Photoastigmatic refractive keratectomy—the cure for astigmatism?

Guest Editorial Photoastigmatic Refractive Keratectomy—the Cure for Astigmatism? Astigmatism exceeding 0.5 diopters (D) is present in 44% of human eyes, 8% of which have 1.5 D or greater cylinder. The tolerance for astigmatism varies greatly between patients and depends on the magnitude and axis of the cylinder. Sensitivity to the cylinder determines in large part a patient’s tolerance for optical correction. Techniques for surgically correcting astigmatism include photoastigmatic refractive keratectomy (PARK) and similar treatment under a corneal flap during laser in situ keratomileusis (LASIK). The excimer laser (193 nm) is already an established refractive surgical tool, but new types, including solid state units, will soon be available. Astigmatic surgery with the excimer laser has progressed through photorefractive keratectomy (PRK) with combined surgical arcuate keratotomies and laser arcuate keratotomies, to attempts at a more physiological correction with pure tissue ablation. PARK has been freely available in Europe for 5 years. United States Food and Drug Administration approval has been received for PARK for up to 4 D cylinder with the Summit, VISX, and Autonomous Technologies lasers, and Nidek is in the process of applying for U S approval for the same procedure. Prerequisites for PARK generally include a stable refraction and the absence of ocular pathology. Pregnancy and systemic conditions, such as collagen vascular disease, should provoke great caution. Soft contact lenses must be removed for at least 2 days and rigid gas-permeable contact lenses for 2 weeks, until stability on repeated topography is demonstrated before PARK is performed. A careful refraction (subjective with blur tests or cycloplegic) is required to accurately determine the axis of the cylinder and the patient’s sensitivity to the cylinder. Discrepancies between manifest and cycloplegic refractions and topography should prompt repeat examination. Corneal topography, patient counseling with visual aids, and assessment of patient expectations are all equally important. The most important guide to determining successful laser settings for the surgeon new to PARK is the help of colleagues experienced with the same laser. Software modifications depend on the laser type, software version, and pulse frequency used, all of which are especially important when treating large cylinders. The minimum magnitude of astigmatism to be treated depends on the surgeon, the laser, the treatment zone size, and the sensitivity of the patient to the cylinder.1 It is useful to mark and verify the cylinder axis with the patient upright to avoid the occasional surprise of cyclotorsion. There is variability in effective output for all lasers (e.g., individual laser fluctuation, variability between lasers from the same manufacturer, errors in calibration, and effects from changes in humidity and temperature). Because calibration is done before treatment, the effective output at the time of treatment is never actually measured. All variables of treatment parameters should be documented in detail because they clearly affect the ability to compare data. Reports on PARK are available from the late 1980s. Choi et al2 reported a surgically induced refractive change of 93.9% (axis error of 5.9 ⫾ 10.2°). Taylor et al3 reported differences between results for PRK and PARK (87% and 68%, respectively, within ⫾ 1 D). Kim et al4 found a wide variation in outcome of PARK (some cylinders even increasing after treatment). More recent reports demonstrate a 36%–98% correction of the total cylinder, depending on the laser and software used (the majority demonstrate about 80% correction).5–9 I demonstrated persistent undercorrections (0.77 D surgically induced astigmatism [SIA] for 1 D cylinder preoperatively and 3.29 D for 5 D preoperatively) in a detailed analysis of 6418 eyes undergoing PARK (presented at the American Society of Cataract and Refractive Surgeons, April 1997). Astigmatic LASIK is now producing similar results.10 Chayet et al11 are achieving excellent results in cases of mixed and hyperopic astigmatism. Recent studies have assessed the effect of axis alignment, changing PARK treatment shapes, and treatment after penetrating keratoplasty. Because different authors tend to group patients and analyze data using nonstandardized parameters, it is difficult to compare studies. In addition, published results are often out of date by the time of publication. Data reported at meetings are useful but lack peer review. Because there are relatively few cases performed with new lasers and software updates, our ability to draw conclusions from many studies is weakened. The nomogram adjustments used for LASIK also vary between lasers and surgeons. When assessing PARK, the magnitudes of the spherical and cylindrical components need to be measured separately from the mean spherical equivalent (MSE) to demonstrate coupling effects. Data should be arranged by pre-existing 1 D MSEs and cylinder groups, and follow-up reporting should be standardized at 12 months. Grouping of data into just a few subgroups can be misleading in terms of the efficacy of treatment. Moves by several journals to standardize the reporting of data will greatly simplify the task of comparing studies for editors and interested readers alike. Studies of the efficacy of PARK often lack useful vector analysis that has taken the magnitude and direction of astigmatic change into account, so it is difficult to tell whether any under- or overcorrections are from problems with the magnitude or the axis of the treatment (a 15° error in axis alignment leads to a 50% reduction in cylinder correction). Calculated indices

The author has no proprietary interest in the development or marketing of any excimer laser and is not a paid consultant to any company.

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Ophthalmology Volume 106, Number 11, November 1999 provide directly comparable measurements between different studies and will help individual centers design changes in techniques.12 Reported PARK complications include haze, halos, loss of best-corrected visual acuity, irregular astigmatism, off-axis treatments, and regression of the cylinder owing to differential healing of the steeper meridian. Healing and stability after PARK are slower than after PRK. The stability of cylinder axis is often a sign of overall stability. The results of PARK rapidly deteriorate with increasing cylinder and involve all measurable parameters (e.g., visual acuity, and patient satisfaction). At low levels of astigmatism, the results of PARK may be comparable with those of spherical PRK. Large studies comparing the results of treating subjective versus keratometric cylinder are needed. The ideal magnitude and axis of cylinder for treatment may be somewhere between the subjective and keratometric values. The total corneal cylinder is a combination of that from the anterior (ACS) and posterior corneal surfaces (PCS), the latter being about 10%, on average, of the total corneal cylinder. It is possible that the most predictable results will be obtained from treating the ACS cylinder only, and patients will have to be advised that they will be left with residual PCS and lenticular astigmatism. There are also changes in the PCS shape after surgery, which have not yet been fully evaluated. These can be readily visualized with slit-scanning topography, but the algorithms for calculating PCS are not fully validated. The PCS will be particularly relevant for intraocular lens power calculations when the prerefractive surgery biometry is unknown. Modifications of algorithms and treatment profiles to improve the results of PARK will continue. However, the benefit of any modifications will need to be demonstrated in large, randomized, controlled trials whose results are analyzed using multivariate analysis or neural networks to assess outcome, as well as vector analysis. The prospect of an accurate and reproducible procedure to correct myopia and astigmatism has been long awaited. As more PARK experience is gained, its proper place in the array of available refractive procedures will become clear. PARK is an emerging technology undergoing active research, much of which remains to be subjected to peer review. Even the present equipment required for PARK, which is considerably more expensive than that required for other refractive procedures, is still evolving and may quickly become outdated as lasers improve. PARK, however, is here to stay, and knowledge about the procedure is necessary to adequately counsel patients about all possible options in refractive surgery. References 1. Shah S, Chatterjee A, Doyle SJ, Bessant DA. Astigmatism induced by spherical photorefractive keratectomy corrections. Ophthalmology 1997;104:1317–20. 2. Choi YI, Min HK, Hyun PM. Excimer laser photorefractive keratectomy for astigmatism. Korean J Ophthalmol 1993;7:20 – 4. 3. Taylor HR, Kelly P, Alpins N. Excimer laser correction of myopic astigmatism. J Cataract Refract Surg 1994;20(Suppl):243–51. 4. Kim YJ, Sohn J, Tchah H, Lee CO. Photoastigmatic refractive keratectomy in 168 eyes: six-month results. J Cataract Refract Surg 1994;20:387–91. 5. Zadok D, Haviv D, Vishnevskia-Dai V, et al. Excimer laser photoastigmatic refractive keratectomy. Eighteen-month follow-up. Ophthalmology 1998;105:620 –3. 6. Colin J, Cochener B, Le Floch G. Excimer laser treatment of myopic astigmatism. A comparison of three ablation programs. Ophthalmology 1998;105:1182– 8. 7. Brodovsky S, Couper T, Alpins NA, et al. Excimer laser correction of astigmatism with multipass/multizone treatment. The Melbourne Excimer Laser Group. J Cataract Refract Surg 1998;24:627–33. 8. Lee JS, Oum BS, Lee BJ, Lee S H. Photorefractive keratectomy for astigmatism greater than ⫺2.00 diopters in eyes with low, high, or extreme myopia. J Cataract Refract Surg 1998;24:1456 – 63. 9. Higa H, Liew M, McCarty C, Taylor H. Predictability of excimer laser treatment of myopia and astigmatism by the VISX Twenty-Twenty. Melbourne Excimer Laser Group. J Cataract Refract Surg 1997;23:1457– 64. 10. Zaldivar R, Davidorf JM, Oscherow S. Laser in situ keratomileusis for myopia from ⫺5.50 to ⫺11.50 diopters with astigmatism. J Refract Surg 1998;14:19 –25. 11. Chayet AS, Magallanes R, Montes M, et al. Laser in situ keratomileusis for simple myopic, mixed, and simple hyperopic astigmatism. J Refract Surg 1998;14(2 Suppl):S175– 6. 12. Alpins NA. A new method of analyzing vectors for changes in astigmatism. J Cataract Refract Surg 1993;19:524 –33.

SUNIL SHAH, FRCOphth, FRCSE Nottingham, England

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