Mixed astigmatism is a challenging form of refractive error to treat with excimer laser technology. Y. Ralph Chu, MD, Medical Director of the Chu Laser Eye Institute in Edina, MN, cites that although there are several studies demonstrating the safety, efficacy, and predictability of treating myopia, hyperopia, as well as myopic and hyperopic astigmatism,1-5 very little has been written about the treatment of mixed astigmatism.6-7 Presently, there are only two excimer laser systems which surgeons can use to treat mixed astigmatism in the United States—Alcon's LADARVision® and VISX's STAR S3 ActiveTrak.TM The Alcon LADARVision® system is the only laser that has received US FDA approval specifically for the treatment of mixed astigmatism. The VISX STAR S3 is FDA approved for the treatment of hyperopic astigmatism, allowing surgeons to treat mixed astigmatism.
ALCON LADARVision®
The LADARVision® excimer laser is a flying small spot laser. Stephen Brint, MD, Associate Clinical Professor of Ophthalmology at Tulane University Medical School in New Orleans, LA, is presently using this laser which has a 0.8-mm Gaussian beam, ablating at a frequency of 60 pulses per second. Dr. Brint explains that the ablation pattern (the beam movement) is computer controlled, and with this single small beam, any ablation pattern can be created without the need for masks, slits, or apertures. The applications are non-sequential—no two spots are immediately adjacent, allowing for tissue relaxation and heat dispersion before the spots in the end are finally overlapped, creating a smooth, “polished” ablation.
Dr. Brint points out that in the case of mixed astigmatism, the ablation pattern is automatically a cross-cylinder type of ablation in which the surgeon can see the increasing density of the shot pattern removing more tissue peripherally along with a smooth blend out to 9.0 mm in the horizontal meridian to steepen it and the increasing density (compactness) of the shot pattern centrally in the vertical meridian to flatten the opposite meridian.
With the LADARVision® platform, the surgeon enters the mixed astigmatic refraction (usually with minimal to no nomogram adjustment), creates the flap, and engages the LADARVision® high-fidelity tracker creating a “freeze frame” image which permits the surgeon to precisely align the horizontal astigmatic reference line allowing exact astigmatic correction right on axis, even with cyclotorsion by the patient.
Dr. Brint concludes, “The key differentiating factor in treating mixed astigmatism with LADARVision®, as opposed to other lasers, is that you simply program the refraction into the laser and the shot pattern is delivered to the cornea in a tissue-sparing, one-pass procedure. There's no need to double card.”
LADARVision® Study Results
Dr. Brint's database showed 27 eyes with mixed astigmatism that had at least 1 month of follow-up. The pre-operative spheres ranged from +0.50 D to +4.50 D and the preoperative cylinders ranged from -1.25 D to -6.0 D. At 1 month postoperative, 66% of the patients had 20/20 vision or better, 81.4% saw 20/25 or better, and 96.2% saw 20/40 or better. One eye had 20/50 vision, however preoperative best-corrected visual acuity was 20/40, and the preoperative prescription was +3.25 D
-5.50 D X15.
VISX STAR S3 ActiveTrakTM
John Doane, MD, a refractive surgeon from Discover Vision Centers of Kansas City, MO, states that the VISX Star S3 ActiveTrakTM laser is extremely flexible with regard to mixed astigmatism. With the VISX Star S3 ActiveTrak,TM one can treat the mixed astigmatic population with either a cross-cylinder approach or with a bitoric approach. Dr. Doane was one of the investigators in the VISX mixed astigmatic LASIK FDA clinical investigations, and states that the results surpassed all FDA thresholds by significant margins and was one of, if not the most, successful trials that VISX has conducted.
Dr. Doane has considerable experience with the bitoric approach. He explains that with this approach, one-half of the astigmatism is treated with minus cylinder ablation and the other half of the astigmatism is treated with a plus cylinder ablation. The third part of the treatment is ablating the spherical equivalent of the original refraction. The key difference with bitoric ablation from cross cylinder is that there can be enhanced smoothness of the overall ablation contour. At the expense of a smoother profile, a deeper central ablation is attained versus the cross-cylinder approach.
In contrast, Dr. Chu believes that the cross-cylinder technique has emerged as the most successful method in which to treat mixed astigmatism with the VISX platform. He explains, “When using this approach to treat mixed astigmatism, the refractive error is considered as two cross cylinders.” The laser treats by steepening one meridian and flattening the other meridian 90o away. Only the cylinder is treated. “The advantages of using this technique include maximum tissue conservation, efficient treatment times, and superior visual results,” Dr. Chu states. Importantly, the surgeon can perform either approach with the VISX laser.
STAR S3 ActiveTrakTM Study Results
Dr. Chu presented a study conducted by Raymond Stein, MD, of the Bochner Eye Institute in Toronto, Canada, which demonstrated the efficacy of the cross-cylinder technique on a large number of eyes.6-7,10 This study evaluated 32 eyes in 20 patients with an average patient age of 32 (range, 19 to 62 years of age). The mean preoperative sphere was +2.09 D (range, +0.50 to +4.25 D). The mean preoperative cylinder was -3.55 D, (range, -1.00 D to -6.00 D). The mean postoperative cylinder was -1.51 D ranging from plano to 1.75 D. Uncorrected visual acuity between 6 to 12 months showed 100% of the patients were 20/40 or better, 78% were 20/30 or better, and 66% were 20/25 or better.6-7,10
Nidek EC-5000
The Nidek EC-5000 has not yet received FDA approval for treating mixed astigmatism. James Shumer, MD, from Eye Surgery Consultants in Mansfield, OH, uses the Nidek system. “We have completed the Nidek hyperopia clinical trial with the FDA, the data are submitted, and that submission was for hyperopia, hyperopia with astigmatism, and mixed astigmatism,” says Dr. Schumer, who treated his mixed astigmatism patients with the cross-cylinder technique. He states that out of all the FDA studies in which he participated, including back to1993—the old Summit PRK, the Technolas PRK, Nidek's PRK studies, myopia, myopia with astigmatism, and LASIK—the mixed astigmatism patients have had the best results of any group that he has treated in any protocol.
In Dr. Schumer's experience, the mixed astigmatism patients are the most stable refractive surgery patients. He explains that when the surgeon performs the treatment for hyperopia, there is an initial overcorrection and then a drift to where the patient becomes stable; the same is true with myopia. “We didn't see that when treating with the cross-cylinder technique on mixed astigmatism patients. If you think about it, it makes sense because when treating hyperopia, you are steepening the cornea, so you are changing the overall shape in a steepened fashion. The cornea is a tissue that likes to be in its original shape,” notes Dr. Schumer. When treating hyperopia or myopia using the cross-cylinder technique, the overall shape across the entire cornea does not change a great deal. “You are only treating pure astigmatism and that is why I believe we do not see these patients drift very much,” he says, “so where they are on the first post-operative week is where they end up. That's both good and bad—if you are on the money, it's great.” The FDA now has the Nidek clinical trials data and Dr. Schumer expects to have approval for the cross-cylinder technique by the end of the year.
SUMMARY
Dr. Chu points out that mixed astigmatism may be seen more commonly in patients who have had previous corneal surgery including refractive or cataract surgery. He and the other surgeons show that although these situations can be challenging, experience shows that performing LASIK to treat mixed astigmatism can produce excellent visual outcomes.
John F. Doane, MD, FACS, is in private practice with Discover Vision Centers in Kansas City, MO, and is Clinical Assistant Professor of the Department of Ophthalmology, Kansas University Medical Center. Dr. Doane is a member of VISX's Speaker's Bureau as well as an investigator for the company. (816) 350-4539; jdoane@discovervision.com
James Shumer, MD, is from Eye Surgery Consultants in Mansfield, Ohio.Dr. Schumer is an investigator for Nidek. (419) 525-3737; Schumer@revisioneyes.com
1. Hardten DR, Lindstrom RL, Chu YR: LASIK for Myopic Astigmatism: Results using the VISX Star and Star S2 lasers. VISX white paper, 1998
2. Doane JF, Morris SB, Denning, JA: Results of LASIK for myopic astigmatism: A comparison between the VISX Star and the VISX Star S2 Excimer laser system. VISX white paper, 1998
3. Jackson WB, Mintsioulis G, Agapitos PJ, Casson E: Long-term stability of PRK for hyperopia using the VISX Star excimer laser system. VISX white paper, 1998
4. Braunstein R, Belin MW, Bowman RW, et al: Hyperopia: Results of a U.S. Multicenter Trial. J Laser Vision Correction of VISX white paper, 1998
5. Chayet AS, Magallanes R, Montes M, et al: Laser in situ keratomileusis for simple myopic, mixed and simple hyperopic astigmatism. J Refract Surg 14:S175-S176, 1998 (suppl)
6. Stein R, Stein H, Cheskes A, Salim G: Treatment of Mixed Astigmatism VISX white paper, 1998
7. Stein R: Cross-cylinder approach a plus for mixed astigmatism. Ophthalmology Times 28-29, 2001
8. Stein R: Surgical technique pearls: Mixed astigmatism. Dallas, TX, American Academy of Ophthalmology, 2000
9. Ma S, Liang K, Schumer DJ, et al: Highlights from the First Annual Nidek Refractive Users Symposium. Review of Ophthalmology 15, 2001 (part 3 of 3)
