Use of Lasers for Vision Correction of Nearsightedness and Farsightedness
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《新英格兰医药杂志》
This Journal feature begins with a case vignette highlighting a common clinical problem. Evidence supporting various strategies is then presented, followed by a review of formal guidelines, when they exist. The article ends with the author's clinical recommendations.
A 32-year-old woman with moderate myopia and mild dry eye has worn soft contact lenses for 12 years. She notes decreased tolerance of the lenses and must remove them after only four to five hours. On examination, her refraction is –4.25 + 1.0 x 90 (–3.75 diopters of spherical equivalent and 1 diopter of astigmatism at 90 degrees) in the right eye and –3.5 + 0.5 x 88 in the left eye, yielding a best corrected visual acuity of 20/20 in each eye. She asks about refractive surgery. What would you advise?
The Clinical Problem
Vision correction for myopia (nearsightedness) with the use of an excimer laser was approved by the Food and Drug Administration in 1995. It is estimated that more than 1.1 million such procedures were performed in 2003 in the United States.
The first procedure performed for the correction of myopia was a form of ablation of the stromal surface, termed photorefractive keratectomy (PRK), in which the epithelium is removed from the center of the cornea, and excimer laser correction is applied to the stromal surface (Figure 1).1 Visual function improves as the epithelial defect heals. PRK has proven to be very effective, with a good ratio of risk to benefit.2 Delayed recovery of vision after PRK and transient discomfort in some patients, as well as the development of visually significant opacity of the corneal stroma (referred to as haze) in a small percentage of cases, led to the development of laser-assisted in situ keratomileusis (LASIK) in the mid 1990s.3
Figure 1. Differences among the PRK, LASIK, and LASEK Procedures.
In PRK (top panel), the central corneal epithelium is removed and stromal tissue is removed from the stromal surface with an excimer laser. The epithelium heals in three to seven days. In LASIK (middle panel), an epithelial–stromal flap is formed with a microkeratome, and a portion of the stromal bed is removed with an excimer laser. In LASEK (bottom panel), an epithelial flap is formed with the use of dilute ethanol and special instruments, and stromal tissue is removed from the stromal bed with an excimer laser. The surgeon hopes to retain the epithelium in LASEK, but in some cases it is lost, and then the procedure is similar to PRK.
The LASIK procedure (Figure 1) is performed with an instrument called a microkeratome or, more recently, with a femtosecond laser and involves forming an epithelial–stromal flap that is attached to the periphery of the cornea by a hinge of uncut tissue. The flap is typically 8 to 10 mm in diameter and 100 to 180 μm in thickness (approximately 15 to 35 percent of the total corneal thickness), but microkeratomes deviate 20 to 30 μm, so flap thickness varies substantially from eye to eye. The flap is lifted, and the high-energy pulses of the excimer laser are applied to the underlying stromal bed so that the ablated cornea overlies the pupil. The flap is then returned to its original position, without the need for sutures. The greater comfort and faster rate of recovery of vision with LASIK than with PRK led to nearly total abandonment of PRK by many refractive surgeons during the late 1990s.
In an attempt to improve on PRK, another method of surface ablation, called laser-assisted subepithelial keratomileusis (LASEK), has been developed.4 In LASEK, an epithelial flap, attached to the cornea by an uncut hinge at the periphery of the epithelium, is formed with a trephine after brief exposure of the area to 10 to 20 percent ethanol (Figure 1). The epithelial flap is lifted, and excimer laser ablation is applied to the surface of the stroma (as in PRK). Then the epithelial flap is replaced. Proponents of LASEK have claimed that there may be faster recovery of vision, less pain, and a decreased tendency for haze than with PRK. Since there are no definitive studies of the outcomes of LASEK, this article focuses on LASIK and PRK, with PRK including LASEK.
Strategies and Evidence
Benefits of PRK and LASIK
In the hands of an experienced and competent surgeon, the vast majority of patients who undergo PRK or LASIK have an improvement in uncorrected visual acuity.5,6,7 The odds of attaining a desired level of uncorrected visual acuity are inversely related to the baseline level of correction. For example, in an eye with 2 diopters of myopia, the likelihood of attaining uncorrected visual acuity of 20/20 is 70 to 80 percent, and the likelihood of achieving 20/40 or better (the visual acuity, with or without corrective lenses, most often required to drive a motor vehicle in the United States and the United Kingdom) is greater than 98 percent. In contrast, in an eye with 9 diopters of myopia, the likelihood of achieving visual acuity of 20/20 is on the order of 40 to 55 percent, and the likelihood of achieving 20/40 is 95 to 98 percent.5,6,7 Substantial improvement over preoperative uncorrected vision is achieved in most eyes after PRK or LASIK, even if a visual acuity of 20/40 is not achieved.6
Depending on the surgeon and the initial level of attempted correction, approximately 5 to 20 percent of eyes will need a reoperation with additional laser ablation (commonly referred to as an enhancement) to achieve the best possible results.8 The results for visual acuity are almost identical with PRK and with LASIK for low-to-moderate myopia (up to approximately 6 diopters) and low hyperopia, or farsightedness (less than 2 diopters).5 Observational studies show that LASIK performed for myopia or hyperopia above these levels is more likely to yield uncorrected visual acuity of 20/20 or 20/40 than is PRK, because the results of PRK have a greater tendency for regression (the loss of the effect of surgery over time).6
In most eyes, corrections of visual acuity with PRK or LASIK are relatively stable over time, and few patients return after one year with a drop in uncorrected visual acuity. In my experience, late refractive instability occurs in less than 0.2 percent of patients. Such a decline is usually due to further lengthening of the eye that is independent of refractive surgery. If the healing epithelium becomes thicker than normal (epithelial hyperplasia) in the early postoperative period, then greater correction, or even mild overcorrection, may occur during the months or years after surgery.
Risks
There are several complications that can occur with PRK and LASIK.5,6,7,9 These include visual disturbances (Figure 2) such as starbursts, halos, distorted images, and multiple images that are most common at night. These aberrations are more likely to occur with excimer laser ablations that are decentered relative to the pupil, and with corrections that are greater than 7 to 8 diopters of myopia or 2 to 3 diopters of hyperopia. In particular, levels of myopia greater than approximately 10 diopters or of hyperopia greater than approximately 4 diopters are associated with a marked increase in visual complications. The occurrence of such complications has been reduced with improvements in laser technology, such as the development of tracking devices that monitor and adjust for eye movements during ablation, and with better screening of patients to rule out corneal topographic abnormalities, corrections that are too high, or other ocular conditions that increase the chances of diminished vision. Although the incidence of these complications has not been well defined prospectively, retrospective studies have suggested an incidence of less than 1 percent among patients who are considered good candidates for surgery. Other complications, such as infection and ectasia (progressive thinning and irregularity of the cornea), occur at rates much lower than 1 percent.
Figure 2. Rare Optical Imperfections after LASIK Procedures.
Panel A shows the letter E as viewed by an eye with few consequential aberrations (optical imperfections) after a LASIK procedure. Panel B shows the letter E as viewed by an eye with severe aberrations after a LASIK procedure. Before the procedure, the eye had serious abnormalities of the corneal topography. This level of distortion is very rare. Panel C shows the letter E as viewed by an eye with a higher order aberration called coma (because items appear to have a tail like that of a comet) after a complicated LASIK procedure. The vision in an eye with this level of coma would probably have substantial starbursts, in which a point of light appears diffuse, with rays extending from the point.
Physicians and patients should consider procedure-specific complications when deciding between approaches. Complications with LASIK that cannot occur after PRK, since there is no flap in the latter,5,6,7,8,9 include free caps (severed flap hinges), short flaps (incomplete flaps that must be recut later), flap striae (wrinkles that may cause irregular astigmatism), and diffuse lamellar keratitis (inflammation in the stromal interface beneath the flap). The risk of loss of two or more lines of best corrected visual acuity on a Snellen chart is 1 to 2 percent greater with LASIK than with PRK, owing primarily to flap complications. LASIK also may induce transient dry eye, a condition related to the neurotrophic effects of cutting the nerves during flap formation. This condition typically resolves six to nine months after LASIK, when the nerves grow back into the flap.
Haze in the anterior stroma that is related to wound healing is very rare after LASIK but occurs in 1 to 2 percent of eyes that have been treated with PRK for more than 6 diopters of myopia.6,7,9 Other advantages of LASIK over PRK include better comfort and vision in the first few days after surgery.
Complications with LASEK tend to be similar to those with PRK, although delayed healing of the epithelium may be more common with LASEK.4 Observational data are inconsistent with regard to whether recovery of vision is faster, pain is less, or haze is less with LASEK than with PRK for similar levels of myopia or hyperopia.
The rates of complications with LASIK seem to be lower for more experienced surgeons than for those who are less experienced.10 Limited training beyond a residency in ophthalmology is required to perform LASIK or PRK. An ophthalmologist can perform these procedures after completing approximately two days of training with the specific laser and microkeratome and having a more experienced surgeon serve as proctor during the first few procedures. However, some residents complete one-year or two-year fellowships that focus on refractive surgery.
Preoperative Screening
Preoperative screening is critical to maximize the likelihood of a good outcome of laser surgery for vision correction. A comprehensive eye examination that includes slit-lamp biomicroscopy and a funduscopic examination should be performed in all candidates. In general, patients should have normal, healthy eyes.
Surgery is typically performed only in patients over 21 years of age, owing to concern about ongoing changes in refractive error. Regardless of the patient's age, the refraction should have been relatively stable during the previous one to two years. This may be difficult to verify, and often physicians must rely on the patient's history. There is no upper age limit for the patient who plans to undergo surgery, if the eye is normal aside from refractive error.
Contraindications to laser surgery include systemic diseases such as rheumatoid arthritis, systemic lupus erythematosus, and other active immune-related disorders that may be associated with healing abnormalities or complications such as corneal melting, which leads to corneal thinning and even perforation. Pregnancy and lactation are contraindications, owing to temporary changes in refraction. Patients with moderate-to-severe dry eye are not considered to be good candidates for LASIK or PRK. Schirmer's test for tear production, in conjunction with slit-lamp biomicroscopy and the history, may be helpful in evaluating patients who have signs or symptoms of dry eye.
Corneal thickness is measured with an ultrasonic pachymeter. The normal thickness of the cornea ranges from 490 to 650 μm.8 The minimal corneal thickness that is adequate depends on the level of intended correction and the procedure that is planned. The corneal thickness should be considered when deciding whether LASIK, PRK, or no surgery is the best choice in a particular eye. In some cases, LASIK is not a good option, even when the corneal topography is normal, on account of inadequate corneal thickness and an inadequate amount of stromal tissue that will remain untouched posterior to the flap after the laser ablation is completed (residual bed thickness). Many surgeons strive to maintain at least 250 μm of untouched posterior stroma, despite the absence of definitive data suggesting that this is the amount necessary to reduce the risk of corneal ectasia.8
The diameter of the pupil in the dark is also relevant. The greater the diameter in the dark, the higher the incidence of glare, halos, and other visual disturbances, especially in eyes with high myopia. The measurements performed during the preoperative eye examination are particularly critical in cases in which high myopia or high hyperopia are present, given the increased associated risk of visual complications.
Topographic maps of the cornea should show no signs of diseases in which progressive thinning of the cornea tends to occur over time. Corneal diseases, such as keratoconus, that are likely to cause progressive corneal thinning and change in the shape of the cornea, are likely to be associated with poor results of PRK or LASIK. Recently, wavefront analysis has been used to identify aberrations affecting the quality of vision throughout the eye, from the tear film to the retina.
Areas of Uncertainty
PRK versus LASIK
Data from prospective clinical trials directly comparing LASIK with PRK are lacking. Retrospective studies suggest that these procedures have similar results for patients with low-to-moderate myopia (up to 6 diopters)6,7 but that for patients with higher levels of myopia, the average uncorrected visual acuity was better after LASIK than after PRK.6 In eyes with low-to-moderate myopia, the advantages of LASIK over PRK are better comfort and vision during the first few days after surgery. However, the added risk from flap complications with LASIK includes loss of two or more lines of best corrected visual acuity on a Snellen chart.
According to retrospective studies with 6 to 12 months of follow-up, PRK and LASIK have similar results for the correction of low hyperopia (less than 2 diopters).6,7,9 Most surgeons use LASIK rather than PRK for patients with hyperopia, owing to the faster recovery of vision and the lower likelihood of regression.
Astigmatism is a refractive error in which the correcting lens has greater curvature in one meridian (e.g., vertically) than it does in another (e.g., horizontally); it is measured in magnitude (diopters) and in direction (axes). Astigmatism blurs both distance and near vision. By definition, regular astigmatism can be corrected with lenses in which the maximal and minimal axes of the astigmatism are 90 degrees apart. Irregular astigmatism cannot be corrected with glasses and requires rigid contact lenses or surgery for treatment. Most eyes have at least a small level of regular astigmatism, along with myopia or hyperopia.6,7 Regular astigmatism up to approximately 6 diopters can be corrected at the same time as myopia or hyperopia with LASIK or PRK.
PRK is the procedure of choice for the correction of myopia or hyperopia in some eyes.11,12 These include eyes that have thin but otherwise normal corneas; eyes with anterior basement-membrane dystrophy, in which the microkeratome used to make the LASIK flap commonly produces a large epithelial defect resulting in delayed recovery of vision; and moderately dry eyes.
Custom Corneal Ablation Procedures
Custom corneal ablation procedures involve the use of wavefront analysis to measure the aberrations of the eye and to direct the laser ablation of the cornea. These procedures are an alternative to traditional laser ablation techniques.13,14,15 However, there are limited data to compare the custom and the traditional approaches. Studies of one widely used laser system seem to show a modest advantage of custom corneal ablation for lower levels of myopia and hyperopia.13,14 In general, early results suggest that the benefit of custom corneal ablation may increase as the level of attempted correction increases. Further development and assessment of these techniques are needed to clarify their role.
Data are also lacking to compare various laser systems and other equipment with regard to the outcome of LASIK or PRK. Limited data suggest that microkeratomes now are superior to those that were first developed for LASIK, but rigorous comparisons of equipment have not been performed.
Guidelines
General policy statements regarding PRK16 and LASIK17 have been published by the American Academy of Ophthalmology (AAO). These statements, based on a review of the literature, indicate that PRK and LASIK are effective and predictable for low-to-moderate myopia (up to 6.0 diopters) with regard to obtaining very good or excellent uncorrected visual acuity and that they are safe in terms of minimal loss of visual acuity in the great majority of eyes. However, the statements also note that the results of PRK and LASIK are more variable in eyes with moderate-to-high myopia (more than 6.0 diopters). Eyes treated with LASIK for low-to-moderate degrees of hyperopia (less than 2.0 diopters) tended to have similar results to eyes treated for low-to-moderate myopia.17 The AAO summary recommendations for LASIK and PRK can be found at www.aao.org/aao/education/library/recommendations/lasik.cfm and www.aao.org/aao/education/library/recommendations/prk.cfm, respectively. The AAO provides no guidelines for LASEK and no guidance on choosing between LASIK and PRK.
Conclusions and Recommendations
PRK, LASIK, and LASEK are relatively effective and predictable surgical procedures for the correction of nearsightedness and farsightedness with or without low-to-moderate astigmatism. LASIK is associated with faster recovery of vision and greater comfort in the early postoperative period but with a somewhat higher risk of complications, such as loss of two or more lines of best corrected visual acuity on a Snellen chart. For levels of myopia from 6 diopters to approximately 10 diopters, LASIK seems to be superior to PRK or LASEK in terms of the predictability of the results and the rate of recovery of vision, although there are no prospective trials directly comparing these procedures. Some patients are not candidates for LASIK owing to inadequate corneal thickness, moderately dry eyes, or other factors.
The patient described in the vignette has relatively low myopia and low astigmatism and should be a reasonable candidate for a laser-ablation procedure. If her corneal thicknesses are in the normal range, LASIK is a good choice, although there is disagreement among surgeons about what constitutes normal corneal thickness. If her corneas are thinner than normal, or if she has mild-to-moderate dry eye, a surface-ablation procedure such as PRK or LASEK is a better choice. I would recommend that the patient consult an experienced surgeon to determine whether either LASIK or PRK is a good choice for her.
I am indebted to Ed Sarver at Sarver and Associates for the images of optical imperfections.
Source Information
From Cole Eye Institute, the Cleveland Clinic Foundation, Cleveland.
Address reprint requests to Dr. Wilson at Cole Eye Institute, the Cleveland Clinic Foundation, 9500 Euclid Ave., Cleveland, OH 44195 or at wilsons4@ccf.org.
References
Hersh PS, Stulting RD, Steinert RF, et al. Results of phase III excimer laser photorefractive keratectomy for myopia. Ophthalmology 1997;104:1535-1553.
Pallikaris IG, Siganos DS. Excimer laser in situ keratomileusis and photorefractive keratectomy for correction of high myopia. J Refract Corneal Surg 1994;10:498-510.
Peyman GA. Excimer laser in situ keratomileusis under a corneal flap for myopia of 2 to 20 diopters. Am J Ophthalmol 1996;122:284-285.
Vinciguerra P, Camesasca FI. Butterfly laser epithelial keratomileusis for myopia. J Refract Surg 2002;18:Suppl 3:S371-S373.
Walker MB, Wilson SE. Recovery of uncorrected visual acuity after laser in situ keratomileusis or photorefractive keratectomy for low myopia. Cornea 2001;20:153-155.
Van Gelder RN, Steger-May K, Yang SH, Rattanatam T, Pepose JS. Comparison of photorefractive keratectomy, astigmatic PRK, laser in situ keratomileusis, and astigmatic LASIK in the treatment of myopia. J Cataract Refract Surg 2002;28:462-476.
El-Agha MS, Bowman RW, Cavanagh D, McCulley JP. Comparison of photorefractive keratectomy and laser in situ keratomileusis for the treatment of compound hyperopic astigmatism. J Cataract Refract Surg 2003;29:900-907.
Ambrósio R Jr, Wilson SE. Complications of laser in situ keratomileusis: etiology, prevention, and treatment. J Refract Surg 2001;17:350-379.
Ambrósio R Jr, Wilson SE. LASIK vs LASEK vs PRK: advantages and indications. Semin Ophthalmol 2003;18:2-10.
Stulting RD, Carr JD, Thompson KP, Waring GO III, Wiley WM, Walker JG. Complications of laser in situ keratomileusis for the correction of myopia. Ophthalmology 1999;106:13-20.
Vinciguerra P, Azzolini M, Airaghi P, Radice P, De Molfetta V. Effect of decreasing surface and interface irregularities after photorefractive keratectomy and laser in situ keratomileusis on optical and functional outcomes. J Refract Surg 1998;14:Suppl 2:S199-S203.
Autrata R, Rehurek J. Laser-assisted subepithelial keratectomy for myopia: two-year follow-up. J Cataract Refract Surg 2003;29:661-668.
Phusitphoykai N, Tungsiripat T, Siriboonkoom J, Vongthongsri A. Comparison of conventional versus wavefront-guided laser in situ keratomileusis in the same patient. J Refract Surg 2003;19:Suppl 2:S217-S220.
Porter J, MacRae S, Yoon G, Roberts C, Cox IG, Williams DR. Separate effects of the microkeratome incision and laser ablation on the eye's wave aberration. Am J Ophthalmol 2003;136:327-337.
Vongthongsri A, Phusitphoykai N, Naripthapan P. Comparison of wavefront-guided customized ablation vs. conventional ablation in laser in situ keratomileusis. J Refract Surg 2002;18:Suppl 3:S332-S335.
Excimer laser photorefractive keratectomy (PRK) for myopia and astigmatism: American Academy of Ophthalmology. Ophthalmology 1999;106:422-437.
Sugar A, Rapuano CJ, Culbertson WW, et al. Laser in situ keratomileusis for myopia and astigmatism: safety and efficacy: a report by the American Academy of Ophthalmology. Ophthalmology 2002;109:175-187.(Steven E. Wilson, M.D.)
A 32-year-old woman with moderate myopia and mild dry eye has worn soft contact lenses for 12 years. She notes decreased tolerance of the lenses and must remove them after only four to five hours. On examination, her refraction is –4.25 + 1.0 x 90 (–3.75 diopters of spherical equivalent and 1 diopter of astigmatism at 90 degrees) in the right eye and –3.5 + 0.5 x 88 in the left eye, yielding a best corrected visual acuity of 20/20 in each eye. She asks about refractive surgery. What would you advise?
The Clinical Problem
Vision correction for myopia (nearsightedness) with the use of an excimer laser was approved by the Food and Drug Administration in 1995. It is estimated that more than 1.1 million such procedures were performed in 2003 in the United States.
The first procedure performed for the correction of myopia was a form of ablation of the stromal surface, termed photorefractive keratectomy (PRK), in which the epithelium is removed from the center of the cornea, and excimer laser correction is applied to the stromal surface (Figure 1).1 Visual function improves as the epithelial defect heals. PRK has proven to be very effective, with a good ratio of risk to benefit.2 Delayed recovery of vision after PRK and transient discomfort in some patients, as well as the development of visually significant opacity of the corneal stroma (referred to as haze) in a small percentage of cases, led to the development of laser-assisted in situ keratomileusis (LASIK) in the mid 1990s.3
Figure 1. Differences among the PRK, LASIK, and LASEK Procedures.
In PRK (top panel), the central corneal epithelium is removed and stromal tissue is removed from the stromal surface with an excimer laser. The epithelium heals in three to seven days. In LASIK (middle panel), an epithelial–stromal flap is formed with a microkeratome, and a portion of the stromal bed is removed with an excimer laser. In LASEK (bottom panel), an epithelial flap is formed with the use of dilute ethanol and special instruments, and stromal tissue is removed from the stromal bed with an excimer laser. The surgeon hopes to retain the epithelium in LASEK, but in some cases it is lost, and then the procedure is similar to PRK.
The LASIK procedure (Figure 1) is performed with an instrument called a microkeratome or, more recently, with a femtosecond laser and involves forming an epithelial–stromal flap that is attached to the periphery of the cornea by a hinge of uncut tissue. The flap is typically 8 to 10 mm in diameter and 100 to 180 μm in thickness (approximately 15 to 35 percent of the total corneal thickness), but microkeratomes deviate 20 to 30 μm, so flap thickness varies substantially from eye to eye. The flap is lifted, and the high-energy pulses of the excimer laser are applied to the underlying stromal bed so that the ablated cornea overlies the pupil. The flap is then returned to its original position, without the need for sutures. The greater comfort and faster rate of recovery of vision with LASIK than with PRK led to nearly total abandonment of PRK by many refractive surgeons during the late 1990s.
In an attempt to improve on PRK, another method of surface ablation, called laser-assisted subepithelial keratomileusis (LASEK), has been developed.4 In LASEK, an epithelial flap, attached to the cornea by an uncut hinge at the periphery of the epithelium, is formed with a trephine after brief exposure of the area to 10 to 20 percent ethanol (Figure 1). The epithelial flap is lifted, and excimer laser ablation is applied to the surface of the stroma (as in PRK). Then the epithelial flap is replaced. Proponents of LASEK have claimed that there may be faster recovery of vision, less pain, and a decreased tendency for haze than with PRK. Since there are no definitive studies of the outcomes of LASEK, this article focuses on LASIK and PRK, with PRK including LASEK.
Strategies and Evidence
Benefits of PRK and LASIK
In the hands of an experienced and competent surgeon, the vast majority of patients who undergo PRK or LASIK have an improvement in uncorrected visual acuity.5,6,7 The odds of attaining a desired level of uncorrected visual acuity are inversely related to the baseline level of correction. For example, in an eye with 2 diopters of myopia, the likelihood of attaining uncorrected visual acuity of 20/20 is 70 to 80 percent, and the likelihood of achieving 20/40 or better (the visual acuity, with or without corrective lenses, most often required to drive a motor vehicle in the United States and the United Kingdom) is greater than 98 percent. In contrast, in an eye with 9 diopters of myopia, the likelihood of achieving visual acuity of 20/20 is on the order of 40 to 55 percent, and the likelihood of achieving 20/40 is 95 to 98 percent.5,6,7 Substantial improvement over preoperative uncorrected vision is achieved in most eyes after PRK or LASIK, even if a visual acuity of 20/40 is not achieved.6
Depending on the surgeon and the initial level of attempted correction, approximately 5 to 20 percent of eyes will need a reoperation with additional laser ablation (commonly referred to as an enhancement) to achieve the best possible results.8 The results for visual acuity are almost identical with PRK and with LASIK for low-to-moderate myopia (up to approximately 6 diopters) and low hyperopia, or farsightedness (less than 2 diopters).5 Observational studies show that LASIK performed for myopia or hyperopia above these levels is more likely to yield uncorrected visual acuity of 20/20 or 20/40 than is PRK, because the results of PRK have a greater tendency for regression (the loss of the effect of surgery over time).6
In most eyes, corrections of visual acuity with PRK or LASIK are relatively stable over time, and few patients return after one year with a drop in uncorrected visual acuity. In my experience, late refractive instability occurs in less than 0.2 percent of patients. Such a decline is usually due to further lengthening of the eye that is independent of refractive surgery. If the healing epithelium becomes thicker than normal (epithelial hyperplasia) in the early postoperative period, then greater correction, or even mild overcorrection, may occur during the months or years after surgery.
Risks
There are several complications that can occur with PRK and LASIK.5,6,7,9 These include visual disturbances (Figure 2) such as starbursts, halos, distorted images, and multiple images that are most common at night. These aberrations are more likely to occur with excimer laser ablations that are decentered relative to the pupil, and with corrections that are greater than 7 to 8 diopters of myopia or 2 to 3 diopters of hyperopia. In particular, levels of myopia greater than approximately 10 diopters or of hyperopia greater than approximately 4 diopters are associated with a marked increase in visual complications. The occurrence of such complications has been reduced with improvements in laser technology, such as the development of tracking devices that monitor and adjust for eye movements during ablation, and with better screening of patients to rule out corneal topographic abnormalities, corrections that are too high, or other ocular conditions that increase the chances of diminished vision. Although the incidence of these complications has not been well defined prospectively, retrospective studies have suggested an incidence of less than 1 percent among patients who are considered good candidates for surgery. Other complications, such as infection and ectasia (progressive thinning and irregularity of the cornea), occur at rates much lower than 1 percent.
Figure 2. Rare Optical Imperfections after LASIK Procedures.
Panel A shows the letter E as viewed by an eye with few consequential aberrations (optical imperfections) after a LASIK procedure. Panel B shows the letter E as viewed by an eye with severe aberrations after a LASIK procedure. Before the procedure, the eye had serious abnormalities of the corneal topography. This level of distortion is very rare. Panel C shows the letter E as viewed by an eye with a higher order aberration called coma (because items appear to have a tail like that of a comet) after a complicated LASIK procedure. The vision in an eye with this level of coma would probably have substantial starbursts, in which a point of light appears diffuse, with rays extending from the point.
Physicians and patients should consider procedure-specific complications when deciding between approaches. Complications with LASIK that cannot occur after PRK, since there is no flap in the latter,5,6,7,8,9 include free caps (severed flap hinges), short flaps (incomplete flaps that must be recut later), flap striae (wrinkles that may cause irregular astigmatism), and diffuse lamellar keratitis (inflammation in the stromal interface beneath the flap). The risk of loss of two or more lines of best corrected visual acuity on a Snellen chart is 1 to 2 percent greater with LASIK than with PRK, owing primarily to flap complications. LASIK also may induce transient dry eye, a condition related to the neurotrophic effects of cutting the nerves during flap formation. This condition typically resolves six to nine months after LASIK, when the nerves grow back into the flap.
Haze in the anterior stroma that is related to wound healing is very rare after LASIK but occurs in 1 to 2 percent of eyes that have been treated with PRK for more than 6 diopters of myopia.6,7,9 Other advantages of LASIK over PRK include better comfort and vision in the first few days after surgery.
Complications with LASEK tend to be similar to those with PRK, although delayed healing of the epithelium may be more common with LASEK.4 Observational data are inconsistent with regard to whether recovery of vision is faster, pain is less, or haze is less with LASEK than with PRK for similar levels of myopia or hyperopia.
The rates of complications with LASIK seem to be lower for more experienced surgeons than for those who are less experienced.10 Limited training beyond a residency in ophthalmology is required to perform LASIK or PRK. An ophthalmologist can perform these procedures after completing approximately two days of training with the specific laser and microkeratome and having a more experienced surgeon serve as proctor during the first few procedures. However, some residents complete one-year or two-year fellowships that focus on refractive surgery.
Preoperative Screening
Preoperative screening is critical to maximize the likelihood of a good outcome of laser surgery for vision correction. A comprehensive eye examination that includes slit-lamp biomicroscopy and a funduscopic examination should be performed in all candidates. In general, patients should have normal, healthy eyes.
Surgery is typically performed only in patients over 21 years of age, owing to concern about ongoing changes in refractive error. Regardless of the patient's age, the refraction should have been relatively stable during the previous one to two years. This may be difficult to verify, and often physicians must rely on the patient's history. There is no upper age limit for the patient who plans to undergo surgery, if the eye is normal aside from refractive error.
Contraindications to laser surgery include systemic diseases such as rheumatoid arthritis, systemic lupus erythematosus, and other active immune-related disorders that may be associated with healing abnormalities or complications such as corneal melting, which leads to corneal thinning and even perforation. Pregnancy and lactation are contraindications, owing to temporary changes in refraction. Patients with moderate-to-severe dry eye are not considered to be good candidates for LASIK or PRK. Schirmer's test for tear production, in conjunction with slit-lamp biomicroscopy and the history, may be helpful in evaluating patients who have signs or symptoms of dry eye.
Corneal thickness is measured with an ultrasonic pachymeter. The normal thickness of the cornea ranges from 490 to 650 μm.8 The minimal corneal thickness that is adequate depends on the level of intended correction and the procedure that is planned. The corneal thickness should be considered when deciding whether LASIK, PRK, or no surgery is the best choice in a particular eye. In some cases, LASIK is not a good option, even when the corneal topography is normal, on account of inadequate corneal thickness and an inadequate amount of stromal tissue that will remain untouched posterior to the flap after the laser ablation is completed (residual bed thickness). Many surgeons strive to maintain at least 250 μm of untouched posterior stroma, despite the absence of definitive data suggesting that this is the amount necessary to reduce the risk of corneal ectasia.8
The diameter of the pupil in the dark is also relevant. The greater the diameter in the dark, the higher the incidence of glare, halos, and other visual disturbances, especially in eyes with high myopia. The measurements performed during the preoperative eye examination are particularly critical in cases in which high myopia or high hyperopia are present, given the increased associated risk of visual complications.
Topographic maps of the cornea should show no signs of diseases in which progressive thinning of the cornea tends to occur over time. Corneal diseases, such as keratoconus, that are likely to cause progressive corneal thinning and change in the shape of the cornea, are likely to be associated with poor results of PRK or LASIK. Recently, wavefront analysis has been used to identify aberrations affecting the quality of vision throughout the eye, from the tear film to the retina.
Areas of Uncertainty
PRK versus LASIK
Data from prospective clinical trials directly comparing LASIK with PRK are lacking. Retrospective studies suggest that these procedures have similar results for patients with low-to-moderate myopia (up to 6 diopters)6,7 but that for patients with higher levels of myopia, the average uncorrected visual acuity was better after LASIK than after PRK.6 In eyes with low-to-moderate myopia, the advantages of LASIK over PRK are better comfort and vision during the first few days after surgery. However, the added risk from flap complications with LASIK includes loss of two or more lines of best corrected visual acuity on a Snellen chart.
According to retrospective studies with 6 to 12 months of follow-up, PRK and LASIK have similar results for the correction of low hyperopia (less than 2 diopters).6,7,9 Most surgeons use LASIK rather than PRK for patients with hyperopia, owing to the faster recovery of vision and the lower likelihood of regression.
Astigmatism is a refractive error in which the correcting lens has greater curvature in one meridian (e.g., vertically) than it does in another (e.g., horizontally); it is measured in magnitude (diopters) and in direction (axes). Astigmatism blurs both distance and near vision. By definition, regular astigmatism can be corrected with lenses in which the maximal and minimal axes of the astigmatism are 90 degrees apart. Irregular astigmatism cannot be corrected with glasses and requires rigid contact lenses or surgery for treatment. Most eyes have at least a small level of regular astigmatism, along with myopia or hyperopia.6,7 Regular astigmatism up to approximately 6 diopters can be corrected at the same time as myopia or hyperopia with LASIK or PRK.
PRK is the procedure of choice for the correction of myopia or hyperopia in some eyes.11,12 These include eyes that have thin but otherwise normal corneas; eyes with anterior basement-membrane dystrophy, in which the microkeratome used to make the LASIK flap commonly produces a large epithelial defect resulting in delayed recovery of vision; and moderately dry eyes.
Custom Corneal Ablation Procedures
Custom corneal ablation procedures involve the use of wavefront analysis to measure the aberrations of the eye and to direct the laser ablation of the cornea. These procedures are an alternative to traditional laser ablation techniques.13,14,15 However, there are limited data to compare the custom and the traditional approaches. Studies of one widely used laser system seem to show a modest advantage of custom corneal ablation for lower levels of myopia and hyperopia.13,14 In general, early results suggest that the benefit of custom corneal ablation may increase as the level of attempted correction increases. Further development and assessment of these techniques are needed to clarify their role.
Data are also lacking to compare various laser systems and other equipment with regard to the outcome of LASIK or PRK. Limited data suggest that microkeratomes now are superior to those that were first developed for LASIK, but rigorous comparisons of equipment have not been performed.
Guidelines
General policy statements regarding PRK16 and LASIK17 have been published by the American Academy of Ophthalmology (AAO). These statements, based on a review of the literature, indicate that PRK and LASIK are effective and predictable for low-to-moderate myopia (up to 6.0 diopters) with regard to obtaining very good or excellent uncorrected visual acuity and that they are safe in terms of minimal loss of visual acuity in the great majority of eyes. However, the statements also note that the results of PRK and LASIK are more variable in eyes with moderate-to-high myopia (more than 6.0 diopters). Eyes treated with LASIK for low-to-moderate degrees of hyperopia (less than 2.0 diopters) tended to have similar results to eyes treated for low-to-moderate myopia.17 The AAO summary recommendations for LASIK and PRK can be found at www.aao.org/aao/education/library/recommendations/lasik.cfm and www.aao.org/aao/education/library/recommendations/prk.cfm, respectively. The AAO provides no guidelines for LASEK and no guidance on choosing between LASIK and PRK.
Conclusions and Recommendations
PRK, LASIK, and LASEK are relatively effective and predictable surgical procedures for the correction of nearsightedness and farsightedness with or without low-to-moderate astigmatism. LASIK is associated with faster recovery of vision and greater comfort in the early postoperative period but with a somewhat higher risk of complications, such as loss of two or more lines of best corrected visual acuity on a Snellen chart. For levels of myopia from 6 diopters to approximately 10 diopters, LASIK seems to be superior to PRK or LASEK in terms of the predictability of the results and the rate of recovery of vision, although there are no prospective trials directly comparing these procedures. Some patients are not candidates for LASIK owing to inadequate corneal thickness, moderately dry eyes, or other factors.
The patient described in the vignette has relatively low myopia and low astigmatism and should be a reasonable candidate for a laser-ablation procedure. If her corneal thicknesses are in the normal range, LASIK is a good choice, although there is disagreement among surgeons about what constitutes normal corneal thickness. If her corneas are thinner than normal, or if she has mild-to-moderate dry eye, a surface-ablation procedure such as PRK or LASEK is a better choice. I would recommend that the patient consult an experienced surgeon to determine whether either LASIK or PRK is a good choice for her.
I am indebted to Ed Sarver at Sarver and Associates for the images of optical imperfections.
Source Information
From Cole Eye Institute, the Cleveland Clinic Foundation, Cleveland.
Address reprint requests to Dr. Wilson at Cole Eye Institute, the Cleveland Clinic Foundation, 9500 Euclid Ave., Cleveland, OH 44195 or at wilsons4@ccf.org.
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Sugar A, Rapuano CJ, Culbertson WW, et al. Laser in situ keratomileusis for myopia and astigmatism: safety and efficacy: a report by the American Academy of Ophthalmology. Ophthalmology 2002;109:175-187.(Steven E. Wilson, M.D.)