Natural history of corneal topography after excimer laser photorefractive keratectomy

PURPOSE: To investigate the natural history of corneal topography after excimer laser photorefractive keratectomy (PRK). DESIGN: A prospective, single center clinical study. PARTICIPANTS: A total of 40 eyes of 34 patients with myopia were studied. INTERVENTION: Excimer laser photorefractive keratectomy and computer-assisted videokeratography were performed. MAIN OUTCOME MEASURES: Preoperative, 1 week, and 1, 2, and 3 months postoperative topography patterns were compared and changes assessed by averaging defined sectors of the ablation zone in individual maps to produce composite "average" topography maps. RESULTS: Corneal topography was generally not homogeneous at 1 week after PRK. By 3 months, there had been considerable smoothing of corneal contour. A general "central island" effect early in the postoperative period flattened over time. When right and left eyes were evaluated independently, a tendency toward a keyhole-semicircular pattern was seen on average; the maximum flattening was nasal and the least flattening was inferotemporal for both right and left eyes. CONCLUSIONS: A tendency, on average, toward central island and keyhole-semicircular patterns is seen early in the postoperative course after PRK. Central islands tend to evolve into the keyhole-semicircular pattern, and the corneal topography in general after PRK tends to smooth considerably with time.     

Changes in corneal epithelial barrier function after excimer laser photorefractive keratectomy

PURPOSE: To use fluorophotometry to measure corneal epithelial barrier function after excimer laser photorefractive keratectomy (PRK). SETTING: Seoul National University Hospital, Seoul, Korea. METHODS: Twenty-five eyes of 21 patients (13 women, 8 men) had PRK to correct myopia. Corneal epithelial healing time was measured and corneal epithelial permeability to sodium fluorescein evaluated by fluorophotometry 1, 2, and 3 weeks after surgery. RESULTS: Epithelial permeability showed a statistically significant increase 1 week after surgery and returned to its preoperative level 1 week later. Comparative studies according to epithelial healing day and corrected diopter showed results that were not statistically significant (P > .05). CONCLUSION: These results suggest that PRK delays complete reconstruction of corneal epithelial barrier function. In humans, the corneal epithelium regained its normal barrier function 2 weeks after PRK. Thus, at least during these weeks, care should be taken to minimize further epithelial trauma.     

Macular hemorrhage after excimer laser photorefractive keratectomy

We describe three patients with high myopia, 13.0 to 20.0 diopters (D), who had typical myopic macular hemorrhages 1 to 6 months after photorefractive keratectomy (PRK). The hemorrhages eventually resolved, with resultant permanent decrease in vision and pigmentary and atrophic changes in the macula. Highly myopic eyes are predisposed to bleeding in areas of lacquer cracks and to myopic choroidal neovascularization formation. Awareness of potential retinal pathology in patients having PRK and follow-up retinal examinations are warranted.     

Real-time confocal microscopic observations on human corneal nerves and wound healing after excimer laser photorefractive keratectomy

PURPOSE: Corneal wound healing after excimer laser photorefractive keratectomy (PRK) passes through a series of characteristic stages which have earlier been defined by means of histological, histochemical, and biochemical approaches. We investigated the potential of confocal microscopy to verify morphological changes in human corneas in vivo after PRK. METHODS: Ten corneas of eight patients that had earlier undergone PRK were examined at different postoperative time points (7 days-34 months). One of the PRK patients was examined sequentially three times. Three additional corneas, which had earlier undergone corneal grafting surgery and then were subjected to excimer laser photoastigmatic keratectomy (PARK), were studied as well. Seven healthy untreated corneas served as controls to define the normal morphology of human cornea. A tandem scanning confocal microscope (TSCM) was used to generate real-time images of the corneas on an S-VHS videotape. The images were either digitized and further processed or the individual video frames were produced with a video printer. RESULTS: Seven days post-PRK in vivo confocal microscopy revealed the presence of morphologically immature surface epithelial cells. Delicate nerves, activated keratocytes and deposition of extracellular light-reflecting scar tissue were perceived. The epithelium appeared normal one month post-PRK. Ongoing activation of the anterior stromal keratocytes along with extracellular scar tissue were detected. We also observed increasing numbers of regenerating subepithelial nerve leashes with somewhat twisted pattern. Highly reflective, presumably activated keratocytes were no longer detected 6-7 months post-PRK. Hypercellularity with scar tissue could still be found up to 30 months post-PRK. Only one cornea examined 34 months post-PRK showed normal keratocyte morphology and recovery of the anterior stroma. However, the morphology of subepithelial nerves was still somewhat abnormal. The two corneal grafts examined 11 or 32 months post-PARK exhibited a normal-appearing epithelium but considerable stromal hypercellularity and extracellular scar deposition. The subepithelial nerves were poorly regenerated in one eye and fairly well detectable in the other. The third graft examined 15 months post-PARK revealed the presence of enlarged surface epithelial cells and dense stromal scarring but no nerves. CONCLUSION: TSCM clinically confirms the earlier histological data on healing of excimer laser wounds. It offers a distinct improvement in the assessment of excimer laser-treated corneas, as it enables cellular details and nerves to be perceived in vivo. In addition the thickness of the stromal scar can be be measured for e.g. planning of phototherapeutic keratectomy.     

The shape of the corneal apical zone after excimer photorefractive keratectomy

Applying an experimental photo-keratoscope, which assesses the shape of cornea within the pupillary region, to a group of subjects who have undergone excimer laser photorefractive keratectomy over a central 4 mm chord diameter of the cornea, we report the shape of the typical cornea within the ablated zone conforms to a steepening ellipse (average shape factor, 1.25). A statistically significant difference in the mean shape factor (asphericity) between the photoablated and the normal cornea (average shape factor 0.89) was not confirmed. However, there is more variability in the shape factors found in the photorefractive keratectomy group compared with normals, within the same distance from the corneal apex. Using the criterion of overlap within two standard deviations, averaging the vertical horizontal meridians, 75% of photorefractive keratectomy eyes fall within the shape factor limits of the normal eye group. In all cases the post-ablated corneal surface was found to be regular in terms of surface quality alone. The excimer photorefractive keratectomy technique is therefore a clinically acceptable method of refractive surgery.     

Fitting contact lenses after excimer laser photorefractive keratectomy for myopia

The number and percentage of C-cells per unit area were investigated in 2-3-year-old sheep by an immunoperoxidase technique, using a digital-image analysis. C-cells were distributed throughout the thyroid lobes but were not present in either the isthmus or the superior and inferior poles of the thyroid. C-cells were more concentrated in the deep central region of the lobes and decreased gradually toward the periphery. There was a high correlation between the number and the percentage of C-cells per unit area of thyroid gland in sheep. Significant differences were not present between male and female sheep of 2-3 years of age.     

Complications of excimer laser photorefractive keratectomy for myopia

PURPOSE: To evaluate the safety and complication rates of excimer laser photorefractive keratectomy (PRK). SETTING: Assutah Laser Center, Tel-Aviv, Israel. METHODS: This retrospective study evaluated the complication rate after PRK in 825 consecutive patients who had PRK for myopia and had a follow-up of at least 12 months. RESULTS: At 12 months postoperatively, 4.0% of patients suffered from overcorrection and 8.6% from undercorrection. Induced astigmatism developed in 1.4% of all operated eyes. Three percent of the patients had haze, and 3.6% reported glare or halos. Twenty-three eyes (2.7%) lost one line or more of best corrected visual acuity (BCVA). Ptosis developed in 0.4% of the eyes, and 3.5% had a significant increase in intraocular pressure resulting from corticosteroid treatment. There were no complications in 678 eyes (82.5%). CONCLUSION: Eighty-two percent of eyes having PRK did not develop complications. In 18.0% one or more complication, mainly undercorrection, overcorrection, or loss of BCVA, occurred.     

The effect of tranilast on subepithelial corneal opacity after excimer laser keratectomy

Recent studies have reported that tranilast inhibited in vitro the proliferation of keratocytes from corneal subepithelial opacities (haze) and collagen synthesis in cultured corneas after excimer laser photorefractive keratectomy (PRK). In this study 0.5% tranilast eye drops, 0.1% betametazone phosphate eyedrops, and a 0.5% tranilast base solution (control) were administered four times daily to rabbits which had undergone PRK. Weekly evaluation of the inhibitory effect of these drugs on haze began two weeks after surgery according to Fantes' classification. 0.5% tranilast suppressed haze from six weeks to thirteen weeks after PRK (p < 0.05). 0.1% betametazone phosphate showed no effect. These results suggested that 0.5% tranilast had a satisfactory therapeutic effect on haze after PRK.     

Topical interferon alpha 2b for corneal haze after excimer laser photorefractive keratectomy. The Melbourne Excimer Laser Group

PURPOSE: To determine whether topical interferon alpha 2b (IFN-alpha) prevents corneal haze after excimer laser photorefractive keratectomy (PRK). SETTING: Tertiary referral ophthalmic hospital. METHOD: A prospective, double-blind, placebo-controlled, randomized study of 31 patients was undertaken. After surgery in a single institution, patients received a drop of either a placebo or IFN-alpha (5 x 10(6) IU/ml) four times daily for 4 weeks. The main outcome measures were corneal haze, refraction, and visual acuity. RESULTS: The major side effect of interferon alpha treatment was a significant delay in epithelial healing by a mean of 2 days. The means of the average post-treatment clinical scores for haze in all patients up to 12 months after surgery were 0.46 +/- 0.25 for the IFN-alpha group and 0.64 +/- 0.43 for the placebo group (P = .20). Of patients with a correction of greater than 5.00 diopters (D), the IFN-alpha group had significantly less haze over the course of the study (0.39 +/- 0.23 versus 0.98 +/- 0.50; P = .03). After 12 months, the mean absolute spherical equivalent in the two groups was not significantly different (1.02 +/- 1.13 D versus 1.44 +/- 2.64 D). There was a tendency toward better uncorrected visual acuity in the INF-alpha group (P < .10, Kolmogorov-Smirnov). CONCLUSION: Topical IFN-alpha may merit further investigation as a treatment to reduce corneal haze after excimer laser PRK for corrections greater than 5.00 D.     

Results of excimer laser photorefractive keratectomy for the correction of myopia

PURPOSE: This report summarizes the authors' 3-year experience with excimer laser photorefractive keratectomy on 240 eyes of 161 patients. METHODS: With constant laser emission parameters, nitrogen flow across the cornea was used on 79 eyes, whereas 161 eyes had no nitrogen flow. Of the 240 eyes, 74 were operated on without suction ring fixation. Postoperative pain management included patching and oral analgesics in 77 eyes and the use of topical diclofenac or ketorolac and a therapeutic soft contact lens in 163 eyes. Follow-up ranged from 1 month (206 eyes) to 36 months (10 eyes). RESULTS: At 3 months, 88% (144 eyes) had uncorrected visual acuity of 20/40 or better; 86% (151 eyes) had corrected visual acuity to within +/- 1 diopter of intended correction, and 10% (17 eyes) lost two or more lines of best-corrected visual acuity. At 12 months, 89% (122 eyes) achieved uncorrected visual acuity of 20/40 or better, 79% (115 eyes) had corrected visual acuity to within +/- 1 diopter of intended correction, and 4% (6 eyes) lost two or more lines of best-corrected visual acuity. At 24 months, 92% (44 of 48 eyes) had uncorrected visual acuity of 20/40 or better, 86% (44 of 51 eyes) had corrected visual acuity to within +/- 1 diopter of intended correction, and 5% (2 eyes) lost two or more lines of best-corrected visual acuity. At 36 months, 90% (9 eyes) achieved an uncorrected visual acuity of 20/40 or better, 90% (9 eyes) had corrected visual acuity to within +/- 1 diopter of intended correction, and no eyes lost two or more lines of best-corrected visual acuity. CONCLUSIONS: The results obtained with one procedure are within accepted standards of accuracy for refractive surgery, and there is the potential for refinement of the final optical correction. Complication rates are low and are not vision threatening. They included increased intraocular pressure, epithelial "map dot" changes, and recurrent corneal erosion syndrome, "central islands," and others. Photorefractive keratectomy appears to be a safe procedure over the short and medium term.     

Effect of surface photorefractive keratectomy and laser in situ keratomileusis on the corneal endothelium

PURPOSE: To investigate endothelial cell loss in pairs of fresh human autopsy globes following high-diopter myopic photorefractive keratectomy (PRK) or laser in situ keratomileusis (LASIK). SETTING: Center for Research on Ocular Therapeutics and Biodevices and Magill Laser Center for Vision Correction, Storm Eye Institute, Charleston, South Carolina, USA. METHODS: In the first part of the study, 12 globes had either -10 diopters (D) multizone surface PRK or -10 D single-zone LASIK. In the second part, three groups of 5 globes each had -15 D, -20 D, or -25 D multizone-blend LASIK procedures. Fellow globes in both groups were used as untreated controls. Corneoscleral buttons were excised from all globes. Following 7 days in corneal organ culture, the endothelial surface was stained with two vital dyes: calcein-AM and ethidium homodimer. Fluorescence microscopy was used to obtain endothelial cell counts. RESULTS: The mean dead cells per square millimeter (cells/mm2) were 0.94 in the -10 D PRK treated corneas compared with 0.91 in the fellow untreated eyes (P = 0.06(. The mean dead cells/mm2 in the -10 D single-zone LASIK-treated corneas and in the fellow untreated eyes were 0.61 (P = 0.88). The mean dead cells/mm2 in the -15 D, -20 D, and -25 D multizone-blend LASIK-treated corneas were 3.08, 2.33, and 5.55, respectively, compared with 3.49, 1.92, and 5.01 in the fellow untreated eyes (P = 0.276, P = 0.339, and P = 0.427, respectively). Dead cell counts for treated and control paired corneas were highly correlated in all treatment groups. CONCLUSIONS: No significant endothelial cell loss occurred after -10 D PRK or LASIK corrections up to -25 D. Although this study has limitations that prevent direct extrapolation to the clinical situation, it does afford a comparable clinical correlate for endothelial cell toxicity following a typical excimer laser ablations.     

Retinoscopy after excimer laser photorefractive treatments

Photorefractive keratectomy (PRK) is capable of changing the refractive power of the human cornea. Retinoscopy is an objective method of measuring the refractive power of the eye. The authors performed PRK in 28 myopic eyes with the Aesculap Meditec MEL 60 ArF excimer laser. During a prospective study, the subjective and objective refractive power of the eyes were assessed and compared 6 months after PRK. The average preoperative refractive error was: -5.11 +/- 2.51 D and -5.08 +/- 2.20 D on the right side (n = 16) and on the left side (n = 12), respectively. In order to obtain the best corrected visual acuity 6 months after PRK, the right and left eyes needed an average of -0.09 +/- 0.26 D and -0.33 +/- 0.64 D postoperative correction, respectively. The postoperative refractive change could be verified with the retinoscopy at +/-0.5 D accuracy. The authors describe a double refractive corneal ring after PRK, which may disturb retinoscopy. The post-PRK refractive change can be verified by retinoscopy. Postoperative funduscopy, fundus photography and possible future retinal procedures were not disturbed by the double corneal refractive rings in myopic eyes.     

Reliability of intraocular pressure measurements after myopic excimer photorefractive keratectomy

OBJECTIVE: To determine the reliability of intraocular pressure (IOP) measurements by Goldmann applanation tonometry and pneumotonometry in eyes treated with excimer myopic photorefractive keratectomy (PRK). DESIGN: A prospective case series. PARTICIPANTS: Forty consecutive eyes treated with PRK were evaluated. INTERVENTION AND MAIN OUTCOME MEASURES: Central and peripheral corneal Goldmann tonometry and pneumotonometry measurements were done before surgery, at 1 week, and at 1 and 3 months after surgery. RESULTS: The IOP by Goldmann tonometry from the central cornea was significantly lower than the peripheral IOP; however, there was no difference between IOP measured from central and peripheral corneas by pneumotonometry, which, in turn, correlated with peripheral Goldmann measurements. There was a trend, but not a statistically significant correlation, between the spherical equivalent of the treatment and the amount of decrease in central Goldmann IOP. CONCLUSIONS: Pneumotonometry measures the IOP reliably after PRK from all parts of the cornea, whereas central Goldmann tonometry underestimates the IOP by 2.40+/-1.23 mmHg.     

Three-dimensional organization of collagen fibrils during corneal stromal wound healing after excimer laser keratectomy

PURPOSE: To investigate the structural changes in corneal stromal collagen fibrils after excimer laser keratectomy in relation to the degree of corneal haze. SETTING: University of Tokyo Hospital, Tokyo, Japan. METHODS: Corneal haze was quantitatively measured by analyzing the light scattering in Scheimpflug images of the corneas of white rabbits after excimer laser keratectomy. Collagen fibril structure was examined using scanning electron microscopy after chemical digestion with sodium hydroxide solution; the same specimens were examined by transmission electron microscopy after re-embedding. RESULTS: Corneal haze reached a peak 4 weeks after excimer laser keratectomy and then gradually decreased. The collagen fibrils of the normal cornea were regularly arranged parallel to the surface of the cornea, with small interfibrillar distances. After excimer laser keratectomy, the arrangement was highly disordered, with increased interfibrillar distances. These structural changes were most prominent 4 weeks after excimer laser keratectomy. CONCLUSION: The structural changes in the collagen fibrils of the corneal stroma, especially the increase in interfibrillar distances and the disordered arrangement, were associated with corneal haze after excimer laser keratectomy.     

Effect of amniotic fluid in corneal sensitivity and nerve regeneration after excimer laser ablation

To examine the effect of topical application amniotic fluids on the recovery of corneal sensitivity and nerve regeneration after excimer laser photokeratectomy, excimer laser was applied to 18 rabbits (VISX 20/20, 5 Hz, 7 microns depth: nine rabbits; 100 microns depth: nine rabbits). Human amniotic fluid (AF) was topically applied to the right eyes (AF group), and a balanced salt solution (BSS) was applied to the left eyes (BSS group). Corneal sensitivity was measured by using a Cochet-Bonnet aesthesiometer after weeks 1, 2, 4, 5, 8, and 12. Nerve-regeneration status was evaluated after weeks 2, 4, 5, 8, and 12 by gold chloride staining. Corneal sensitivity was initially subnormal and recovered close to the normal level at week 8. The sensitivity was higher in the AF group than in the BSS control group, except at week 2. Subepithelial nerve regeneration at the laser site was detected both at week 4 in the AF group and at week 5 in the BSS group. There was no significant correlation between the nerve-regeneration state and sensitivity changes. Long striated nerve regeneration from the deep stromal nerve was remarkable at the third month, especially in the BSS group. The BSS group showed more marked scarring of the superficial stroma, compared with the AF group. The nerve regeneration at the scar site was discontinued and delayed. Subepithelial leash nerves in the AF group were more abundant than those in the BSS group. The recovery of sensitivity and nerve regeneration were faster in the AF cornea than in the BSS cornea. These results suggest that the factors in AF helped the recovery of corneal sensitivity, nerve regeneration, and reduced scar formation.     

Lack of diurnal variation in vision, refraction, or keratometry after excimer laser photorefractive keratectomy

PURPOSE: To examine whether diurnal variation in visual acuity, spherical equivalent, keratometry, or intraocular pressure occurs in patients who have undergone photorefractive keratectomy. METHODS: Twenty-nine post-photorefractive keratectomy patients underwent morning and evening measurements of visual acuity, spherical equivalent, keratometry, and intraocular pressure at intervals between 5 and 13 months after photorefractive keratectomy. RESULTS: There was no statistically significant post-photorefractive keratectomy diurnal variation in visual acuity, spherical equivalent, or keratometry in the group studied. Diurnal variation in post-photorefractive keratectomy intraocular pressure was not significantly different between patients' treated and untreated eyes. CONCLUSION: Photorefractive keratectomy does not appear to result in diurnal variation in visual acuity, spherical equivalent, or keratometry post-operatively, nor does it seem to effect normal diurnal variation in intraocular pressure.     

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.     

Results of photorefractive keratectomy for myopia with the technolas keracor 116 excimer laser

BACKGROUND: The refractive results of first consecutive 157 myopic eyes undergoing photorefractive keratectomy with the Technolas Keracor 116 excimer laser are presented with a 6-month follow up. METHODS: Preoperative refractive errors (spherical equivalent) ranged between -1.75 diopters (D) and -20.00 D (mean: -7.19 +/- 3.73). A single ablation zone ranging from 4.5 mm to 6 mm was used. In all eyes except for those with myopia over -15 D, emmetropia was a goal. Postoperatively patients used fluoromethalone drops for 1 month. RESULTS: Treated eyes were divided into four groups based on preoperative myopia. Group 1 contained 15 eyes with myopia less than -3 D (93% were within +/- 1 D postoperatively). In Group 2 there were 53 eyes with preoperative myopia between -3.25 D and -6.00 D (64.1% were within +/- 1 D postoperatively). Group 3 consisted of 53 eyes with preoperative myopia between -6.25 D and -10.00 D (41.5% were within +/- D postoperatively). Group 4 contained 36 eyes with myopia over -10.00 D (13.8% were within +/- 1 D of attempted correction and 11.1% were within +/- 1 D postoperatively). CONCLUSIONS: Our study suggests that excimer laser photorefractive keratectomy using a 4.5 to 6.0 mm ablation zone effectively reduces myopia; however, predictability decreases as the attempted correction increases.     

Epithelial removal with the excimer laser (laser-scrape) in photorefractive keratectomy retreatment

1. A study was made relating the involvement of alpha-adrenoceptors in the cardiovascular responses to intracerebroventricular (i.c.v.) injection of B-HT 920, a clonidine-type drug, in conscious sham-operated and sinoaortic-denervated rats. 2. Wistar rats were used, 7 days after the sham operation or sinoaortic denervation. For i.c.v. injection of drugs, a guide cannula had been previously implanted in the left lateral ventricle. 3. In sham-operated rats, cardiovascular responses to B-HT 920 (10-60 microg) were increased blood pressure and bradycardia; but, in sinoaortic-denervated rats, after the pressor response, a decrease in blood pressure also was seen. The responses to this agent were greater in sinoaortic-denervated rats than in sham-operated animals. Treatment with the alpha2-adrenoceptor antagonist yohimbine (30 microg), the imidazoline receptor antagonist idazoxan (15 microg) and the alpha1A-adrenoceptor antagonist 5-methylurapidil (15 microg) blocked the responses to B-HT 920 (30 microg). The alpha1-adrenoceptor antagonist prazosin (15 microg) and the alpha1B-adrenoceptor antagonist chloroethylclonidine (100 microg) did not modify the responses to agonist. 4. Sinoaortic denervation enhances the cardiovascular responses to B-HT 920. Moreover, the effects of i.c.v. administration of B-HT 920 could be mediated by several types of brain receptors: imidazoline receptors and alpha1A- and alpha2-adrenoceptors.