Corneal Topography and Tomography

What does corneal topography map, and what technology does it use?

Maps the surface curvature of the anterior cornea only, using projection rings (mires). Examples: Medmont, Atlas.

What does corneal tomography map, and what technology does it use?

Maps both anterior AND posterior corneal surfaces, generating a 3D representation using Scheimpflug technology. Examples: Pentacam, Galilei.

What is the Scheimpflug principle?

Because the cornea is non-planar (curved), tomographers deliberately tilt the object, lens, and image planes so they all intersect at one point — the Scheimpflug intersection — allowing production of a sharp, in-focus image.

What does the Pentacam do mechanically?

It takes cross-section images throughout the cornea, rotating around the eye to build a full 3D tomogram.

What does corneal tomography (a 'tomogram') let you evaluate?

Front and back corneal surfaces, the anterior chamber, and sometimes the crystalline lens.

What 5 categories of data does corneal tomography provide?

Anterior curvature (topography), corneal thickness, anterior chamber depth, anterior and posterior elevation data, and densitometry (amount of corneal/lens opacification).

Name 4 clinical applications of corneal tomography.

1) Detect/manage corneal ectatic conditions (keratoconus, PMD), 2) screen for refractive surgery, 3) preoperative planning for cataract surgery, 4) postoperative suture removal after PK.

What are the 4 basic maps generated by tomography?

1) Axial/sagittal curvature, 2) corneal thickness, 3) elevation of the anterior surface, 4) elevation of the posterior surface (both anterior/posterior compared to a best-fit sphere).

On an axial/sagittal curvature map, what do warm and cool colors represent?

Warm colors = steeper cornea. Cool colors = flatter cornea. Values are dioptric power at each point on the anterior surface.

What are the 3 symmetrical bowtie shapes, and when are they normal?

Round, oval, and symmetric bowtie. Considered normal if Kmax is less than 47 D.

What Kmax values indicate moderate vs high risk of ectasia in symmetrical shapes?

Kmax > 47 D = moderate risk of ectasia. Kmax > 50 D = high risk of ectasia.

What defines an asymmetrical shape, and when is it likely normal?

Superior or inferior steepening, or an asymmetric bowtie (with inferior or superior steepening). Likely normal if the I-S (inferior-superior) value is less than 1.5 D.

What I-S value carries a risk of ectasia in asymmetrical shapes?

An inferior-superior difference greater than 1.5 D.

What defines a skewed radial axis (skew shape), and what angle threshold matters?

The bowtie is not present along the same axis inferiorly and superiorly. If the skewed axis is < 22 degrees, likely normal; if > 22 degrees, risk of ectasia.

What are the 4 'special' axial map shapes, and what do they generally indicate?

Irregular, irregular, butterfly, and crab claw patterns — almost always abnormal and indicate risk of ectasia.

Summarize the HIGH risk axial map shapes.

All special shapes; skew shapes with Srax (skew radial axis) > 22 degrees; symmetrical shapes with Kmax > 50 D.

Summarize the MODERATE risk axial map shapes.

Asymmetric shapes with I-S difference > 1.5 D; symmetrical shapes with Kmax between 47-50 D.

On a corneal thickness map, what do cool vs warm colors mean, and what does orange/red indicate?

Cool colors = normal/thick cornea. Warm colors = thin cornea. Orange/red = higher risk of ectasia.

What inferior-superior thickness asymmetry is normal vs concerning?

< 30 µm difference = normal. > 30 µm difference = suggests ectasia.

What 4 questions should you ask when reviewing a corneal thickness map?

Where is the thinnest point? What is the value at the thinnest point? What is pupil-center pachymetry? Is there inferior-superior asymmetry?

What thickness pattern suggests Pellucid Marginal Degeneration on a thickness map?

A 'Bell sign' — inferior thinning pattern on the corneal thickness map.

What thickness pattern suggests Keratoglobus?

Global (diffuse) thinning across the entire cornea, rather than a focal area.

What is a Best-Fit Sphere (BFS)?

A computer-generated reference sphere (the 'orange line' / reference point, like sea level) that the actual corneal shape (the 'green line') is compared against. Red arrows = elevations (+ values); blue arrows = depressions (- values).

What do elevation maps show, and what do warm vs blue colors mean?

They compare the cornea's shape to a computer-generated BFS (central 8mm zone), revealing areas of elevation and depression on both anterior and posterior cornea. Warm colors = elevation; blue colors = depression.

On a standard elevation map, how is an area of keratoconus displayed?

Areas aligned with the reference (yellow line) get a value of zero. In keratoconus, the cone projects above the reference sphere, showing as a red (+) elevated area with very high positive values.

What is the Enhanced Best-Fit Sphere (Enhanced BFS), and why is it used?

It ignores the areas of high elevation (the cone) to give a more accurate BFS and avoid missing small areas of abnormal elevation. It excludes the central 3.5-4mm zone of the cornea.

What normal elevation values apply to front vs back elevation maps, and what difference is acceptable between them?

Front: normal is 12 µm or less. Back: normal is 17 µm or less. The difference between front and back should be less than 5 µm.

What is the Belin/Ambrosio Enhanced Ectasia Display (BAED) used for?

It uses the Enhanced BFS to provide elevation data and screen for corneal ectasia.

What three maps make up the BAED elevation comparison, and how are they related?

Standard elevation map (compared to BFS), exclusion elevation map (compared to Enhanced BFS), and a difference map, which subtracts the standard map from the exclusion map values.

On the BAED difference map, what do green, yellow, and red mean?

Green =

What does the Corneal Thickness Spatial Profile (CTSP) describe?

The average progression of corneal thickness starting from the thinnest location out to the periphery, analyzed at concentric rings (2mm, 4mm, 6mm, etc.) from the thinnest point.

On the CTSP/PTI graphs, what do the black dotted lines and red line represent?

Black dotted lines = normative data (center = normal, above/below = 2 standard deviations away). Red line = the patient's actual data.

What is the Percentage Thickness Increase (PTI)?

The percent of progression of the CTSP — essentially the rate at which thickness increases from the thinnest point outward.

What does a 'rapid slope' pattern on the CTSP/PTI graph indicate?

The red (patient) line leaves the normative data before the 6mm zone — indicates risk of ectasia.

What do S-shape, flat-shape, and inverted-shape CTSP/PTI curves indicate respectively?

S-shape = abnormal. Flat shape = edematous cornea (e.g., Fuchs' dystrophy, guttae). Inverted shape = pellucid marginal degeneration.

Define K1, K2, and Km.

K1 = flat meridian curvature in the central 3mm. K2 = steep meridian curvature in the central 3mm. Km = the mean of K1 and K2.

What is Kmax, and what value signals ectasia risk?

Kmax is the maximum curvature reading on the anterior surface. Greater than 47.2 D signals risk of ectasia.

What does the Q value indicate, and what ranges are normal vs abnormal?

Q value indicates the general shape of the anterior corneal surface. Between 0 and -1 = normal prolate shape. Positive values = oblate shape. Less than -1 = hyperprolate (likely ectasia).

What two things should you verify first before interpreting any Pentacam scan?

1) Verify the patient and which eye (date of birth must be accurate since it's compared to age-based normative data). 2) Confirm the quality and centration of the scan (Quality Specification).

What does 'QS: OK' mean vs. 'Data Gaps,' 'Fix,' or 'Model'?

OK = good scan quality. 'Data Gaps,' 'Fix,' or 'Model' indicate the scan was affected by eye movement, blinking, etc., and should be repeated.

What thinnest-point pachymetry values indicate high vs moderate risk of ectasia?

< 470 µm = high risk of ectasia. 470-500 µm = moderate risk.

How is keratoconus defined, and what is it characterized by?

A corneal ectatic disorder characterized by progressive thinning and steepening. Topography/tomography is used for screening, diagnosis, and monitoring progression.

What are the classic topographic findings in keratoconus?

Inferior steepening with correlating corneal thinning, posterior elevation, and high irregular astigmatism.

List the values that should raise clinical suspicion for keratoconus.

Central K > 48.7 D (forme fruste range: K 47.2-48.7 D); I-S value > 1.2 D; astigmatism > 2.5 D; posterior elevation > 18-20 microns.

How is Pellucid Marginal Degeneration (PMD) defined?

A corneal ectatic disorder characterized by peripheral and inferior thinning.

What are the classic topographic findings in PMD?

A crab-claw pattern, inferior and peripheral thinning, protrusion of the cornea above the area of thinning, and a bell-shaped corneal thickness map.