Melanie C. Corbett

Corneal Optical Aberrations Induced by Photorefractive Keratectomy

BACKGROUND Photorefractive keratectomy causes marked alteration to anterior corneal topography, and is likely to induce major changes to the optical aberrations of the eye. METHODS Six diopters (D) of myopia correction was attempted on one eye of 50 patients, randomly allocated to one of three different treatments: 5-mm or 6-mm single ablation zone, or a double ablation (multizone; -5.00 D correction over 4.6 mm and -1.00 D over 6 mm). Topographic data was used to estimate corneal aberration coefficients. These were compared for effect of ablation zone, before and 1 year after photorefractive keratectomy. The coefficients were used to derive modulation transfer functions for the anterior corneal surface. RESULTS Corneal spherical aberrations and coma-like aberrations both increased significantly following photorefractive keratectomy (p < 0.001). The mean spherical aberration coefficient increased from 0.36 +/- 0.11 before, to 0.91 +/- 0.37 after treatment, while the mean coma-like aberration coefficient changed from 0.28 +/- 0.16 before, to 0.60 +/- 0.31 after treatment. Ablation zone form had a significant effect on spherical aberration (p = 0.030), but not for coma (p = 0.96). The spherical aberration coefficient increased least for the 6-mm ablation (by 0.38 +/- 0.17), compared with the 5-mm ablation (0.69 +/- 0.45) and the multizone (0.62 +/- 0.38). Corneal modulation transfer functions were reduced significantly following the photorefractive procedure. The effect was greatest for large pupil diameters and for spatial frequencies between 2 and 15 cycles per degree. CONCLUSIONS Corneal modulation transfer function calculations suggest that a significant loss of visual performance should be anticipated following photorefractive keratectomy, the effect being greatest for large pupil diameters. Results for three ablation zones show that induced aberrations are least for the largest (6 mm) ablation zone.

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.

The Effects of Topical Corticosteroids and Plasmin Inhibitors on Refractive Outcome, Haze, and Visual Performance after Photorefractive Keratectomy: A Prospective, Randomized, Observer-masked Study

BACKGROUND This study of 86 patients with 12 months of follow-up was designed to determine whether topical corticosteroids or plasmin inhibitors have an effect on the outcome of photorefractive keratectomy. METHODS Patients were allocated randomly to either steroid (0.1% fluorometholone for 6 months), plasmin-inhibitor (aprotinin 40 IU/ml for 3 weeks), or control (no treatment) groups and underwent either -3.00- or -6.00-diopter (D) corrections. RESULTS With -3.00-D corrections, the mean refractive change was significantly greater at 3 and 6 months (P < 0.05) in the steroid group compared with the control group. When steroids were discontinued, the difference became insignificant within 3 months. Similarly, with -6.00-D procedures the mean refractive change was greater at 6 weeks and 3 and 6 months (P < 0.01), but the refractive change again became insignificant 3 months after stopping steroid treatment. Four patients treated with steroids had a hyperopic shift greater than +2.00 D of that intended at 12 months. Similar overcorrections were not noted in the other treatment groups. There were no differences in refractive outcome between the aprotinin and control groups at any stage. With -6.00-D procedures, objective measurements of haze were significantly greater in the aprotinin group compared with the control group at 9 and 12 months (P < 0.05). With this exception, there were no differences in haze, forward or backward scatter of light, best-corrected visual acuity, or halo measurements between the groups. CONCLUSIONS Corticosteroids can maintain a hyperopic shift during their administration, but this effect is reversed on cessation of treatment. Objective tests have shown that steroids have no effect on corneal haze or visual performance after PRK. There is no justification for routinely submitting all patients to long-term steroid regimens and their associated side effects. Treatment with aprotinin produced no beneficial effect on refractive outcome, and haze was greater in the -6.00-D procedures. The concept of modulating the plasminogen activator/plasmin system to regulate wound healing after PRK is discussed.

A Prospective, Randomized, Double-masked Trial to Evaluate the Role of Topical Anesthetics in Controlling Pain after Photorefractive Keratectomy

PURPOSE To investigate the role of 1% tetracaine in controlling pain after photo-refractive keratectomy (PRK) and determining its effect, if any, on epithelial healing, refractive outcome, and visual performance. METHODS In this study, 44 patients were randomized to receive either Gutt. 1% tetracaine or placebo after undergoing PRK. Drops were instilled at 30-minute intervals during waking hours for 24 hours postoperatively. In addition, all patients received two coproxamol (paracetamol + dextropropoxyphene) tablets every 6 hours for 2 days. Visual Analogue Pain Charts were used to record pain levels for 4 days after surgery. Serial digitized retro-illumination photography was used to assess rates of epithelial healing, and surface epithelial quality was monitored using topography. At fixed intervals over a 6-month period visual performance was assessed by measuring refractive outcome, best-corrected visual acuity, objective haze, halo, and glare. RESULTS Patients in the tetracaine group had significantly less pain (P < 0.0001). Both groups demonstrated full epithelial closure within 72 hours. Similar numbers of patients in both groups at 1 week showed topographic irregularity that completely resolved by 1 month. No statistically significant difference was seen in any of the parameters monitoring visual performance. CONCLUSIONS Tetracaine in conjunction with coproxamol is effective in reducing pain after PRK without adversely affecting corneal wound healing or visual performance.

DISTURBANCES IN NIGHT VISION AFTER EXCIMER LASER PHOTOREFRACTIVE KERATECTOMY

Eighty-four patients with up to -6.00 dioptres of myopia underwent photorefractive keratectomy (PRK), using 5.00 mm ablation zones. Three months post-operatively 38 (45%) complained of disturbances in night vision, compared with 21 (25%) pre-operatively. In the majority, these disturbances were regarded as negligible. However, 9 (11%) reported significant problems, defined as an inability to drive safely at night with the treated eye. At 12 months, 32 patients (38%) complained of impaired night vision, 4 (5%) of whom had significant problems. A series of measurements were performed to investigate the origins of these disturbances, especially in patients reporting significant problems. Visual impairment from forward scattered light was investigated using a computerised technique. Back scattered light was measured with a charge coupled device–camera system and a computer program was used to assess the degree of halation around a bright light source on a high-resolution monitor. Pupillary diameters were measured by infrared television pupillometry. At 6 months, those reporting a starburst effect around lights at night had small hyperopic shifts, minimal halos and high forward and back light scatter measurements. Patients who reported halo phenomena had large hyperopic shifts, little light scatter and large pupillary diameters. Of the 4 patients who reported significant disturbances at 12 months, all had persistent halo problems. Those with starburst effects in the early postoperative period noticed an improvement with time as their corneal haze gradually improved. Perturbations of night vision after PRK manifest as starbursts and halos around lights. Disturbances in corneal transparency appear to be responsible for starburst effects and are usually transient. Halos are myopic blur circles and may be persistent in a small number of individuals. All patients should be informed pre-operatively of the possible consequences of disturbances in night vision.

Do topical corticosteroids have a role following excimer laser photorefractive keratectomy

The article reviews the evidence for and against the use of topical corticosteroids following PRK. Two significant problems after PRK are the development of corneal haze and unpredictability of the refractive outcome. These arise from changes in the anterior stroma, the deposition of new subepithelial tissue, and individual variations in the wound healing response. In rabbits, corticosteroids reduced corneal haze by limiting the synthesis of subepithelial collagen. However, in humans, controlled trials showed that corticosteroids had no lasting effect on either haze or regression, and were associated with an unacceptably high incidence of unwanted effects. The difference between species probably related to the relative absence of collagen and predominance of glycosaminoglycans during corneal wound healing in humans. Some human studies have shown rapid changes in refraction occurring within days of stopping or restarting corticosteroids. This suggests that their transient hyperopic effect is probably mediated by changes in the hydration of the tissue. Therefore, the role of corticosteroids in PRK is very limited. There is no justification for their routine use after PRK for low or moderate myopia. If corticosteroids have a role in improving haze or refractive outcome in selected patients, a means for their early identification must be found.

Biologic and Environmental Risk Factors for Regression after Photorefractive Keratectomy

BACKGROUND/PURPOSE After photorefractive keratectomy (PRK), the final refraction in the majority of patients is near the intended refraction. However, a significant proportion of patients show regression, which is commonly associated with corneal haze. This study aims to determine whether biologic and environmental factors influence the outcome of PRK. METHODS One hundred patients prospectively underwent -3.00- or -6.00-diopter corrections. Three months after surgery, 100% of patients answered a questionnaire that recorded their biologic characteristics, medical and ocular history, and postoperative exposure to physical and chemical insults. Multivariable analysis identified those factors associated with regression. RESULTS Multivariable analysis showed that the risk of regression was increased significantly in patients who underwent higher dioptric or smaller diameter treatments (P = 0.045) and in those who had had regression after treatment of the first eye (P = 0.019). There was no difference between the sexes, but regression was 13.5 times more likely in females who were taking oral contraceptives. Regression was increased in those exposed to solar radiation (odds ratio, 7.6; 95% confidence interval, 1.82-32.22) and sun beds (odds ratio, 2.4; 95% confidence interval, 0.64-9.39), and was significantly greater in patients with ocular-surface disorders (on univariable analysis only; P = 0.034). Regression was not associated with previous contact lens wear, swimming, cigarette smoking, or minor ocular trauma. CONCLUSION The questionnaire identified biologic and environmental risk factors for regression. The biologic risk factors may be used preoperatively to counsel patients and, where necessary, advise them against surgery. The environmental risk factors are largely avoidable by modification of patient behavior.

Keratocyte density and size in conscious humans by digital image analysis of confocal images.

Purpose Confocal microscopy can give images of high magnification and resolution in undisturbed living tissue. It provides new information about the cellular structure of the cornea. Our aim was to measure the density, size and distribution of keratocytes.Methods Healthy cornea in four subjects was examined using tandem scanning confocal microscopy. Methods for digital analysis of images were developed.Results Keratocyte density in confocal cross-sections was greatest immediately under Bowmans membrane (maximum 800 cells/ mm2) and decreased sharply towards posterior cornea (minimum 65 cells/mm2). Cross-sectional cell size ranged from 78 to 211 µm2, but did not correlate with depth in the tissue.Conclusions Results are consistent with those of earlier work using histological and biochemical techniques in isolated tissue. The methods we have developed enable studies of ongoing processes in conscious humans and can be used to examine diseased tissue as well as the response to injury.

Loss and recovery of corneal sensitivity following photorefractive keratectomy for myopia

BACKGROUND Photorefractive keratectomy affects corneal innervation in a new, and drastic, way. This inevitably results in a significant loss of corneal sensitivity. This paper investigates the pattern of sensitivity loss and recovery following PRK for low to moderate myopia. METHODS Patients were recruited for two separate studies. Longitudinal Study: 10 patients, treated with a -6.00 D/6 mm PRK ablation, were examined over a 1-year period. Transverse Study: A comparison was made between 35 non-contact lens wearers, 80 contact lens wearers, and 60 patients who underwent five different PRK treatments, 1 year previously. Corneal sensitivity was assessed using the Non-Contact Corneal Aesthesiometer at four corneal locations: centrally, temporally, medially, inferiorly, on all patients. RESULTS Longitudinal Study: Corneal sensitivity was significantly reduced at week 1, with a further significant reduction at week 2. A gradual recovery in sensitivity then followed to reach preoperative levels by 1 year. Transverse Study: There was no significant difference in corneal sensitivity found as a result of different PRK ablation depths. The depth of ablation was not a factor in corneal sensitivity recovery in low to moderate myopic corrections. Corneal sensitivity in the PRK treated eyes was significantly lower than in the control groups. CONCLUSION The immediate loss of corneal sensitivity after surgery was due to the total removal of the corneal epithelial nerve supply and a substantial portion of the underlying stromal nerves. The further decrease at week 2 was probably due to the new epithelium acting as barrier to stimulation. The gradual recovery of corneal sensitivity was most likely caused by epithelial reinnervation within the ablation zone, but this was still below normal levels at 1 year after surgery. Corneal nerve function demonstrates a biphasic pattern of loss and recovery following the excimer laser correction of low to moderate myopia.

Effect of ablation profile on wound healing and visual performance 1 year after excimer laser photorefractive keratectomy.

BACKGROUND: Early photorefractive keratectomy ablations were of limited diameter and depth to maintain the integrity of the globe and to minimise postoperative haze. This study evaluated the effects of deeper, larger diameter wounds on refractive stability and corneal haze, and investigated the effects of ablation profile on wound healing and visual performance. METHODS: One hundred patients undergoing -3.00D and -6.00D corrections were randomised to receive 5 mm, 6 mm, or multizone treatments. The multizone treatment was 6 mm in diameter, but only the depth of the 5 mm treatment. Outcome was measured by Snellen visual acuity, residual refractive error, objective techniques for haze and halos, pupil diameter, subjective night vision, and requirement for retreatment. RESULTS: Overall, the results of 6 mm treatments were superior to those of 5 mm and multizone treatments: they had a smaller hyperopic shift (p < 0.01), a more predictable (p < 0.001) and stable refractive outcome, less haze (p < 0.05), smaller halos (p < 0.05), fewer subjective night vision problems, and fewer patients required retreatment. CONCLUSIONS: Analysis of these data and a literature review of corneal wound healing demonstrated that the improved outcome associated with the 6 mm beam did not relate to the depth of ablation. The factor with greatest apparent influence on the development of haze and regression was the slope of the wound surface over the entire area of the ablation. Tapering the wound edge provided no additional benefit, and contributed to night vision problems. It is, therefore, recommended that small diameter or multizone treatments should not be used in low and moderate myopia.