Expanding OCT to the Anterior Segment

Refractive surgery and glaucoma patients can benefit from new OCT applications.

BY RENÉ LUTHE, SENIOR ASSOCIATE EDITOR

Optical coherence tomography (OCT) was originally developed for posterior imaging and was launched in the U.S. market in 2002. Recently, investigators and industry have developed applications for the anterior segment as well. According to clinicians experienced with these new uses, OCT delivers significant benefits for refractive surgery, as well as the treatment of cataract and glaucoma. Clinicians experienced with the technology and its more recent applications explain here how OCT for the anterior segment can boost surgical success.

Improving Refractive Surgery Results

OCT's advantages for anterior segment surgery are not yet fully appreciated by many surgeons, according to David S. Rootman, M.D., of the Toronto Eye Surgery Centre and associate professor at the University of Toronto. For one thing, he notes, OCT machines are not yet that prevalent in those practices. "I think it is also sometimes difficult to convince anterior segment surgeons that there is value in it. When we examine the cornea at the slit lamp, we're essentially using a dynamic form of tomography and most of the time the structures we're looking at are transparent or slightly opaque or translucent," he says. "So, for diagnosis, there are few occasions in which OCT is absolutely critical."

However, an area where anterior segment OCT is "absolutely critical," the experts interviewed for this article agree, is in refractive surgery procedures. Here, surgeons have expanded the use of OCT to improve screening, management and follow-up.

Roxana Ursea, M.D., assistant professor of ophthalmology and vision science at the University of Arizona College of Medicine in Tucson, says that she uses OCT frequently for pre- and post-refractive patients. "The entire LASIK flap can be fully visualized," she says. In her case, she uses the Visante OCT (Carl Zeiss Meditec, Dublin, Calif.). "The beauty of the machine is that it gives you very accurate measurements of the anterior chamber width, the anterior chamber depth, corneal thickness and curvature, the crystalline lens vault — and I'm really excited to use it in refractive patients." Because it is non-contact, OCT also eliminates patient discomfort and prevents the possibility of anterior chamber distortion, as well as provides a faster scan, she notes.

Another advantage to OCT for refractive patients, Dr. Ursea says, is that it features a color mode, which provides better identification of the flap. However, she adds, the colors included in the feature do not correlate with the anatomical structures.

"I think the most important [application] is after refractive surgery, when you're trying to determine if there's enough residual tissue to do an enhancement," Dr. Rootman says. "OCT is really one of the best ways to get an accurate measurement of the flap thickness and the residual stromal bed. It's been helpful with patients that come to you from other places and you have no records and no documentation of what was done before."

Dr. Ursea notes that anterior segment OCT also enables surgeons to visualize placement of intracorneal rings and objectively measure the depth at which they are located.

Additionally, OCT's ease of use means that technicians can quickly learn to perform scans.

Combination Software on the Horizon

What current OCT, and other imaging technologies, have not been able to provide refractive surgeons, however, is an improved ability to screen for keratoconus before surgery. A new combination Visante OCT and Atlas Topographer could soon rectify this current limitation, though. Jack Holladay, M.D., clinical professor of ophthalmology at Baylor College of Medicine and in practice at the Holladay LASIK Institute in Houston, reports having worked with Carl Zeiss Meditec for the last several months on the integration of the company's Atlas Topographer with the Visante OCT. The result, he says, is a "comprehensive unit that provides significantly information about the anterior segment." Visante Software Version 3 integrates anterior corneal topography from the Atlas models 993, 995 and 9000 with enhanced pachymetry from the Visante.

"Integrating these two instruments can provide you with several new maps of the anterior segment that can measure not only the front surface with topography, but — by taking the thickness data that you get from the Visante — they can subtract it from the height data generated by the Atlas and come up with posterior floats, which is the most sensitive way of picking things up like keratoconus," Dr. Holladay says (Figures 1 and 2). Visante Software Version 3 is scheduled to be launched in the first quarter of 2009.

Figure 1. Normal corneal with-the-rule astigmatism. Note that "Simulated Keratometry" and values over the "4.5 mm Zone" are almost identical. The 3.2 mm keratometry ring and the 4.5 mm zone are not different in "normals" because there is very little variation in power over the 4.5 mm zone. In the 1st column, note the Tangential Map's much greater detail vs. the Axial Power Map in showing anterior corneal geometry. The Relative Pachymetry Map shows that the entire cornea is uniformly thinner than normal (~8 to 10%) corresponding to the minimum central pachymetry of 481 microns. The Anterior and Posterior Elevations relative to a Toric Ellipsoid are normal with no areas within the 8 mm zone above 5 microns.

Figure 2. Keratoconus suspect. The "Simulated Keratometry" and values over the "4.5 mm Zone" are different. The 3.2 mm keratometry ring and the 4.5 mm zone are much different in "abnormal" corneas because there is a significant variation in power over the 4.5 mm zone. The Tangential Map is much more detailed than the Axial Power Map in showing anterior corneal geometry. The actual center of the ‘nipple’ is seen clearly on the Tangential Map. The Relative Pachymetry Map shows that the entire cornea is uniformly thinner than normal (~10%) corresponding to the minimum central pachymetry of 441 microns. Notice, however, that the area of the ‘nipple’ is even thinner. The Anterior and Posterior Elevations relative to a Toric Ellipsoid are abnormal with the areas of elevations at the same hot spot as on the Tangential Map and Relative Pachymetry Map. Elevations above 10 microns using the Toric Ellipsoid in the 8 mm zone are abnormal.

Medico-Legal Considerations

Given its increased sensitivity for detecting forme fruste keratoconus, this new combination unit will help surgeons avoid potential legal trouble by identifying patients who are not appropriate for refractive surgery, Dr. Holladay says.

"If you don't detect that person with keratoconus, every one of those cases would pay for the unit three or four times over," he explains. "For those people with keratoconus, there's no excuse, legally, for missing it — the technology was there at the time, and now the patient is blind. I think almost every physician would think that the unit was a good investment if it's protecting you from something that's potentially catastrophic, such as a patient who develops post-LASIK ectasia. Return on investment would occur pretty quickly for major refractive surgery centers."

Dr. Rootman agrees about OCT's utility in avoiding medico-legal issues. "You want to ensure that there's sufficient tissue left if you're doing an enhancement," he says, "and if the patient were then to develop ectasia, you could show that you were within a reasonable limit that is acceptable to most refractive surgeons," he says.

Other Corneal Applications

Clinicians also rely on OCT imaging of the anterior segment for effective management of other corneal procedures. Dr. Ursea reports that she also uses OCT to image the anterior segment anatomy and pathological conditions present.

"I find it beneficial in cases of trauma that need to go to the operating room, or cases with a very small pupil in which you cannot really see behind it, you can prevent surprises in the operating room just by taking a look with the Visante and seeing what exactly the anatomy is to avoid possible complications," she says.

Dr. Rootman finds OCT helpful for situations involving corneal ulcers, which obscure details beneath the cornea. "The OCT may be able to give you some information as to what's going on behind an opaque cornea, because the energy penetrates better than visible light energy," he says.

The only limitation Dr. Ursea finds is that of imaging behind the iris plane. "Visualization of the iris and ciliary body lesions or tumors is not possible due to the poor penetration," she explains. "The barrier is the iris pigment epithelium, which prevents imaging of the zonules and ciliary processes."

Dr. Rootman reports that OCT imaging of the anterior segment has many research applications, as well. "We've used it to understand what is happening in the corneal transplant wound that isn't readily apparent when you look at the patient with a slit lamp. We've been able to look at the back surface of a cornea, look at the posterior wound and to document mismatches and gapes and things like that, and this can help us understand better the effect on corneal astigmatism," he explains (Figures 3, 4). "We have also been able to look at different wound configurations such as top-hat, mushroom and zigzag configuration. You can really see what is going on in terms of alignment of the wound and how well it's healing."

Figure 3. A transplant wound with an internal mismatch on the left side, typical in keratoconus, where the peripheral cornea is often quite thin.

Figure 4. An internal step on the right side of the PKP wound, and a peripheral anterior synechia on the left side.

Complicated Cataracts

Philip Paden, M.D., of Paden Eye Care Center in Medford, Ore., believes that OCT is significantly helpful in his high-volume diabetic and macular degeneration cataract practice. For these cases, he uses OCT for its original purpose — checking on the state of the posterior segment. He uses Heidelberg's Spectralis OCT (Vista, Calif.). The most important issue in removing diabetic cataracts, Dr. Paden says, is to make certain that there is not even a threat of macular edema in the eye. OCT's ability to detect fluid within the inner retinal layers is essential.

"There's a long history of diabetics having cataract surgery done expertly, the IOL looks beautiful, and they see worse," explains Dr. Paden. "That's because they develop worsening macular edema as the cataract surgery heals; the front of the eye is fine and the back of the eye can't see the detail that it could have before the surgery."

OCT is also particularly helpful with what he terms a new syndrome. "It is fluid accumulations between the photoreceptors and the RPE," Dr. Paden says. "So it's not classic cystoid macular edema, but it is macular edema underneath the photoreceptor of the outer retinal layers. And this shows up on high-resolution OCT better than anything else."

Helping the Glaucoma Practice

OCT enables the glaucoma specialist to perform precise angle evaluation. "You can actually measure the degree of the angle before and after a laser procedure," says Dr. Ursea.

Additionally, recent surgical devices such as canaloplasty and suprachoroidal stents and shunts elevate the importance of preoperative anatomical assessments, according to Ike Ahmed, M.D. of the University of Toronto. "What OCT allows us to do is to have very quick, high-resolution and convenient imaging in the space that we're working in, which is otherwise a pretty difficult area to visualize," he says. "So it's given us the ability not only to image but also determine the appropriate indication per patient, as well as the ability to assess location of implantation. Postoperatively, it allows us to assess the effectiveness of the procedure as far as position and what's happening to the canal."

Dr. Ahmed sees OCT as a great improvement over gonioscopy, which is not as well standardized, among other limitations. "Unfortunately, gonioscopy is not done by a number of practitioners, it's not done well, or the patients don't have access to those who can do it well, especially around the world. OCT allows us to have a very nice, objective manner of seeing the angle," Dr. Ahmed says.

And while he cautions that researchers still have a lot to discover about validating the findings of OCT in regard to glaucoma, he says that the technology has also made a significant impact in differentiating various forms of angle-closure mechanisms. In patients whom he had thought to have open-angle glaucoma, he says that OCT has revealed them to have more of a narrow angle or to have angle-closure mechanism.

"That's quite revealing because it can change the management," Dr. Ahmed explains. "For example, it can encourage us to remove a lens earlier, like a cataract or even a clear lens of somebody who has elevated pressures of progressive glaucoma in order to open up the angle, where the benefit of gonioscopy may be more questionable. We've now developed diagnostic guidelines in terms of imaging the angle and elucidation of the mechanism. Any practitioner needs to first determine if this is an angle-closure mechanism when they look at someone with glaucoma."

While it is difficult to assess this clinically, he says that imaging helps the clinician reach more confident conclusions in this matter. "It's not going to replace the clinical examination of the optic nerve, but it does help us to have a quantifiable baseline to follow as well."

Patient Education Benefits

Both Dr. Paden and Dr. Ahmed view OCT imaging as a powerful tool in improving patient compliance in the treatment of ocular disease such as glaucoma and macular degeneration. "It's been extremely good in getting the patient and the patient's family on board, to show them that this is not smoke and mirrors," Dr. Paden says. "OCT enables you to say ‘Here's the problem, and here's the improvement that you can see.’ That part of it is actually fun, to see them light up."

"When we advocate a certain procedure, for example, laser iridotomy, patients sometimes don't quite understand why they need a laser," Dr. Ahmed says. "It helps if you can show them the results and say, ‘Look at your angle, and the spacing in your outflow canal. Here is somebody who would be considered to be open-angle, and look at the difference.’"

Though OCT's expansion into anterior segment applications is still new, these clinicians expressed confidence that it would win over ophthalmologists. Dr. Ursea said that while the benefits are not yet fully appreciated, she noticed increased interest during the recent AAO conference. Expect more awareness as it moves into the mainstream. OM