Nearly half of the population of North Amercia

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1 Quality of Vision and Freedom from Optical Correction after Refractive Surgery George O. Waring III, MD, FACS, FRCOphth Nearly half of the population of North Amercia has myopia; the prevalence of myopia in other populations ranges from 2 to 60%, as McCarty and colleagues document in this issue of the Journal (pp ). The ideal goal of refractive surgery for such individuals is to eliminate the need for distance optical correction and to allow them to see as well as or better than they could with spectacles or contact lenses. In 1997, we have only partially achieved that goal. In this issue of the Journal (pp ), Werblin 1 opines that the goal of refractive surgery is 20/20 or better uncorrected visual acuity and patients will not be happy until refractive surgery is able to satisfy this need. After contemporary refractive keratotomy, photorefractive keratectomy (PRK), excimer laser in situ keratomileusis (LASIK), intrastromal corneal ring segment insertion, and phakic intraocular lens (IOL) implantation, approximately 50 to 60% of patients with myopia of diopters (D) or less, see 20/20 or better uncorrected at approximately 1 year after surgery, but well over 90% could see 20/20 or better before surgery with optical correction. At the 20/40 level, commonly referred to as driver s license vision, we are doing much better over 90% of operated eyes achieve this level. A significant proportion of patients wear an optical correction after refractive surgery either part-time (eg, night driving) or full-time. For example, in the PERK study at 6 years after surgery, 36% (167) of patients younger than 40 years of age wore lenses for distance or near vision. 2 Unfortunately, published data on how many patients wear spectacles and contact lenses after refractive surgery is almost nonexistent a remarkable indictment of clinical reporting from the refractive surgical community, since the most important criterion of success for patients who have refractive surgery is freedom from corrective lenses. It is not only the refractive outcome after surgery that determines spectacle wearing status, but also the stability of refraction over time. Changes in refraction may occur from the surgical procedure itself. A hyperopic shift after radial keratotomy has been well documented 3 ; shorter corneal incisions may decrease the hyperopic shift 4 but long-term follow-up data at 3 to 10 years have not been published. Sawelson and colleagues 5 attributed the hyperopic shift to physiological changes in the refractive state of the eye rather than to continued flattening of the cornea, a contention refuted by the data of Ellingsen and colleagues 6 in this issue of the Journal (pp ). After photorefractive keratectomy, a loss of initial effect (myopic regression) occurs commonly but is highly variable with different excimer lasers and surgical techniques. Emerging data on LASIK demonstrate considerable stability of refractive outcome. Synthetic procedures such as the intrastromal corneal arc segments and phakic IOLs produce quite stable refractions over many years. 7,8 The changes in refraction over time may occur from normal physiological aging, as presented by Ellingsen and colleagues. 6 Current concepts usually state that simple myopia is stable by the late teens or early 20s, and this is commonly the earliest age at which refractive surgery is recommended. These findings are based on studies carried out 30 to 50 years ago Ellingsen and colleagues 6 point out that simple myopia increases on the average by D in a patient s 20s, D in a patient s 30s, and D in a patient s 40s, but then switches to a hyperopic shift with increases of D in the patient s 50s, and D in the 60s. This lends importance to informing patients before surgery that their eyes may change with age and that they may have to wear spectacles some of the time for Journal of Refractive Surgery Volume 13 May/June

2 best distance vision in the future. Even a perfect plano result shortly after refractive surgery is likely to undergo refractive shifts with time including the shift toward presbyopia in a patient s 40s. Refractive surgeons and their staffs should create realistic expectations for patients by not promising freedom from glasses but rather emphasizing a decreased dependence on glasses. I personally never promise a patient that I can completely eliminate the need for distance glasses or contact lenses with refractive surgery and always emphasize the possible need for part-time use of spectacles: Our goal is to get you out of glasses and contact lenses as much as possible, but you may have to wear spectacles about 10% of the time for example, for driving at night. This communication is important, not only in the surgeon s consulting room, but also in public communications, news articles, and advertisements. If refractive surgeons hold out the promise of total freedom from optical correction, ignoring the occasional need for glasses (and the need for reading glasses in presbyopes), our trustworthiness will be called into question by the public, the news media and our colleagues; our patients will be disappointed feeling both a sense of failure about the surgery and a sense of betrayal about their surgeon. Refractive surgery will develop a bad reputation, with the opposite effect wished by those proffering over-zealous advertising a decrease in refractive surgery business. Observations about the quality of a patient s vision after refractive corneal surgery are difficult to make, because we have no reliable way to measure quality of vision clinically, in spite of our attempts to use contrast sensitivity and glare tests. It is a well published fact that over 90% of patients who have refractive corneal surgery would elect to have it again, would recommend it to a friend, and express overall satisfaction with the results. But, such positive conclusions are often punctuated by a patient adding, But, I wish I saw as sharply as I did with my glasses...i wish I didn t see halos and lines and ghost images. A recent patient of mine had bilateral simultaneous LASIK to correct a refractive error in right eye: x 90 and left eye: D sphere. At 6 months the outcome in his right eye was x 90 with 20/16 uncorrected visual acuity, and in the left eye, plano with 20/12 uncorrected visual acuity. He was very satisfied with his miraculous sight but complained persistently of a slight blurriness and smudginess in his vision. I spent a long time discussing why he did not see as sharply as he did with his contact lenses before surgery. It is not difficult to understand such a decline in the quality of vision because every time we do refractive corneal surgery, we create an abnormally shaped cornea, converting the normal prolate shape that is steeper in the center and flatter in the periphery, to an oblate shape that is flatter in the center with an inflexion zone of change in refractive power paracentrally. Even the intrastromal corneal ring segments, which retain a prolate shape, change the physiologic contours of the cornea. These new corneal shapes, with their varying refractive power, coupled with whatever opacities result from the corneal surgery, create optical aberrations and intraocular light scattering that increase as the pupil dilates, as clearly described by Applegate and Howland in this issue (pp ) 13 and by Oliver and colleagues (pp ). 14 Refractive surgeons have continuously devised new methods to reduce these optical aberrations: 1) larger diameter central clear zones and a reduced number of incisions in refractive keratotomy, 2) larger diameter ablation zones with more gradual edge taper in photorefractive keratectomy and LASIK, and 3) larger diameter corneal flaps without sutures and more refined methods of handling flaps in keratomileusis. Further refinements are needed to eliminate optical aberrations. Phakic intraocular lenses hold the promise of decreasing optical aberrations because the corneal contours remain normal. An IOL has the potential of minimizing optical aberrations and intraocular light scattering if it has a large diameter optic (ideally 7 mm), if it remains properly centered on the pupil (to keep the edge behind the iris), if it is manufactured to exquisite optical tolerance, and if it is foldable to reduce the size of the surgical wound and the amount of induced astigmatism. Refractive surgeons, clever researchers, and inventive manufacturers will create improved techniques of refractive surgery that are adjustable enough to refine a patient s refraction many times after the initial surgery, thereby ensuring that the patient does not need to wear a distance optical correction. Synthetic procedures such as synthetic epikeratoplasty, intrastromal corneal rings, and phakic intraocular lenses are already heading in that direction. Flying spot lasers that can custom shape a cornea based on spacially resolved refractometry are on the horizon. Such new techniques should create better corneal contours, decrease optical aberrations, and improve patients quality of vision. These observations lead to two conclusions. First, as refractive surgeons succeed in developing better 214 Journal of Refractive Surgery Volume 13 May/June 1997

3 techniques to treat ametropias, we must not let our enthusiasm cloud clear vision of the goal: visual function equal to or better than that achieved with spectacles and contact lenses. Second, we must fulfill our obligation to communicate honestly and thoroughly with patients so they have realistic expectations about postoperative use of spectacles and postoperative quality of vision using current techniques. REFERENCES 1. Werblin TH. 20/20 how close must we get? J Refract Surg 1997;13: Bourque LB, Lynn MJ, Waring GO III, Cartwright C, Prospective Evaluation of Radial Keratotomy Study Group. Spectacle and contact lens wearing six years after radial keratotomy in the Prospective Evaluation of Radial Keratotomy study. Ophthalmology 1994;100: Waring GO III, Lynn MJ, McDonnell PJ, PERK Study Group. Results of the Prospective Evaluation of Radial Keratotomy (PERK) study 10 years after surgery. Arch Ophthalmol 1994;112: Lindstrom RL. Minimally invasive radial keratotomy: Mini-RK. J Cataract Refract Surg 1996;21: Sawelson H, Marks RG. Ten year refractive and visual results of radial keratotomy. Ophthalmology 1995;102: Ellingsen KL, Nizam A, Ellingsen BA, Lynn MJ. Age-related refractive shifts in simple myopia. J Refract Surg 1997;13: Nosé W, Neves RA, Burris TE, Schanzlin DJ, Belfort R. Intrastromal corneal ring: 12-month sighted myopic eyes. J Refract Surg 1996;12: Baikoff G, Joly P. Comparison of minus power anterior chamber intraocular lenses and myopic epikeratoplasty in phakic eyes. Refract Corneal Surg 1990;6: Brown EVL. Net average yearly shifts in refraction in atropinized eyes from birth to beyond middle life. Arch Ophthalmol 1938;19: Exford J. A longitudinal study of refractive trends after age forty. Am J Optom 1965;42: Hirsch MJ. Changes in refractive state after the age of fortyfive. Am J Optom 1958;35: Bucklers M. Changes in refraction during life. Brit J Ophthal 1953;37: Applegate RA, Howland HC. Refractive surgery, optical aberrations, and visual performance. J Refract Surg 1997;13: Oliver KM, Hemenger RP, Corbett MC, O Brart DPS, Verma S, Marshall J, Tomlinson A. Corneal optical aberrations induced by photorefractive keratectomy. J Refract Surg 1997;13: Steep Central Islands: Have We Finally Figured Them Out? Ronald R. Krueger, MD Of all the potential complications associated with excimer laser photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK), steep central islands demonstrated on videokeratography are perhaps the most frustrating. These corneal anomalies are typically evidenced topographically as islands of red, yellow, and green amid a sea of blue, and tend to be associated with loss of spectacle-corrected visual acuity and image ghosting. 1 Curiously, they have not been restricted to these procedures, but also occur after automated lamellar keratoplasty (ALK). 2 What are these islands? and How do they get there? They seem to appear almost immediately following PRK, with the highest incidence noted within the first postoperative week, and gradually diminish in prevalence during the ensuing months. 1 Their incidence depends on the defining criteria, which range from 1 to 3-mm in diameter and from 1.00 to 3.00 diopters (D) in power, and varies according to the type of excimer laser used, ablation zone diameter and corneal topographic device by which they are recorded. In one study of eyes treated with both the Schwind Keratom I (Schwind, Kleinostheim, Germany) and Summit Omnimed (Summit, Waltham, MA) using a 6-mm ablation zone diameter, steep central islands were discovered at Journal of Refractive Surgery Volume 13 May/June

4 Figure 1: A) Videokeratography of a steep central island after excimer laser PRK has insufficient information to identify the island when using the EyeSys CAS (only 0.50 D of steepening). B) In comparison, the steep central island is clearly delineated by the Technomed C-Scan (more than 4.00 D of steepening). (Courtesy of Theo Seiler, MD, PhD, Dresden, Germany) A B 1 month in 81% of eyes (50 of 62) when using the Technomed C-scan (Technomed, Baesweller, Germany), and 35% of eyes (22 of 62) when using the EyeSys CAS (EyeSys, Houston, Texas) 3 (Fig 1). These findings demonstrate the variability of corneal topography in identifying this condition, and highlight the frequent presence of steep central islands from excimer laser systems with standard, clinically-accepted ablation zone size. Steep central islands were first observed in patients treated with the VisX 20/20 excimer laser in early U.S. clinical trials. The VisX laser used a larger and more uniform ablation zone diameter than its contemporary, the Summit Excimed. However, when Summit expanded its zone and uniformity of treatment in the Summit Omnimed and Apex lasers, it too created steep central islands. Today, there are over ten commercially available excimer lasers, which can be divided into three groups: wide beam, scanning slit, and flying spot lasers. Although eyes 216 Journal of Refractive Surgery Volume 13 May/June 1997

5 A B Figure 2: High speed photography of wide beam (A) and small spot (B) excimer laser ablation. The wide beam ablation illustrates a low pressure zone (arrow) behind the plume which encases a small percentage of the ablation by-products, leading to deposition, shielding, and steep central island formation. (Courtesy of Thomas Neuhann, MD, Munich, Germany) treated with any one of these lasers could potentially develop steep central islands, it is predominantly the wide beam group (VisX, Summit, Schwind, and Technolas Keracor 116) that demonstrate the highest incidence. Several theories have been proposed to explain why wide beam lasers with a large ablation zone diameter are more prone to the formation of steep central islands. The most accepted explanations have been the shock wave/fluid theory whereby acoustic waves during ablation lead to a central accumulation of stromal fluid which shields further ablation, and the vortex plume theory whereby the ablation products in the ejected plume interfere with the subsequent pulse, shielding further ablation centrally. 1 Inconsistencies exist in these theories, and recently, Thomas Neuhann proposed a new explanation: a high pressure microexplosion on the ablated surface results in overlying low pressure behind the plume which draws air inward, creating turbulence and rotors in the center of a wide beam ablation (Fig 2). 4 The rotors are self contained whirls of air that carry and encase material within them and keep it close to the surface where it is deposited when the low pressure neutralizes. The deposit creates a shielding layer of one-tenth the magnitude of the depth removed and attenuates the ablation centrally with each subsequent pulse. 4 Although numerous authors have identified the presence of steep central islands and have proposed theories regarding their origin, few have provided a reproducible solution. In this issue of the Journal of Refractive Surgery, Shimmick and co-authors 5 (pp ) attempt to define clearly the variables associated with steep central islands and provide a model for modifying the ablation algorithm to eliminate their formation. Using optical profilometry, they demonstrated that steep central islands occur not only as central under-ablation (less than the expected ablation rate), but also peripheral over-ablation (more than the expected ablation rate) of enucleated porcine and live rabbit corneas, as well as polymethylmethacrylate (PMMA) plastic. This spatial variation in ablation, which increases with ablation diameter independent of the dioptric correction, is modeled to modify the ablation algorithm to correct for these conditions in a clinical setting. Although these authors have contributed greatly to the improvement of the ablation algorithm with the VisX excimer laser (VisX, Santa Clara, Calif), other wide beam excimer lasers (Summit, Schwind, Technolas) must adopt these principles of relative under- and overcorrection to develop their own algorithms or anti-central island software. Steep central islands, when they do occur, are relevant to visual function and require careful topographic analysis. Fortunately, the majority resolve with time due to the smoothing effect of epithelial Journal of Refractive Surgery Volume 13 May/June

6 hyperplasia in the midperipheral over-ablated zone. Steep central islands that persist beyond 1 year can be corrected by one of several strategies of excimer laser retreatment to improve visual function. 3,6,7 But treating the symptom of impaired visual function is not as effective as dealing with the source of steep central islands. Consequently, applying the principles of spatial variation within the ablation algorithm espoused by Shimmick and co-authors 5 might make steep central islands a thing of the past. REFERENCES 1. Krueger R, Saedy N, McDonnell P. Clinical analysis of steep central islands after excimer laser photorefractive keratectomy. Arch Ophthalmol 1996;114: Price F. Central islands of corneal steepening after automated lamellar keratoplasty for myopia. J Refract Surg 1996;12: Krueger R, Seiler T. PRK postoperative and complication management. In: Talamo JH, Krueger RR, (eds). The Excimer Manual: A Clinician s Guide to Excimer Laser Surgery. Boston, MA; Little, Brown and Co; 1997: Neuhann T. The origin of central islands. In: Lindstrom RL (ed). Laser Refractive Surgery. Video Journal of Cataract and Refractive Surgury 1996: Shimmick JK, Telfair WB, Munnerlyn CR, Bartlett JD, Trokel SL. Corneal ablation profilometry and steep central islands. J Refract Surg 1997;13: Castillo A, Romero F, Martin-Valverde JA, Diaz-Valle D, Toledano N, Sayagues O. Management and treatment of central steep islands after excimer laser photorefractive keratectomy. J Refract Surg 1996;12: Krueger RR, Tersi I, Seiler T. Corneal iron ring associated with steep central islands after excimer laser photorefractive keratectomy. J Refract Surg, in press. 218 Journal of Refractive Surgery Volume 13 May/June 1997