Evaluation of corneal structures in myopic eyes more than twenty-two years after photorefractive keratectomy

The aim of this study is to evaluate corneal epithelial thickness (CET), corneal densitometry (CD) in 84 myopic eyes (57 patients) more than 22 years after photorefractive keratectomy, using anterior segment-optical coherence tomography (AS-OCT) and Scheimpflug imaging system. The CET was significantly higher in all operated eyes than in unoperated eyes in central sector. A statistically significant increase in CD in corneal anterior layer of central sector was shown in groups of operated eyes with greater ablation depth respect to unoperated eyes. While there was no significant difference in CD between the operated eyes groups with lower ablation depth and unoperated eyes. A significant trend toward higher values in anterior CD with deeper ablations in central sector was found. These noninvasive imaging techniques allow to better understand the corneal remodeling process after photoablation and to monitor the patients over time.

IOL power calculation after corneal refractive surgery

PURPOSE

To describe the different formulas that try to overcome the problem of calculating the intraocular lens (IOL) power in patients that underwent corneal refractive surgery (CRS).

METHODS

A Pubmed literature search review of all published articles, on keyword associated with IOL power calculation and corneal refractive surgery, as well as the reference lists of retrieved articles, was performed.

RESULTS

A total of 33 peer reviewed articles dealing with methods that try to overcome the problem of calculating the IOL power in patients that underwent CRS were found. According to the information needed to try to overcome this problem, the methods were divided in two main categories: 18 methods were based on the knowledge of the patient clinical history and 15 methods that do not require such knowledge. The first group was further divided into five subgroups based on the parameters needed to make such calculation.

CONCLUSION

In the light of our findings, to avoid postoperative nasty surprises, we suggest using only those methods that have shown good results in a large number of patients, possibly by averaging the results obtained with these methods.

Changes in contrast sensitivity after Artisan lens implantation for high myopia

PURPOSE

To determine the effects of Artisan lens implantation on contrast sensitivity.

DESIGN

Prospective consecutive interventional case series.

PARTICIPANTS AND CONTROLS

Forty-nine eyes of 30 patients with myopia and myopia with astigmatism, who underwent implantation of the Artisan iris-fixated phakic intraocular lens. Preoperative testing served as the control.

INTERVENTION

Implantation of the Artisan phakic intraocular lens to correct myopia.

MAIN OUTCOME MEASURES

Refractive predictability and Snellen visual acuity were evaluated preoperatively and at least 4 months postoperatively. Additionally, photopic and mesopic contrast sensitivities were measured at 1.5, 3, 6, 12, and 18 cycles per degree, with and without glare testing.

RESULTS

The mean preoperative spherical equivalent (SE) was -12.16 diopters (D) (range, -6.88 to -18.00). The mean postoperative SE was -0.46+/-0.58 D (range, +0.50 to -1.75). Ninety percent of eyes were within 1.00 D of the predicted result, and 39% gained > or =1 lines of best-corrected visual acuity (BCVA). When compared with preoperative measurements, postoperative contrast sensitivity was increased under photopic conditions and slightly decreased under mesopic conditions. Adverse events were one wound leak requiring resuturing in the immediate postoperative period and one subluxed lens after significant blunt trauma. No eyes lost > or =2 lines of BCVA.

CONCLUSIONS

Artisan implantation for the correction of high myopia seems to be a predictable procedure. Increases in photopic contrast sensitivity values after implantation of this phakic intraocular lens stand in distinction to the decreases in photopic contrast sensitivity previously reported after LASIK correction of this degree of myopia.

Quality of vision following clinically successful penetrating keratoplasty

PURPOSE

To evaluate visual function following clinically successful penetrating keratoplasty (PKP).

SETTING

Department of Ophthalmology, Ege University, School of Medicine, Izmir, Turkey.

METHODS

Patient group (PG) included 9 patients (12 eyes) who had clinically successful PKP in our department. The control group (CG) included 12 people (18 eyes) who had no ocular disease other than refractive errors. Those with a visual acuity level less than 20/25 were not included in the study. Contrast sensitivity levels and light threshold values of the central retina were measured; scanning-slit corneal topography-pachymetry and aberrometric analysis were performed.

RESULTS

There were no statistical difference in terms of age (32.55 years +/- 9.25 (SD) in PG, 36.75 +/- 5.85 years in CG; P =.53), cylinder power in plus form (2.60 +/- 1.25 diopter (D) in PG, 2.79 D +/- 2.51 D in CG; P =.88), and spherical equivalent of refractive errors (-3.66 +/- 3.57 D in PG, -5.52 +/- 3.37 D in CG; P =.29) between the PG and CG. Cambridge low-contrast grating scores were 96.5 +/- 41.1 in grafted eyes and 148 +/- 27.7 in CG (P =.004). Central retinal light sensitivity was measured as 29.91 +/- 2.39 db in PG and 33.08 +/- 1.56 db in CG (P =.001). In corneal topographic analysis, mean kappa intercept was 0.69 +/- 0.37 mm in PG and 0.55 +/- 0.24 mm in CG (P =.20). Lower-order Zernike root mean squares (RMS) were 7.30 +/- 3.89 microm for PG and 8.58 +/- 3.46 microm for CG (P =.37). However, higher-order Zernike RMS were 2.15 +/- 0.78 in PG and 0.38 +/- 0.10 in CG, which is a statistically significant difference (P<.001).

CONCLUSIONS

Even though the clinically successful PKP patients have correctable amount of spherocylindrical refractive errors with spectacle lenses, they still have reduced visual quality because of the significantly high amount of higher- order aberrations when compared with naturally occurring refractive errors.

Mesopic Contrast Sensitivity Function After Laser in situ Keratomileusis

PURPOSE

To evaluate contrast sensitivity under mesopic conditions in patients who have undergone uncomplicated bilateral laser in situ keratomileusis (LASIK) for myopia.

METHODS

Best-corrected monocular contrast sensitivity was measured with the Stereo Optical F.A.C.T. chart at 1.5, 3, 6, 12, and 18 c/deg in 20 patients (20 left eyes) before and after bilateral LASIK (with the Nidek EC-5000 excimer laser and the Automated Corneal Shaper). Mean preoperative refractive error was -6.40 +/- 1.28 D (range -4.00 to -8.00 D) and postoperative mean refractive error was -0.41 +/- 0.45 D (range -0.75 to +0.50 D). Contrast sensitivity function was measured before and 6 months after surgery using four different chart luminances: 85, 5, 2.5, and 0.1 cd/m2, the first being a photopic level and the rest mesopic. Log contrast sensitivity values at each spatial frequency were used for statistical analysis and normalized values for graphical representation.

RESULTS

Contrast sensitivity of postoperative LASIK eyes did not differ from preoperative values at a photopic level (85 cd/m2). However, under mesopic conditions (5 cd/m2 or less) a statistically significant reduction (P<.01) in contrast sensitivity was found at high spatial frequencies (12 and 18 c/deg), although no significant contrast sensitivity differences were observed at low and middle spatial frequencies (P>.01 for 1.5, 3 and 6 c/deg).

CONCLUSIONS

LASIK induced significant reductions in contrast sensitivity under mesopic conditions only at high spatial frequencies, even though the photopic contrast sensitivity function was normal.

Mesopic Contrast Sensitivity Function After Excimer Laser Photorefractive Keratectomy

PURPOSE

To evaluate contrast sensitivity under mesopic conditions in patients who had undergone uncomplicated excimer laser photorefractive keratectomy (PRK) for myopia.

METHODS

Monocular contrast sensitivity function was measured with the Stereo Optical F.A.C.T. chart in 26 patients who had received PRK using the Nidek EC-5000 excimer laser system. Mean preoperative refractive error was -6.23 +/- 1.69 D (range, -4.00 to -8.25 D); postoperatively, mean refractive error was -0.36 +/- 0.58 D (range, -0.75 to +0.50 D). Contrast sensitivity function was measured 6 months after surgery using four different chart luminances: 85, 5.0, 2.5, and 0.1 cd/m2, the first being a photopic level and the rest mesopic. A control group of eight emmetropic subjects was also studied to allow comparison of results for statistical purposes.

RESULTS

Logarithmic values of contrast sensitivity at each spatial frequency were used for statistical analysis and normalized values were used for graphical representation. The results showed a statistically significant reduction (P < .01) in contrast sensitivity for the PRK patients in comparison with the control group under mesopic conditions for each spatial frequency tested (1.5, 3, 6, 12, and 18 c/deg), although no significant contrast sensitivity differences were observed between PRK and control groups at the photopic (85 cd/m2) level (P > .01 for all frequencies).

CONCLUSION

Photorefractive keratectomy can induce significant reductions in contrast sensitivity under mesopic conditions, even though the photopic contrast sensitivity function is normal.

Image Quality and Visual Performance in the Peripheral Visual Field Following Photorefractive Keratectomy

PURPOSE

A theoretical and experimental study was performed to assess the possible effects of photorefractive keratectomy (PRK) on retinal image quality and thresholds in the peripheral visual field.

METHODS

Simple optical calculations suggest that although the quality of the retinal image at the fovea of the postoperative PRK eye may be comparable to that in an emmetropic eye, images in the peripheral field may be markedly worse, since peripheral ray bundles may pass partly through ablated and partly through unablated cornea, giving a simultaneous-vision bifocal effect. This would be expected to create an annular zone of confusion, so that light from two different directions in object space arrives at the same point on the retina. The position of this zone and its width are a function of the ablation zone geometry, the attempted correction, and the pupil diameter, but the major effects typically occur at field angles between about 40 degrees and 60 degrees. To explore this effect, Goldmann static quantitative perimetry was carried out along the 0 degree to 180 degrees meridian in six patients who had undergone myopic PRK and eight emmetropes.

RESULTS

The results showed that thresholds for the PRK group at field angles from 40 degrees to 60 degrees were significantly higher than those of the emmetropic group (P < .01). These findings arise because the retinal images in the peripheral field of originally myopic, postoperative PRK patients are, in general, significantly degraded in comparison with those of emmetropes.

CONCLUSION

Although under photopic conditions, retinal image quality close to the visual axis in patients who have been corrected by PRK is similar to that in emmetropes, it may be markedly worse in the peripheral visual field.

Choice of Spatial Frequency for Contrast Sensitivity Evaluation After Corneal Refractive Surgery

PURPOSE

To study the utility of measurements of contrast sensitivity at different spatial frequencies as an index of visual recovery following refractive surgery.

METHODS

Contrast sensitivity at 1.5, 3, 6, 12, and 18 c/deg was measured with the Stereo Optical FACT chart in 20 patients after photorefractive keratectomy (PRK) using the Nidek EC-5000 excimer laser system, and in 18 patients following laser in situ keratomileusis (LASIK). Contrast sensitivity was measured preoperatively and 1, 3, 6, and 12 months after surgery.

RESULTS

Results showed a statistically significant reduction (P<.01) in contrast sensitivity at all spatial frequencies in PRK patients during the first and third month, but contrast sensitivity recovered to preoperative values by 6 months after surgery (P>.1). In LASIK patients, decreased contrast sensitivity values 1 month after surgery were also obtained at all spatial frequencies. After 3 months, contrast sensitivity at 1.5 and 3 c/deg had recovered and did not differ significantly from preoperative values (P>.1), although contrast sensitivity at other frequencies remained reduced (P<.01). At 6 and 12 months, contrast sensitivity at all spatial frequencies did not differ from that obtained preoperatively (P>.1).

CONCLUSIONS

Contrast sensitivity measurements at 6 and 12 c/deg appear to be most useful in the assessment of patients who have undergone laser refractive surgery because defocus and optical aberrations primarily affect the higher spatial frequencies.

Visual performance following photoastigmatic refractive keratectomy: a prospective long-term study

PURPOSE

To prospectively determine the long-term effect of excimer laser photoastigmatic refractive keratectomy (PARK) on visual performance using psychophysical assessments and to identify predictors of poor performance on the psychophysical assessments.

SETTING

University-based hospital, Stanford University, Stanford, California, USA.

METHODS

Ninety-three eyes of 56 patients with a mean of -4.98 diopters +/- 1. 80 (SD) of primary compound myopic astigmatism had PARK using the Summit Apex Plus excimer laser and an erodible mask system. Patients were prospectively evaluated 1, 3, 6, 9, 12, 18, and 24 months following the procedure. Primary outcome variables included changes in the contrast sensitivity function curve (3.0, 6.0, 12.0, 18.0 cycles per degree) under 2 standard illuminance conditions (scotopic and photopic) and changes in the best spectacle-corrected visual performance under scotopic, photopic, and glare conditions.

RESULTS

A relative decline in the contrast sensitivity function curve occurred in the early postoperative period under both scotopic and photopic conditions. This was most pronounced under photopic illuminance and at the low spatial frequencies at the 6 month visit. By 1 year, however, the mean contrast sensitivity at all spatial frequencies and all illuminance conditions had returned to the preoperative level. Further improvements beyond the preoperative level may be related to the independent analysis of retreatment eyes beyond 6 months. A higher level of attempted correction of the spherical equivalent was predictive of an elevated scotopic contrast threshold at the extreme spatial frequencies 6 months after PARK (P <.05). The attempted level of astigmatic correction was predictive of a poor best corrected visual performance under scotopic conditions at 1 month (P <.05). This effect was only temporary and by postoperative month 3, there was no predictive effect of preoperative astigmatism (P >.05).

CONCLUSIONS

Psychophysical assessments may be a more sensitive indicator of decreases in visual performance following excimer laser refractive surgery. The attempted level of correction of spherical equivalent and astigmatism may adversely affect early scotopic visual performance. Decreases in visual performance are temporary, return to normal by 12 months, and remain stable 24 months following PARK.

Improvement of visual function with glare testing after photorefractive keratectomy and radial keratotomy

PURPOSE

To evaluate the effect of a glare source on visual function in patients after photorefractive keratectomy and radial keratotomy.

METHODS

Thirteen patients (22 eyes) who underwent photorefractive keratectomy and 20 patients (40 eyes) who underwent radial keratotomy were evaluated in this cross-sectional study. LogMAR visual acuity and contrast sensitivity were measured. Pupils were measured with the Rosenbaum card. A halogen/tungsten glare source approximated the luminance of headlights of an oncoming car at 100 feet.

RESULTS

In the photorefractive keratectomy and radial keratotomy groups, pupils were significantly smaller (P<.01) and the pupillary clearance of the ablation zone in photorefractive keratectomy and the clear zone in radial keratotomy were significantly larger under the glare condition (P<.01). In the photorefractive keratectomy group, visual acuity and contrast sensitivity under the glare condition were significantly higher than in the no-glare condition (P = .02). In the radial keratotomy group, contrast sensitivity under the glare condition was significantly higher than under the no-glare condition (P = .001 to .003).

CONCLUSIONS

After photorefractive keratectomy or radial keratotomy, the traditional glare source constricted the pupil and partially masked the optical aberrations, which resulted in an improvement in visual function. A "pupil-sparing" aberration test is needed for evaluation of visual function after refractive surgery.

Visual performance after photorefractive keratectomy with a 6-mm ablation zone

PURPOSE

To prospectively examine the effect of photorefractive keratectomy with a 6-mm ablation zone on best-spectacle-corrected visual performance.

METHODS

A prospective study was conducted of 164 eyes of 164 patients with an average (+/-SD) of -4.02 +/- 1.74 diopters (range, -0.63 to -8.38 diopters spherical equivalent). Best-spectacle-corrected high-contrast and low-contrast visual acuity (18% Weber contrast) was measured with both natural and dilated pupils. Patients were tested preoperatively and at 3, 6, and 12 months after photorefractive keratectomy. Photorefractive keratectomy was performed with an argon fluoride excimer laser. Fifty-five eyes of 55 patients also underwent astigmatic keratotomy.

RESULTS

Twelve months after photorefractive keratectomy, best-spectacle-corrected high-contrast visual acuity with natural pupils showed no significant change from preoperative values; mean (+/-SD) change was 0.004 +/- 0.10 logMAR (t = 0.45, P = .65). Best-spectacle-corrected low-contrast visual acuity with natural pupils was significantly reduced compared to baseline; mean (+/-SD) change was 0.04 +/- 0.13 logMAR (t = 3.3, P = .001). The low-contrast loss was larger (1.5 lines) with dilated pupils; mean (+/-SD) change was 0.13 +/- 0.15 logMAR (t = 9.31, P < .001). Greater losses in dilated low-contrast visual acuity were associated with concurrent astigmatic ketatotomy (t = 2.28, P = .025) and corneal haze of grade 1 or greater (t = 2.71, P = .005).

CONCLUSIONS

Reductions in visual performance occur after photorefractive keratectomy with a 6-mm zone. These changes are greatest for low-contrast visual acuity with dilated pupils. Corneal haze and concurrent astigmatic keratotomy are associated with greater losses in low-contrast visual acuity. Best-spectacle-corrected low-contrast visual acuity is a sensitive measure for evaluating visual performance after refractive surgery.

Measuring contrast sensitivity in aretinopathic patients with Insulin Dependent Diabetes Mellitus

Contrast sensitivity measurements were obtained from 66 patients with Insulin Dependent Diabetes Mellitus (IDDM) and no visible diabetic retinopathy (Group A, n = 30 prepubescent and pubescent subjects; Group B, n = 36 postpubescent subjects). Contrast thresholds were determined for stationary and 6.87 Hz phase-alternating gratings at eight spatial frequencies from 0.18 to 15.7 c/deg. Data from each group of diabetic patients was compared with data from puberty-matched normal subjects. Mean value of the last four determinations of glycohemoglobin Alc (HbAlc) was used as a index of glycemic control. We found that patients with IDDM and no retinopathy had normal contrast sensitivity and that contrast thresholds were not significantly related to sexual maturity, duration of diabetes and glycemic control.

Corneal Asphericity in Eye Bank Eyes Implanted with the Intrastromal Corneal Ring

OBJECTIVE

To evaluate the effects of the intrastromal corneal ring, a device developed to reduce myopia, on corneal asphericity in a large set of eye bank eyes.

METHODS

Forty-one deturgesced eye bank eyes were implanted with intrastromal corneal rings of five different thicknesses, ranging from 0.25 mm to 0.45 mm. Corneal asphericity, before and after implantation, was examined using two different metrologies. Corneal asphericity profiles were produced from dioptric power data collected from videokeratography. To statistically assess the corneal asphericity differences between exam times for each intrastromal corneal ring thickness, dependent sample confidence intervals (95%) were calculated for the mean differences between preoperative and postoperative measures for each topographic diameter zone. Laser holographic interferometry was used to inspect corneal asphericity in one eye bank eye case study for four intrastromal corneal ring sizes. Wave unit map and geometric zonal spot ray tracing analyses derived from laser holographic interferometry topography were surveyed.

RESULTS

Videokeratographic analysis suggested that preoperative corneal shape was prolate, i.e., flattened from central to paracentral cornea. Corneal shape became more prolate with intrastromal corneal ring implantation for all intrastromal corneal ring thicknesses. Laser holographic interferometry demonstrated that prolate asphericity was preserved with the intrastromal corneal ring sizes tested and that optical collection efficiency of the cornea was not diminished.

CONCLUSION

Using two different measurement techniques, this eye bank eye study demonstrated that intrastromal corneal rings maintain prolate corneal asphericity.

Near vision contrast sensitivity after photorefractive keratectomy

BACKGROUND

To evaluate near vision contrast sensitivity as a measure of visual performance after photorefractive keratectomy (PRK).

SETTING

LSU Eye Center, New Orleans, Louisiana.

METHODS

Using Holladay Contrast Acuity Test cards, near (reading) vision for five levels of contrast sensitivity was evaluated in a cross section of 53 eyes of 31 patients 25 to 732 days after PRK. Twenty-four normal eyes of 22 myopic patients served as controls.

RESULTS

Near contrast sensitivity decreased at all tested contrast levels for approximately 7 months after PRK and then returned to baseline. This phenomenon paralleled the fluctuation in best corrected distance Snellen acuity.

CONCLUSIONS

These preliminary results indicate that Snellen visual acuity and near contrast sensitivity returned to baseline within 1 year after PRK.

An in vivo investigation of the structures responsible for corneal haze after photorefractive keratectomy and their effect on visual function

PURPOSE

To make serial measurements of corneal haze and microscopic anatomy after photorefractive keratectomy (PRK) and compare the results with visual function measured at the same time points in the same single group of human subjects.

METHODS

Ten patients underwent -6.00-diopter, 6-mm PRK. The patients were reviewed frequently for 12 months. Corneal haze was measured objectively in two ways: (1) an opacification index was determined from the variance in digitized retroillumination images; and (2) light reflected and scattered back from the cornea was assessed by gray-scale analysis of video slit images. In vivo confocal microscopy recorded the anatomic changes occurring in the cornea, and computer analysis of the images quantified the keratocytes and subepithelial deposit. Visual performance was assessed by Snellen visual acuity, contrast sensitivity, and glare-induced visual dysfunction.

RESULTS

In the first week, epithelial irregularity resulted in a transient reduction in all aspects of visual function. In the first month, keratocyte disturbances reduced contrast sensitivity at high frequencies and produced glare. Over the next couple of months, the subepithelial deposit resulted in a more prolonged loss of contrast sensitivity at low frequencies and glare-induced visual dysfunction due to the scattering of light. In several patients, these visual defects persisted after 1 year.

CONCLUSIONS

Epithelial and keratocyte disturbances only transiently affect visual function. The subepithelial deposit is more persistent and can have a lasting effect on visual performance. Therefore, attempts to improve the visual outcome of PRK must be aimed at controlling the synthesis of subepithelial material.

Refractive outcome and corneal topographic studies after photorefractive keratectomy with different-sized ablation zones

BACKGROUND

Discrepancies may still occur between planned and actual refractive correction in eyes undergoing photorefractive keratectomy (PRK). The authors have evaluated the use of an enlarged ablation zone.

METHOD

A computerized corneal analysis system has been used to compare the changes of the anterior surface of the cornea and the refractive changes before and 1, 6, and 12 months after PRK in 113 patients (119 eyes) treated with an excimer laser. The patients were divided into two groups: those treated with a mask with a 5-mm window (59 eyes), and those with a new mask with different window openings according to the degree of refraction at the corneal apex, starting from 5 mm in diameter for treatments less than 6.5 diopters (D) and from 7 mm in diameter for higher treatments (60 eyes). In the first group, treatment ranged from -2.5 to -16 D (mean +/- standard deviation. -8.5 +/- 3.24 D); in the second group, it ranged from -1 to -14 D (-7.8 +/- 3.06 D). Treatments were evaluated with a chi-square test.

RESULTS

In the first group of eyes, 46% were within +/-1 D at 1 month, 37% at 6 months, and 39% at 12 months. In the second group of eyes, 73% were within +/-1 D at 1 month, 60% at 6 months, and 58% at 12 months. The comparison between these data and corneal topographic changes shows that both are more stable and predictable with the new mask compared with the 5-mm mask (P = 0.002, 0.02, 0.04, at 1, 6, and 12 months, respectively).

CONCLUSIONS

The use of larger ablation zones improves the predictability and stability of refractive changes.

Preliminary results of photorefractive keratectomy in active-duty United States Navy personnel

PURPOSE

To evaluate the safety, efficacy, and quality of vision after photorefractive keratectomy (PRK) in active-duty military personnel.

METHODS

Photorefractive keratectomy (6.0-mm ablation zone) was performed on 30 navy/marine personnel(-2.00 to -5.50 diopters [D]; mean, -3.35 D). Glare disability was assessed with a patient questionnaire and measurements of intraocular light scatter and near contrast acuity with glare.

RESULTS

At 1 year, all 30 patients had 20/20 or better uncorrected visual acuity with no loss of best-corrected vision. By cycloplegic refraction, 53% (16/30) of patients were within +/- 0.50 D of emmetropia and 87% (26/30) were within +/- 1.00 D. The refraction (mean +/- standard deviation) was +0.45 +/- 0.56 D (range, -1.00 to 1.63 D). Four patients (13%) had an overcorrection of more than 1 D. Glare testing in the early (1 month) postoperative period demonstrated increased intraocular light scatter (P<0.01) and reduced contrast acuity (with and without glare, (P<0.01). These glare measurements statistically returned to preoperative levels by 3 months (undilated) and 12 months (dilated) postoperatively. Two patients reported moderate to severe visual symptoms (glare, halo, night vision) worsened by PRK. One patient had a decrease in the quality of night vision severe enough to decline treatment in the fellow eye. Intraocular light scatter was increased significantly (>2S D) in this patient after the procedure.

CONCLUSIONS

Photorefractive keratectomy reduced myopia and improved the uncorrected vision acuity of all patients in this study. Refinement of the ablation algorithm is needed to decrease the incidence of hyperopia. Glare disability appears to be a transient event after PRK. However, a prolonged reduction in the quality of vision at night was observed in one patient and requires further study.

Refractive and visual results and patient satisfaction after excimer laser photorefractive keratectomy for myopia

A study of excimer laser photorefractive keratectomy was performed on 108 patients. Variation in individual refractive outcome was noted, particularly for the higher levels of correction. At 12 months, of those with up to 3 dioptres of myopia, 93% were within 1 dioptre of emmetropia, 100% achieved an acuity of 6/12 unaided, 69% were very pleased with their result, and 10% were very disappointed. Of those with between 3.1 and 6 dioptres of myopia, the respective figures were 75%, 73%, 65%, and 11%. For those with over 6 dioptres of myopia the respective figures were 46%, 58%, 38%, and 23%. Some patients were disappointed despite having achieved good unaided visual acuity. Forty seven per cent of patients lost at least one Snellen line of best corrected acuity. Glare was a problem for some of the time in over 60%. Perceived distortion of vision was a problem for all of the time in 11% to 31% of cases depending on the initial level of myopia. In some cases the refraction continued to change between 12 months and 20 months.

Excimer laser photorefractive keratectomy

Excimer laser photorefractive keratectomy has been used for the correction of myopia, hyperopia and astigmatism. This laser removes tissue through a process termed photoablative decomposition, in which incident photon energy is sufficient to break molecular bonds. Selective removal of tissue across the anterior corneal surface results in a change in anterior corneal curvature. The surgical outcome may be influenced also by interindividual variability in wound healing and pharmacologic interventions. The nature of the excimer laser-tissue interaction, and clinical outcomes of predictability, stability and complications of surgery for myopia are discussed in detail.

A multicenter trial of photorefractive keratectomy for residual myopia after previous ocular surgery. Summit Therapeutic Refractive Study Group

PURPOSE

The Summit Therapeutic Refractive Clinical Trial is a nine-center prospective, nonrandomized, self-controlled trial to assess the efficacy, stability, and safety of using a standardized technique of excimer laser photorefractive keratectomy (PRK) to correct residual myopia in eyes with previous refractive surgery or cataract surgery.

PATIENTS AND METHODS

Eligible eyes with a mean residual myopia of -3.7 +/- 1.8 diopters (D) (range, -0.63 to -11.00 D) underwent PRK with a 193-nm excimer laser for myopic corrections between -1.50 and -7.50 D. Standardized settings were used for the ablation zone, ablation rate, repetition rate, and fluence. One hundred seven of the first 114 treated eyes were examined 1 year after PRK, with 98% of eyes having had refractive keratotomy and 2% having had cataract surgery.

RESULTS

One year postoperatively, the mean manifest spherical equivalent refraction was -0.6 +/- 1.4 D (range, -6.50 to 2.50 D); 63% of eyes were within +/-1.00 D of the attempted correction; and uncorrected visual acuity was 20/40 or better in 74% of eyes. Twenty-nine percent of eyes lost two or more Snellen lines of best-corrected visual acuity, and central corneal haze was moderate or severe in 8% of eyes.

CONCLUSIONS

Excimer laser PRK is effective in reducing residual myopia after previous refractive and cataract surgery. However, it is less accurate than PRK in eyes that did not undergo surgery and is more likely to cause a loss of best-corrected visual acuity 1 year after treatment.