17 - Corneal Topography and Tomography

How did corneal curvature assessment evolve into modern corneal topography?

• Early methods viewed reflections from the ocular surface to estimate corneal shape.

• Javal developed ophthalmometry, which became keratometry.

• Placido introduced concentric rings and analyzed the reflected ring pattern.

• Gullstrand added a camera to the Placido system (photokeratoscopy), allowing objective recording and analysis.

• Modern corneal topography combines Placido rings, video capture, and computer software.

What are the major methods used to evaluate corneal shape and curvature today?

• Keratometry: measures central corneal curvature.

• Placido-based corneal topography: maps a larger corneal surface area.

• Tomography: provides 3D corneal analysis, including anterior and posterior surfaces.

• OCT: can assess corneal structure and shape.

• Orbscan (slit-scanning) was an older tomography method and is now rarely used.

What is the major limitation of keratometry and how does corneal topography overcome it?

• Keratometry measures curvature only in the central 3-4 mm of the cornea.

• Topography evaluates a much larger area, approximately 8-10 mm of the cornea.

• This provides a more complete assessment of corneal shape and irregularities.

Why is corneal topography especially useful for contact lens fitting?

• Topography evaluates the cornea over an 8-10 mm diameter, which closely corresponds to the area covered by many gas permeable (GP) lenses.

• This allows practitioners to assess the entire fitting surface rather than relying only on central keratometer readings.

What are the major goals when learning corneal topography and tomography?

• Axial maps (overall curvature patterns)

• Tangential maps (localized curvature changes)

• Elevation maps (height relative to a reference surface)

• Refractive maps (corneal optical power)

• Keratoconic indices (metrics used to detect and monitor keratoconus)

Which map types and indices are considered the core outputs of corneal topography/tomography?

Axial, Tangential, Elevation, Refractive maps, and Keratoconic indices.
These are the primary tools used to characterize corneal shape, detect irregularities, and identify conditions such as keratoconus.

Where does the keratometric constant 337.5 come from, and how is it used?

Keratometers use a standardized corneal refractive index of 1.3375.

337.5 is the keratometric conversion constant used to convert corneal radius (mm) to dioptric power.

What is the relationship between corneal radius of curvature and corneal power?

• Smaller radius = more curved cornea = steeper cornea = higher dioptric power.

• Larger radius = flatter cornea = lower dioptric power.

• Radius and power are inversely related:

• D= 337.5 / r (mm)

What does corneal topography measure, and what does it not measure?

• Corneal topography measures corneal curvature.

• It converts curvature/radius into dioptric power maps.

• Traditional topography does not directly measure elevation.

• Elevation analysis is a feature of tomography, not simple Placido-based topography.

How many scans should typically be obtained with Medmont topography, and why?

• Medmont is a Placido-ring topographer and is tear film dependent.

• Tear film instability can affect measurements.

• Therefore, obtain 3 serial scans to ensure repeatability and reliability.

Which corneal imaging systems are tear-film dependent versus tear-film independent?

• Medmont (Placido topography) → Tear-film dependent.

• Pentacam (Scheimpflug tomography) → Tear-film independent.

When should repeated scans be obtained for Medmont, OPD, and Pentacam?

• Medmont: Routinely obtain 3 scans for reliability.

• OPD: Can be repeated as needed, often used for entrance testing.

• Pentacam: One scan is usually sufficient; repeat only if the scan appears unreliable or reveals suspicious findings.

What are common clinical uses of the OPD?

• Often used for entrance testing.

• Combines several measurements in one device (topography, autorefraction, keratometry, pupillometry, aberrometry).

• Commonly not relied upon for GP base curve selection.

How does radius of curvature relate to corneal steepness and dioptric power?

• Smaller radius = steeper cornea = greater dioptric power.

• Larger radius = flatter cornea = lower dioptric power.

How does Placido cone size affect topographic data collection?

Small cones

• Collect more data points

• Provide better accuracy

• More difficult to properly capture

Large cones

• Easier to align and capture

• Generally collect fewer data points

What is the tradeoff between small and large Placido cones in corneal topography?

• Small cone: Better resolution and accuracy but more challenging acquisition.

• Large cone: Easier acquisition but potentially less detailed data.

What are the major map types available in corneal topography/tomography?

The primary outputs include:

• Reliability indices (scan quality assessment)

• Axial map

• Tangential (instantaneous) map

• Refractive map

• Elevation map

• Difference map

What do reliability indices tell you on a corneal topography scan?

Reliability indices indicate whether the scan quality is sufficient for interpretation by assessing factors such as:

• Image quality

• Alignment/fixation

• Completeness of captured data

• Repeatability

What does the color scale represent on corneal topography maps?

Colors represent relative corneal curvature/power:

• Red = steepest

• Orange = relatively steep

• Yellow = moderately steep

• Green = intermediate/average

• Blue = flattest

On a corneal topography map, what does a red area indicate compared with a blue area?

• Red area = increased curvature, smaller radius of curvature, higher dioptric power (steeper cornea)

• Blue area = decreased curvature, larger radius of curvature, lower dioptric power (flatter cornea)

What is a common mistake when interpreting corneal topography colors?

Assuming colors represent corneal thickness or elevation.

For standard curvature maps:

• Red/Orange = steeper curvature

• Blue = flatter curvature

The colors represent relative curvature (or power), not necessarily thickness or elevation. Different map types may use the colors differently, so always identify the map type before interpretation.

What are the primary map types displayed on a corneal topography report?

The four major displays commonly reviewed are:

• Axial Power Map

• Tangential (Instantaneous) Power Map

• Elevation Map

• Shape Factor (e²) Analysis

Each provides different information about corneal shape and curvature.

What is the difference between axial and tangential power maps?

Axial Map

• Measures curvature relative to the visual/optical axis.

• Produces a smoother, less detailed map.

• Best for identifying overall corneal shape and astigmatism patterns.

• Most commonly used clinically.

Tangential Map

• Measures local or instantaneous curvature.

• More sensitive to abrupt curvature changes.

• Reveals localized abnormalities more clearly.

• Better for detecting small cones and early ectatic changes.

High-yield:
Axial = overall pattern
Tangential = local detail

Which map is best for detecting localized corneal abnormalities such as early keratoconus?

Tangential (instantaneous) maps are superior because they:

• Display local curvature changes more accurately.

• Are less smoothing-dependent than axial maps.

• Better reveal the true size and location of ectatic regions.

What information does an elevation map provide that curvature maps do not?

• Curvature maps show how steep or flat the cornea is.

• Elevation maps show how high or low the corneal surface sits relative to a reference surface (best-fit sphere or other reference shape).

Key distinction:

• Topography → curvature

• Elevation mapping → height

Match each map type with its primary clinical purpose.

Map Type	Primary Use
Axial Power	Overall corneal shape and astigmatism
Tangential Power	Localized curvature changes, cone detection
Elevation	Height relative to a reference surface
Shape Factor (e²)	Corneal asphericity/prolateness assessment

Why should multiple map types be reviewed together rather than relying on a single map?

Each map provides different information:

• Axial → global curvature pattern

• Tangential → localized curvature detail

• Elevation → surface height changes

• Shape factor (e²) → corneal asphericity

Reviewing all helps improve detection of subtle corneal abnormalities and reduces the risk of missing early ectatic disease.

What are the key features and clinical uses of a tangential (instantaneous) map?

• Also called the instantaneous map.

• Measures localized corneal curvature/power.

• Curvature is calculated from the tangent to the corneal surface.

• Most sensitive topography map.

• Best for detecting small, localized changes.

What is tangential map particularly useful for?

• Keratoconus diagnosis

• Ortho-K follow-up

• Monitoring corneal reshaping treatments

What are the key characteristics of an axial map?

• Shows the overall shape of the cornea

• Uses averaged ("smoothed") data

• Measures curvature relative to the optical axis

• Most accurate centrally

• Less accurate in the peripheral cornea

What are commonly axial maps used for?

• General topography interpretation

• Corneal GP fitting

• Scleral lens base curve selection

What information does an elevation map provide?

An elevation map shows the height of the corneal surface relative to a reference surface (usually a best-fit sphere).

• Positive elevation = surface protrudes above the reference.

• Negative elevation = surface lies below the reference.

• Provides the most accurate representation of actual corneal shape.

Why are elevation maps particularly useful for irregular corneas?

Elevation maps:

• Best represent true corneal shape.

• Help determine lens selection for irregular corneas.

• Can predict fluorescein patterns prior to lens fitting.

• Aid evaluation of ectatic disorders such as keratoconus.

How is elevation determined in topography versus tomography?

Topography

• Does not directly measure elevation.

• Uses mathematical algorithms to estimate elevation relative to a best-fit sphere.

Tomography

• Directly measures elevation.

• Still displays results relative to a best-fit sphere or similar reference surface.

High-yield:
Topography estimates elevation.
Tomography measures elevation.

Which map is considered the most accurate representation of actual corneal shape?

Elevation map

Because it evaluates corneal height relative to a reference surface rather than simply describing curvature.

Which map should you choose for the following clinical scenarios?

• Overall corneal shape / GP base curve selection → Axial map

• Early keratoconus detection → Tangential map

• Ortho-K follow-up → Tangential map

• Assessing true corneal shape → Elevation map

• Predicting fluorescein patterns on irregular corneas → Elevation map

hat is eccentricity (e), and what does it describe about the cornea?

• Eccentricity (e) describes how much a corneal shape deviates from a perfect sphere.

• It is derived from conic section geometry.

• A sphere has:

  • e = 0

  • Q = 0

  • SF = 0

• Normal corneas have positive eccentricity because they flatten toward the periphery.

What is the difference between prolate and oblate corneal shapes?

Prolate Cornea

• Steeper centrally

• Flatter peripherally

• Normal corneal shape

• Q < 0

Oblate Cornea

• Flatter centrally

• Relatively steeper peripherally

• Often seen after myopic refractive surgery

• Q > 0

What is the purpose of a refractive power map?

• Estimates the quality of vision through the pupil.

• Accounts for the eye's refractive characteristics rather than simply curvature.

When is a refractive power map most clinically useful?

Most useful for:

• Refractive surgery screening

• Post-LASIK/PRK evaluation

• Assessing optical quality through the pupillary zone

Less useful for:

• Routine GP fitting

• Initial topography interpretation

Why is tear film quality important when interpreting Placido-ring corneal topography?

• Placido-based topography measures reflections from the tear film.

• An irregular or unstable tear film distorts the reflected rings (mires).

• Distorted mires can create inaccurate curvature maps and misleading interpretations.

What do good versus poor mires look like on corneal topography?

Good mires

• Sharp, clear, concentric rings

• Uniform spacing

• Reliable topographic data

Poor mires

• Distorted, broken, irregular, or blurred rings

• Often caused by tear-film instability

• May produce unreliable maps

What is the IS (Inferior-Superior) Index?

The IS Index measures the difference between:

• Average inferior corneal power

• Average superior corneal power

It helps identify inferior steepening, a classic feature of keratoconus.

What is the Surface Asymmetry Index (SAI)?

The SAI measures overall corneal asymmetry by comparing:

• Corresponding corneal points

• Located on the same chord

• 180° apart

Higher values indicate increasing asymmetry.

What SAI value is considered suspicious for corneal disease?

SAI > 1 suggests abnormal corneal asymmetry and possible disease.

What is the Surface Regularity Index (SRI)?

The SRI quantifies corneal surface regularity.

• Higher values indicate greater irregularity.

• Increases with increasing asymmetry.

• Often correlates with reduced optical quality.

What SRI value is considered abnormal?

SRI > 0.80 is considered abnormal.

What are the major Scheimpflug imaging devices used in corneal tomography?

• Pentacam (Oculus)

• Galilei Dual Scheimpflug Analyzer (Ziemer)

Both are tomographic devices that provide 3D corneal analysis, including anterior and posterior corneal measurements.

Which imaging technology is used by the Pentacam and Galilei?

Both devices use Scheimpflug imaging, a camera technique that captures cross-sectional images of the anterior segment and allows reconstruction of the cornea in three dimensions.

What measurements can the Pentacam provide?

• Corneal pachymetry (thickness)

• Topography

• Anterior corneal curvature

• Posterior corneal curvature

• Astigmatism

• Lens imaging using Scheimpflug technology

What information can tomography provide that Placido topography cannot directly measure?

Tomography can directly evaluate:

• Anterior corneal curvature

• Posterior corneal curvature

• Corneal elevation

• Corneal thickness (pachymetry)

Placido topography primarily measures anterior surface curvature.

How does a Pentacam acquire its data?

• The camera system rotates around the eye.

• Acquires approximately 50 images in about 2 seconds.

• Reconstructs a 3D model of the anterior segment from multiple cross-sectional images.

Why is Pentacam considered less affected by tear-film quality than Placido topography?

• Placido systems rely on reflected mires from the tear film.

• Pentacam acquires tomographic cross-sectional images using Scheimpflug imaging.

• Therefore, Pentacam measurements are relatively tear-film independent and often require fewer repeat scans.

What are the major advantages of Pentacam for keratoconus detection?

Pentacam can evaluate:

• Posterior corneal steepening

• Elevation abnormalities

• Thinnest pachymetry location

• Corneal thickness progression

• Anterior and posterior curvature changes

What map types are available on a Pentacam tomographic analysis?

Pentacam provides:

• Instantaneous (Tangential) map

• Axial map

• Elevation maps

  • Anterior elevation

  • Posterior elevation

How much of the eye can be kept in focus using Scheimpflug imaging?

The depth of focus extends from:

• Anterior cornea through

• Posterior lens

This allows accurate imaging of multiple ocular structures within a single scan.

What is a key advantage of Pentacam elevation maps over standard Placido topography maps?

Pentacam can directly evaluate:

• Anterior elevation

• Posterior elevation

• Corneal thickness (pachymetry)

This allows earlier detection of ectatic disease because posterior corneal changes may appear before obvious anterior curvature abnormalities.

What is the Best Fit Sphere (BFS) in Pentacam analysis?

The Best Fit Sphere (BFS) is a reference sphere used to compare corneal elevation.

• Created from the central 8 mm of the cornea.

• Elevation maps display how much the cornea rises above or falls below this reference surface.

• Used as the baseline for anterior and posterior elevation analysis.

What is the difference between a Regular BFS and an Enhanced BFS?

Regular BFS

• Calculated using the entire central 8 mm cornea.

• Includes the cone/ectatic area.

• The steep cone makes the BFS appear steeper.

Enhanced BFS

• Removes the 3.5 mm zone surrounding the thinnest point before calculating the BFS.

• Excludes the cone.

• Produces a flatter reference sphere.

Why does the Enhanced BFS improve detection of keratoconus?

Because the cone is removed from the reference calculation:

• The reference sphere becomes flatter.

• The ectatic area stands out more prominently.

• Elevation differences become easier to recognize.

Clinical pearl: Enhanced BFS is more sensitive for detecting subtle ectatic changes.

What anterior elevation values at the thinnest point are considered abnormal on Enhanced BFS analysis?

Anterior Elevation

• > 6 μm at the thinnest point → seen in <5% of normal individuals

• > 8 μm at the thinnest point → seen in <0.5% of normal individuals

What posterior elevation values at the thinnest point are considered abnormal on Enhanced BFS analysis?

Posterior Elevation

• > 13 μm at the thinnest point → seen in <5% of normals

• > 18 μm at the thinnest point → seen in <0.5% of normals

Which is generally more sensitive for early ectasia detection: anterior or posterior elevation abnormalities?

Posterior elevation abnormalities are typically more sensitive.

Key cutoffs:

• Anterior: >6 μm, >8 μm

• Posterior: >13 μm, >18 μm

What is the Corneal Thickness Spatial Profile (CTSP)?

The CTSP evaluates how corneal thickness changes from the thinnest point toward the periphery.

• Uses concentric measurements around the thinnest point.

• Helps detect abnormal thinning patterns.

• Important in keratoconus and ectasia screening.

How is the CTSP generated in Belin/Ambrosio analysis?

• The thinnest point (TP) is identified.

• 22 imaginary concentric circles are created around that point.

• Thickness is analyzed in 0.4 mm increments moving outward from the TP.

What is Percent Increase in Thickness (PIT)?

PIT measures how rapidly the cornea thickens away from the thinnest point. Abnormal PIT values suggest abnormal thickness progression and possible ectatic disease.

What does the Belin/Ambrosio Enhanced Ectasia Display (BAD-D) evaluate?

The BAD-D combines multiple tomographic parameters:

• Anterior elevation

• Posterior elevation

• Pachymetric progression

• CTSP

• PIT

• Location of the thinnest point

What is the KISA% index and what components make it up?

KISA% is a keratoconus screening index that combines four parameters:

• K = K value (corneal power)

• I-S = Inferior-Superior asymmetry

• AST = Regular corneal astigmatism

• SRAX = Skewed Radial Axis (irregular astigmatism)

What KISA% value is highly suggestive of keratoconus?

KISA% > 100 = keratoconus

What is BAD-D (Belin/Ambrosio Deviation Value)?

BAD-D is the overall Pentacam ectasia screening score that combines multiple abnormalities into a single value.

It incorporates:

• Anterior elevation

• Posterior elevation

• Pachymetric progression

• Thinnest pachymetry

• Thinnest point displacement

What are the five D-values used to generate the BAD-D score?

• Df = Anterior elevation deviation (front)

• Db = Posterior elevation deviation (back)

• Dp = Pachymetric progression deviation

• Dt = Thinnest pachymetry deviation

• Da = Thinnest point displacement deviation

The overall BAD-D combines all five values and compares them to a normative database.

How is BAD-D interpreted clinically?

BAD-D Value	Interpretation
< 1.6	Normal (white)
≥ 1.6	Suspicious (yellow)
≥ 2.6	Abnormal/pathologic (red)

What is the Index of Surface Variance (ISV), and when is it abnormal?

• Measures overall corneal surface irregularity/variance.

• Elevated values indicate increasing irregularity.

• Abnormal when ISV > 41

What is the Index of Vertical Asymmetry (IVA), and when is it abnormal?

• Measures superior-inferior asymmetry.

• Useful for detecting keratoconus-related asymmetry.

• Abnormal when IVA > 0.32

What are the pathologic cutoff values for ISV, IVA, KI, and CKI?

Index	Pathologic Value
ISV	> 41
IVA	> 0.32
KI	> 1.07
CKI	> 1.03

What do KI and CKI measure?

• KI (Keratoconus Index) → General index of keratoconic change.

  • Abnormal: > 1.07

• CKI (Central Keratoconus Index) → Compares central and peripheral curvature to identify central cone formation.

  • Abnormal: > 1.03

What do the Pentacam indices IHA, IHD, and Rmin represent, and when are they abnormal?

• IHA = Index of Height Asymmetry

  • Abnormal: > 21

• IHD = Index of Height Decentration

  • Abnormal: > 0.016

• Rmin = Smallest radius of axial curvature

  • Abnormal: < 6.71 mm

Pattern: Greater asymmetry/decentration and smaller radius values suggest keratoconus.

What are the two major methods used for corneal tomography?

1. Scheimpflug imaging (e.g., Pentacam)

2. Anterior Segment OCT (e.g., Anterion)

Both provide 3D corneal analysis, including elevation and pachymetry.

How does tomography differ from traditional topography?

Topography

• Evaluates primarily anterior corneal curvature

• Placido-ring based

• Tear-film dependent

Tomography

• Creates a 3D reconstruction

• Measures both anterior and posterior surfaces

• Provides pachymetry maps

• Detects ectatic changes earlier

What should raise suspicion for early keratoconus when topography appears normal?

A patient may report:

• "Shadowing"

• "Ghosting"

• "Doubling of letters"

• Reduced quality of vision despite good Snellen acuity (often 20/20)

If a Pentacam is unavailable and topography appears normal, what additional test can help detect early keratoconus?

Perform aberrometry (such as with the OPD).

Look for:

• Increased higher-order aberrations (HOAs)

• Especially coma

Which higher-order aberration is most strongly associated with keratoconus?

Coma

• Often increases before obvious clinical signs appear.

• May explain ghosting, shadowing, or doubled letters.

• Can be detected on wavefront aberrometry even when topography is inconclusive.

What does the Belin ABCD Keratoconus Staging System evaluate?

The ABCD system stages keratoconus using:

• A: Anterior radius of curvature

• B: Posterior radius of curvature

• C: Corneal thickness at the thinnest point (pachymetry)

• D: Best-corrected distance visual acuity (BCDVA)

Unlike older systems, it incorporates posterior corneal changes and pachymetry.

What corneal parameters are used to define progression in the Belin ABCD system?

Progression is defined by change in two of three parameters:

• Anterior corneal surface

• Posterior corneal surface

• Thinnest pachymetry

What are the Stage 0 ABCD criteria for keratoconus?

Stage 0 (normal):

• Anterior radius > 7.25 mm (<46.5 D)

• Posterior radius > 5.90 mm (<57.25 D)

• Thinnest pachymetry > 490 μm

• BCDVA = 20/20

How do ABCD staging parameters change as keratoconus worsens?

With increasing stage:

• Anterior radius decreases (cornea steepens)

• Posterior radius decreases (back surface steepens)

• Pachymetry decreases (cornea thins)

• Visual acuity worsens

What are the severe-stage (Stage 4) ABCD findings in keratoconus?

Stage 4:

• Anterior radius < 6.15 mm (>55 D)

• Posterior radius < 4.95 mm (>68.5 D)

• Pachymetry ≤ 300 μm

• BCDVA < 20/400