Journal of Cataract & Refractive Surgery
Volume 31, Issue 6 , Pages 1104-1113, June 2005

Excimer laser photorefractive keratectomy for hyperopia: 7.5-year follow-up

From the Department of Ophthalmology, St. Thomas' Hospital, London, United Kingdom

Accepted 13 October 2004.

Article Outline

Purpose

To report the results of a long-term prospective study to evaluate refractive stability and safety of hyperopic photorefractive keratectomy (H-PRK).

Setting

Department of Ophthalmology, St. Thomas' Hospital, London, United Kingdom.

Methods

Twenty-one patients (49%) (40 eyes) of cohort of 43 patients who participated in 1 of the first clinical trials of H-PRK were assessed at a mean follow-up of 90.7 months (range 62 to 106 months). The H-PRK was performed using a Summit Technology Apex Plus Excimer laser (Summit Technology, Inc.). The mean preoperative spherical equivalent refraction (SEQ) was +4.70 diopters (D) (range +2.00 to +7.50 D). Patients were allocated to 1 of 4 treatment groups based on their preoperative refraction and received 1 of the following spherical corrections: +1.50 D, +3.00 D, +4.50 D, or +6.00 D.

Results

At 7.5 years, the refractive correction remained stable with a mean difference in SEQ between 1 year and 7.5 years of +0.28 D. The mean manifest SEQ was +0.83 D (range +5.00 to −3.00 D). Sixty-seven percent of eyes having corrections of +1.50 D and +3.00 D were within ±1.00 D of the predicted correction. Predictability was poorer with +4.50 D and +6.00 D corrections, with 40% of eyes within ±1.00 D of that expected. An improvement in uncorrected near acuity was achieved in 35 eyes (87.5%), and 35 eyes (87.5%) showed an improvement in uncorrected distance acuity from preoperative levels. Best spectacle-corrected visual acuity (BSCVA) was unchanged or improved from preoperative values in 25 eyes (62.5%). Three eyes (8%) lost 2 lines of Snellen BSCVA, which in 2 cases was attributable to cataract formation. A peripheral ring of haze, 6.5 mm in diameter, appeared in most eyes. Its intensity was greatest at 6 months and then diminished with time. In 10 eyes (25%), remnants of the haze ring were evident at 7.5 years and subepithelial iron rings, 6.5 mm in diameter were evident in 26 eyes (70%). No patient complained of night-vision problems and no eye developed ectasia.

Conclusions

In H-PRK, refractive stability achieved at 1 year was maintained up to 7.5 years with no evidence of hyperopic shift, diurnal fluctuation, or late regression. Peripheral corneal haze decreased with time but was still evident in a number of eyes at the last follow-up visit.

 

Despite the development of numerous technologies, the surgical correction of hyperopia remains a challenge. Procedures such as thermokeratoplasty, hexagonal keratotomy, keratophakia, and keratomileusis have met with limited success including reports of poor predictability, refractive stability, and sight-threatening complications.1, 2, 3, 4, 5 The development of excimer laser technology over the past 2 decades has heralded a new era in refractive surgery. Clinical trials have demonstrated the efficacy of photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK) for the correction of low, moderate, and high degrees of myopia and astigmatism.6, 7, 8, 9, 10, 11, 12, 13 Although results for hyperopic corrections have been encouraging, with very few reports of sight-threatening complications, predictability compared to myopic treatments has been poorer. Studies of hyperopic PRK (H-PRK) and hyperopic LASIK (H-LASIK) have demonstrated acceptable efficacy for corrections up to +4.00 diopters (D), but with limited predictability for higher-order corrections.15, 16, 17, 18, 19, 20

For any surgical procedure, it is essential to constantly monitor long-term stability and efficacy. To date, the longest prospective follow-up studies for excimer laser refractive surgery are mainly confined to myopic PRK and are limited to 6 years.21, 22 With such limited follow-up, the long-term refractive and mechanical stability of the cornea after excimer laser surgery is uncertain. Possibilities of chronic stromal remodeling and late regression of the induced refractive correction cannot be excluded. Therefore, we reevaluated our original cohort of patients who participated in 1 of the first clinical trials of H-PRK in the United Kingdom.16, 23 This present study reports the findings in 21 of the original 43 patients followed up for a mean of 90.7 months (7.5 years) (median 92.0 months, range 62.0 to 106.0 months).

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Patients and Methods 

Cohort and Patient Assessment 

Details of the materials and methods used in this study have been published.16, 23 Following ethics committee approval, a cohort of 43 patients had H-PRK from October 1995 to December 1996. Twenty-one patients (40 eyes) in this cohort attended for long-term follow-up and are the subject of this current paper. The mean age at entry into the study of these 20 patients was 52.9 years (range 22.0 to 70.0 years). Twelve patients were women and 9 were men. The mean preoperative manifest distance refraction (spherical equivalent) was +4.70 D (range +2.00 to +7.50 D). The mean follow-up was 90.7 months (range 62.0 to 106.0 months, median 92.0 months).

Patients were originally recruited from a database of volunteers. Prior to participation, they were counseled to discuss the experimental nature of the procedure. All were over 18 years old. Those with preexisting ocular pathology, previous anterior segment surgery, diabetes, or connective tissue disorders were excluded.

Preoperatively, refraction, autorefraction, keratometry, biomicroscopy, tonometry, and mydriatic fundoscopy were performed. During subjective refraction, reliance was placed on fogging techniques, in particular the +1.00 D blur test, to ensure that an accurate end point was reached consistently. This test assumes that when a further, unnecessary +1.00 D is added, the patient will be “fogged” to approximately 20/60. If, with this extra +1.00 D, Snellen acuity is significantly better than 20/60, too much minus has been prescribed and the spherical component can be revised accordingly. Corneal topography was assessed using a computerized photokeratoscope (TMS-1, Computed Anatomy, Inc.).

Laser 

An SVS Apex Plus laser (Summit Technology, Inc.) with an emission wavelength of 193 nm, a pulse repetition rate of 10 Hz, and a fixed radiant exposure of 180 mJ/cm2 at the cornea was used. This laser uses an ablatable mask with an Axicon to create a hyperopic correction with an optical zone of 6.00 mm and an overall diameter of 9.50 mm.

The ablatable mask is a small, precisely shaped poly-(methyl methacrylate) button mounted in the center of a quartz substrate, which is loaded into a cassette and placed in the path of the excimer laser beam. It is used to transfer the shape of its surface contour to the anterior corneal surface and has been described.16 For hyperopic corrections, the ablatable masks have a convex surface shape, the curvature of which depends on the degree of hyperopia to be corrected and is based on treatment algorithms developed by Munnerlyn.24

The Axicon is a lens–prism combination constructed from quartz and placed within a cassette, which supports the Axicon in path of the beam. It is used to refract the circular 6.50 mm beam cross-section into a 1.50 mm wide annulus with an inner diameter of approximately 6.50 mm and an outer diameter of 9.50 mm to create a 1.50 mm transition zone around the central hyperopic correction.

Surgical Technique 

The aim of the study was not to fully correct each patient's refractive error but to assess the predictability, safety, and stability of H-PRK. On the basis of their preoperative refraction, patients were assigned to 1 of 4 treatment groups: +1.50 D, +3.00 D, +4.50 D, or +6.00 D. Patients in each group received an identical correction, and therefore emmetropia was not the primary aim. The mean attempted correction in the 21 patients (40 eyes) was +4.275 D.

Treatments were performed by a single surgeon (D.O'B.). The eye to be treated was aligned beneath the laser aperture. Topical amethocaine 1% and pilocarpine 4% were instilled. The patient practiced fixating on a target light within the laser aperture, and the noise and burn-like smell were demonstrated by exposing the epithelium to a short series of pulses. The laser was programmed for the hyperopic correction, and the ablatable mask was fixed in its cassette and placed in the cassette port. A lid speculum was inserted, and an area of central corneal epithelium 11.00 mm in diameter was removed with a #64 Beaver blade. Care was taken not to remove epithelium over the limbus. After epithelial debridement, the mask correction was performed. Once completed, the mask cassette was removed and replaced with the Axicon cassette. A further series of laser pulses was delivered to fashion the blend zone around the hyperopic correction. Alignment of the eye was facilitated by 2 helium–neon lasers, 1 at each side of the laser aperture. The intersection of the beams was maintained by the surgeon on the corneal apex over the entrance pupil center, which was facilitated by observing the locations of the beams on the iris and maintaining them at the 3 o'clock and 9 o'clock positions.

Postoperative Treatment and Assessment 

Topical chloramphenicol 1% ointment was applied immediately after the procedure. Oral analgesics were prescribed, and then the eye was padded overnight. Chloramphenicol eyedrops were administered 4 times a day from the time of removing the pad for 10 days. No corticosteroids were used postoperatively.

Postoperative examinations were carried out at 1 week, 2 weeks, 4 weeks, 3 months, 6 months, 9 months, and 12 months; the mean follow-up was 90.7 months (range 62.0 to 106.0 months). At each visit, a full refraction, slitlamp biomicroscopy, and corneal topography were performed. Corneal topography was assessed using the TMS-1 a computerized photokeratoscope at 1 month, 3 months, 6 months, and 12 months. At the last postoperative visit, corneal topography was performed using the Keraton Scout Corneal Analyzer (Optikon 2000) with a 28-mire cone. This was used to analyze higher-order aberrations of the anterior corneal surface. The root mean square (RMS) of total higher-order aberrations, spherical aberration (SA), coma, and trefoil were recorded on a standard 3 mm and 7 mm pupil. To assess disturbances in peripheral corneal transparency, a subjective grading of stromal haze was made at each visit. This was based on the following criteria: 0=no haze; 0.5=trace/just perceptible; 1=easily seen with slitlamp; 2=moderate haze; 3=pronounced haze, iris details visible; 4=scarring, iris details obscured.

Statistical Analysis 

To investigate vectoral astigmatic change in the manifest refraction, vector analysis was performed in all eyes according to the system described by Retzlaff et al.25 Student t tests were used to compare refractive changes and vectoral astigmatic changes preoperatively and postoperatively. Results with P<.05 were considered statistically significant.

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Results 

Refractive Outcome 

The mean changes in refraction with time are shown in Figure 1 and Table 1. In most eyes, there was an overcorrection during the first few months after surgery that diminished toward emmetropia by 3 to 6 months (Figure 1).

Table 1. Mean preoperative refraction and refractive change after H-PRK.
Postop
ParameterPreop1 Month3 Months6 Months12 Months90.7 Months
Spherical equivalent refraction (D)4.70−1.24−0.350.070.550.83
Standard deviation (D)1.801.421.341.601.681.86

At 12 months postoperatively, the mean spherical equivalent was +0.55 D (range −2.75 to +5.125 D) with a mean achieved correction of −4.15 D; 65% of eyes were within ±1.00 D of the intended correction and 42.5%, within ±0.50 D.

At a mean follow-up of 90.7 months (7.5 years), the mean spherical equivalent was +0.83 D (range −3.00 to +5.00 D) with a mean achieved correction of −3.87 D; 52.5% of eyes were within ±1.00 D of the intended correction and 27.5%, within ±0.50 D.

Of eyes having low-order hyperopic treatments of +1.50 D and +3.00 D, 67% were within ±1.00 D of the intended correction at 7.5 years and 27% were within ±0.50 D. Predictability was poorer for +4.50 D and +6.00 D corrections, with 40% of eyes within ±1.00 D of that expected and 24%, within ±0.50 D. Figure 2 shows a scattergram of the achieved versus attempted corrections in the 4 treatment groups at a mean follow-up of 7.5 years.

Figure 3 shows the refractive change with time for patients in the cohort over 50 years of age (mean 59.6 years, range 52.0 to 70.0 years) and under 50 years of age (mean 43.8 years, range 22.0 to 49.0 years) at the time of surgery. Preoperatively, there was no difference in spherical equivalent refractive error or attempted refractive correction between these 2 groups. At 1 month, there was no difference in refractive outcome. However, at 3 months, 6 months, 12 months, and the last follow-up visit, refractive outcome was significantly greater in patients over the age of 50 than in those under 50 years of age (P<.05) (Figure 3).

Refractive Stability 

In the first few months after surgery, there was an overcorrection, with most eyes settling toward emmetropia by 6 months (Figure 1 and Table 1). Refractive stability did not appear to be achieved until the 12th postoperative month. Thereafter, the refractive correction remained relatively stable.

Between 3 months and 6 months, when most eyes were overcorrected and settling toward emmetropia, the mean change in the achieved refractive correction was +0.42 D (range −0.625 to +2.625 D) (P<.001). From 6 to 12 months, regression of the intended correction continued, with a mean change in achieved refractive correction of +0.48 D (range −0.875 to +2.50 D) (P<.0005). Between 12 months and a mean of 7.5 years, however, the mean change in the achieved refractive correction was only +0.28 D (P<.05) (range −2.25 to +1.50 D), which is consistent with the normal age-related physiological hyperopic shift in refractive status of between +0.25 D to +0.50 D expected in middle-aged individuals over a time period of 6.5 years.26, 27, 28 In 4 eyes (10%), the change in spherical equivalent refractive error was +1.00 D or greater between 12 months and 7.5 years. In the remaining 36 eyes (90%), the change was less than +1.00 D and in 10 eyes (25%), there was a small myopic shift.

Astigmatic Change and Vector Analysis 

The mean preoperative manifest refractive cylindrical correction was −0.80 D ± (SD) 0.63 at a mean axis of 76 degrees. At 7.5 years months, the mean manifest refractive cylindrical correction was −1.2625 ± 0.79 D at a mean axis of 91 degrees. In 35 eyes (87.5%), the change in manifest refractive cylinder was 1.00 D or less. In no patient was the change in manifest refractive cylinder greater than 2.00 D. Vector analysis demonstrated a mean change in the manifest refractive cylinder at 7.5 years of 0.97 D (range 0.00 to 2.22 D), with a change of >1.00 D in 19 eyes (47.5%). In 15 eyes that received low-order hyperopic corrections of +1.50 D and +3.00 D, the mean change in manifest refractive cylinder on vector analysis was 0.64 D. This was significantly less than in the 25 eyes that had higher-order corrections of +4.50 D and +6.00 D, in which the mean change was 1.17 D (P<.001).

Analysis of 19 eyes in the cohort with preexisting with-the-rule astigmatism demonstrated a mean change on vector analysis at 7.5 years of 0.86 D, which was similar to 14 eyes with against-the-rule astigmatism preoperatively, in which the mean change was 1.04 D (P>.3). Seven eyes had no refractive astigmatic error preoperatively. Vector analysis at 7.5 years demonstrated mean induced cylindrical correction of 1.14 D. The axis of the induced cylinder in these 7 eyes was with-the-rule in 3 and against-the-rule in 4.

Uncorrected Visual Acuity 

Uncorrected near visual acuity was improved in 35 eyes (87.5%) at 7.5 years compared to preoperative levels. It was N12 or better in 22 eyes (55%). Uncorrected distance acuity was improved in 35 eyes (87.5%) at 7.5 years compared to preoperative levels. It was >20/40 in 23 eyes (57.5%), >20/25 in 13 eyes (32.5%), and equal to or greater than 20/20 in 6 eyes (15%). Two treated eyes were amblyopic and did not have a visual potential of 20/60 preoperatively.

Best Spectacle-Corrected Visual Acuity 

At 7.5 years, best spectacle-corrected visual acuity (BSCVA) was unchanged or improved in 26 eyes (65%) compared to preoperative levels. It was reduced by 1 line in 12 eyes (30%). Two eyes (5%) lost 2 lines of BSCVA, which in 1 case was attributable to cataract formation. No eyes lost more than 2 lines of BSCVA.

Between 12 months follow-up and 7.5 years, BSCVA was unchanged or improved by 1 line in 32 eyes (80%). Seven eyes (17.5%) lost 1 line, 3 of which had early cataract formation, and 1 eye (2.5%) lost 2 lines.

At 7.5 years, 38 eyes (95%) could read N5 or better with appropriate reading spectacle correction at 7.5 years. Two eyes with preexisting ambylopia were unable to achieve N5.

Corneal Haze 

No eye showed any disturbance of central corneal transparency. A peripheral ring of haze, 6.5 mm in diameter, appeared in most eyes 1 month postoperatively. Its intensity was greatest at 6 months and then diminished with time. In 10 eyes (25%), remnants of the haze ring were still evident at 7.5 years. All eyes with haze remnants had higher-order +4.50 D or +6.00 D corrections. Subepithelial iron rings, 6.5 mm in diameter, were evident in 28 eyes (70%) at the last follow-up visit. Iron rings were present in all eyes (100%) with +6.00 D corrections, 55% with +4.50 D, 70% with +3.00 D, and 20% with +1.50 D treatments.

Corneal Topography and Higher-Order Aberrations 

We have previously published data investigating anterior corneal aberrations induced by H-PRK demonstrating the induction of negative spherical aberrations. Analysis of higher-order aberrations of the anterior corneal surface for a 7 mm pupil diameter with the Optikon Scout system at 7.5 years showed high values for root mean square (RMS) of total higher-order aberrations (Table 2). The mean RMS values were 2.03 μm (range 0.466 μm to 4.43 μm). These changes were largely due to high values for coma (mean 1.19 μm, range 0.015 to 2.90 μm) and negative forth order spherical aberration (mean −1.1 μm, range 0.251 to −2.41 μm). The RMS values for high-order aberrations, coma, and negative 4th order spherical aberration were greater in eyes that received high-order hyperopic corrections of +4.50 D and +6.00 D than in those with lower-order corrections of +1.50 D and +3.00 D (P<.03).

Table 2. Measurements of high-order aberrations of the anterior corneal surface with 3.0 mm and 7.0 mm standard pupil sizes.
Aberration (μm)
Pupil (mm)Root Mean SquaredComaTrefoil4th Order Sph Aberration
3.00.160.080.080.01
7.02.031.190.47−1.09

Analysis of higher-order aberrations of the anterior corneal surface for a 3 mm pupil diameter revealed measurements within normal limits, with no differences between the treatment groups.

Complications 

A list of complications at 7.5 years is shown in Table 3. No eyes were found to have late sight-threatening complications.

Table 3. Numbers of eyes with complications 7.5 years after H-PRK in all 4 treatment groups.
Treatment Groups (Eyes Treated)
Complication+1.50 D (n=5)+3.00 D (n=10)+4.50 D (n=11)+6.00 D (n=14)
Peripheral haze +1 or greater∗0028
Reduced BCVA of 1 line2244
Reduced BCVA of 2 lines0101
Astigmatic change >1 D (vector analysis)23411
Astigmatic change >2 D (vector analysis)0011
Age-related cataract0202

BCVA=best corrected visual acuity

∗None had haze greater than 2+

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Discussion 

Refractive Outcome 

This cohort of patients was 1 of the first to have H-PRK as part of a clinical trial in the United Kingdom. Treatments were conducted with a broad-beam laser using a prototype delivery system and first-generation algorithms. At a mean follow-up of 7.5 years, H-PRK, using this system, offered reasonable predictability for the correction of low-order hyperopic refractive errors of +1.50 D and +3.00 D, with 67% of eyes within ±1.00 D of that intended. However, predictability for higher-order corrections was limited (Figure 2). These results are similar to those in other published studies of H-PRK and H-LASIK, which demonstrate reasonable predictability for low to moderate hyperopic corrections but less satisfactory results for higher-order errors.14, 15, 16, 17, 18, 19, 20

Clinical studies of PRK and LASIK for myopia demonstrate similar tendencies, with predictability varying inversely with the degree of attempted correction.7, 8, 9, 10, 11, 12, 13 However, compared to myopic corrections, predictability for comparable hyperopic refractive errors is poorer. Previously, we reported an improvement in the predictability of myopic PRK with an increase in the diameter of the ablation zone from 5.00 mm to 6.00 mm29, 30 and illustrated the importance of the wound diameter and edge profile in the outcome of PRK. By optimizing such parameters in H-PRK and H-LASIK, it may be possible to improve the predictability of higher-order hyperopic corrections. In the literature, the use of larger optical and ablation zones in H-LASIK appears to offer better predictability and refractive stability.18, 31

One interesting finding in our study was the difference in refractive outcome between patients over 50 years of age and under 50 years of age at the time of surgery (Figure 3). Although 1 might expect less aggressive corneal wound-healing responses in an older population, resulting in less regression of effect, the differences were quite dramatic, especially considering the limited numbers in the study (N=40). This suggests that age may be an important factor influencing the outcome of H-PRK.

Refractive Stability 

We previously published data concerning refractive stability after H-PRK in the immediate and early postoperative periods in this cohort of patients at 6 months and 24 months follow-up.16, 23 In summary, after H-PRK, all treatments were accompanied by a myopic shift in the early postoperative period, the magnitude of which increased with the degree of attempted correction. This was followed by a period of regression towards emmetropia, which stabilized between 6 months and 12 months, with the time course being longest for higher-order corrections. While the standard deviations of the postoperative refractive outcomes have been found to increase with an increase in attempted correction, no significant further regression of the attempted correction was encountered between 12 months and 2 years.23

The aim of the present study was to determine the long-term refractive and mechanical stability of the cornea after H-PRK. Since the inception of excimer laser refractive surgery, long-term regression, persistent haze, and a putative decrease in biomechanical corneal strength have been cited as possible late complications and have identified the need for long-term follow-up studies.32, 33, 34 Studies of myopic PRK have demonstrated refractive stability between 3 months and 9 months, depending on the degree of attempted myopic correction, with no regression of effect with up to 5 to 6 years.21, 22 The present study demonstrates good refractive stability between 12 months and a mean of 7.5 years after H-PRK, with no significant evidence of late regression, inferring that chronic stromal remodeling and corneal ectasia are unlikely events in H-PRK. The mean age of our cohort of patients at time of surgery was 52.9 ± 10 years, and therefore the natural history of age-related refractive changes needs to be considered when assessing refractive stability.26, 27, 28 Bengtsson and Grødum,26 in a cross-sectional study, demonstrated a physiological true hyperopic shift of 0.6 D every 10 years between 50 and 70 years. Similarly, a trend toward hyperopic shift between 50 and 65 years, followed by myopic drift after 65 years, was demonstrated in the Blue Mountains Eye Study27 and the study by Saunders.28 In this present study, between 3 months and 12 months postoperatively, there was a mean refractive change of +0.900 D (range −0.875 to +4.124 D), which represented a true regression of effect (Figure 1). However, between 12 months and a mean follow-up of 7.5 years, the refractive change was only +0.28 D. This falls well within the range of the normal age-related hyperopic shift and strongly suggests that stability of refraction is achieved in H-PRK after 12 months. It is of interest that in a long-term, 5-year study of a cohort of 33 patients (47 eyes) who had H-LASIK at our institution using the same laser delivery system, treatment nomograms, and range of refractive correction,35 stability did not appear to be achieved between 1 year and 5 years. In these eyes, the mean refractive change between 1 year and 5 years was +0.53 D (range −1.25 to +3.13 D), with over 50% experiencing a hyperopic drift of over +0.50 D and 28% a drift of over +1.00 D. This apparent difference in long-term refractive stability between H-PRK and H-LASIK is of great interest and requires further investigation.

Astigmatic Change and Vector Analysis 

In myopic PRK, the induction of astigmatism and an increase in preexisting astigmatism have been reported.7, 8, 9, 10 Typically, alterations are small and not clinically significant. In this present study, although the change in the mean preoperative manifest refractive cylindrical correction at 7.5 years was only −0.46 D, with a change of less than −1.00 D in nearly 90% of eyes, vector analysis demonstrated a mean change of almost 1.00 D.

Astigmatic changes in both hyperopic and myopic PRK are difficult to explain. It can be hypothesized that they may result from observer error, decentration of the ablation zone, misalignment of laser optics, variability in laser fluencies, irregular epithelial and stromal wound healing, and/or the induction of higher-order aberrations. It is of note that such changes were of a greater magnitude with higher-order corrections, where variation in laser fluency, wound healing, and the induction of higher-order wavefront anomalies play a more important role.

Visual Outcome 

The mean age of our patients at the time of surgery was over 50 years, with only 26 under 40 years of age. Most were therefore presbyopic as well as hyperopic and without refractive correction had no clear point of focus for distance or near. This may explain why (although in this pilot study the aim was not to fully correct each patient's refractive error and predictability with this prototype system and first generation algorithms was limited) patient satisfaction was still high. At 7.5 years, uncorrected near acuity had improved in almost 90% of eyes, with over 50% able to read N12 or better unaided. Similarly distance acuity had improved in almost 90%, with nearly 60% able to see 20/40 or better unaided. Even in that were eyes were overcorrected and became myopic, patients had a clear point of focus without spectacle correction for near and typically were pleased. It is for this reason that despite the relatively poor predictability compared to myopic corrections, 39 (91%) of the original cohort of 43 patients elected to have both eyes treated.23

In H-PRK, the central corneal stroma is relatively spared, with the main site of corneal ablation in the periphery. It is not surprising, therefore, that in the absence of serious adverse events such as decentration or central stromal inflammation/infection, significant disturbances in BSCVA after H-PRK are rare. In this present study, 2 eyes lost 2 lines of BSCVA. This was attributable to cataract formation in 1 case. No eyes lost more than 2 lines.

It is of note that there were no significant changes in BSCVA between 12 months and 7.5 years with 80% of eyes either unchanged or showing an improvement in BSCVA. Late perturbations of corneal transparency or biomechanical stability of the cornea affecting visual performance were not evident.

Disturbances in Corneal Transparency 

Photorefractive keratectomy is performed in healthy eyes, hence any deterioration in postoperative corneal transparency is of concern. In myopic PRK, subepithelial haze develops over the central cornea by the 4th week, with maximal disturbances at 3 to 6 months.7, 8, 9, 10 As the main site of ablation is in the corneal periphery in H-PRK, impairment of transparency of the axial cornea is not readily apparent; we have previously reported that after 6 months, no disturbances of the axial cornea are detectable either by slitlamp examination or objective measurements.16, 17, 18, 19, 20, 21, 22, 23 As far as the corneal periphery is concerned, we have reported the appearance of a ring of haze, 6.5 mm in diameter, by the 4th week, which reaches maximal intensity at 3 months to 9 months and diminishes at 12 months to 24 months.16, 17, 18, 19, 20, 21, 22, 23 At 7.5 years, the intensity of the haze ring has continued to diminish although in 25%, remnants of peripheral haze remain, especially in higher-order corrections.

At 7.5 years, faint subepithelial iron rings, 6.5 mm in diameter were evident in 28 eyes (70%) and were more common with +6.00 D corrections. The mechanism of iron deposition is unknown; it may perhaps result from the deposition of iron pigments derived from the tear film in the subepithelial layers or may be due to interference with epithelial cell migration in the areas of deepest stromal ablation interfering with the normal cell maturation. This interesting phenomenon is seen after myopic and hyperopic treatments as well as after both LASIK and PRK. The rings appear to be clinically insignificant.

Corneal Aberrations 

We previously reported the induction of negative spherical aberration after H-PRK in patients in this cohort.33 These findings were confirmed in this present study in which analysis of higher-order aberrations of the anterior corneal surface for a 7.0 mm pupil revealed significant negative 4th-order spherical aberration in most eyes. No patients, however, reported any problems with night vision, which would be expected with the induction of negative rather than positive spherical aberration. Total RMS values were generally high and accompanied by elevated measurements for coma and trefoil. These changes were generally more evident with high-order corrections. Despite the high RMS values, however, visual performance was generally good, with little loss of BSCVA. This probably reflects the generally small pupils in our patients, the vast majority of whom were over 45 years of age. Measurements of higher-order aberrations for 3.0 mm pupils were typically within normal limits. The optical zone size in this study was 6.5 mm. The use of larger optical zone treatments for hyperopia should be associated with a reduction in higher-order wavefront anomalies induced by H-PRK and merits further investigation.

Late Complications 

No late stage complications were found in this study. Early cataracts of were present in 4 eyes (10%), and 2 patients (5%) of the original cohort of 4316, 23 had cataract extraction during the follow-up period. This is not unexpected considering that some of the patients in our cohort were in their 70s at the time of surgery and would be expected to develop senile lenticular changes. None of the eyes with cataract seemed to experience a myopic shift due to lenticular index myopia. Dry eyes or night-vision disturbances were not reported. No patient experienced any late problems with epithelial instability.

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Conclusions 

In conclusion, we have shown that refractive stability was maintained for up to 7.5 years after PRK for mild to moderate hyperopia. Peripheral corneal haze decreased with time but was still evident in a number of eyes at the last follow-up visit. There was no evidence of late regression or disturbances in corneal transparency. There were late no sight-threatening complications.

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 Supported by the Iris Fund for Prevention of Blindness in relation to both the purchase and maintenance of the laser.Professor Marshall was a consultant for Summit Technology. No other author has a proprietary or financial interest in any material or method mentioned.

PII: S0886-3350(04)01136-8

doi:10.1016/j.jcrs.2004.10.051

Journal of Cataract & Refractive Surgery
Volume 31, Issue 6 , Pages 1104-1113, June 2005

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