Diagnostics

Using corneal topography

How to make the most out of this indispensable corneal imaging technique.

BY KAREN BLUM, CONTRIBUTING EDITOR

Traditionally used as a tool to measure the shape of the cornea, corneal topography has become indispensable in many ophthalmology practices.

Corneal topography is a noninvasive medical imaging technique that scans the cornea to show any astigmatism or any irregularities on its surface.

“It’s like giving you a topographic map of the cornea — it will tell you the hills and valleys, where it’s steep and where it’s flat, and whether it’s smooth all over,” says Zoe Lazarou, OD, BS, acting chief of optometry at the Baltimore VA Medical Center. “If you’re planning to operate on someone, you don’t want to have any surprises, and if you’re the one getting operated on, you don’t want to have any surprises, either.”

Topography provides a scan of the cornea to show any irregularities on the corneal surface. The topographic map above shows keratoconus, a condition in which the cornea thins and begins to bulge. Image taken with the Nidek OPD-Scan III.IMAGE COURTESY OF MARCO

Topography uses

Topographers are commonly used for a variety of situations. These include diagnosing and monitoring patients with keratoconus, a progressive eye disease in which the cornea becomes more cone-shaped instead of round; helping ophthalmic surgeons plan where to make incisions in patients with astigmatism; studying astigmatism; determining when to remove sutures after surgery; monitoring conditions like pterygium (an abnormal growth of tissue onto the cornea) and their effects; and fitting contact lenses. Also, with recent FDA approvals, certain topographers can assist surgeons with customizing vision correction treatment during LASIK surgery.

While many topographers are standalone instruments, topography may be integrated into instruments that include other diagnostic capabilities. (See the sidebar “Topographer manufacturers,” page 17.)

With the emergence of widespread keratorefractive surgery, corneal topography has become an essential tool for screening patients and to evaluate outcomes, says Stephen Klyce, PhD, an adjunct professor of ophthalmology at Mount Sinai School of Medicine in New York, N.Y. Cataract surgery is rapidly moving into the refractive surgery realm, Dr. Klyce says, “with patient expectations ever more optimistic to achieve functional uncorrected vision.”

Along with this trend, newer premium intraocular lenses (IOLs) aim to enhance vision beyond the limits of regular spherical IOLs, but their use and success “is predicated on a low amount of higher order corneal aberrations,” he says. Several corneal topographers can measure and report these aberrations, helping ophthalmologists determine which patients qualify for premium IOLs.

Ophthalmic Consultants of Boston uses corneal topography for a wide range of purposes, says Lena Entin, BA, COT, ROUB, clinical supervisor of the practice’s testing department. Along with those previously mentioned, Ms. Entin says they monitor induced astigmatism from chalazion, a benign nodule that can grow in the eyelid.

Types of topographers

Today’s topographers fall into three classes, says William Trattler, MD, an ophthalmologist with the Center for Excellence in Eye Care in Miami: Placido disc, Scheimpflug, and color light-emitting diode (LED) reflection. Placido disc machines have a cone of alternating light and dark concentric rings; these are projected onto the cornea, and the image reflected back is recorded and measured. This method helps when evaluating patients before photorefractive keratectomy procedures to correct myopia, hyperopia, and astigmatism, and before cataract surgery, Dr. Trattler says.

“We’ve found that about 25% of our patients scheduled for cataract surgery have abnormal topography. We often evaluate patients with completely normal-appearing corneas on slit lamp who have significant irregularities on topography, such as early keratoconus or irregular corneal astigmatism, so identifying an eye with an abnormal shape ahead of time is very helpful in order to set the proper expectations for surgery.”

Scheimpflug devices can measure more depth and can be used to assess corneal topography, anterior chamber depth, as well as the density of the crystalline lens. These are helpful in managing care for keratoconus patients due to their very steep, irregular shaped corneas, Dr. Trattler says.

LED light devices illuminate the cornea, precisely measuring the axis and magnitude of astigmatism while providing high-resolution images that can be used for cataract surgery planning, Dr. Trattler says. Also, the images can be imported into a microscope used during surgery to help align the placement of toric IOLs, he says.

Getting optimal results

Corneal topographers are very user-friendly machines, Ms. Entin says. “It takes less than a week to teach ophthalmic personnel to perform accurate topography.” Usually optometrists, or ophthalmic technicians who work with surgeons, operate the machines.

Before operating the machine, fully inform patients on what to do, Ms. Entin says. “Patients need to understand how important it is to look straight ahead at the [fixation point] and keep the eyes wide open for a few seconds so the eyelids don’t get in the way of capturing the image.”

Sarah Glass, BS, COA, a technician and ophthalmic equipment coordinator at Surgical Eye Care PA, a group practice in Wilmington, N.C., agrees. Prior to the test, she and her colleagues tell patients, “It makes a map of your cornea, the surface of your eye, showing us any astigmatism or irregularities. Astigmatism simply means that some eyes are more football-shaped while others are baseball-shaped.”

To obtain a good quality exam, have the patient seated comfortably and relaxed, Dr. Lazarou advises. Line up the patient so that his/her pupil is in line with the fixation target. Also, to get the most cornea exposed, ensure that patients with deep-set eyes or a big brow tilt their chin forward or back as needed. And, make sure tear film is spread out because patients with dry eye may have a scan that doesn’t detect some spots. In some patients, using artificial tears before topography may help.

“My tip would be to have them lined up, focused, and then I tell them to blink three to four times to spread out the tears and then open wide, then take the scan,” Dr. Lazarou says. “Take at least two or three scans to make sure it’s repeatable.”

Certain eye conditions require some creativity to get a good image, Ms. Glass says. With strabismus, occluding the fellow eye allows the patient to focus on the central fixation light, she says. With severe dermatochalasis, or excess skin around the eyes, it may be necessary to have a team member roll excess skin away from the eye using a cotton swab or finger, and tape it if necessary.

In a typical day, Dr. Trattler says at least half of the patients in his office have some type of corneal mapping, whether prior to cataract surgery, prior to LASIK, prior to corneal collagen cross-linking for keratoconus, or postoperatively. “The key thing is that this is something we do all day, every day — not just once in a while,” he says. OP

Dr. Klyce has been a consultant to Abbott Medical Optics, Alcon, Nidek, Oculus, Tomey, and Topcon. Dr. Trattler has a financial interest with Oculus, I-Optics, AMO, and B&L.