| Home Features Departments News & Trends  |
Topography Mapping The CorneaCorneal topography has reached a level of sophistication that makes it indispensable to the anterior segment practice.Leslie
Sabbagh, Current Technologies Four different methods capture corneal topography: placido disc (video keratoscopy); fluorescein profilometry (stereo photogrammetry); scanning slit technology; and interference techniques. The first three are in commercial use, and the last, which uses phase-modulated laser holography, never entered clinical use because interferometry is “too accurate,” says Stephen D. Klyce, PhD. “The range of corneal power or curvature that it measures is exceedingly accurate, but only for extremely small ranges.” Most corneal topography devices use placido disc technology, considered the gold standard. It is available in two styles: a cone projection type (e.g., Tomey) and a large face plate type (e.g., Orbscan). The cone type has an advantage over the face-plate type because it fits into the orbit better and can measure corneal topography out to the limbus. Due to its construction, the larger face-plate system has a smaller coverage area. Although placido disc technology is “very accurate, the quality of its resolution varies, as does ease of use,” says Dr. Klyce, of the LSU Health Sciences Center and Tulane University. To date, 10 companies make the placido type: Alcon Labs, Alliance Medical, Dicon, EyeTec, Humphrey Instruments, Medmount, Oculus, Technomed Tech, Tomey and Topcon.  Two companies use fluorescein profilometry: Par Vision Systems and Euclid System Corp. Fluorescein is instilled in the tear film; then standard elevation techniques determine the topography. The machines take two simultaneous pictures at different angles, then use the equation for stereography to calculate exactly the surface elevation, says Dr. Klyce. Clinicians must apply quality control to ensure accurate representation of corneal topography. Here, inspection of the mire tracking (right panel) shows that the apparent central irregular astigmatism in the color map (left panel) is due to a mistracking artifact. This was probably due to eye movement during the exam. “The advantages are that you can measure out onto the sclera, and you can measure very irregular corneas,” he says. “That makes the system excellent for following patients after penetrating keratoplasties.” The disadvantage, he adds, is that it is not as sensitive in measuring small distortions. Nor is it non-invasive. “You must instill a drop of fluorescein into the tear film, and that can alter tear volume and chemistry, which can lead to tear film breakup and artifactual irregular astigmatism.” He believes fluorescein profilometry has a role in measuring very irregular corneas, “when you need to measure the relationship between cornea and the pericorneal sclera if, for example, you’re making scleral contact lenses.” Scanning Slit For now, only one company, Bausch & Lomb Surgical, parent to Orbtek, the developers of the technology, has marketed scanning slit corneal topography. Its device, the Orbscan, performs 140 slit scans of the cornea during its capture of the data.  The advantage is that the slit scans can be “pasted” together to measure the anterior and posterior corneal surface curvatures. Although it’s much smaller than the anterior surface’s average of 48 D of power, the posterior surface does have some refractive power, from -4 to -5 D, Dr. Klyce notes. A measure of the posterior surface is valuable to help determine posterior keratoconus. Corneal topographers may use artificial intelligence to detect keratoconus. Here, two independent methods interpret the topography of an eye with inferior steepening. Trained to report only clinical keratoconus, the Klyce/Maeda method remains silent. The more sophisticated Smolek/Klyce method—trained to recognize the characteristics of a keratoconus suspect’s corneal topography in addition to clinical keratoconus—reports keratoconus suspect for this cornea. The clinician must use differential diagnosis to determine whether this inferior steepening is due to pseudo-keratoconus from contact lens molding. The first model, the Orbscan I, uses scanning slit technology to measure not only corneal pachymetry, but also corneal surface topography. But “it wasn’t adequate for anterior surface topography. In response, the company developed the Orbscan II which has the same scanning slit system and the addition of placido disc keratoscopy to measure anterior surface curvature,” he says. Do You Need One? The decision to invest in a topography system is driven by your sub-specialty. If you’re a clinician who works with the anterior segment, then it’s hard to justify not purchasing a unit, especially with prices as low as $6,000 to $7,000. “ Any anterior segment clinician should have a topography unit, whether you fit contact lenses, perform cataract surgery or transplant corneas,” Dr. Klyce says. Maureen Lundergan, MD, of the University of Utah Medical School, uses the Tomey and Humphrey placido disc systems and the Orbscan scanning slit unit. The Tomey, she says, was chosen for its “excellent clinical track record”; the Humphrey, “so we could get experience with their elevation.” But she relies almost entirely on the Orbscan. She feels it provides all of the information needed to make a good decision about patients’ candidacy for refractive surgery and solves problems in all areas of anterior segment clinical and surgical practice. “I believe that the information about the shape of the posterior cornea, along with data about the shape of the anterior cornea and overall pachymetry, are very important,” says Dr. Lundergan. “The Orbscan II allows us to pick up cases of posterior keratoconus and other subtle corneal irregularities that may preclude a good refractive outcome. It can also help us to better understand some of the visual aberrations that our patients experience.” In addition to screening refractive surgery patients, the devices are becoming the standard of care, from fitting contact lenses in difficult cases to diagnosing corneal pathology to assessing central corneal power for intraocular lens calculations. In refractive surgery, for example, the scanning slit system, if accurate, is important because it measures corneal thickness. Pachymetry has always been a critical point for refractive surgeons in avoiding inadvertent penetration into the anterior chamber. Now, with LASIK, they need to leave enough tissue in the stromal bed to avoid ectasia and other problems.
Dr. Lundergan believes that the corneal topographer is a major diagnostic tool in determining patients’ candidacy for surgery. With it, she can not only evaluate the overall shape of the cornea, but tell at a glance whether there is enough corneal tissue to perform the correction required. The Orbscan II also gives information on pupil size and the white-to-white diameter, which, she says, helps in selecting the ring size of the keratome. It also locates the visual axis relative to the pupil, which is very important in centering laser treatment. Avoiding Artifacts Computer programs are created to be very smart, but they can’t recognize, and account for, every problem. Take, for example, patients with irregular tear films or dry-eye conditions. Corneal topography should be the first exam performed, before giving dilating drops and taking intraocular pressures, “because you don’t want to disturb the tear film,” says Dr. Klyce. Tear film breakup not only diminishes visual acuity, it also causes mistracking of the mires. When mires are mistracked by the computer program, artifacts in the topography occur that look like significant irregularities, and sometimes are suggestive of corneal pathology, such as keratoconus. Dr. Lundergan agrees: “This is primarily an issue with placido topography and is important to consider. It does not seem to be important in the elevation data acquired by the slit scan.” Another key point, Dr. Klyce notes, is that clinicians should “use a fixed, standard scale for the color map, one in which the colors always represent the same power. Using an adaptive scale—such as that generally used to demonstrate corneal topographers—can amplify clinically insignificant detail, and cause confusion in making a correct interpretation. When we developed the original color map, we used colors and power intervals that would hide clinically insignificant topographic details, yet emphasize topographic features that represented abnormalities.” Dr. Lundergan feels there is less likelihood for clinical errors if a standard scale is used and adds that the operator can use the changes in color scale and customize it for a particular patient’s eye or clinical problem. Accuracy Not all corneal topographers are created equal, even among the same class of machine. Some placido disc devices, for example, have thick mires and do not take closely spaced readings in a radial fashion along the semi-meridian. “If the points taken along cornea surface are spaced widely apart, that machine will have less sensitivity and resolution than one in which the distance between the data points is less,” Dr. Klyce says. In addition, there is the issue of validation: No universal standards for corneal topography units exist yet. With no single standard, clinicians must look to the peer-reviewed literature for validation of each device. Dr. Lundergan notes that clinically small differences, such as simulated K-readings from one device to another, are less important in decision making than is generally considered. It is at least as important to have a variety of corroborating data, allowing the clinician to make a good clinical judgment on any individual case, she contends. “An example is tear-film abnormality masquerading as keratoconus; this might be more easily interpreted if we also have elevation maps of the anterior and posterior surfaces combined with the pattern of corneal thickness changes. All of this information together makes it much clearer whether the patient truly has keratoconus,” she says. Contact Lens Concerns Discontinuation of contact lens wear at least two weeks before the first topography is critical to achieve accurate readings. Often contacts can often produce corneal molding, Dr. Klyce warns, and it is advisable to have contact lens wearers stay out of their lenses for an additional two to three weeks for a repeat topography. “Generally, if topography looks normal and the change in refraction is less than .25 D between visits, you can assume the cornea is stable,” he says. The caution here is that clinicians need to individualize how they approach refractive surgery candidates. “  Topography is just one piece
of information that we use to determine if a patient is ready for surgery. Even
then, many surgeons would agree that enhancement rates are higher for contact
lens wearers than for spectacle wearers,” Dr. Lundergan says. She has
rigid gas permeable wearers stop wearing their lenses at least two weeks before
the initial visit, and all patients must return for one or two more intervals
until they are relatively stable. A normal cornea with a slight amount of asymmetry (indicated by SAI) is shown in the left panel using the fixed standard “Klyce/Wilson“ scale. Using a self-adapting scale that is built into most corneal topographers, makes this corneal topography look abnormally steep (right panel). Correct scale choice is essential to avoiding misinterpretation in corneal topography. Patients with naturally occurring corneal astigmatism may present with inferior steepening, called pseudokeratoconus, if the condition is induced by contact lens wear. If the condition is true keratoconus, the asymmetry often becomes more pronounced two to three weeks after the first exam. But, if the inferior steepening will be less pronounced in that time, it is probably due to corneal molding. No matter what the situation, though, ophthalmologists should wait for corneal stability before suggesting a permanent refractive modality. In a study of symptomatic contact lens wearers, Dr. Klyce and associates found that patients wearing PMMA and rigid gas permeable lenses took as long as six months to stabilize after lens use was discontinued. Even soft contact lens wearers had slow resolution, he warns. Future Developments Corneal topographers are fairly sophisticated and reliable devices routinely used in the clinic. Several manufacturers are developing wavefront sensors—devices that can measure the total aberrations of the eye. Questions about this new technology, such as accuracy, spatial resolution and the effect of accommodative state and luminance, remain. Wavefront sensing holds great promise for the refractive surgical correction of not just sphere and cylinder, but the higher order aberrations such as spherical aberration and coma as well, Dr. Klyce notes. A topography/wavefront device, coupled to and driving the strategy for tissue removal by a small-beam scanning excimer laser with the aim of producing aberration-free vision, is an exciting research avenue, he says. Neither Dr. Klyce nor Dr. Lundergan has any financial interest in the products mentioned. |
|