For decades, ophthalmologists used 2-D, Placido-based topography systems for surface evaluation of the cornea. Tomography, however, which uses optical cross-sectioning to generate 3-D reconstructions of the anterior segment, has greater potential for the diagnosis, prognosis, and treatment planning of ectatic diseases, said Michael W. Belin, MD, at the 2011 annual meeting of the American Academy of Ophthalmology (AAO).
In a nutshell, 3-D Scheimpflug tomography provides information on both the anterior and posterior surfaces, unlike Placido-based systems, which describe the anterior surface only. With information on both anterior and posterior elevation, tomography can also generate a full pachymetric map.
Scheimpflug tomography provides greater coverage of the cornea compared with Placido-based imaging, which is limited to about 60% of the cornea for two reasons. One limitation of Placido systems is strictly anatomic: It is impossible to design a cone that will cover the entire cornea because of facial features such as the nose and the brow. The second limitation is geometric and optical: Light cannot bounce off the peripheral cornea and come back to a central camera. Scheimpflug imaging is not reflective and therefore capable of much greater corneal coverage, at times extending out to the limbus, Dr. Belin said.
Compared with a Placido-based system, the advantages of 3-D tomography include information about the posterior cornea, the ability to generate full pachymetric maps, and access to greater coverage of both anterior and posterior surfaces. As Dr. Belin explained, these capabilities aid in the diagnosis of keratoconus, pellucid marginal degeneration (PMD), and post-LASIK ectasia.
“Effective screening requires that you look not only at the anterior surface, but that you look at both pachymetric distribution and the posterior surface,” Dr. Belin said. Specifically, to diagnose early keratoconus, the practitioner must look for abnormalities on the posterior surface, which are positive islands of elevation; therefore, one of the best screening indices is the absolute elevation value at the thinnest point on the cornea, he explained.
To demonstrate the effectiveness of 3-D tomography in the diagnosis of keratoconus, Dr. Belin presented a case example of a patient with unilateral keratoconus. The patient was examined using topography with automatic detection. Anterior surface mapping showed one of the patient’s eyes to be highly abnormal. However, the patient’s fellow eye appeared normal in terms of anterior curvature, elevation, and curvature indices.
The patient was then evaluated using 3-D tomography. On the Belin/Ambrosio display, nearly every parameter for the abnormal eye was outside the normal limits, which was approximately 12 standard deviations from the norm, Dr. Belin said. Changes on the anterior surface, the posterior surface, and pachymetric distribution were present. However, this was unsurprising, as the eye was already known to be abnormal. Full tomographic evaluation of the patient’s socalled normal eye, however, revealed a positive island off the best-fit sphere on the posterior surface.
“If you only looked at anterior curvature, you would view this eye as normal,” Dr. Belin said. “Most of you would say, ‘That’s interesting, but I would not have touched that patient because the other eye was so abnormal.’ That may be true, but what would happen if both eyes looked like the [so-called] normal eye, and all you had was curvature?”
Furthermore, some patients with keratoconus retain good visual acuity. Historically, keratconus was diagnosed when the patient came to the ophthalmologist’s office complaining of decreased vision. By the time these patients were examined, they already had changes on the anterior surface. With refractive surgery, so-called normal patients are being examined with advanced technology that previously was reserved for patients with known disease, revealing a number of individuals with subclinical or asymptomatic keratoconus. Subclinical keratoconus is not suspect, Dr. Belin said; it is a true disease. These corneas are abnormal based on posterior elevation and pachymetric progression; however, they have normal anterior curvature, so the patients retain good vision. Structurally, the posterior surface may be a more important indicator of pathology than the anterior surface. As a refracting surface, however, the posterior surface contributes little to the overall refractive power because it is a cornea-aqueous interface, with little change in index of refraction, in comparison with the air-cornea interface at the anterior surface.
Scheimpflug-based elevation data provides greater coverage of the cornea, which is advantageous in the diagnosis of PMD, Dr. Belin said. The classic description of PMD is a band of inferior thinning 1 to 2 mm from the limbus. This area is where the pathology of PMD is; however, it is not analyzed by Placido-based systems, which are limited to covering 60% of the cornea. The best way to truly diagnose PMD is to look at a complete limbal-to-limbal corneal thickness map, he said.
The ability of 3-D tomography to image the posterior corneal surface also accounts for its advantage over Placido-based analysis for diagnosing post-LASIK ectasia. Early diagnosis is now even more crucial with the ability to perform corneal collagen crosslinking (CXL) to prevent disease progression, Dr. Belin said. Ideally, post- LASIK ectasia would be detected before the patient has major changes on the anterior surface. To look for subtle changes associated with post-LASIK ectasia, one cannot look at the anterior surface because it has been altered. Pachymetric map indices cannot be used either, as they rely on algorithms based on a normal population, and post-LASIK patients have abnormal pachymetric progression. The normal postoperative posterior surface, however, should remain unaltered.
The best way to evaluate a patient for post-LASIK ectasia is with a subtraction map of the posterior corneal surface, which is created by taking the post-LASIK posterior elevation and subtracting the preoperative posterior elevation. If the difference represents a significant change, it is an indication of post-LASIK ectasia, Dr. Belin said. He suggested that Pentacam (Oculus Optikgeräte GmbH) users generate a follow-up map to evaluate the posterior surface of all post-LASIK patients. If the patient’s changes are significant, he or she may be a good candidate for CXL, as the surgeon would want to stabilize the patient before he or she becomes symptomatic.
Overall, 3-D tomography enables a more detailed characterization of the cornea, making it an essential tool in evaluating patients with keratoconus, PMD, and post- LASIK ectasia, Dr. Belin concluded.