Keratoconus, the most common corneal ectasia, is a progressive corneal Figure 8. Keratoconus on Pentacam. The anterior axial map shows. To assess the effect of age on elevation and pachymetric Pentacam keratoconus (KC) detection indices, and the need to adjust normative. Dr. Holladay says the changes ac-companying keratoconus are According to Dr. Holladay, the Pentacam, Galilei and Visante Omni all pick.
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Performing laser refractive surgery on an individual who is in the early stages of keratoconus—or any other corneal thinning disorder—is a bad idea; the consequences of missing this disease can be devastating for both the patient and the surgeon.
The problem, as every refractive surgeon knows, is determining whether a patient is headed down that path when the signs of developing disease are still barely detectable. In this situation, technology is a powerful aid. But in many cases, the signs indicating early corneal disease are subtle and ambiguous. Here, three experts share their advice for making the most accurate determination about the presence of a corneal thinning disorder when the evidence produced by this technology is less than conclusive.
Given the importance of technology in this situation, a key issue is determining which technology is most likely to provide the kind of solid evidence of early disease that surgeons need. Topography, long relied on in this situation, models the anterior corneal surface; tomography provides detailed information about corneal structure. Klyce, PhD, adjunct professor of ophthalmology at Mt. At the same time, tomography has proven itself to be extremely valuable.
Tomography not only measures the front surface curvature, but the posterior surface curvature and corneal pachymetry as well. Today, I believe anyone doing refractive surgery needs to have a tomographer for screening. Holladay says the changes ac-companying keratoconus are usually more visible on the posterior surface of the cornea.
Instead of being six to eight layers thick, it ends up being only four or five layers thick. The result is that the elevation on the front of the cornea is not as great as the elevation on the back. That’s why the changes you find on the posterior surface are much more sensitive indicators of change than the changes on the anterior surface.
Holladay, the Pentacam, Galilei and Visante Omni all pick up posterior surface changes quite well. One way they do so is by comparing the posterior corneal surface to a hypothetical toric ellipsoid as a reference surface. These devices are all set up to make this kind of comparison. Apparently, most surgeons today are not relying on topography alone. So I think most of the higher-volume surgeons have realized that there’s a lot more to evaluate than just the anterior surface.
The Power of Pachymetry. Of course, when screening for corneal thinning disorders, measuring corneal thickness is important. The traditional way to evaluate corneal thickness is with an acoustic pachometer, by taking central, superior, inferior, temporal and nasal readings.
Corneal Imaging: An Introduction
Those five readings are usually adequate for finding areas in which the cornea is thinner than usual. Of course, many of the more recent instruments can also measure corneal thickness. You can also use a global pachymetry map to kkeratoconus for the radial distribution of corneal thickness.
Some published data suggest that there’s a correlation between keratoconus and how thickness changes from the center to the edge.
Pentacam top indices for diagnosing subclinical and definite keratoconus
In terms of technology, devices using Scheimpflug imaging may be advantageous. Klyce warns, however, that the technology isn’t flawless. Because of the way the topographic data is derived, jeratoconus placido device can be as much as 20 times more sensitive to topographic contours than a tomographer.
Despite the greater topographic accuracy of placido technology, Dr. Klyce notes that in some situations tomography is more advantageous for measuring the topographic component.
That’s because on very distorted corneas, placido devices have keratocconus of data dropout. The rings, or mires, tend to get very close together and finally merge when a cornea is steep or distorted.
Ideally, a pentxcam would use a placido device to get very sensitive anterior surface measurements, and a tomographer, such as a scanning Scheimpflug device, for analyzing more distorted corneas.
Choosing the Right Color Scale.
When interpreting a scan accurately is crucial, the way the data is represented onscreen can make a huge difference. Klyce and his team developed the color-coded contour map that’s now the international standard for displaying corneal topography.
It becomes very confounding for the clinician. In the end, many clinicians pick the default scale or play around with the options, trying to make sense of it.
Furthermore, many topographers are shipped keratocouns a so-called ‘adaptable’ scale, in which every cornea is presented with its own color scale. This complicates matters further. He notes that the international standard scale he helped develop is available in most topographers today. The Relative Pachymetry Map. As any weather buff knows, there are many ways to integrate and present available information, and the way you choose to analyze and present it can make all the difference in how useful it is at the practical level.
In that spirit, Dr. Holladay says he’s helped to develop a new map that displays key information in a novel way, called the relative pachymetry map. So, we changed the scale to reflect what percent of that normal thickness is found at each point. A completely normal map would be all green. If the measured thickness is 10 percent thinner than normal at a given point, a warmer color at that point indicates percent. So the map shows whether—and by how much—the corneal thickness deviates from normal.
Any deviation greater than 3 percent is significant, and anything over 6 percent is absolutely too thin. Taking this idea further, Dr.
Xnd has helped to develop new onscreen reports for the Pentacam and the Visante Omni that incorporate this information.
The middle column shows the pachymetry map on top and the relative pachymetry map on the bottom. On the right, the elevation of the front surface relative to the reference surface is on top, and the posterior surface relative to its reference surface is at the bottom.
Then you look at the relative pachymetry map and say, aha, that same spot is a little thinner than it should be. Then you look ppentacam the posterior float map that compares the elevation to the toric ellipsoid. The following strategies may help you make the tough calls in day-to-day practice: That’s where the greatest refractive index change occurs, and that’s the surface from which the placido disc is reflected. Many times you’ll see a topography scan that looks a little irregular, and you’ll notice that amd pupil is 8 mm wide.
In that situation it’s pretty clear that the technicians started the dilating drops before they did corneal topography. However, on tomography you may note some against-the-rule astigmatism and inferior steepening that’s very mild, slight changes in the relative pachymetry map and posterior elevation map—suspicious, but not diagnostic. What should you do? If keratoconhs patient has early keratoconus, you’ll see changes in the astigmatism correction in his glasses that reveal increasing asymmetrical against-the-rule astigmatism.
In contrast, most people who have high degrees of astigmatism but no keratoconus have close to the same amount of astigmatism in each eye over time, and it’s symmetrical. It can qnd like an asymmetric bow tie, for example. A symmetric psntacam tie is normal, regular corneal astigmatism.
Pentacam Analysis of Keratoconus Indices in Normal Patients | IOVS | ARVO Journals
Keratoconus also can show up as a truncated bow tie—a very short, steep bow tie in the center of the cornea. It can also resemble a moderate amount of corneal astigmatism, but with skewing of the radial axis, so the bow tie looks a little bit weepy or saggy.
See sample map, p. If it’s elevated, look for other signs of keratoconus. Also, check for differences in central corneal thickness between the two eyes. The two eyes should be symmetrical in both corneal topography and pachymetry. If they’re not, that’s generally a sign of abnormality and a risk factor for developing complications after refractive surgery.
Khachikian and I published a study two years ago evaluating pachymetric asymmetry between fellow eyes,” he says. For example, if someone has an intraocular pressure of 9 mmHg in both eyes, we’d think little of it.
If the pressure is 18 in both eyes, we probably wouldn’t think twice about it, as long as everything else was normal. But if the pressure is 9 in one eye and 18 in the other, that’s a red flag. If that’s the case, you can discard that exam, or at least be aware that the abnormality is an artifact and not use it in your evaluation. By the time we get to age 60 or 70, it’s more like plastic. So if a year-old shows signs of keratoconus, his risk is much greater than that of a year-old with the same signs.
Holladay says he gets three to 10 e-mails a day from doctors asking for help interpreting scans.
It turned out that the cleaning crew had bumped the cone on the topographer and dislodged it a little one night. If it changes markedly, then something is wrong and the unit needs to be looked at. Or, if scans all begin to seem suspicious, you can use the calibration surface that comes with the instrument.
Managing Contact Lens Patients. Because wearing contact lenses is well-known as a source of corneal distortion, having the patient stop wearing the lenses before making final measurements for surgery is crucial.
How long should the surgeon wait? Many surgeons, if it’s a spherical soft lens, would wait only a few days. But if the lenses are rigid, the patient will need to be out longer. If a map shows distortion or abnormalities, keep the pa-tient out of the lenses and repeat the map until you get a stable reading.
Journal of Ophthalmology
If the corneal shape is abnormal—for example, if we find inferior steepening that mimics keratoconus but is actually contact lens warpage—we ask patients to keep their lenses out and come back in two to three weeks, until the topography normalizes.
These strategies may help: How can we tell if warpage is caused by the contact lens? The only sure way is to discontinue lens wear for a while and see what the cornea does. However, I’ve never seen a case of keratoconus in which the posterior surface changes didn’t exceed those on the anterior surface. With contact lenses, you commonly see the changes on the anterior surface.
Finding the latter is one indicator that this may be contact lens warpage.