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Up Front | Jun 2003

Refractive Challenge

A 60-year-old white female was referred to me for a LASIK consultation with a general complaint of mildly decreased vision in both eyes and glare and ghosting in her right eye that were most apparent in dim illumination. The patient had undergone PRK on her right eye in January 1994 and on her left eye in May 1994. Preoperatively, her degree of myopia was approximately -5.00 D with no astigmatism, and both eyes had a BCVA of 20/20. The PRK procedure itself and the immediate postoperative course reportedly were routine and uncomplicated.

Upon her presentation to my office, the patient's right eye had a UCVA of 20/50, which improved to 20/40 with a refraction of +0.50 -1.00 X 180. The quality of this vision was poor and not sharp. By contrast, her left eye had a UCVA of 20/30 that improved to a clear 20/20 with a refraction of +1.50 -1.50 X 155. Central pachymetry as measured by the Orbscan topographer (Bausch & Lomb Surgical, San Dimas, CA) and confirmed ultrasonically was 544 µm OD and 550 µm OS. When measured with a Colvard pupillometer (OASIS Medical, Inc., Glendora, CA), her maximum pupil size in dim illumination was 5.5 mm OU, and neither cornea showed any evidence of haze. The remainder of the examination of each of her eyes was normal.

Axial views from a Humphrey ATLAS Topography System (Carl Zeiss Meditec Inc., Dublin, CA) showed a grossly decentered ablation in her right eye and a well-centered ablation in her left eye (Figure 1).
1. Would you re-treat the right eye with either PRK or LASIK and use elevation-based software designed to address decentrations?
2. Perform PRK or LASIK on her right eye and use a wavefront-guided technique?1
3. Perform a transepithelial phototherapeutic keratectomy followed by an eccentric PRK on the right eye?2
4. Consider placing an arcuate incision 180º across the axis of decentration in her right eye?3

After much discussion with the patient, and in large part due to her excellent pachymetry measurements, I decided to re-treat her right eye using the VISX Custom Contoured Ablation Pattern (Custom-CAP; VISX, Inc., Santa Clara, CA) and LASIK. I chose to re-treat her left eye by simply addressing the residual refractive error with a LASIK procedure.

When evaluating the elevation maps of the patient's right eye, I determined that the maximum difference from high to low across a 6.5-mm central corneal diameter was 31 µm. Using the VisionPro software of the ATLAS Topographer, I formulated a treatment plan to compensate for these differences in elevation in order to create the net effect of centering the overall ablation profile. Multiple attempts to create a smooth corneal contour resulted in a treatment table consisting of four myopic spheres that ranged in size from 2.3 to 3.4 mm and in depth from 7 to 19 µm. Once I had selected the appropriate treatment shapes, the software simulated the intended ablation, and I generated a postoperative elevation map in order to verify the resulting corneal contour (Figure 2).

After marking the limbus of the patient's right eye at the 3-o'clock and 9-o'clock positions at the slit lamp, my staff brought the patient into the laser suite and positioned her. I used the 180-µm head and 9.5-mm ring of a Hansatome microkeratome (Bausch & Lomb Surgical) to create a successful keratectomy for each of her eyes. Next, I engaged the three-dimensional tracking system of the Star S3 (VISX, Inc.), which delivered a total of 210 pulses at 6.0 Hz to the patient's right eye. I then treated her left eye at 10 Hz for its residual refractive error.

On postoperative day 1, the patient's UCVA was 20/20- OD, and she showed an improvement on axial topography of the ablation zone centration (Figure 3). At her 1-week follow-up visit, the UCVA in her right eye was 20/25, and she reported a significant reduction in her preoperative symptoms of glare and ghosting. A refraction of +0.75 -0.75 X 100 improved the visual acuity of this eye to 20/20 and sharpened the quality of her vision. I anticipate addressing the residual refractive error in the patient's right eye at approximately the 3-month point depending upon its refractive stability. Her left eye had a UCVA of 20/20 on all follow-up visits after undergoing an uneventful LASIK retreatment.

As tracking systems have become an integral part of excimer laser platforms, true decentrations with visual consequences have fortunately become increasingly less common. Nevertheless, decentered ablations can still occur due to such factors as improper patient alignment under the laser that results in a tilted head, a poorly calibrated tracking system, a fixation error on the part of the patient, intraoperative corneal hydration issues, or a lack of laser beam homogeneity. It is commonly believed that decentrations of greater than 1 mm can result in visual sequelae, the significance of which generally depends primarily on pupil size.4 Obtaining wavefront images and data on eyes that have undergone prior surgery can oftentimes be quite difficult, thereby eliminating the possibility of wavefront-guided treatment, as in this case.

In December 2001, the FDA granted VISX, Inc., a humanitarian device exemption (HDE) for its Custom-CAP software. An HDE application resembles a pre-market approval, but it is exempt from the effectiveness requirements that are part of the latter. An approved HDE authorizes marketing of a humanitarian use device, which is intended for the treatment of conditions that affect fewer than 4,000 individuals in the US per year. Surgeons who wish to use a humanitarian use device must obtain approval from an Institutional Review Board.

Carl Zeiss Meditec Inc. sells Custom-CAP software as VisionPro Ablation Planning software, which runs exclusively on the Humphrey ATLAS Corneal Topography Systems (model 993 or 995). A VISX Star S3 ActiveTrak laser platform must also be used. The VisionPro software is broadly indicated for the treatment of asymmetrical ablation patterns from previous laser refractive surgery and specifically for patients who manifest one or more of the following: reduced BSCVA, debilitating glare, monocular diplopia, and/or debilitating halos. Either PRK or LASIK may be used as the retreatment procedure.

This elevation-based treatment requires at least a 6-µm preoperative difference on the elevation topography, from the lowest to the highest point, across either a central 6.5-mm diameter or the patient's pupil diameter, whichever is larger. Once this elevation criterion is fulfilled, perhaps the single most important objective in preoperative planning is to impress upon the patient that the goal of Custom-CAP is to regularize the corneal surface and not to simultaneously treat the eye's residual refractive error. The surgeon needs to present this treatment option as a planned two-step procedure, the timing of which is quite variable from patient to patient.

Once he has selected an appropriate candidate, the physician can plan and simulate the proposed treatment at the ATLAS using the VisionPro software. I have found the software itself to be remarkably flexible and straightforward to use, and it allows me maximal control when I am selecting various surfaces to correct a decentration. Surfaces are programmed by depth, and choices include a myopic sphere, cylinder, or ellipse. The specific topographic decentration that the surgeon is trying to correct is what determines the actual size and orientation of each surface. The surgeon can program a total of up to 20 surfaces, an ability that can result in quite a complex treatment profile. Perhaps the most surgeon-friendly feature of the software is its ability to compensate for the supine treatment position of the patient by using x/y offsets; this capability makes the delivery of information from the topographer to the actual patient under the laser more direct. As in the case detailed in this article, I believe that this technology provides surgeons with an enormous opportunity to please patients whose prior refractive surgery resulted in a less than optimal outcome.

Stephen C. Coleman, MD, is Director of Coleman Vision in Albuquerque, New Mexico. He does not hold a financial interest in any technologies or companies mentioned herein. Dr. Coleman may be reached at (505) 821-8880; stephen@colemanvision.com.
1. Kanellopoulos AJ, Mrochen M. Wavefront-guided LASIK treats decentered ablations. Review of Refractive Surgery. 2003;4:23-28.
2. Alkara N, Genth U, Seiler T. Diametral ablation—a technique to manage decentered photorefractive keratectomy for myopia. J Refractive Surg. 1999;15:436-440.
3. Pallikaris IG, Siganos DS. A new optical effect of arcuate keratectomy. In: Pallikaris IG, Siganos DS, eds. LASIK. Thorofare, NJ: Slack Inc.; 1998:297-302.
4. Tsai Y, Lin JM. Ablation centration after active eye-tracker-assisted photorefractive keratectomy and laser in situ keratomileusis. J Cataract Refract Surg. 2000;26:28-34.
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