I remember the excitement as a senior resident in 1994 experiencing the wealth of corneal imaging introduced by the novel — at the time — scanning-slit Orbscan (Bausch + Lomb) three-dimensional imaging device for the cornea!^1,2 Twelve years later, my colleagues and I at The Laservision.gr Clinical and Research Eye Institute in Athens, Greece, started using the newly introduced Pentacam (Oculus) Scheimpflug-tomography device used globally by clinicians for the assessment of the cornea and anterior segment and evaluating cornea curvature and cornea thickness.^3-8

Corneal tomography as captured and reported by the Pentacam delivers a great wealth of useful information on corneal and anterior chamber properties. We will describe herein a basic approach to understand and start using them in your clinical practice today!^9-16

So let’s get started on how to read and interpret Pentacam images.

THE FOUR MAPS REFRACTIVE REPORT

Overview

The four maps refractive report (Figure 1) provided by the device is likely the most popular diagnostic test for cornea tomographic normality. Parameters include the corneal curvature, which is noted as “cornea front.” However, it is the total cornea power that we see in the first group of numbers under the demographic data of the patient that determines which eye was studied, the evaluation date, the evaluation time and the date of birth of the individual that had the exam (top left).

Corneal front

The data numbers under “corneal front” (Figure 1A) are corneal curvature data in arc and in keratometric diopters. K1 in this specific example is 41.6 D. This is the total cornea keratometric power, not the front corneal curvature. K2, the second keratometry, is 43.2 D. K median (of the two) is the Km, which here is noted as 42.4 D. The astigmatism is calculated with the device to be -1.16 D at 168º, which makes the steep axis at 78º. In the small icon (top left of this group of data), a red meridian line shows the steep axis, and the blue meridian line is the flat axis.

The device offers an indication of the capture quality: It checks “OK” when clarity of the Scheimpflug images used to calculate these measurements are acceptable.

The device displays a value for eccentricity measured at 7 mm, which is an important value for corneal optics. Eccentricity of the cornea documents the change of corneal curvature from the center to the periphery — which means how aspheric (prolate) or non-aspheric (oblate) the cornea is. Corneal asphericity helps in real life to reduce chromatic aberrations and improve low light and night vision clarity.

The eccentricity here is noted as 0.44. We know the asphericity is the square of that with an opposite +/- sign. We can therefore retro-calculate asphericity in this cornea is -0.19 (0.44 x 0.44). This is close to the average cornea sphericity for most corneas, which have an asphericity of -0.2 to -0.3.

In summary: Measuring total cornea power — the front surface of the cornea as well as the posterior cornea surface — gives more accurate total keratometric data for those cases that deviate from the usual (ie, mild keratoconus, post-corneal refractive surgery, corneal scars, etc.).

A reminder: Being off by even 1 D in effective keratometry will miscalculate our IOL by about 1 D. I think we all agree that a refractive “surprise” of being off our refraction target after cataract surgery by even 1 diopter is “a lot.”

Corneal posterior

Figure 1B shows posterior cornea data. In similar fashion to the anterior, the left column is in radial numbers and the right column in diopter power. In this case, the K1 is -5.9 D, the K2 -6.5 D for an average of -6.2 D, slightly more minus than the average of the human cornea. We can now understand better how the mean total keratometry of 42.4 D is calculated here. The anterior should have been +48.6 D. By subtracting the posterior cornea power (-6.2 D), we end up with a total cornea power of +42.4 D.

The posterior cornea curvature power measurements also point out posterior cornea astigmatism of 0.5 D; this is not negligible, especially if we are attempting accuracy in a toric IOL calculation.

The QS box says “OK” conforming the quality of the scans are reliable, and the posterior cornea eccentricity is 0.69 D — again within normal range. The posterior curvature values are also noted in mm radius values, additionally to diopters in the next column (Rf, Rs, Rm, Rper and Rmin).

Cornea pachymetry

In Figure 1C, we see cornea pachymetry at the pupillary center — in this example case, 527 microns. Next to it are numbers for the x in mm and y in mm. These represent the distance between the cornea vertex and the center of the pupil, which is defined as the dotted line on all of the maps in the relative center of the image. The x and y describe the angle kappa, which is 60 microns on the x horizontal axis and 0 microns — so there is no angle kappa — on the vertical y axis.

The pachymetry at the apex (the steepest corneal point) is 527 microns, similar with the pachymetry in the pupillary center, which corresponds with the fact that we have a very small angle kappa in this example.

Kmax “front” is noted as 43.7 D on the top left axial sagittal map. As we already discussed, this value actually represents the total (front + back) maximum cornea power point of the central 9 mm of the cornea measured and illustrated here.

Anterior chamber data

In Figure 1D, we can see cornea volume, anterior chamber volume and depth. If we were using these data to consider a phakic IOL (Staar Surgical just received FDA approval for its Toric EVO design), these data would help determine whether this eye would be a good candidate to implant such a phakic IOL (min. 2.8 mm chamber depth is needed).

This section also offers IOP measurement adjustment based on corneal pachymetry: The device calculates in this example that we have to add 1.3 mm Hg in the measured IOP.

KPD stands for keratometric power deviation and compares the anterior curvature in diopters in the sagittal (axial) curvature map to the true net corneal refractive power. Also, angle is the measurement of the anterior chamber angle in degrees — 40 degrees here documents a wide angle.

Finally, we have the pupillary diameter. In this example, 2.72 mm is very important because this pupil diameter documents the pupil size when assessed with a light that the device emits and the ambient light of the room. If the ambient room light conditions is standardized, the pupil size noted here can be used as comparison for small, medium or larger pupillary apertures. We can correlate the pupillary aperture centration to the vertex (angle kappa and the respective mesopic and scotopic pupil expected). We have reported how this measurement changes after cataract surgery and its potential significance.¹⁷

The four color maps

Now for everybody’s favorite part: the color maps! Figure 1E is the sagittal curvature map. You can choose in the device which one of these maps you would like to be displayed in which position. The scale coded in colors for this map is on the left, and you can correlate the value of each color with the respective colors on the map.

You can also determine the “size” of the scale “steps.” Each “step”on map 1E — as you can see on the color scale on its left side — is 2.00 D. So, these maps give broad surface detail; we have to adjust the steps to a smaller step to see finer detail — if so desired — in the cornea curvature and this can be easily adjusted in the device display settings.

At the top right (Figure 1F) is the anterior elevation map. The color scale to its right demonstrates the deviation of each point in microns from the best-fit-sphere of the anterior surface (another demonstration of the normal mid periphery corneal asphericity we mentioned earlier).

The bottom right (Figure 1G) highlights the posterior elevation map, as if we have turned the cornea inside out and we are looking directly onto the endothelial surface relative elevation (concavity). Again, here the color scale to its right demonstrates the deviation of each point in microns from the best-fit-sphere of the posterior surface. Its correlation, symmetry and normality are important for evaluating the normality of the cornea.

Finally, the total cornea pachymetry map (Figure 1H) is, in my opinion, the most important in screening for keratoconus. The color scale to its left demonstrates the thickness of each point in microns. Even in cases with high astigmatism — over 3 diopters — the thickness maps normally display two or three thickness colors: dark blue in the periphery, lighter blue in the mid-periphery and green in the center all changing in concentric circles. Thickness steps in this map are normally round, few (2-3 colors) and centered. Oval-shaped and eccentric thickness changes in this map and in more than 3 steps (more colors, for example, deep blue, light blue, green, yellow and brown) are not centered and, when skewed inferotemporally, usually illustrate abnormal corneas.

There are many algorithms on this — the Belin-Ambrosio is probably the most widely used, contributed by Drs. Michael Belin and Renato Ambrosio. Besides the raw data, they have introduced an algorithm for the gradient of how a cornea becomes “thicker” from its center to its periphery, helping to differentiate normal from suspicious for ectasia.

Each one of these maps has a color scale next to it on the left or right. This helps the reviewer have a quick qualitative impression of each map. Once experienced with what is normal, borderline and abnormal, close review of each map and its specific qualitative measurements of course provides a more in-depth evaluation. An experienced Pentacam clinician can use the qualitative aspect of these maps to assess the normality and the specific progress of a patient within seconds.^18-25

‘SLICING’ THE ANTERIOR SEGMENT

Scheimpflug tomography captures and processes optical “slices” of the cornea and the anterior segment using the physics optical technique of Scheimpflug in honor of the physicist Scheimpflug, who had first described this concept of imaging in the early 1900s.

By reviewing this sample image of the “raw” Scheimpflug capture (Figure 2), the device enables us to validate the light “slicing” the anterior segment. The red curved line represents the anterior corneal curvature, the green curved line is respectively the posterior corneal curvature and the “dark” space under them is the anterior chamber, as it has minimal reflectivity. The yellow line centrally under that is the anterior crystalline lens surface, and the white “curly” line to the side of the yellow line is the iris surface.

 FIGURE 2. Sample image showing “raw” Scheimpflug capture

The device uses many of these “slices” to create and calculate a three-dimensional model of the cornea and the anterior chamber measurements.

A pearl here is that the “vertical” Scheimpflug image confirms whether the capture was taken without any eyelids or lashes within the image, an important point to establish accuracy by the examiner.

From the usual 12-mm diameter of the cornea, the Pentacam maps and data illustrate the important central 9 mm.

Having “clear” from eyelid and eyelashes, vertical Scheimpflug enhances the validity of the measurement. Depending on the clinical setting, the device offers the option to capture 25 or 50 such Scheimpflug images then uses very sophisticated software to calculate through those Scheimpflug images.

The plethora of measurements that we see in the Pentacam data and maps resonate the complexity and accuracy that this device offers within a few seconds required to acquire each test.

A CASE STUDY

Now, let’s test what we learned so far: Does this post-LASIK case (Figure 3) demonstrate ectasia?

 FIGURE 3. Does this post-LASIK case demonstrate ectasia?

1. The sagittal curvature (top left map) demonstrates significant inferior steepening, which suggests ectasia.
2. The anterior elevation-map (top right) corresponds with the curvature map as the cornea is elevated inferior to its center.
3. The posterior elevation map (lower right) surprisingly appears normal.
4. The pachymetry map (lower left) illustrates that the thinnest part of the cornea is slightly superior to the center of the cornea, which does not correspond with the steepest point defined by the dioptric power map (top left).

Since the steepest point of the cornea in this example is not the thinnest and as the posterior elevation maps are nearly normal, the anterior elevation irregularity is contributing to the inferior steepening due to a decentered mixed astigmatic correction, not due to ectasia. OM

REFERENCES

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A. John Kanellopoulos, MD, is the medical director of The Laservision.gr Clinical and Research Eye Institute in Athens, Greece, and serves as a clinical professor of ophthalmology at NYU Grossman School of Medicine. Contact him at ajkmd@mac.com, via phone 917-770-0586 or on Instagram @BrilliantVisionMD.
Dr. Kanellopoulos reports no relevant financial disclosures.