenesis and treatment paradigms. Eye. 2021. PMID: 33899681.
Lee BJH, Tey KY, Cheong EZK, et al. Anterior-segment optical coherence tomography angiography: applications for the cornea and ocular surface. Medicina. 2024;60(10):1597. PMID: 39459384.