Part 2 | How to Read Corneal Topography and Pentacam Tomography in Keratoconus and Corneal Thinning Disorders
- David B. Sabin
- 5 hours ago
- 11 min read
Corneal topography and tomography are essential tools for diagnosing and monitoring keratoconus, pellucid marginal degeneration, post-surgical corneal ectasia, and other disorders that alter the shape or thickness of the cornea.
Although the colorful maps may initially appear complicated, they become much easier to interpret when they are reviewed in a consistent order. The goal is not to search for one abnormal number. Instead, the clinician should determine whether the corneal curvature, elevation, and thickness maps tell the same structural story.
A reliable interpretation combines the imaging results with the patient’s refraction, visual acuity, retinoscopy, slit-lamp examination, symptoms, age, contact lens history, and previous scans.
Corneal Topography Versus Corneal Tomography
The terms topography and tomography are sometimes used interchangeably, but they describe different types of corneal imaging.
Corneal topography
Placido-disc corneal topography analyzes reflections from rings projected onto the tear film. It primarily measures the curvature and optical power of the anterior corneal surface.
Topography is especially useful for identifying:
Regular and irregular astigmatism
Inferior or asymmetric corneal steepening
Skewed astigmatic axes
Contact lens–induced corneal warpage
Keratoconus patterns
Corneal shape for specialty contact lens fitting
Corneal tomography
The Pentacam uses a rotating Scheimpflug camera to construct a three-dimensional model of the anterior segment. In addition to anterior curvature, it evaluates:
Anterior corneal elevation
Posterior corneal elevation
Corneal thickness throughout the cornea
The location of the thinnest point
Pachymetric progression
Anterior chamber measurements
Multiple keratoconus and ectasia screening indices
This is important because early ectatic disease may involve abnormal posterior elevation or an abnormal thickness distribution before dramatic anterior steepening becomes visible. Tomography therefore provides information that cannot be obtained from Placido topography alone.
Step 1: Confirm That the Scan Is Reliable
Never interpret an abnormal map before checking scan quality.
Poor fixation, blinking, eyelid interference, an unstable tear film, missing data, excessive eye movement, corneal scarring, or poor alignment may produce misleading curvature and thickness measurements.
Placido topography is particularly dependent on a smooth tear film because the instrument is analyzing reflected rings. Dry eye may create broken rings, irregular islands of steepening, or inconsistent measurements between scans.
Before accepting the result:
Review the device’s quality specification.
Examine the raw Placido rings or Scheimpflug images.
Repeat scans that appear poorly centered or incomplete.
Compare at least two reproducible measurements when the result will affect treatment.
Ask about recent rigid, scleral, hybrid, or soft contact lens wear.
Contact lens warpage can produce asymmetric bow-tie patterns, inferior steepening, or other findings that resemble early keratoconus. The cornea may need time without contact lens wear before its natural shape can be accurately assessed.
Step 2: Read the Axial or Sagittal Curvature Map
The axial curvature map is usually the easiest map to recognize. Warm colors generally represent steeper corneal power, while cool colors represent flatter areas.
The axial map provides a smooth, broad overview of corneal shape. It is helpful for recognizing:
Regular symmetric bow-tie astigmatism
Asymmetric bow-tie astigmatism
Inferior steepening
Skewed radial axes
Central or paracentral steepening
Flattening after refractive surgery
General cone location
A normal astigmatic cornea often displays a relatively symmetric bow tie. Keratoconus more commonly produces an asymmetric bow tie, focal inferior or inferotemporal steepening, or a bow tie in which the superior and inferior axes do not line up.
However, inferior steepening alone does not diagnose keratoconus. Normal corneas, contact lens warpage, poor tear film quality, decentered scans, and other corneal conditions may produce similar patterns.
The axial map also tends to average curvature over a larger area. This makes it visually smooth, but it may exaggerate the size of a cone and may not precisely identify its true location.
Important keratometry values
Common measurements include:
K1: The flattest principal corneal meridian.
K2: The steepest principal corneal meridian.
Km: The mean corneal curvature.
Kmax: The steepest measured point on the anterior cornea.
Kmax is useful, but it should not be treated as the complete definition of keratoconus severity or progression. It is a single-point measurement and may be influenced by measurement noise, tear-film abnormalities, scar tissue, cone location, and the device’s sampling method.
A stable Kmax does not guarantee that the posterior cornea or pachymetric profile is stable.
Step 3: Use the Tangential Map to Localize the Cone
The tangential, instantaneous, or local curvature map calculates curvature at each individual point without assuming that the cornea is part of a sphere.
Compared with the axial map, the tangential map:
Shows more localized curvature changes
Defines the size and position of the cone more precisely
Better separates a small central cone from a broad inferior cone
May reveal localized irregularities that appear smoothed on the axial map
The tangential map often looks more dramatic or “noisy” because it emphasizes local changes. For this reason, it should be interpreted alongside the smoother axial map rather than by itself.
Step 4: Evaluate the Anterior Elevation Map
Elevation maps do not directly show corneal power. They show how high or low the corneal surface lies relative to a computer-generated reference surface, usually a best-fit sphere.
Positive elevation indicates that an area sits above the reference surface. Negative elevation indicates that it sits below it.
In keratoconus, the anterior elevation map may reveal a localized positive island near the cone. The location of this elevation should generally correspond with the region of steepening on the curvature map and the region of thinning on the pachymetry map.
Elevation values depend heavily on:
The reference surface selected
The diameter used to calculate the reference surface
Whether the map is centered on the corneal apex, pupil, or thinnest point
The color scale
The device and software version
For that reason, elevation values from different devices or differently configured maps should not be directly compared without understanding how each map was generated.
Step 5: Examine the Posterior Elevation Map
The posterior corneal surface is especially important when screening for ectasia.
Because the back of the cornea is not measured by traditional Placido topography, an abnormal posterior elevation pattern may provide additional evidence of early structural change. A focal posterior elevation that corresponds with anterior steepening and localized thinning strengthens the suspicion for keratoconus.
Posterior elevation should not be interpreted as an isolated number. A borderline posterior map without corresponding curvature, pachymetric, clinical, or longitudinal findings may not represent true ectasia.
The most meaningful finding is agreement among the maps:
Anterior steepening
Corresponding anterior or posterior elevation
A displaced thinnest point
Abnormal pachymetric progression
Consistent clinical findings
Step 6: Read the Pachymetry Map
The pachymetry map shows corneal thickness in micrometers across the entire measured cornea.
Do not look only at the central corneal thickness. Record:
Corneal thickness at the pupil center
Corneal thickness at the apex
Minimum or thinnest corneal thickness
Location of the thinnest point
Distance between the thinnest point and the corneal center
Pattern of thickness progression toward the periphery
A naturally thin cornea is not necessarily ectatic. Some healthy corneas are thinner than average but maintain a relatively symmetric shape and normal progression of thickness from the center toward the periphery.
Keratoconus is more suspicious when the cornea demonstrates localized thinning associated with matching steepening and elevation, rather than simply having a low central thickness.
The thinnest point in keratoconus is commonly displaced inferiorly or inferotemporally. In pellucid marginal degeneration, thinning is usually located farther inferiorly and more peripherally. In keratoglobus, the thinning tends to be more diffuse.
Step 7: Evaluate Pachymetric Progression
A normal cornea gradually becomes thicker from the center toward the periphery. An ectatic cornea may thicken more rapidly or asymmetrically as measurements move away from the thinnest point.
The Pentacam evaluates this pattern using measurements such as:
PPImin: Minimum pachymetric progression index.
PPIavg: Average pachymetric progression index.
PPImax: Maximum pachymetric progression index.
A higher or unusually asymmetric pachymetric progression pattern may support the presence of ectasia.
Ambrósio Relational Thickness
ARTmax combines the minimum corneal thickness with the maximum pachymetric progression index:
ARTmax = thinnest pachymetry ÷ PPImax
A cornea that is thin but thickens normally toward the periphery may have a less concerning ARTmax than a similarly thin cornea with rapid peripheral thickness progression.
ARTmax should still be interpreted within the complete scan. Device thresholds are screening aids, not independent diagnoses.
Step 8: Read the Belin–Ambrósio Enhanced Ectasia Display
The Belin–Ambrósio Enhanced Ectasia Display, commonly called the BAD display, combines elevation and thickness information to identify corneas that differ from the device’s normative database.
The display compares the cornea with a standard best-fit sphere and an enhanced best-fit sphere.
The enhanced reference surface excludes an area around the thinnest point when calculating the new reference surface. In an ectatic cornea, removing the cone region may flatten the reference surface and make localized elevation easier to identify.
The BAD display commonly includes:
Df: Deviation of the anterior elevation.
Db: Deviation of the posterior elevation.
Dp: Deviation of pachymetric progression.
Dt: Deviation of minimum corneal thickness.
Da: Deviation related to displacement of the thinnest point.
BAD-D: A combined final deviation score.
The Pentacam uses a traffic-light system. Green generally represents values within the normative range, yellow indicates a suspicious or borderline result, and red indicates a greater deviation from the normative database.
On many Pentacam software versions, individual values become yellow at approximately 1.6 standard deviations and red at approximately 2.6 standard deviations. These colors must be interpreted according to the specific software version, patient population, scan quality, corneal diameter, and complete clinical picture.
BAD-D is useful for ectasia screening, but it is not a stand-alone diagnosis. False-positive or borderline results can occur, including in otherwise normal corneas with smaller corneal diameters.
Step 9: Review the Topometric Indices
The Pentacam may also provide several anterior surface indices:
ISV — Index of Surface Variance: Reflects overall corneal surface irregularity.
IVA — Index of Vertical Asymmetry: Compares superior and inferior curvature.
KI — Keratoconus Index: Evaluates curvature relationships associated with keratoconus.
CKI — Central Keratoconus Index: Emphasizes central steepening.
IHA — Index of Height Asymmetry: Evaluates superior–inferior elevation differences.
IHD — Index of Height Decentration: Evaluates vertical decentration of elevation data.
These measurements may support a diagnosis, especially when several are abnormal in a consistent pattern. ISV, IVA, IHD, BAD-D, and other indices have demonstrated value in identifying clinical and subclinical keratoconus, but their diagnostic performance varies among populations and disease stages.
Step 10: Understand the ABCD Keratoconus Classification
The Belin ABCD system evaluates keratoconus using four separate domains:
A — Anterior curvature: Anterior radius of curvature within a zone centered on the thinnest point.
B — Posterior curvature: Posterior radius of curvature within the same region.
C — Corneal thickness: Thinnest pachymetry.
D — Distance visual acuity: Best-corrected distance visual acuity.
Each component is graded independently from 0 through 4.
This system provides a more complete description than older classifications based mainly on anterior keratometry and corneal thickness. It also prevents a clinician from assuming the disease is stable merely because Kmax has not changed.
The ABCD Progression Display can demonstrate significant posterior corneal progression even when anterior curvature and Kmax remain relatively stable.
Recognizing Common Corneal Thinning Patterns
Keratoconus
Typical findings include:
Focal central, paracentral, or inferotemporal steepening
Asymmetric bow-tie pattern
Skewed radial axes
Localized anterior or posterior elevation
Inferior or inferotemporal displacement of the thinnest point
Abnormal pachymetric progression
Increasing irregular astigmatism
Elevated coma and other higher-order aberrations
The curvature, elevation, and thinning abnormalities should usually correspond spatially.
Pellucid marginal degeneration
Pellucid marginal degeneration typically produces a narrow band of inferior peripheral thinning, with the greatest steepening located above the area of thinning.
The anterior curvature map may display a crab-claw, kissing-dove, or butterfly pattern. However, the crab-claw pattern is not specific to pellucid marginal degeneration. Inferior keratoconus may produce a similar appearance.
The distinction requires careful evaluation of the true location of thinning, posterior elevation, slit-lamp findings, and complete tomographic pattern.
Keratoglobus
Keratoglobus produces more generalized corneal protrusion and diffuse thinning that may extend toward the limbus. The pattern is less focal than classic keratoconus, and the peripheral cornea may be particularly thin.
Post-LASIK or post-PRK ectasia
Post-refractive surgery ectasia may produce:
Progressive inferior or central steepening
Increasing irregular astigmatism
Posterior elevation
Progressive thinning
Reduced best-corrected vision
Changes within or adjacent to the previous ablation zone
The patient’s preoperative maps, treatment parameters, residual stromal bed, surgical history, and serial postoperative imaging are critical to interpretation.
Terrien marginal degeneration
Terrien marginal degeneration typically causes peripheral thinning, frequently superiorly, and may be associated with vascularization, lipid deposition, and high against-the-rule or oblique astigmatism.
Corneal imaging helps document the induced shape change, but the diagnosis depends heavily on the slit-lamp appearance.
How to Determine Whether Keratoconus Is Progressing
Progression should not be diagnosed from a single scan.
Serial examinations should be performed with:
The same imaging device when possible
Similar map settings and color scales
Reliable scan quality
Comparable contact lens conditions
Documentation of refraction and corrected visual acuity
A widely used consensus framework defines ectatic progression as repeatable change beyond the normal noise of the testing system in at least two of these domains:
Progressive anterior corneal steepening
Progressive posterior corneal steepening
Progressive thinning or an abnormal increase in the rate of thickness change from the periphery toward the thinnest point
Kmax may contribute to the assessment, but a change in Kmax alone should not automatically define progression. Posterior curvature, ABCD parameters, minimum thickness, pachymetric progression, refraction, visual acuity, and clinical findings should also be reviewed.
Common Interpretation Mistakes
Diagnosing keratoconus from a red color
A red area simply represents the upper end of the selected scale. Changing the scale can change the color without changing the cornea.
Looking only at Kmax
Kmax does not evaluate the posterior surface or describe how thickness changes across the cornea.
Calling every thin cornea keratoconus
A uniformly thin but symmetric cornea may be normal. Ectasia usually creates an abnormal relationship among curvature, elevation, thickness location, and thickness progression.
Ignoring contact lens warpage
Contact lens molding may mimic keratoconus or hide its true severity.
Comparing different devices as though they are identical
Different instruments use different measurement technologies, reference surfaces, algorithms, zones, and normative databases.
Ignoring the location of the abnormality
The location of the steepest point, highest elevation, and thinnest pachymetry should be compared. A convincing ectatic pattern usually demonstrates anatomical agreement among these findings.
Treating BAD-D as a diagnosis
BAD-D is a screening and decision-support measurement. The final diagnosis remains clinical.
A Practical Corneal Tomography Interpretation Template
A concise clinical interpretation may be documented as follows:
Scan quality: Acceptable and reproducible.
Anterior curvature: Inferotemporal asymmetric steepening with a skewed bow-tie pattern.
Tangential curvature: Localized inferotemporal cone.
Anterior elevation: Focal positive elevation corresponding with the area of steepening.
Posterior elevation: Abnormal focal posterior elevation in the same region.
Pachymetry: Inferotemporally displaced thinnest point with abnormal thickness progression.
Indices: Elevated ISV, IVA, IHD, pachymetric progression, and BAD-D.
ABCD classification: Record the individual A, B, C, and D stages.
Clinical impression: Tomographic findings consistent with keratoconus. Compare with previous examinations to determine stability or progression.
The Most Important Principle: Look for Agreement
A corneal topography or Pentacam interpretation is strongest when multiple independent measurements agree.
In keratoconus, the clinician may see:
Steepening on the curvature map
Elevation in the same region
Localized thinning near that region
Abnormal progression of thickness toward the periphery
Increased irregular astigmatism or coma
Consistent slit-lamp, retinoscopy, refraction, or visual acuity findings
When the maps disagree, the first step is not to force a diagnosis. Recheck the scan quality, ocular surface, contact lens history, map scale, and clinical examination.
Corneal imaging provides a detailed structural picture, but proper interpretation requires pattern recognition, reproducibility, clinical correlation, and comparison over time. This systematic approach helps detect early ectatic disease, distinguish keratoconus from other corneal thinning disorders, monitor progression, guide specialty contact lens fitting, and identify patients who may require further evaluation for corneal cross-linking or other treatment.
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