Excimer laser photorefractive keratectomy (PRK) for myopia and astigmatism1

Ophthalmic Procedure Preliminary Assessment

Excimer Laser Photorefractive Keratectomy (PRK) for Myopia and Astigmatism American Academy of Ophthalmology The purpose of the Committee on Ophthalmic Procedures Assessment is to evaluate on a scientific basis new and existing ophthalmic tests, devices, and procedures for their safety, efficacy, clinical effectiveness, and appropriate uses. Evaluations include examination of available literature, epidemiologic analyses when appropriate, and compilation of opinions from recognized experts and other interested parties. After appropriate review by all contributors, including legal counsel, assessments are submitted to the Academy’s Board of Trustees for consideration as official Academy policy. Ophthalmology 1999;106:422– 437

Introduction The field of ophthalmology has been evolving at a rapid pace as new technology continues to be integrated into everyday clinical practice. The process of integration is a long, complex one that involves scientific innovators, industry, clinical investigators, and regulatory agencies. The dissemination of accurate, prompt, and comprehensive information is critical in order for practicing ophthalmologists to evaluate new advances fairly. The American Academy of Ophthalmology (AAO) implemented the Ophthalmic Procedure Preliminary Assessment (OPPA) in 1996 to evaluate new and rapidly evolving technology. The goal of the OPPA is to review the scientific literature to distill what is well established about the technology and to help define and refine the important questions to be answered by future investigations, recognizing that emerging technology is characterized by rapid change and expanding clinical indications. The process for creating this assessment on excimer laser photorefractive keratectomy (PRK) involved writing an outline, which was reviewed by the seven-member Ophthalmic Procedures Assessment (OPA) committee. The peer-reviewed literature was analyzed and all possible relevant articles were selected, 285 in all. Members of the committee evaluated the articles for relevance to this OPPA and divided them into high, medium, and low categories based on the quality of study methodology. A methodologist reviewed all of the relevant articles and assigned a good, fair, or poor quality rating. A high-quality study is one in which the design of the study allowed the issue to be addressed, and one that was performed on the population of interest, executed in such a manner as to produce accurate and reliable data, and analyzed using appropriate statistical methods. Less highly-ranked studies lack one or more of Prepared by the Committee on Ophthalmic Procedures Assessment Refractive Surgery Panel, Christopher J. Rapuano, MD, Chair, and approved by the American Academy of Ophthalmology’s Board of Trustees December 14, 1998.

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these components. High-quality studies were given more weight for the purpose of this OPPA. Ophthalmic organizations and industry groups with interests in PRK also were contacted for their input. OPA committee members and other AAO committees reviewed drafts of this OPPA prior to formal approval by the Board of Trustees. PRK is a procedure that is used to treat myopia and astigmatism by surgically modifying the anterior surface of the cornea to flatten the corneal curvature. The Academy selected excimer laser PRK as the topic for this OPPA because it is a relatively new procedure that is used widely around the world and becoming increasingly popular in the United States. The procedure received Food and Drug Administration (FDA) premarket approval for one laser in the fall of 1995 and for a second laser in the spring of 1996 to treat myopia, and for one laser in the spring of 1997 and another in early 1998 to treat astigmatism. A third laser received FDA approval in late 1998 for myopia and astigmatism correction. However, both hardware and software are still undergoing technical adjustments, and the surgical technique and postoperative care for the myopic patient are still being refined. There are multiple refractive surgery alternatives to PRK. Some options have been extensively studied, such as radial keratotomy,1 and many have undergone less scientific scrutiny, such as ALK, laser in-situ keratomileusis (LASIK), intracorneal ring, clear lens extraction, and phakic intraocular lenses. More accurate and reproducible data on all these refractive surgery choices will only improve our ability to treat patients in the best way possible. This OPPA on PRK for myopia and astigmatism will review the following: 1. Excimer laser technology, animal studies, and early human studies to describe the basis for early optimism regarding PRK 2. Current indications for PRK 3. Results of appropriate peer-reviewed literature on PRK 4. Various adjunctive treatments after PRK 5. Complications of PRK

American Academy of Ophthalmology 䡠 Ophthalmic Procedure Preliminary Assessment Table 1. FDA Approved Indications for Excimer Laser PRK

Company Summit

Approval Date 10/95

Degree of Myopia (D) ⫺1.5 to ⫺7.0 D spherical equivalent at the corneal plane ⫺1.0 to ⫺6.0 D spherical equivalent at the corneal plane ⱕ ⫺6.0 D at the spectacle plane

VISX

5/96

VISX

4/97

VISX

1/98

ⱕ ⫺12.0 D at the spectacle plane

Summit

3/98

ⱕ 6.0 D at the spectacle plane ⫺1.0 to ⫺10.0 D at the spectacle plane

Autonomous Technologies

11/98

Degree of Refractive Cylinder (D)

Min. Age (yrs)

Notes

ⱕ 1.5 D allowed but not treatable

21

6.0 mm ablation zone

ⱕ 1.0 D allowed but not treatable

18

6.0 mm ablation zone

⫺0.75 to ⫺4.0 D at the spectacle plane ⫺0.75 to ⫺4.0 D at the spectacle plane ⱕ ⫺4.0 D at the spectacle plane ⱕ ⫺4.0 D at the spectacle plane

21

Minimal ablation zone size 6.0 ⫻ 4.5 mm

21

Multizone treatment 5.0, 5.5, 6.0 mm potential

21

Combined spherical equivalent to be ⱕ ⫺6.0 D 6.0 mm ablation zone for spherical myopia. 5.5 ⫻ 7.5 mm ablation zone for myopic astigmatism. Maximal treatable spherical equivalent is ⫺10.0 D

21

D ⫽ diopters.

6. Ongoing studies involving PRK 7. Important questions about PRK to be answered by future investigations

Technological and Historical Background The term “excimer” is a contraction of “excited dimer,” an energized molecule of two like components, which was the original concept of how these rare gas-halide lasers functioned. Clinical excimer lasers use argon and fluorine gases to generate ultraviolet light with a wavelength of 193 nm. The energy from this wavelength of light is high enough to break the molecular bonds in the cornea and remove tissue. While various wavelengths of light can be produced depending on the gases used (e.g., 248 nm for krypton-fluorine and 351 nm for xenon-fluorine), the 193 nm wavelength is best for ablating corneal tissue. The ablation results in submicron tissue removal per laser pulse with minimal to no damage to the remaining tissue. Early studies of PRK using the excimer laser on rabbits2 and monkeys3–5 demonstrated relatively predictable and reproducible corneal flattening without significant corneal haze formation, and widespread excitement about the potential for this emerging technology ensued. The FDA premarket approval process for PRK began in the late 1980s using a limited number of patients in blind, and subsequently poorly sighted, eye studies. It progressed slowly to include more patients at more centers, culminating in Phase III studies in the mid-1990s that involved hundreds of patients with 1 to 2 years of follow-up. Two excimer laser companies were involved in these early studies: Summit Technology, Inc. (Waltham, MA) and VISX, Inc. (Santa Clara, CA). Summit Technology, Inc. received FDA premarket approval in October 1995 to use excimer laser PRK to treat myopia, and VISX, Inc. received it in May 1996. Both approvals were for 6 mm diameter ablation zones. The

final FDA-approved indications were slightly different for the two companies, based on their somewhat different clinical trials. Since these first FDA approvals, both VISX and Summit received FDA approval to treat astigmatism, VISX received approval to treat higher degrees of myopia, and Autonomous Technologies (Orlando, FL) received approval to treat myopia and astigmatism (see Table 1). Nidek, Chiron, and Meditec are among companies that are also undertaking FDA-sanctioned studies to treat myopia and astigmatism using excimer laser PRK. Earlier clinical investigators used small ablation zones, typically 4.3 to 4.5 mm in diameter, because ablation depth increases geometrically (related to the square of the ablation zone diameter) as the size of the ablation zone increases. Consequently, smaller ablation zones require much shallower ablations to achieve the same refractive correction, and many investigators believed that the less tissue removed the better. The general formula depth 共microns兲 ⫽

共ablation diameter 共mm兲兲2 ⫻ diopters 3

is used to estimate the amount of tissue to remove centrally (Munnerlyn CR, Koons SJ, Marshall J. Photorefractive keratectomy: a technique for laser refractive surgery. J Cataract Refract Surg 1988; 14:46-52. Seiler T, McDonnell PJ. Excimer laser photorefractive keratectomy. Surv Ophthalmol 1995; 40:89-118). Epstein et al6 presented encouraging results using 4.3 to 4.5 mm diameter ablation zones at the AAO Annual Meeting in 1993. However, it became clear that glare and halos could be substantial problems for patients with large pupils, and for many patients under low light conditions. In response physicians began using larger ablation zones, initially 5.0 and 5.5 mm in diameter and more recently 6 mm in diameter and larger. Multiple-zone and multiple-pass PRK techniques are also being studied and may yield better clinical results,

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Ophthalmology Volume 106, Number 2, February 1999 Table 2. Results of PRK Submitted for FDA Approval Results

Summit

Follow-up (yrs) Number of eyes Preoperative myopia range (D) Postoperative refraction ⫾ 1 D of intended correction (%) UCVA ⱖ20/40 (%) 6 mo/12 mo ⱖ20/20 (%) 6 mo/12 mo Loss BSCVA ⬎2 lines (%) 6 mo/12 mo 2 lines (%) 6 mo/12 mo Overcorrection ⬎ 1 D (%) ⬎5 mmHg IOP elevation (%) 6 mo/12 mo 2⫹ stromal haze (%) 6 mo/12 mo Requiring treatment (%)

1 398 ⫺1.5 to ⫺7.8 95–96.7 95/99 65/81 2.1/0 4.7/1.2 3.3–5.0 1.8/0 2.3/0

VISX 2 480 ⫺1 to ⫺6.5 99.6 (2 yr) 93/95/94 (6 mo/12 mo/2 yr) 56/64/58 (6 mo/12 mo/2 yr) 0.4/0 (1 yr/2 yr) 1.8/0.2 (1 yr/2 yr) 0.4 (2 yr) 1.8/3.6 (1 yr/2 yr) 0.6/0.2 (1 yr/2 yr) 3.6

D ⫽ diopters; UCVA ⫽ uncorrected visual acuity; BSCVA ⫽ best spectacle-corrected visual acuity. Unless noted, results are reported for a 1-year follow-up period.

particularly with higher myopes.7–10 Multizone techniques are designed to reduce the depth of the ablation while maintaining a large total ablation zone. This technique may have particular benefit for high myopes, who require deeper ablation depths. In the multipass techniques treatment is divided into segments and performed one segment after the other. This technique may yield a smoother ablation bed and better clinical results.

Results of Excimer Laser Photorefractive Keratectomy A large number of articles on the results of excimer laser PRK have been published in the peer-reviewed literature. Many of the studies described results utilizing machines that are no longer in use (e.g., the Taunton excimer laser), newer machines that are not FDA approved and consequently not readily available in the United States (e.g., Chiron, Nidek, and Novatec), or older techniques such as using nitrogen gas flow over the cornea during the ablation (as VISX did initially) or ablation zones less than 6.0 mm in diameter. The following discussion concentrates on the data that Summit and VISX provided the FDA from their clinical trials, which were the basis for FDA approval, and more recent 6.0 mm diameter ablation zone studies. The Autonomous Technologies LADARVision small spot excimer laser with an integrated infrared eye tracking system received FDA approval too recently to be fully incorporated into this document. Table 2 summarizes the data submitted to the FDA by Summit and VISX. Summit reported PRK results for ⫺1.5 to ⫺7.8 diopters (D) of myopia with 1.5 or less D cylinder to the FDA as part of their premarket approval requirements. They used a 6.0 mm diameter ablation zone on 398 eyes that were followed for 1 year. VISX reported results to the FDA on 480 eyes that had undergone PRK for ⫺1 to ⫺6.5 D of myopia with 1 or less D cylinder with a 6.0 mm diameter ablation zone, and included 2 years of follow-up data.

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Additionally, VISX reported that between 3.1% and 5.4% of eyes had an increase of 1 to 2 D of cylinder in the follow-up period, and no eyes had greater than 2 D increase in cylinder. Abnormal glare testing was noted in 1.6% of eyes at 1 year and in no eyes at 2 years. However, 3.9% of patients reported a significant increase in difficulty with night vision at 2 years. The current clinical excimer lasers use a minimum of a 6.0 mm diameter ablation zone to treat low to moderate myopia, primarily because refractive results are superior and there are fewer subjective complaints of night vision problems when compared to smaller ablation zones. O’Brart et al11 performed a prospective, randomized, doublemasked study using a Summit laser and compared clinical results of 5.0 mm diameter and 6.0 mm diameter ablation zones. They concluded that 6.0 mm diameter ablation zones resulted in less initial overcorrection, improved refractive predictability and stability, less corneal haze, and fewer problems with postoperative glare and halos compared to the 5.0 mm diameter ablation group. In a later study that included further follow-up, the authors reported that the 6.0 mm diameter ablation zone group required fewer retreatments compared to the 5.0 mm diameter ablation group.12 In addition, they found the same benefits resulted from a 6.0 mm diameter single-zone ablation compared with a 6.0 mm diameter multizone treatment. In this study, the optical zone was provided by the 4.5 mm diameter component of the multizone treatment. The 6.0 mm diameter component of the treatment was used to smooth the transition between treated and untreated cornea. Table 3 describes the results of several PRK studies using a 6.0 mm diameter single-zone ablation or multizone technique with at least a 1-year follow-up. Using a VISX laser with a maximum ablation zone diameter of 6 mm, Kremer et al18 treated 92 eyes with low (ⱕ 1 D), moderate (1.25 to 2.50 D), and high (2.75 to 5.0 D) astigmatism (see Table 4). The VISX excimer laser uses elliptical ablation patterns to correct astigmatism, with the smallest diameter of the ellipse set at a minimum of 4.5 mm.

American Academy of Ophthalmology 䡠 Ophthalmic Procedure Preliminary Assessment Table 3. Summary of Results of PRK to Treat Myopia

Study Alio et al8* Alio et al8 Higa et al9†

Original Eyes (no.) 3000

“

Laser VISX 20-20 “

1218

VISX 20-20

Williams7

281

Williams7‡

30

McCarty et al13§ “

347

VISX 20-20 VISX 20-20 VISX 20-20 “

“

240 58

“

Snibson et al14㛳

208

VISX 20-20

Maguen et al15¶ “

240

VISX 20-20 “

“

“

“

Talley et al16# Schallhorn et al17**

91 30

Ablation Zone (mm)

Followup (yrs)

Followup no. (%)

Preop. Myopia (ⴚD)

6.0; 5.0, 6.0 4.5, 5.0 6.0 “

1

1414 (47)

1–14

2

326 (11) 980 (81)

“

6.0; 5.0, 6.0 4.5, 5.0 6.0 5.0, 5.5 6.0 5.0, 5.5, 6.0 6.0

1

5.0, 6.0

1

4.5, 5.0 6.0 6.0; 5.0, 6.0 4.5, 5.0 6.0 6.0

1

% 20/20 or Better UCVA

% 20/40 or Better UCVA

% Loss >2 Lines BSCVA 1

1–19

34

75

7

% Retreated

% within ⴞ1.0 D

% Incr. IOP

37

94

1

36

97

6

72

2

26

6–10

31

89

12

77

5

2

33

10–26

18

42

18

48

5

1

274 (79) 189 (79) 41 (72) 150 (72)

⬍5

47

87

4

5

87

4

5–10

25

71

9

13

65

3

⬎10

2

27

12

19

39

7

1–15

39

77

6

19

81

3

1–6

37

89

4

79

11 anytime

“

48

92

5

83

“

80

90

0

90

1–7.5

80

98

3

93

2–5.5

100

100

0

1

1

“

2

“

“

3

VISX 2015 Summit Omni med

6.0–7.0

1

6.0

1

149 (62) 59 (25) 10 (4) 85 (93) 30 (100)

0

87

D ⫽ diopter; UCVA ⫽ uncorrected visual acuity; BSCVA ⫽ best spectacle-corrected visual acuity; IOP ⫽ intraocular pressure. * 2 ptosis ⬎ 1.0 mm, 29% central islands. † 2 retinal detachments, 9% glare. ‡ 7% central islands. § 1 infectious keratitis, 1 iritis, 1 poor night vision. 㛳 12% central islands at 3 months. ¶ 1 corneal infiltrate, 4 recurrent corneal erosions, 1% central islands at 6 months. # 3% central islands at 6 months. ** 1 glare disability, no central islands.

In total, 86% of patients studied achieved UCVA of 20/35 or better at 1 year after PRK. Of the remaining, 12% reached 20/40 and 2% achieved 20/50 UCVA. The maximum deviation of cylindrical axis from pre- to postoperative was 15 degrees. In a recent study, Colin et al19 treated myopic astigmatism with three different ablation programs, the VISX 20/20 in either sequential mode (58 eyes) or elliptical mode (54 eyes), and the Technolas-Chiron Keracor 116 hybrid scanning laser (38 eyes). Their findings of more favorable results with the elliptical treatment suggest that the laser and ablation algorithm may contribute to the clinical outcome. There is no coupling effect in astigmatic PRK, unlike in astigmatic keratotomy (AK), where the meridian opposite

the AK incisions steepens, resulting in no major change in spherical equivalent. In fact, there tends to be a hyperopic shift in spherical equivalent that is compensated for in the laser software. Long-term follow-up is not available for PRK using a 6.0 mm diameter ablation zone. Stephenson et al20 reported 6-year follow-up data after PRK using a Summit laser with a 4.0 mm diameter ablation zone. Ninety-one percent of patients who underwent a ⫺2.00 D correction and 76% of patients who received a ⫺3.00 D correction were within ⫾ 1 D of the intended refraction at 6 years. About half of the ⫺4.00 D to ⫺5.00 D group were within ⫾ 1 D of the intended refraction at 6 years. They found no regression after 6 to 12 months (depending on the level of myopia), no

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Ophthalmology Volume 106, Number 2, February 1999 Table 4. One-year Results of PRK for Primary Astigmatism Using a VISX Laser Level of Astigmatism

Number of eyes Reduction of astigmatism (%) UCVA 20/35 or better (%)

Low ⱕ1 D

Moderate 1.25–2.5 D

High 2.75–5.0 D

28 48 89

44 68 82

20 81 90

D ⫽ diopter; UCVA ⫽ uncorrected visual acuity.

hyperopic shift, no progressive ectasia, and no diurnal fluctuation. Corneal haze, which was mild at 1 year, continued to fade. No other ocular side effects were noted.

cluded that “there is no justification for routinely submitting all patients to long-term steroid regimens and their associated side effects.” Corbett et al24 reviewed the literature on the pros and cons of postoperative topical steroids and came to a similar conclusion. On the other hand, Fagerholm et al25 used 4.3 and 4.5 mm diameter ablation zones and treated first eyes with dexamethasone for 3 months, but not second eyes. They found that there was less refractive regression at 9 months in the steroid-treated group. Marques et al26 attempted to reverse myopic regression with dexamethasone drops four times a day. In 16 of 18 (78%) eyes, the regression was reversed with steroid drops and there was an associated improvement in corneal haze and BSCVA. The long-term stability of this reversal needs further evaluation. The debate over the effectiveness of long-term use of steroids after PRK continues, especially in light of known steroid complications such as glaucoma and cataract formation.

Postoperative Management Postoperative treatment after PRK varies greatly from surgeon to surgeon. Both Summit and VISX recommend topical antibiotics until the epithelial defect heals, but additional medications and treatments are more controversial. Many physicians use a bandage soft contact lens and/or topical nonsteroidal anti-inflammatory (NSAID) agents to reduce postoperative pain. It is not known if a tightly fitted or a loosely fitted contact lens yields more comfort and better healing. One study using a ProTek bandage soft contact lens after PRK found a statistically significant decrease in pain in the 24 study patients compared to the 23 control patients.21 However, one patient in the contact lens group developed a severe Staphylococcus aureus corneal ulcer, and two patients developed culture-negative infiltrates. Additionally, seven patients requested early contact lens removal because of pain from a tight lens. The bandage soft contact lens is typically removed 2 to 3 days postoperatively. Topical NSAIDs are effective for pain reduction and are usually prescribed two to four times a day for a day or two. However, they can predispose to sterile corneal infiltrates and are generally prescribed with topical steroids as a preventive measure. There is much less agreement as to whether or not the long-term use of topical steroids after PRK improves haze and refractive outcomes. Gartry et al22 performed a ⫺3 D or ⫺6 D PRK treatment on 113 eyes using a 5.0 mm ablation zone. They randomized eyes to receive dexamethasone or placebo drops for 3 months. At 6 weeks, the steroid group had significantly more refractive effect from the PRK, but this difference disappeared by 6 months. They found no difference in corneal haze between the two groups at any point in the study. O’Brart et al23 also performed a prospective, randomized masked study on 86 patients on the effect of a 6-month fluorometholone taper after PRK. They treated ⫺3 D and ⫺6 D myopes with a 5.0 mm diameter ablation zone. In the 1-year follow-up, they found that topical steroids could maintain a hyperopic shift while being used, but that it was not sustained after discontinuation. They found no difference in corneal haze and visual performance between the steroid and no-steroid groups. The authors con-

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Corneal Topographic Analysis Over the past decade, clinicians have found computerized videokeratographic analysis of corneal curvature to be extremely helpful both before and after refractive surgery. Prior to refractive surgery, corneal topography helps determine whether a cornea is regular or irregular, possibly demonstrating signs of contact lens warpage or early keratoconus. After refractive surgery, it can be used to evaluate centration of the ablation, irregular astigmatism, and central islands. Lin27 reviewed corneal topography of 502 consecutive PRK eyes at 1 month, 322 eyes at 3 months, 203 eyes at 6 months, and 92 eyes at 1 year. He used a VISX laser and primarily a 6 mm diameter ablation zone, and he found a mean ablation zone decentration of 0.34 mm. He characterized the topographic patterns as uniform, keyhole, semicircular, and central island (see Figures 1– 4). Central islands, which were found in 26% of eyes at 1 month but in only 2% at 1 year, were associated with decreased BSCVA. Hersh and Schwartz-Goldstein et al28,29 also characterized topographic patterns and analyzed ablation centration results after PRK with a Summit laser using 4.5 and 5.0 mm diameter ablation zones. At 1 year, they found 59% of corneas with a homogeneous topographic pattern and 41% with varying degrees of topographic irregularities, but no central islands. Ablation zone decentration averaged 0.46 mm. There was a trend toward decreased UCVA and increased glare and halos with increased severity of ablation zone decentration. Hersh et al30 later evaluated corneal topographic patterns after PRK with a Summit laser using a 6.0 mm diameter ablation zone in 98 eyes of 90 patients. At 1 year, they again found 59% of corneas with regular patterns and 41% with irregular patterns. Central islands were detected before 3 months, but not between 3 and 12 months postoperatively. Older age and higher attempted corrections were both associated with more irregular patterns. Glare and halo measurements were less severe than in the 4.5 to 5.0 mm diameter treatment zone group.

American Academy of Ophthalmology 䡠 Ophthalmic Procedure Preliminary Assessment

Top left, Figure 1. Subtraction topography shows a uniform central ablation pattern. The attempted correction was ⫺4.50 D. At the first postoperative month, uncorrected visual acuity was 20/20; best spectacle-corrected visual acuity was 20/20 with ⫹0.50. Surface regularity index preoperatively was 0.69; at 1-month postoperatively, the surface regularity index was 0.54. Bottom left, Figure 2. Subtraction topography shows a semicircular ablation pattern. The attempted correction was ⫺3.50 D ⫺0.75 ⫻ 120. At the first postoperative month, uncorrected visual acuity was 20/25; best spectacle-corrected visual acuity with ⫹0.50 was 20/20. Surface regularity index preoperatively was 0.42; at 1-month postoperatively, the surface regularity index was 0.47. Top right, Figure 3. Subtraction topography shows a keyhole ablation pattern. The attempted correction was ⫺6.50 D. At the first postoperative month, uncorrected visual acuity was 20/15; best spectacle-corrected visual acuity with plano was 20/15. Surface regularity index preoperatively was 0.28; at 1-month postoperatively, the surface regularity index was 0.44. Bottom right, Figure 4. Subtraction topography shows a central island ablation pattern. The attempted correction was ⫺5.00 D. At the first postoperative month, uncorrected visual acuity was 20/40; best spectacle-corrected visual acuity with ⫹0.50 ⫺1.25 ⫻ 160 was 20/30. Surface regularity index preoperatively was 0.23; at 1-month postoperatively, the surface regularity index was 0.72. Source: Lin DT. Corneal topographic analysis after excimer photorefractive keratectomy. Ophthalmology 1994; 101:1432–9. Reprinted courtesy of Ophthalmology.

Two studies evaluated central island formation after PRK specifically.31,32 The exact definition of the term “central island” varies in the literature, but a common definition is a topographic area at least 1.5 mm in diameter and at least 3 D steeper than the surrounding zone. Krueger et al31 followed 49 eyes after PRK with a VISX laser (6 mm diameter ablation zone) using a TMS (Computed Anatomy, New York, NY) corneal topography unit. Some eyes were treated with nitrogen gas flow over the cornea during the procedure. They found central islands in 71% of eyes at 1 week, 51% at 1 month, 20% at 3 months, and 11% at 1 year.

There were fewer central islands in the eyes treated with the nitrogen gas flow. Decreased BSCVA was associated with the presence of central islands. Levin et al32 performed corneal topography (TMS) 3 months after PRK for myopia and/or astigmatism using a VISX laser (6 mm diameter ablation zone for myopia ⱕ ⫺5 D, and multizone treatments for ⬎ 5 D) on 156 of 420 (37%) eyes. Sixty-seven percent had islands in the central 3 mm, but these islands were not associated with decreased vision. There was no association of central islands with type of procedure (myopia or myopic astigmatism), age, sex, or

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Ophthalmology Volume 106, Number 2, February 1999 amount of attempted correction. There were fewer central islands in eyes where the aspiration was placed closer to the cornea. Often central islands are associated with mild to moderate decreased BSCVA, and there also may be monocular diplopia or ghosting. Central islands are often discovered within the first few months after surgery when the visual acuity is not as good as expected. Most often central islands resolve, both symptomatically and topographically, over several months. If the central island does not resolve and the topography is stable for several months, it may be correctable with the phototherapeutic keratectomy (PTK) mode using adjustable transition zones. The exact cause of central islands is unknown, although it is suspected that they occur when less ablation than intended is achieved centrally. They are seen more frequently with 6.0 mm diameter ablation zones than with 4.0 and 5.0 mm diameter ablation zones. The decreased central ablation may be due to partial absorption of the incoming laser light by the plume (debris) from the previous laser pulses.33 It also may be due to the laser shock wave redistributing fluid centrally, partially impeding central ablation.31 Techniques to reduce the incidence of central islands are to “pretreat” with additional pulses centrally (Summit), or to redistribute the same number of pulses from peripherally to centrally (VISX) to compensate for the relative central underablation seen in treated tissues. Grimm et al34 did a computerized topographic analysis of 17 eyes and reported an association between localized post-PRK haze and corneal steepening. In addition, they reported that clear areas of cornea within the ablation zone corresponded to corneal flattening based on topographic analysis. This retrospective, descriptive study is important because it provides the foundation upon which to build a prospective study that can correlate clinical healing patterns and corneal topographic patterns better.

Complications Seiler et al35 prospectively studied PRK complications in 193 eyes of 146 patients for up to 2 years postoperatively. Kim et al36 retrospectively reviewed PRK complication data on 2920 eyes of 1821 patients, also up to 2 years postoperatively. These two studies, the Summit and VISX FDA submission data, the Phase III results using the Summit ExciMed UV 200 LA Excimer Laser,37 and other smaller studies and reports are the primary sources for this section.

Early Postoperative Complications Delayed Epithelialization. Re-epithelialization generally occurs within 3 to 4 days after PRK, depending on the size of the initial epithelial defect. Seiler et al35 found that all but 4 of the 193 eyes re-epithelialized within 4 days of surgery. Neither the VISX nor Summit FDA data revealed any eyes with delayed re-epithelialization. Subepithelial Infiltrates. Subepithelial infiltrates (SEIs) have been reported after PRK. In a survey of refractive surgeons, Teal et al38 observed that SEIs occur in approx-

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imately 1 in 300 treated eyes. When a corneal infiltrate is encountered after PRK, infection should be highly suspected even though it is most likely sterile in origin. If infiltrates are significant, corneal cultures may be warranted and intensive antibiotic therapy may be instituted. SEIs have been seen much more frequently since NSAIDs have been used for postoperative pain, and most commonly when the NSAIDs have been used without topical steroids. Consequently, steroids are generally used in conjunction with NSAIDs.39 In its FDA data, VISX reported three eyes (0.4%) with small corneal infiltrates during the first postoperative week, all associated with the use of a bandage soft contact lens. All three were managed medically with no loss of BSCVA. Bacterial Keratitis. One of the most serious complications of PRK is bacterial keratitis. Wee et al40 reported a Pseudomonas aeruginosa corneal ulcer 2 days after PRK in an eye treated postoperatively with a bandage soft contact lens, light patching, and a chloramphenicol drop administered once immediately after surgery and once on the first postoperative day. The eye was treated medically and recovered 20/30 uncorrected vision. Amayem et al41 reported a Staphylococcus epidermidis corneal ulcer 2 days after PRK in an eye treated postoperatively with fluorometholone, tobramycin, and a bandage soft contact lens. The ulcer was treated medically, and the eye recovered 20/20 UCVA. They also reported multiple corneal infiltrates and a large paracentral corneal abscess 6 days after PRK in an eye treated with diclofenac, chloramphenicol, fluorometholone, and a bandage soft contact lens. Corneal cultures were negative. The eye responded to medical therapy, but it was left with a moderate corneal scar and UCVA of 20/150. A chronic corneal ulceration due to topical anesthetic abuse after PRK that eventually required a corneal transplant was reported by Kim et al.42 Topical anesthetic abuse should be considered in any eye that has delayed re-epithelialization after PRK. Sampath et al43 reported a Streptococcus pneumoniae corneal ulcer 9 weeks after PRK in a patient treated with topical steroids 4 times a day. The infection resolved with antibiotic treatment, but the patient was left with a dense central corneal scar and poor visual acuity. An atypical mycobacterium keratitis after PRK was described by Brancato et al.44 Two days after PRK, the cornea had completely re-epithelialized and was subsequently treated with dexamethasone drops four times daily. One month later the eye became irritated after the patient swam in the sea, and 10 days later it was diagnosed with a 6.0 mm central ulceration. It was cultured, grew Mycobacterium chelonae and responded to medical treatment. One year postoperatively, the UCVA was 20/25, correcting to 20/20. In its FDA submission data, Summit did not report any cases of bacterial keratitis, and other than the three corneal infiltrates mentioned, VISX also did not report any cases of bacterial keratitis.

Late Postoperative Complications Refractive. Refractive complications include undercorrection, overcorrection, and induced astigmatism. While un-

American Academy of Ophthalmology 䡠 Ophthalmic Procedure Preliminary Assessment Table 5. Overcorrection Following PRK

Laser

Follow-up (eyes)

Diameter of Ablation Zone (mm)

1

Summit

161

4.5–5.0

Vajpayee et al45

1

VISX

504

6.0

Loewenstein et al46 FDA submission FDA submission Phase III Trial37

1 2 1 1 2

Summit VISX Summit Summit Summit

825 542 398 599 612

5.0 6.0 6.0 4.5, 5.0 4.5, 5.0

Follow-up (yrs)

Seiler et al35

Study

Reported Results % > 1 D Overcorrection % ⬎ 1 D overcorrection at 1 year by degree Mild (ⱕ ⫺3 D) Mod (⫺3 to ⫺6 D) 0 4 % ⬎ 1 D overcorrection at 1 year by degree Low (ⱕ ⫺5 D) High (⫺5 to ⫺10 D) 7 9 4 1.3 3–5 16 12

of myopia High (⬎ ⫺6 to ⫺9 D) 4 of myopia* Extreme (⬎ ⫺10 D) 11

D ⫽ diopter. * Percent of eyes remaining overcorrected at 12 months that were observed to be overcorrected at 1 month (n ⫽ 116).

dercorrections are not infrequent, overcorrections are more serious (see Results section). Studies that have reported on overcorrection following PRK are summarized in Table 5. Astigmatism was generally unchanged from the preoperative value after PRK. At 1 year, Seiler et al35 found ten eyes (5.6%) had between 0.75 and 1.5 D increase in astigmatism in the same axis as preoperatively. At 2 years, the increase in astigmatism was noted in only three of these ten eyes, ranging from 0.75 to 1.0 D of cylinder increase. Two eyes (1.1%) lost 0.75 D of cylinder at 1 and 2 years. No progressive hyperopic shift was noted between 1 and 2 years. One D or greater myopic shift was found in 20% of the highly myopic eyes. The VISX FDA results showed an increase of 1 or more D of cylinder in 3.0% and none with 2 or more D increase in cylinder at 2 years. The Phase III Summit trials showed 24.3% of eyes had a decrease in refractive astigmatism of more than 0.25 D, 56.6% changed by 0.25 D or less, and 19.1% had an increase in refractive astigmatism of more than 0.25 D.37 Loss of Best Spectacle-corrected Visual Acuity. Seiler et al35 found a two-line decrease in BSCVA in two eyes (1.1%) and a two-line increase in BSCVA in six eyes (3.4%) at 1 year. Kim et al36 found a two-line decrease in eight of 2920 eyes (0.27%) at between 10 and 20 months postoperatively.36 Causes of decreased BSCVA included corneal haze, irregular astigmatism (possibly from decentered ablation zones), cataract, and subretinal neovascular membranes. Two-line loss of BSCVA under glare conditions was investigated by Seiler et al.35 At 1 year they found no eyes with a two-line loss of BSCVA in the mildly myopic group, 5% of eyes in the moderately myopic group, and 19% in the highly myopic group. The VISX FDA data revealed 0.2% of eyes with two or more lines of loss of BSCVA at 2 years, while the Summit FDA data demonstrated 1.2% of eyes with two or more lines of loss of BSCVA at 1 year. The Phase III Summit data showed 2.9% of eyes lost two or more lines of BSCVA at 1 year and 6.9% at 2 years.37 Loewenstein et al46 found a two-line decrease in BSCVA in five eyes (0.61%) and a three-line decrease in three eyes (0.36%) at 1 year. Decentration. Centration of the PRK procedure is a critical aspect of the surgical technique utilizing patient or

surgeon fixation. It is believed that centration is important in optimizing the patient’s visual performance and minimizing side effects such as glare and halo. Centration can vary because the pupil does not dilate or constrict symmetrically. Decentrations among five study centers compared in one study (using 4.5 and 5.0 mm ablation zones) were significantly different, ranging from 0.00 to 1.44 mm. Greater decentrations were associated with worse uncorrected postoperative vision and lower patient satisfaction.47 Other Visual Symptoms. Symptomatic problems with hazy vision and halos, especially under low-light conditions, have been attributed to PRK. O’Brart et al48,49 evaluated disturbances of night vision after PRK with 4 and 5 mm diameter ablation zones. With 5 mm diameter ablation zones, 38 of 85 (45%) patients complained of slight disturbance in night vision, while 9 of 85 (11%) complained of significant problems. When comparing patients who had one eye treated with a 4 mm diameter ablation zone and the fellow eye with a 5 mm zone, the 5 mm zone eye had fewer problems with halos. Ghaith et al50 found decreased contrast sensitivity and increased glare disability after PRK with a Summit laser using both 5.0 and 6.5 mm diameter ablation zones. However, the objective findings did not correlate well with patients’ subjective symptoms. Currently, 6 mm diameter or larger ablation zones are generally used to treat myopia, making some of these numbers obsolete. However, the concept of night-vision problems remains, especially in patients with large pupils. Pupil size should be evaluated preoperatively, and patients should be informed of increased problems associated with large pupils. Also, the VISX laser uses an elliptical ablation pattern to correct astigmatism, the short axis of which can reach as small as 4.5 mm in length. This smaller ablation zone may increase the chance of symptomatic glare and halos in those patients treated for astigmatism. The VISX low-myopia PRK FDA data revealed abnormal glare testing in 1.7% of eyes at 1 year, but none in eyes at 2 years. The VISX myopic astigmatism FDA data revealed increased difficulty with night vision in 28 of 108 (26%) eyes at 1 year and 19 of 82 (23%) eyes at 2 years. Eighteen of 108 (17%) at 1 year and 13 of 82 (16%) of eyes at 2 years noted increased sensitivity to bright lights. The Phase III Summit data showed that at 2

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Ophthalmology Volume 106, Number 2, February 1999 Table 6. Grades of Corneal Haze Grade

Description

0 0.5⫹ 1⫹ 1.5⫹ 2⫹ 3⫹ 4⫹ 5⫹

Clear, no haze Barely detectable or trace Mild, not affecting refraction Mildly affecting refraction Moderate, refraction possible but difficult Opacity prevents refraction, anterior chamber easily visualized View of anterior chamber difficult Unable to view anterior chamber

Source: Braunstein RE, Jain S, McCally RL, et al. Objective measurement of corneal light scattering after excimer laser keratectomy. Ophthalmology 1996; 103:439 – 43.

years, 23.7% of patients reported less halo, 26% reported no change, and 50.1% reported worsening of halo.37 After surgery, the glare index averaged 1.53 at 2 years (reported on a scale of 0 to 5 where 0 represented no glare and 5 represented the worst glare). Haze. Corneal haze/scar is one of the most important short- and long-term complications of PRK. Haze is typically graded as shown in Table 6. Most patients develop detectable haze within 3 to 12 weeks after PRK. Reports indicate that it typically reaches trace or 1⫹ levels, stabilizes, and then subsides and often disappears. Corneal haze is associated with refractive regression and decreased uncorrected and best-corrected visual acuity. Eccentric corneal haze can induce irregular astigmatism. In general, 2⫹ haze is considered clinically significant. Table 7 summarizes studies that reported haze after PRK. An objective method to quantify corneal haze and correlate it with clinical outcome would be a welcome addition to the science of refractive surgery. Maldonado et al51 used computerized analysis of standardized slit-lamp photographs to objectively quantify haze in 40 eyes after PRK for high myopia (⫺6 to ⫺22 D). They found a moderate correlation between attempted correction and postoperative haze, and they noted a tendency for the central haze to clear faster than the peripheral haze over time. Using a scatterometer to measure back-scattered light from the cornea in 26 PRK and 8 PTK patients, Braunstein et al52 found light scattering increased after PRK and decreased after PTK. They demonstrated a better correlation between increased corneal light scattering and decreased visual acuity than between clinical haze grading and visual acuity. Ablation depths greater than 80 microns (needed to correct for higher refractive errors) produced significantly greater light scattering and clinical haze than lesser ablation depths. Of more concern is the report by Meyer et al,53 which described five eyes of four patients that developed localized corneal scars, decreased visual acuity, and increased myopia that occurred between 5 and 33 months postoperatively. Such effects are termed late-onset corneal haze. Lipshitz et al54 reviewed their first 1000 patients who underwent PRK for moderate and high myopia (⫺4.25 to ⫺16 D) and who had clear corneas for the first 4 months. They found 18 eyes of 17 patients with late-onset corneal haze that appeared

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between 4 and 12 months postoperatively, causing decreased BSCVA and regression. These authors suggest that excessive ultraviolet light exposure may be one causative factor. The general impression of refractive surgeons has been that PRK haze begins to improve between 3 and 6 months postoperatively, and if it is not significant by 6 months, it will not become so. However, surgeons and patients need to continue to monitor treated eyes for changes in visual acuity, because in the reported cases, late-onset haze developed after patients reported minimal or no haze for at least 4 months following treatment. Several studies have investigated adjunctive treatment to reduce corneal haze after PRK. Gillies et al55 demonstrated reduced corneal haze after PRK with the use of interferon alpha 2b in a small preliminary study of 31 patients. Myers and Thom et al56,57 detected reduced corneal haze in rabbits after PRK when anti-TGF-beta was given. Endothelial Cell Loss. Another important safety issue is endothelial cell health after PRK. Four studies examined a total of 149 eyes pre- and post-PRK to evaluate changes in endothelial cell densities and morphology.58 – 61 They all found no significant changes to the endothelial cell layer. They also found slight improvements in endothelial cell morphology, perhaps attributable to discontinuation of contact lens wear. The VISX FDA subset data on 23 eyes revealed similar results. Globe Integrity. A safety concern following incisional refractive surgery, such as radial keratotomy, is integrity of the globe after trauma. Burnstein et al62 performed deep PRK and PTK on human cadaver and porcine eyes. They concluded that, at ablation depths commonly used clinically, excimer laser PRK does not weaken the cornea and predispose it to globe rupture. Peacock et al63 performed LASIK, ALK, PRK, and RK on cadaver eyes and subjected them to increasing levels of trauma. They found that LASIK, ALK, and PRK eyes were similar to normal eyes, while RK eyes ruptured with significantly less trauma than did normal eyes. Elevated Intraocular Pressure. Seiler et al35 found a 26% to 32% incidence of intraocular pressures (IOPs) elevated by 6 mmHg or greater at some point in the follow-up period after PRK in eyes with between 1 and 9 D of preoperative myopia. They found the same increase in IOP in 6 of 12 eyes treated for myopia greater than 9 D. The postoperative regimen was dexamethasone 0.1% eyedrops following closure of the epithelium. All IOPs normalized within 2 weeks when topical beta-blockers were used and steroids were reduced or discontinued. Kim et al36 found significantly elevated IOPs at 9 months in four eyes of three patients (0.14%) who had received topical corticosteroid eyedrops (0.25% prednisolone acetate) after complete corneal epithelium healing. All four eyes developed optic nerve damage and visual field loss, prompting trabeculectomies in one patient (two eyes) who did not realize the importance of routine follow-up examinations. The VISX FDA data had 0.8% of eyes with more than 10 mmHg of IOP elevation at 6 months, but none at 12 or 24 months. The Summit FDA data had 1.8% of eyes with more than 5 mmHg of elevation in IOP at 6 months, but none at 1 year. In both studies, the standard steroid was fluorometholone 0.1% QID, which was tapered off completely after 4 months. The IOP of patients treated with topical steroids needs to be monitored regularly.

American Academy of Ophthalmology 䡠 Ophthalmic Procedure Preliminary Assessment Table 7. Summary of Studies Reporting Haze Following PRK Laser

Diameter of Ablation Zone (mm)

Kim et al36 Seiler et al35

Summit Summit

5 4.5–5.0

FDA submission

VISX

6.0

FDA submission

Summit

6.0

Phase III37

Summit

4.5–5.0

Study

Reported Results 2⫹ haze in 11 eyes (0.38%) with preoperative spherical equivalents between ⫺5.13 and ⫺13.5 D 2⫹ haze in 10 eyes (5%)* occurred at: 1–3 months postoperatively in 6 eyes 3–6 months postoperatively in 3 eyes after 12 months postoperatively in 1 eye ⱖ 2⫹ haze in 1.3% of eyes at 3–6 months 2⫹ haze in 0.2% of eyes at 2 years 2⫹ haze in 2.3% of eyes at 6 months 2⫹ haze in 0% of eyes at 1 year 2⫹ haze in 6.7% of eyes at 1 year 2⫹ haze in 3.3% of eyes at 2 years

D ⫽ diopter. * Nine eyes had attempted corrections greater than ⫺6 D.

Decreased Accuracy in Measuring Intraocular Pressure. The measurement of IOP using applanation tonometry appears to be lower than expected after PRK. Schipper et al64 have found that Goldmann applanation tonometry can be 0.5 to 3 mmHg lower than expected. Faucher et al65 found a 2.4 mmHg decrease in Goldmann applanation tonometry after PRK when compared to preoperative measurements. Mardelli et al66 found a 0.5 mmHg decrease in Goldmann applanation pressure after PRK when compared to unoperated fellow eyes at 1 year. These authors postulate that with loss of Bowman’s layer, the thinned central cornea may affect the accuracy of the applanation tonometer. Postoperative Ptosis. Postoperative ptosis has been noted to occur after PRK. Topical steroids are associated with ptosis, which generally resolves once the steroids are discontinued. Ptosis may also be due to a levator dehiscence, possibly from the lid speculum, which may be permanent. Retreatment. It is estimated that between 10% and 20% of patients will have a significant amount of residual myopia and thus will be candidates for retreatment.67 The indications for retreatment are typically undercorrection (the desired correction was never achieved) and regression (the desired correction was achieved, but myopia redeveloped), with or without significant corneal haze. It appears that retreatment is better if performed no sooner than 3 months after the original surgery. In addition, the refraction needs to be stable, that is, within 0.50 D for at least 2 to 3 months before a retreatment. If the retreatment is primarily for haze, it is often best to wait longer than 6 months, since haze tends to clear with time. Numerous authors have described their techniques and results of PRK retreatments.67–73 The surgical techniques were similar to primary PRK treatments, except that more eyes underwent epithelial removal with the laser instead of manual debridement. In general, the clinical results were good, although not as good as results from primary procedures. A prospective randomized, masked trial using different retreatment algorithms studied 106 patients retreated with the VISX 20/20 laser and an ablation diameter of 6 mm.67 The initial mean refraction was ⫺6.73 D and the mean refraction before retreatment was ⫺3.14 D. After retreat-

ment, the mean refraction was ⫺1.75 D. Important risk factors in predicting a poorer outcome after retreatment were initial high myopia, regression over ⫺3.5 D, significant anterior stromal haze after the first PRK, and loss of BCVA after the first PRK. Indeed, it was suggested that patients with these risk factors should be retreated with great caution (if at all) because of risks of further regression, loss of visual acuity, and haze. Pop and Aras69 reported repeat PRK treatments on 90 eyes with myopic regression using a VISX laser with a 6.0 mm diameter ablation zone between 6 and 14 months after the primary PRK. They divided their patients into those with minimal to no haze and those with mild to moderate haze. The outcomes for the minimal-to-no-haze group were significantly better than for the mild-to-moderate-haze group. Forty percent of eyes in the haze group were overcorrected by more than 1 D, but they did have less corneal haze and better BSCVA after the retreatment. However, 11% of the population with marked haze lost two or more lines of BSCVA compared to their original value before any PRK. One study of 980 of 1218 consecutive eyes found that 13 eyes were retreated twice because of refractive regression.68 The study suggested that eyes with higher amounts of myopia and astigmatism were more likely to require multiple retreatments. Following the second retreatment, 12 out of 13 eyes attained a BSCVA of 20/40 or better. The amount of astigmatism corrected was improved in 10 of the 13 eyes. A multivariate analysis of the Summit Phase III trials suggested that eyes with higher attempted corrections need a longer time to stabilize. Thus, for patients with higher degrees of myopia, the interval between first and second eye treatments may be relatively longer.70 Table 8 summarizes results of selected additional studies.

Summary In summary, many of the complications seen with PRK are self-limited or treatable, for example, pain, delayed epithelialization, corneal haze, and elevated IOP. Importantly, a relevant complication for the patient to consider is undercorrection and residual myopia and the subsequent need to retreat. The most serious but rarer risks to consider are

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Ophthalmology Volume 106, Number 2, February 1999 Table 8. Results of Selected Retrospective Studies of PRK Retreatment Results After Retreatment

Study

Preop. Mean Spherical Equiv. (D)

Pop et al69

⫺2.13 ⫾ 1.19

56

Pop et al69

⫺3.95 ⫾ 1.92

34

Snibson et al71

⫺2.88 ⫾ 1.49

645

58 (9)

Rozsival et al72

⫺0.88 ⫾ 1.24

566

48 (9)

Matta et al73

⫺2.61 ⫾ 1.62

247

25 (10)

Original No.

Retreated No. (%)

Laser VISX 20/20 VISX 20/20 VISX 20/20 Coherent Schwind Keratom II VISX 20/20

Follow-up Period (mos)

Within ⫾ 1.0 D of Correction

%ⱖ 20/20 or 20/25 UCVA

%ⱖ 20/40 UCVA

%ⱖ2 Lines Loss of BCVA

% Haze ⱖ2

12

84

56

84

0*

0*

12

50

56

76

4*

26*

12

69

64

3

3

6

75

26

83

6

5

12

82

55

77

0

0

D ⫽ diopter; UCVA ⫽ uncorrected visual acuity; BCVA ⫽ best corrected visual acuity. * These results are based on 6 months’ follow-up.

bacterial keratitis, corneal ulcers, and subepithelial infiltrates. It is clear from the description of these complications and their management that PRK requires a comprehensive medical understanding of corneal disease, physiology, use of steroids, and other anti-inflammatory agents, and the use of beta-blockers and other antiglaucoma medications.

Future Developments in Excimer Laser Photorefractive Keratectomy Many of the newest technological developments in PRK (including scanning slit lasers and flying-spot lasers that are being used extensively outside the U.S.) are not widely available in the U.S. because they have not received FDA premarket approval. Scanning slit lasers generate a slit beam that is smaller than broad-beam lasers. The slit beam is scanned over the surface to alter the lasing profile, improving the smoothness of the ablated cornea and allowing for larger-diameter ablation zones. Flying-spot lasers use the slightly longer wavelengths of solid-state lasers in fundamental mode, but they require a tracking mechanism because of the very high frequency and precise placement required for the beam.74 These lasers may be more accurate and efficient and ultimately less costly, because they require much less laser energy to produce each pulse. In addition, the software for flying-spot lasers can be modified to generate various ablation shapes, which can be used to treat astigmatism, hyperopia, and even irregular corneas. Currently, wide-area ablation lasers, such as Summit and VISX, are also actively involved in investigations of hyperopic PRK. In July 1998, the FDA Ophthalmic Devices Advisory Panel recommended approval of the VISX excimer laser to treat hyperopia. In November 1998, VISX received FDA approval to treat from 1 to 6 D of hyperopia. LASIK is a newer procedure that combines ALK and PRK. A microkeratome is used to create an 8 to 11 mm

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diameter circular flap of superficial cornea (approximately 160 microns in depth), which is left on a hinge. The flap is lifted and the excimer laser is used to ablate the stroma, after which the flap is replaced. The advantages of LASIK over PRK are claimed to include less postoperative pain, reduced need for prolonged steroid therapy, absence of large postoperative epithelial defects, fewer central islands, faster visual recovery, and less corneal haze. Reported disadvantages include difficulties in using the microkeratome, problems with flap creation with the microkeratome, poor flap healing, deeper stromal laser ablations, lack of standardized nomograms for tissue ablation, and epithelial growth under the flap. Evidence on the safety and efficacy of LASIK has been accumulating,75 however, the long-term stability and optical quality of LASIK are still not known.76 A randomized, prospective clinical trial was performed to compare PRK and LASIK for moderate to high myopia (⫺6 to ⫺15 D), and is summarized in Table 9.77 Two hundred twenty eyes were entered and followed up to 6 months, receiving a one-pass multizone excimer laser ablation using the Summit Apex excimer laser. One hundred five were randomized to PRK and 115 to LASIK. Although improved UCVA was seen with LASIK in the early postoperative period, there were no statistically significant differences in the 6-month outcomes found between PRK and LASIK patients. The study was limited by short follow-up and relatively few poor outcomes. It was suggested that the issues of a greater incidence of undercorrection in the LASIK group and greater loss of BSCVA in the PRK group be investigated further. A prospective, multicenter randomized trial also examined the corneal topography results of PRK and LASIK.78 A total of 64 eyes were treated with PRK and 54 eyes were treated with LASIK, using a single-pass, multizone ablation with the Summit Apex excimer laser. At 1 month, 63.3% of the eyes in the PRK group were found to be optically irregular (central island, keyhole, semicircular, or irregularly irregular) compared to 19.6% of the eyes in the LASIK group (P ⬍ 0.001). At 3 months, 36.7% of the eyes in the

American Academy of Ophthalmology 䡠 Ophthalmic Procedure Preliminary Assessment Table 9. Results of PRK vs LASIK in a Randomized Prospective Study ●

Results Outcomes

PRK

LASIK

1 day 20/20 or better visual acuity 1 day 20/40 or better visual acuity 6 month 20/20 or better visual acuity 6 month 20/40 or better visual acuity 6 month ⫾ 1.0 D of attempted correction % 6 month ⫾ 0.5 D of attempted correction Mean predictability (D) Average regression from 1 to 6 months (D) % 1 month BSCVA loss of ⱖ 2 lines % 6 month BSCVA loss of ⱖ 2 lines

0.0 4.5 19.1 66.2

10 68.6 26.2 55.7

57.4

40.7

29.4 ⫺0.77 ⫾ 1.0

27.1 ⫺1.43 ⫾ 1.22

0.89 35 11.8

0.55 14.3 3.2

% % % % %

D ⫽ diopter; BSCVA ⫽ best spectacle-corrected visual acuity. Source: Hersh PS, Brint SF, Maloney RK, et al. Photorefractive keratectomy versus laser in situ keratomileusis for moderate to high myopia. Ophthalmology 1998; 105:1512–23.

PRK group and 10.3% in the LASIK group were determined to be optically irregular (P ⫽ 0.004). This suggests a finding of more regular corneal topography after LASIK compared to after PRK. The relative indications for LASIK and PRK in the treatment of myopia and astigmatism await further definitive evaluation, particularly for low to moderate myopia. VISX is conducting ongoing studies on hyperopic astigmatism and mixed astigmatism (myopic astigmatism and hyperopic sphere). Summit is conducting an ongoing study on hyperopia with astigmatism LASIK PRK.

Important Issues to Be Addressed Despite all the published peer-reviewed studies on PRK, numerous questions remain: ● ●

●

● ●

●

What is the long-term stability of eyes many years after PRK? What is the maximum amount of myopia and of astigmatism effectively treated with PRK? What is the maximum amount of myopia and of astigmatism safely treated with PRK? What is the best method to remove the epithelium? Is a manual debridement, laser-scrape, or transepithelial approach best? Should epithelial toxic solutions, such as alcohol or 4% lidocaine, be used prior to debridement? Should a brush be used to remove the epithelium? Does chilled saline pre- and/or post-PRK reduce corneal haze and improve visual outcomes? Do the benefits of pain reduction from bandage soft contact lens use after PRK outweigh the risks? What is the best contact lens to use, a steeper or a flatter one? How long should it be kept in place? What should the role of NSAIDs be post-PRK? Which

●

● ●

●

●

●

● ● ● ●

● ● ●

one of the many available NSAIDs is best? How often, and for how long, should they be used? Should topical steroids be used post-PRK? If so, which one, how frequently, and for how long? What is the best method to quantify post-PRK corneal haze? What is the actual incidence and timetable of clinically significant corneal haze? What is the longterm risk of scar? Are adjunctive treatments to reduce corneal haze effective? Are they ready for human trials? What risk factors are associated with poorer clinical outcomes or complications, and what can be done to ameliorate them? How well are we assessing visual function post refractive surgery? Is it sufficient to use Snellen visual acuity? Is contrast sensitivity better? If so, which contrast sensitivity method is best? Are there even better methods? What subjective measures of visual function and satisfaction can be used to quantify the patient’s experience? How important is pupil size? How is it best measured? Is an infrared pupillometer under different standardized light conditions required? Is the “gold-standard” Placido-disc corneal topography the best to evaluate corneas after refractive surgery? Is an elevation-based system better? Would a pachymetry map be beneficial? What is the best way to test and measure glare disability after PRK? Would ray tracing and topographically controlled ablations result in more accuracy with fewer side effects? What is the optimal ablation diameter and profile? When and how is repeat PRK treatment best given? Does RK or LASIK offer a better alternative to PRK retreatment? Will other procedures such as LASIK replace PRK? Are patients receiving proper informed consent prior to PRK? Does PRK improve patients’ quality of life?

Conclusion PRK is a well-accepted procedure in the United States and around the world, backed by a large volume of published research. The techniques used in the published research and described in this report are already superseded by newer techniques that require quantitative evaluation. Many of the earlier complications and poorer visual function results have been largely resolved by using a larger ablation zone, a multizone technique, and other improvements since the initial FDA trials. It appears to be a safe and effective procedure for the treatment of low to moderate degrees of myopia and astigmatism. Results for high degrees of myopia are associated with poorer outcomes, that is, longer stabilization periods, greater need for retreatment, and increased loss of lines of BSCVA. Retreatment after PRK for low to moderate myopia appears to be safe and effective and decreases residual refractive error. However, there appear to be higher risks associated with PRK retreatment for high

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Ophthalmology Volume 106, Number 2, February 1999 myopia. Emphasis should be placed on enhancing the initial surgical outcome through efficacious PRK techniques. Newer and better lasers, evolution of techniques and postoperative treatments, and more effective methods with which to evaluate patients will certainly emerge from continuing research in the U.S. and around the world. More diligent follow-up after PRK is also important for a more complete evaluation. Research is needed to answer questions about measurement of visual function, optimal abla-

tion diameter, and specific indications and risk factors associated with poorer outcomes with PRK. It is also important to standardize terminology and to arrive at a consensus on the most relevant outcome measures in order to compare results across studies.79 Finally, because PRK is an elective procedure, it is paramount to have a thorough discussion with the patient of expectations, usual visual activities and function, and risks and benefits so that he/she is fully informed and can make an appropriate decision.

Preparation was coordinated by the Committee on Ophthalmic Procedures Assessment Refractive Surgery Panel. *Proprietary Interests Original draft by: Committee on Ophthalmic Procedures Assessments Refractive Surgery Panel

Christopher J. Rapuano, MD, Chair

C3

Peter J. Agapitos, MD William W. Culbertson, MD Vincent P. de Luise, MD Douglas D. Koch, MD Alan Sugar, MD

C5 N N N N

Edited by:

Susan Garratt

N

Managing Editors

Flora Lum, MD Nancy Collins, RN, MPH Margo Leslie

N N N

Approved by:

Board of Trustees, December 14, 1998

*Proprietary Interests Stated CATEGORY

ABBREV

SPECIFIC FINANCIAL INTERESTS

Product Investor

P Pc I

Consultant

Ic C____

Financial interest in equipment, process, or product presented. Such interest in potentially competing equipment, process, or product. Financial interest in a company or companies supplying the equipment, process, or product presented. Such interest in a potentially competing company. Compensation received within the past three years for consulting services regarding the equipment, process, or product presented. Such compensation received for consulting services regarding potentially competing equipment, process, or product.

Cc____

None

434

C1 C2 C3 C4 C5 C6 C7 C8 N

or or or or or or or or

Cc1 Cc2 Cc3 Cc4 Cc5 Cc6 Cc7 Cc8

Examples of compensation received include: 1. Retainer 2. Contract payments for research performed 3. Ad hoc consulting fees 4. Substantial non-monetary prequisites 5. Contribution to research or research funds 6. Contribution to travel funds 7. Reimbursement of travel expenses for presentation at meetings or courses 8. Reimbursement of travel expenses for periods of direct consultation No financial interest. May be stated when such interests might falsely be suspected.

American Academy of Ophthalmology 䡠 Ophthalmic Procedure Preliminary Assessment

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Related Academy Materials Focal Points: Clinical Modules for Ophthalmologists (available by yearly subscription) L’Esperance FA. Choosing the Appropriate PRK Patient (Module 9, 1998). Sher NA. Postoperative Management of the PRK Patient (Module 10, 1998).

Information Statement Use of Laser Surgery in Ophthalmology (1993). Use of Unapproved Excimer Lasers for Refractive Surgery (1997). Patient Information Booklet Photorefractive Keratectomy: PRK (1996). Patient Information Pack PRK Information Pack (1996). Patient Information Slide Script Refractive Surgery (1997). Patient Information Videotape PRK: Treatment for Myopia and Astigmatism (1997). Policy Statement Laser Surgery (1996). To order any of these materials, please call the Academy’s Customer Service number, 415/561-8540.

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