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PRK after 12 years: Good news for patients
Editorial PRK after 12 Years: Good News for Patients Roger F. Steinert, MD - Irvine, California After gaining a basic understanding of the principles of laser vision correction, some of the most frequent questions posed by prospective patients to a refractive surgeon are “Will the procedure last?” “What about long-term problems?” “Isn’t this procedure fairly new?” Until now, refractive surgeons have had only 2 published studies in the peer-reviewed literature with 6-year follow-up upon which to base statements made to those patients.1,2 In this issue, Rajan et al3 have, for the first time, provided follow-up that exceeds one decade for patients who underwent myopic photorefractive keratectomy (PRK). Their patients were treated in one of the original PRK trials in 1990, utilizing the broad beam UV 200 Excimer laser (Summit Technology, Inc., Waltham, MA) with a 4-mm ablation zone, correcting up to ⫺7 diopters (D). Although the small optical zone and higher corrections in this study were associated with an initial hyperopic overcorrection and subsequent myopic regression over the first postoperative year, the current study shows stability of the correction thereafter, most notably between the 6- and 12-year follow-up intervals. This finding is particularly remarkable in view of the documentation of bilateral myopic progression in 6 patients between ages 28 and 36 who had undergone unilateral treatment, thereby permitting analysis of the unoperated eye as a control. Beyond the importance of demonstrating the fundamental stability of the laser-induced optical correction, several long-term issues merit comment. Corneal haze was universally present in the early postoperative period and was documented to decline rapidly through the first postoperative year and then slowly through the sixth year. Little further change occurred between the sixth and 12th years, and no patients developed late haze or worsening of haze. Sporadic case reports of late-onset haze remain concerning, but this study reassures us that this phenomenon is sporadic and rare. Night vision disturbances are not surprising in this cohort, given the small optical zone, particularly for patients having corrections in the higher dioptric range. The analysis of subjective symptoms in this report is further hampered by the study protocol, which permitted undercorrection and, in many cases, a large amount of residual myopia in many of the patients. Notably, however, the 8 patients who reported important night vision disturbances at 6 years experienced subjective improvement in their night vision at the 12-year follow-up. Whether this represents improvement due to a progressive remodeling healing response in the transition from the optical zone to the peripheral cornea or is due to psychophysical adaptation is unknown. Also reassuring was the absence of any evidence of progressive corneal ectasia, development of recurrent erosions, or other degenerations. Two patients in the cohort © 2004 by the American Academy of Ophthalmology Published by Elsevier Inc.
reported the onset of dry eyes at 12 years. In the absence of an adequate control group, it is impossible to know whether this phenomenon is significant, particularly in view of the small number of patients and the natural history for an increase in incidence of dry eyes with age. Certainly there are important limitations in our ability to generalize from the results of this single study. The fundamental technique of PRK has evolved in a number of important ways since this 1990 early investigation. Most notably, the 4-mm optical zone was abandoned rapidly due to night vision disturbances. In addition, most excimer lasers in clinical use today employ a scanning spot or slit for some or all of the treatment. In addition, LASIK is the dominant laser vision correction procedure today, and laser epithelial keratomileusis is competing with PRK for those patients having surface treatment. Of course, because most surgical procedures evolve, follow-up data of a decade or more will usually derive from a procedure no longer applied with an identical technique. We have no choice but to utilize the outcomes from obsolete procedures to extrapolate the likely outcomes from newer methods. Perhaps the largest caveat stems from the reality that a 4-mm optical zone results in a profound reduction in the depth of tissue removal relative to a larger optical zone. As taught by the Munnerlyn formula,4 the depth of ablation increases as a square of the diameter. Therefore, with a 4-mm optical zone, the maximum depth of tissue removal in the current study, for a ⫺7-D correction, would be only 37.3 m, whereas with a 6-mm optical zone (often exceeded in today’s treatments, with or without wavefront-guided custom procedures) the depth would be 84 m. In any long-term study, the completeness of follow-up and the reasons for patient dropout are a major concern. In this study, 68 of 120 patients (56.6%) were available for the 12-year follow-up visit. The only information provided about the characteristics of the group lost to follow-up is that the refractive error mean and range did not statistically significantly differ from those of the follow-up cohort. A 12-year follow-up rate exceeding 50% is considered acceptable. With lack of further information about the patients lost to follow-up, however, we do not know whether major bias has been introduced. Patients with poor outcomes may become disgruntled and refuse further follow-up. Conversely, unhappy patients may persist and return at a higher rate than patients who perceive that they are doing well, with no need for the inconvenience of later examinations. A more detailed analysis of the characteristics of the patients not available for follow-up, compared with those in the current cohort, would help address these concerns. The prospective evaluation of radial keratectomy study, a benchmark for careful long-term follow-up, utilized a number of aggressive measures to promote a high rate of follow-up, including active tracking of the patients on a yearly basis, ISSN 0161-6420/04/$–see front matter doi:10.1016/j.ophtha.2004.07.001
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Ophthalmology Volume 111, Number 10, October 2004 sending birthday cards, and an understanding by the patients that a decade of follow-up was expected at the time when the patients initially enrolled in the study.5 Despite these limitations, the long-term study reported by Rajan et al represents a major body of work addressing an area of vital concern to refractive surgeons and refractive surgery candidate patients. The diligence of this group in providing both 6- and 12-year follow-up data is commendable, and serves as a stimulus to other investigators who are in a position to follow their lead and strengthen our body of knowledge. Although it is perhaps more exciting to report on new cutting-edge procedures, attention to the long-term sequelae of these procedures is an obligation to our patients and provides an opportunity to maximize the quality of the outcomes with which our patients will live for the rest of their lives.
1800
References 1. Stephenson CG, Gartry DS, O’Brart DP, et al. Photorefractive keratectomy. A 6-year follow-up study. Ophthalmology 1998; 105:273– 81. 2. Sekundo W, Bonicke K, Mattausch P, Wiegand W. Six-year follow-up of laser in situ keratomileusis for moderate and extreme myopia using a first-generation excimer laser and microkeratome. J Cataract Refract Surg 2003;29:1152– 8. 3. Rajan MS, Jaycock P, O’Brart D, et al. A long-term study of photorefractive keratectomy. 12-year follow-up. Ophthalmology 2004;111:1813–24. 4. Munnerlyn CR, Koons SJ, Marshall J. Photorefractive keratectomy: a technique for laser refractive surgery. J Cataract Refract Surg 1988;14:46 –52. 5. Waring GO III, Lynn MJ, McDonnell PJ. Results of the prospective evaluation of radial keratotomy (PERK) study 10 years after surgery. Arch Ophthalmol 1994;112:1298 –308.
reported the onset of dry eyes at 12 years. In the absence of an adequate control group, it is impossible to know whether this phenomenon is significant, particularly in view of the small number of patients and the natural history for an increase in incidence of dry eyes with age. Certainly there are important limitations in our ability to generalize from the results of this single study. The fundamental technique of PRK has evolved in a number of important ways since this 1990 early investigation. Most notably, the 4-mm optical zone was abandoned rapidly due to night vision disturbances. In addition, most excimer lasers in clinical use today employ a scanning spot or slit for some or all of the treatment. In addition, LASIK is the dominant laser vision correction procedure today, and laser epithelial keratomileusis is competing with PRK for those patients having surface treatment. Of course, because most surgical procedures evolve, follow-up data of a decade or more will usually derive from a procedure no longer applied with an identical technique. We have no choice but to utilize the outcomes from obsolete procedures to extrapolate the likely outcomes from newer methods. Perhaps the largest caveat stems from the reality that a 4-mm optical zone results in a profound reduction in the depth of tissue removal relative to a larger optical zone. As taught by the Munnerlyn formula,4 the depth of ablation increases as a square of the diameter. Therefore, with a 4-mm optical zone, the maximum depth of tissue removal in the current study, for a ⫺7-D correction, would be only 37.3 m, whereas with a 6-mm optical zone (often exceeded in today’s treatments, with or without wavefront-guided custom procedures) the depth would be 84 m. In any long-term study, the completeness of follow-up and the reasons for patient dropout are a major concern. In this study, 68 of 120 patients (56.6%) were available for the 12-year follow-up visit. The only information provided about the characteristics of the group lost to follow-up is that the refractive error mean and range did not statistically significantly differ from those of the follow-up cohort. A 12-year follow-up rate exceeding 50% is considered acceptable. With lack of further information about the patients lost to follow-up, however, we do not know whether major bias has been introduced. Patients with poor outcomes may become disgruntled and refuse further follow-up. Conversely, unhappy patients may persist and return at a higher rate than patients who perceive that they are doing well, with no need for the inconvenience of later examinations. A more detailed analysis of the characteristics of the patients not available for follow-up, compared with those in the current cohort, would help address these concerns. The prospective evaluation of radial keratectomy study, a benchmark for careful long-term follow-up, utilized a number of aggressive measures to promote a high rate of follow-up, including active tracking of the patients on a yearly basis, ISSN 0161-6420/04/$–see front matter doi:10.1016/j.ophtha.2004.07.001
1799
Ophthalmology Volume 111, Number 10, October 2004 sending birthday cards, and an understanding by the patients that a decade of follow-up was expected at the time when the patients initially enrolled in the study.5 Despite these limitations, the long-term study reported by Rajan et al represents a major body of work addressing an area of vital concern to refractive surgeons and refractive surgery candidate patients. The diligence of this group in providing both 6- and 12-year follow-up data is commendable, and serves as a stimulus to other investigators who are in a position to follow their lead and strengthen our body of knowledge. Although it is perhaps more exciting to report on new cutting-edge procedures, attention to the long-term sequelae of these procedures is an obligation to our patients and provides an opportunity to maximize the quality of the outcomes with which our patients will live for the rest of their lives.
1800
References 1. Stephenson CG, Gartry DS, O’Brart DP, et al. Photorefractive keratectomy. A 6-year follow-up study. Ophthalmology 1998; 105:273– 81. 2. Sekundo W, Bonicke K, Mattausch P, Wiegand W. Six-year follow-up of laser in situ keratomileusis for moderate and extreme myopia using a first-generation excimer laser and microkeratome. J Cataract Refract Surg 2003;29:1152– 8. 3. Rajan MS, Jaycock P, O’Brart D, et al. A long-term study of photorefractive keratectomy. 12-year follow-up. Ophthalmology 2004;111:1813–24. 4. Munnerlyn CR, Koons SJ, Marshall J. Photorefractive keratectomy: a technique for laser refractive surgery. J Cataract Refract Surg 1988;14:46 –52. 5. Waring GO III, Lynn MJ, McDonnell PJ. Results of the prospective evaluation of radial keratotomy (PERK) study 10 years after surgery. Arch Ophthalmol 1994;112:1298 –308.