Direct objective quantification of corneal haze after excimer laser photorefractive keratectomy for high myopia

Mitomycin C for the prevention of corneal haze in photorefractive keratectomy: a meta-analysis and trial sequential analysis

PURPOSE

To evaluate the anti-haze effect and visual outcome after intraoperative mitomycin C (MMC) use during photorefractive keratectomy (PRK) in myopia or myopic astigmatism patients.

METHODS

We searched in PubMed, EMBASE, Cochrane Library and Google Scholar comprehensively to obtain studies comparing the clinical effects after PRK with and without MMC published until February 2020. Meta-analysis of primary outcome (corneal haze rate) and secondary outcomes [predictability, efficacy, safety and corneal endothelial cell density (ECD)] were conducted. We used trial sequential analysis (TSA) in an effort to collect firm evidence supporting our conclusion.

RESULTS

Of the included 11 randomized controlled trials, five cohort and one case-control studies, 3536 eyes (2232 and 1304 in the MMC and control groups, respectively) were enrolled for meta-analysis. The TSA disclosed strong evidence of decline in corneal haze rate in the MMC group compared with that of the control group. In the subgroup analysis of duration, MMC seemed to reduce corneal haze rate in early-onset and late-onset haze. Predictability of refraction and visual acuity were greater in the MMC groups, not significantly though. The proportion of patients losing at least two lines of best corrected visual acuity postoperatively in the MMC groups was lower than that in the control groups. The corneal postoperative ECD showed no significant difference between the MMC and control groups.

CONCLUSION

Our meta-analysis revealed that MMC is an important anti-haze agent in PRK for reducing both early- and late-onset haze and can also help improving predictability of refraction and subjective postoperative visual acuity.

Resolvin D1 attenuates the inflammatory process in mouse model of LPS-induced keratitis

The aim of this study was to investigate the effects of the lipid mediator Resolvin D1 in experimental keratitis. C57BL/6J mice were injected with lipopolysaccharide (2 µg/eye), and after 24 hours, the corneal damage was assessed. Clinical score was quantified, and corneal inflammatory biomarkers were detected by immunohistochemistry. A robust accumulation of sub‐epithelial macrophages and polymorphonuclear leucocytes, chemokine (C‐X‐C motif) ligand 1 (also known as keratinocyte‐derived chemokine), interleukin‐10 and promoters of apoptosis was also observed in lipopolysaccharide‐treated mice. Formyl peptide receptor 2 corneal expression was also assessed. The corneal stroma treated with lipopolysaccharide was characterized by presence of macrophages of M1‐like subtype and immature fibroblastic cells, marked with Ki67, not fully differentiated in fibroblasts. Indeed, the staining of the cornea with anti‐vimentin antibodies, a marker of differentiated myofibroblasts, was very faint. Resolvin D1 attenuated all the inflammatory parameters assessed in the present study, except for IL‐10. In conclusion, the data presented here seem to be consistent with the hypothesis that Resolvin D1 protected the cornea from the lipopolysaccharide‐induced keratitis by acting on several inflammatory components of this damage, pivoted by Formyl peptide receptor 2 (FPR2) activation and macrophages‐leucocytes activity.

Incidence and quantification of corneal haze by Pentacam Scheimpflug densitometry following photorefractive keratectomy for myopia in virgin and post corneal transplant eyes with dark irides

PURPOSE:

To assess the incidence and quantification of corneal haze after myopic photorefractive keratectomy in virgin and post corneal transplant eyes with dark irides.

METHODS:

In this retrospective study at the tertiary eye hospital, the corneal haze was evaluated by slit-lamp and quantified by Pentacam Scheimpflug densitometry after myopic photorefractive keratectomy with mitomycin C in virgin eyes (group I) and post corneal transplant eyes (group II) with dark irides over 12 months.

RESULTS:

Group I compromised 126eyes from 77patients (35males and 42females) aged 31.31 years (24.11–36.18 years), and group II compromised 44eyes from 33patients (18males and 15females) aged of 32.25 years (26.15–36.66 years). The incidence of corneal haze was 9.5% (95%CI: 5.0%–16%) and 6.8% (95%CI: 1.4–18.7%) in group I and II respectively (P = 0.587). The corneal densitometry was 22.69 ± 8.28GSU preoperatively and 17.98 ± 3.13GSU at 12 months postoperatively (P = 0.010) in group I. The corneal densitometry was 21.86 ± 6.22GSU preoperatively and 21.23 ± 4.29GSU at 12 months postoperatively (P = 0.815) in group II. High corneal maximal densitometry was associated with the thin central corneal thickness (P = 0.027), the presence of haze (P = 0.028), post-keratoplasty (P = 0.004), steep keratometry (P = 0.035).

CONCLUSION:

The incidence of corneal haze was comparable in virgin and post corneal transplant eyes. The Pentacam Scheimflug densitometry can be helpful in the diagnosis of corneal haze; however, the cutoff values need to be studied further in larger studies.

The corneal fibrosis response to epithelial-stromal injury

The corneal wound healing response, including the development of stromal opacity in some eyes, is a process that often leads to scarring that occurs after injury, surgery or infection to the cornea. Immediately after epithelial and stromal injury, a complex sequence of processes contributes to wound repair and regeneration of normal corneal structure and function. In some corneas, however, often depending on the type and extent of injury, the response may also lead to the development of mature vimentin+ α-smooth muscle actin+ desmin+ myofibroblasts. Myofibroblasts are specialized fibroblastic cells generated in the cornea from keratocyte-derived or bone marrow-derived precursor cells. The disorganized extracellular matrix components secreted by myofibroblasts, in addition to decreased expression of corneal crystallins in these cells, are central biological processes that result in corneal stromal fibrosis associated with opacity or "haze". Several factors are associated with myofibroblast generation and haze development after PRK surgery in rabbits, a reproducible model of scarring, including the amount of tissue ablated, which may relate to the extent of keratocyte apoptosis in the early response to injury, irregularity of stromal surface after surgery, and changes in corneal stromal proteoglycans, but normal regeneration of the epithelial basement membrane (EBM) appears to be a critical factor determining whether a cornea heals with relative transparency or vision-limiting stromal opacity. Structural and functional abnormalities of the regenerated EBM facilitate prolonged entry of epithelium-derived growth factors such as transforming growth factor β (TGF-β) and platelet-derived growth factor (PDGF) into the stroma that both drive development of mature myofibroblasts from precursor cells and lead to persistence of the cells in the anterior stroma. A major discovery that has contributed to our understanding of haze development is that keratocytes and corneal fibroblasts produce critical EBM components, such as nidogen-1, nidogen-2 and perlecan, that are essential for complete regeneration of a normal EBM once laminin secreted by epithelial cells self-polymerizes into a nascent EBM. Mature myofibroblasts that become established in the anterior stroma are a barrier to keratocyte/corneal fibroblast contributions to the nascent EBM. These myofibroblasts, and the opacity they produce, often persist for months or years after the injury. Transparency is subsequently restored when the EBM is completely regenerated, myofibroblasts are deprived of TGFβ and undergo apoptosis, and the keratocytes re-occupy the anterior stroma and reabsorb disordered extracellular matrix. The aim of this review is to highlight factors involved in the generation of stromal haze and its subsequent removal.

Contact lens wearing and chronic cigarette smoking positively correlate with TGF-β1 and VEGF tear levels and impaired corneal wound healing after photorefractive keratectomy

PURPOSE

To study the correlation of gender, contact lens (CL) wearing, chronic drinking and chronic smoking with wound healing cytokine levels and corneal recovery after photorefractive keratectomy (PRK).

MATERIALS AND METHODS

One hundred and twenty-eight age-matched patients (180 eyes) undergoing PRK were enrolled. PDGF, EGF, VEGF, HGF and TGF-β(1) protein levels were measured in tears by enzyme-linked immunosorbent assay either preoperatively or 2, 7 and 15 days after PRK. Patients were seen between one day and five days postoperatively for the evaluation of epithelial healing. Delayed re-epithelialization was defined as healing after day 5. All patients were followed for haze formation for a minimum of three months.

RESULTS

All cytokines increased significantly during the first two postoperative days (p < 0.001). PDGF, EGF, HGF decreased to the preoperative levels by day 7, whereas TGF-β1 and VEGF remained elevated over the entire period of observation of 15 days, although to a lesser extent than the second day after surgery, in CL-wearers and smokers, respectively (p < 0.01). The Pearson correlation analysis showed that: (i) CL-wearing positively correlated with TGF-β1 amounts, while chronic smoking positively correlated with VEGF production; (ii) CL-wearing and TGF-β1 amount were found to be associated with early haze formation, whereas chronic smoking and VEGF level with delayed re-epithelialization. No association was found between gender or alcohol consumption and cytokine levels or wound healing.

CONCLUSIONS

Our findings highlight for the first time the important role that cigarette smoking and CL wearing may have in altering the tear cytokine network and impairing corneal epithelial wound repair after surgical injury.

Effect of nepafenac sodium 0.1% on delayed corneal epithelial healing and haze after photorefractive keratectomy: retrospective comparative study

PURPOSE

To assess delayed epithelialization and corneal haze related to nepafenac ophthalmic suspension 0.1% (Nevanac) use after photorefractive keratectomy (PRK).

SETTING

Private practice, Beverly Hills, California, USA.

METHODS

This retrospective comparative chart review comprised 69 eyes (44 patients) that were divided into 2 treatment groups that were not statistically significantly different in age or preoperative spherical equivalent. The nepafenac group consisted of 34 eyes (22 patients) that received nepafenac 0.1%, moxifloxacin, and fluorometholone postoperatively. The non-nepafenac group included 35 eyes (22 patients) that received moxifloxacin and fluorometholone only. Patients were seen between 1 day and 5 days postoperatively for evaluation of epithelial healing and haze formation. Delayed epithelialization was defined as healing after day 5. All patients were followed for haze formation for a minimum of 3 months.

RESULTS

Statistical analysis showed no difference between the nepafenac and non-nepafenac groups in delayed epithelialization (P = .61, chi-square test). Neither group had significant corneal haze.

CONCLUSION

Nepafenac did not appear to delay corneal epithelial healing or contribute to haze formation after PRK.

Advances in technologies for laser-assisted in situ keratomileusis (LASIK) surgery

Laser-assisted in situ keratomileusis (LASIK) has become the most widely used form of refractive surgery today. The objective of this surgical technique is to modify the anterior corneal shape by ablating tissue from the stroma by means of the excimer laser after creating a hinged corneal flap. This way, we are able to change the refractive status of the patient, providing better unaided vision. Continuous improvements in the original technique have made the surgical procedure safer, more accurate and repeatable. These progressions are due to the development of novel technologies that are the responsible for new surgical instrumentation, which makes the surgical procedure easier for the surgeon, and better excimer laser ablation algorithms, which increase the optical quality of the ablation and thus the safety of the vision correction procedure. This article aims to describe the more relevant advances in LASIK that have played an important role in the spread and popularity of this technique.

Optical Quality Analysis After Surface Excimer Laser Ablation: The Relationship Between Wavefront Aberration and Subepithelial Haze

PURPOSE

To investigate the relationship between mild and moderate corneal haze and the distribution of higher order wavefront aberrations after photorefractive keratectomy (PRK).

METHODS

Thirty-six eyes from 18 patients who underwent PRK were divided into two groups: 10 eyes with corneal haze and 26 eyes without corneal haze (control). All eyes were evaluated up to 6 months after PRK. Wavefront aberrations were measured using a psychophysical wavefront sensor and the NIDEK OPD-Scan. Topography, point spread function, and modulation transfer function maps were obtained from the OPD-Scan.

RESULTS

The mean total higher order aberration was slightly higher in the corneal haze group than in the control group. This difference was not statistically significant. The mean third order coma aberrations were higher and mean fourth order spherical aberrations were lower in the haze group compared with the control group, although neither difference attained statistical significance. The t test values were 1.05, -0.38, -1.10, -0.08, and -0.23, when comparing the mean third, fourth, fifth, sixth, and seventh order aberrations, respectively. None of these differences attained statistical significance. In terms of Zernike coefficients, Z-1 and Z1 showed greater mean root-mean-square (RMS) in the haze group (0.33 and 0.35 microm, respectively) than those for the control group (0.26 and 0.23 microm, respectively) (t=0.71 and P=.49; t=0.84 and P=.43, respectively). However, ZO had lower RMS in the haze group (0.18 microm) than in the control group (0.28 microm). This difference also was not statistically significant.

CONCLUSIONS

In this study comparing the optical aberrations of eyes with and without corneal haze after PRK, corneal haze did not affect the magnitude and distribution of higher order aberrations in a predictable manner.

Repeatability and Reproducibility of Posterior Corneal Curvature Measurements by Combined Scanning-Slit and Placido-Disc Topography after LASIK

OBJECTIVE

To assess the repeatability and reproducibility of posterior corneal curvature (PCC) measurements made by combined scanning-slit/Placido-disc topography (Orbscan II) after LASIK.

DESIGN

Experimental instrument validation study.

PARTICIPANTS

We recruited 22 consecutive postmyopic LASIK patients for the repeatability study and another 50 consecutive postmyopic LASIK patients for the reproducibility study.

METHODS

To analyze intrasession repeatability, 1 examiner measured 22 postmyopic LASIK eyes 10 times successively in the shortest time possible. To study intersession reproducibility, the same operator obtained measurements from another 50 eyes with stable refraction in 2 consecutive visits at the same time of the day between 6 and 9 months after myopic LASIK. We explored any association between residual stromal bed thickness and measurement variability.

MAIN OUTCOME MEASURES

Orbscan II scanning-slit PCC data, precision, within-subject coefficient of variation (CV(w)), limits of agreement (LoA), and intraclass correlation coefficient (ICC).

RESULTS

For intrasession repeatability, precision was 0.067 mm (best-fit sphere [BFS]), 0.110 diopters (D; power within 5 mm), 0.158 D (power within 3 mm), and 0.46 (eccentricity). Repeatability was high for PCC BFS and power measurements within 3-mm and 5-mm zones (CV(w) ranged from 0.5%-1.2%) but poor for eccentricity data (CV(w), 31.6%). Correspondingly, ICCs ranged from 0.89 to 0.98 for PCC BFS and power, and the ICC was 0.20 for PCC eccentricity values. For intersession reproducibility, on average, no difference in PCC measurements could be found, indicating that when there is variability, it is due to random factors. The width of the 95% LoA between sessions was clinically acceptable for BFS (0.25 mm) and power (0.4 D [within 5 mm] and 0.6 D [within 3 mm]). Similarly, ICCs indicated good intersession reliability for BFS and power (0.98, 0.96, and 0.85 for BFS, power within 5 mm, and power within 3 mm, respectively) but poor reliability for eccentricity (0.59). Repeatability and reproducibility were unrelated to stromal bed thickness.

CONCLUSIONS

Orbscan II provides reliable post-LASIK PCC data for symmetrical parameters (BFS and power), independent of the residual stromal bed thickness, but is unreliable for measurements that are radially asymmetrical (eccentricity). Orbscan II is useful for monitoring the PCC after LASIK once the early postoperative period is over.

Identical excimer laser PTK treatments in rabbits result in two distinct haze responses

PURPOSE

To obtain objective light-scattering measurements to test a hypothesis that identical PTK treatments cause distinct low- and high-level light-scattering responses in rabbit corneas.

METHODS

An excimer laser was used to produce identical 6-mm diameter phototherapeutic keratectomy treatments (PTK) in 32 pigmented rabbits. Eyes were treated by performing a 40-microm epithelial ablation, followed by a 100-mum stromal PTK. Objective scattering measurements were made before treatment, weekly up to 5 weeks, and then biweekly to 9 weeks. Confocal microscopy was performed on several corneas at 4 and 7 weeks.

RESULTS

Mean scattering levels split into distinct low- and high-scattering groups 2 weeks after treatment and remained distinct until week 7 (P < 0.003). Scattering in the low group reached a broad peak that lasted from weeks 2 to 4 at approximately 3 times the pretreatment level. Scattering in the high group peaked at 3 weeks at approximately 12 times the pretreatment level. Scattering levels diminished after reaching their peaks. Confocal images showed a band of highly reflective material in the anterior stroma that extended much deeper in corneas from the high group. The reflective band in the highly scattering corneas obscured the posterior stroma from view for up to 5 weeks.

CONCLUSIONS

Quantitative scattering data obtained with the scatterometer suggest that identical PTK treatments indeed result in distinct low- and high-level light-scattering responses in rabbits.

Correlation Between TGF-β1 in Tears and Corneal Haze Following LASEK and Epi-LASIK

PURPOSE

To compare the incidence and degree of corneal haze formation following laser subepithelial keratomileusis (LASEK) and epithelial laser in situ keratomileusis (epi-LASIK), and examine its correlation with tear film transforming growth factor-beta1 (TGF-beta1) levels.

METHODS

This prospective, interventional, clinical trial included 20 eyes (20 patients) randomly assigned to undergo LASEK or epi-LASIK. The level of TGF-beta1 in tear fluid was measured preoperatively and 1, 3, and 5 days postoperatively. Corneal haze was graded at 1 and 3 months after surgery, and the relationship with TGF-beta1 levels was determined.

RESULTS

Mean preoperative spherical equivalent refraction was -4.50 +/- 1.44 diopters (D) (range: -1.50 to -6.00 D) for LASEK eyes and -4.90 +/- 1.26 D (range: -1.75 to -6.00 D) for epi-LASIK eyes. Although mean corneal haze scores at 1 month were significantly higher in LASEK-treated eyes than in epi-LASIK treated eyes (P=.031), these scores were similar at 3 months (P=.608). Tear fluid TGF-beta1 levels were similar in LASEK and epi-LASIK eyes before surgery (P=.458) and significantly higher in the LASEK group at 1, 3, and 5 days postoperatively (P=.015, P=.023, and P=.039, respectively). A positive correlation was noted between tear TGF-beta1 levels on the first postoperative day and the degree of corneal haze at 1 month (r=0.501, P=.016).

CONCLUSIONS

Less corneal haze was noted after epi-LASIK than LASEK. A positive correlation between corneal haze and tear fluid TGF-beta1 levels on the first postoperative day suggest a possible mechanism for the observed difference.

Posterior Corneal Curvature Changes after Undersurface Ablation of the Flap and In-the-Bed LASIK Retreatment

OBJECTIVE

To analyze LASIK retreatment-induced changes in the posterior corneal curvature (PCC) with undersurface ablation of the flap (UAF) and in-the-bed techniques.

DESIGN

Nonrandomized, comparative, interventional study.

PARTICIPANTS

Forty-six eyes with a residual spherical equivalent refraction between -0.37 and -2.75 diopters (D) and astigmatism between 0.0 and -1.25 D were included prospectively. In 23 eyes, the calculated postenhancement flap thickness exceeded 150 microm using micropachymetric optical coherence tomography, whereas with further ablation of the bed, the residual bed thickness (RBT) would have been <250 microm, or <55% of the pre-LASIK central pachymetry. In another 23 eyes, RBT allowed the planned ablation for a calculated post-retreatment RBT exceeding 250 microm, >55% of the pre-LASIK central pachymetry.

INTERVENTION

Eyes with insufficient RBT for further ablation underwent UAF retreatment, whereas those with adequate RBT received conventional in-the-bed LASIK retreatment. Examinations were performed before retreatment and 3 and 6 months postoperatively. No eye was lost to follow-up.

MAIN OUTCOME MEASURES

Micropachymetry, Orbscan II scanning-slit PCC data, and visual acuity (VA).

RESULTS

The groups did not differ in age, intraocular pressure, or retreatment ablation depth, but the UAF eyes had a lower mean pre-retreatment RBT (270.7+/-25.4 microm) than conventional enhancement eyes (353.0+/-41.5 microm) (P = 0.001). Eyes undergoing UAF had no significant change in PCC, whereas eyes undergoing conventional retreatment had an increase in the posterior corneal power within the central 3-mm zone (P = 0.008) 3 months after retreatment. No significant changes occurred thereafter. The amount of change in posterior corneal power within the 3-mm central zone from before to after retreatment differed significantly between the groups (mean difference, 0.135 D; 95% confidence interval, 0.022-0.248 D; P = 0.02). No keratectasia developed clinically, and no retreated eye lost or gained > or =2 lines of best-corrected VA. Six months after retreatment, the efficacy and safety indices for the UAF procedure were 0.96 and 1.01, respectively, and 1 and 1.06 for conventional LASIK enhancement.

CONCLUSION

Undersurface ablation of the flap retreatment appears to have less potential for changing the posterior corneal surface than conventional LASIK enhancement and can help reduce the likelihood of retreatment-induced keratectasia.

Grading of corneal transparency

AIM

To examine the academic literature on the grading of corneal transparency and to assess the potential use of objective image analysis.

METHOD

Reference databases of academic literature were searched and relevant manuscripts reviewed. Annunziato, Efron (Millennium Edition) and Vistakon-Synoptik corneal oedema grading scale images were analysed objectively for relative intensity, edges detected, variation in intensity and maximum intensity. In addition, corneal oedema was induced in one subject using a low oxygen transmissibility (Dk/t) hydrogel contact lens worn for 3h under a light eye patch. Recovery from oedema was monitored over time using ultrasound pachymetry, high and low contrast visual acuity measures, bulbar hyperaemia grading and transparency image analysis of the test and control eyes.

RESULTS

Several methods for assessing corneal transparency are described in the academic literature, but none have gained widespread use in clinical practice. The change in objective image analysis with printed scale grade was best described by quadratic parametric or sigmoid 3-parameter functions. 'Pupil image scales' (Annunziato and Vistakon-Synoptik) were best correlated to average intensity; however, the corneal section scale (Efron) was strongly correlated to variations in intensity. As expected, patching an eye wearing a low Dk/t hydrogel contact lens caused a significant (F = 119.2, p < 0.001) 14.3% increase in corneal thickness, which gradually recovered under open eye conditions. Corneal section image analysis was the most affected parameter and intensity variation across the slit width, in isolation, was the strongest correlate, accounting for 85.8% of the variance with time following patching, and 88.7% of the variance with corneal thickness.

CONCLUSION

Corneal oedema is best determined objectively by the intensity variation across the width of a corneal section. This can be easily measured using a slit-lamp camera connected to a computer. Oedema due to soft contact lens wear is not easily determined over the pupil area by sclerotic scatter illumination techniques.

Evaluation of the prophylactic use of mitomycin-C to inhibit haze formation after photorefractive keratectomy in high myopia: a prospective clinical study

Background

To study the effect of prophylactic application of mitomycin-C on haze formation in photorefractive keratectomy (PRK) for high myopia.

Methods

Fifty-four eyes of 28 myopic patients were enrolled in this prospective study. All eyes were operated by PRK followed by 0.02% mitomycin-C application for two minutes and washed with 20 ml normal saline afterwards. All eyes were examined thoroughly on the first 7 days and one month after surgery; 48 eyes (88.9%) at 3 and 6 months postoperatively. Hanna grading (in the scale of 0 to 4+) was used for assessment of corneal haze.

Results

The mean spherical equivalent refraction (SE) was -7.08 diopters (D) ± 1.11 (SD) preoperatively. Six months after surgery, 37 eyes (77.1%) achieved an uncorrected visual acuity (UCVA) of 20/20 or better, all eyes had a UCVA of 20/40 or better and 45 (93.7%) eyes had an SE within ± 1.00D. One month postoperatively, 2 eyes (3.7%) had grade 0.5+ of haze, while at 3 and 6 months after surgery no visited eye had haze at all. All eyes had a best corrected visual acuity (BCVA) of 20/40 or better and there were no lost lines in BCVA by 6 months after surgery. In spatial frequencies of 6 and 12 cycles per degree contrast sensitivity had decreased immediately after PRK and it had increased 1.5 lines by the 6^th postoperative month compared to the preoperative data.

Conclusions

The results show the efficacy of mitomycin-C in preventing corneal haze after treatment of high myopia with PRK. This method- PRK + mitomycin-C – can be considered an alternative treatment for myopic patients whose corneal thicknesses are inadequate for laser in situ keratomileusis (LASIK). However, the results should be confirmed in longer follow-ups.

Laser Epithelial Keratomileusis With Mitomycin C: Indications and Limits

PURPOSE

To assess the advantages and safety of using mitomycin C (MMC) with laser epithelial keratomileusis (LASEK) by measuring high order aberrations of the cornea.

METHODS

We analyzed the results of 86 eyes that had LASEK plus a brushstroke of MMC 0.01%. Another group of 100 eyes had LASEK only (control group). Mean follow-up was 312 days for the MMC group and 774 days for the control group. A Nidek EC-5000 laser was used in all eyes. We measured mean refractive error, best spectacle-corrected visual acuity (BSCVA), haze, and high order aberrations before and at 1 month and nearly 1 year after LASEK.

RESULTS

Mean baseline spherical equivalent refraction was -6.80 +/- 3.00 D for the MMC group and -6.00 +/- 3.10 D for the control group (P=.34). In eyes with low myopia (-1.20 to -10.80 D), a single ablation zone of 6 mm with a transition zone of 8 mm was used, and in the higher myopia group (-5.60 to -15.50 D), smoothing and a cross sphere procedure (+0.75 D) were used. Mean spherical equivalent refraction after LASEK for low myopia in the MMC group at last examination (mean follow-up 312 days) was +0.50 +/- 0.93, and 0 +/- 0.34 D for the control group. Mean spherical equivalent refraction after LASEK for high myopia in the MMC group at last examination (mean follow-up 312 days) was +0.50 +/- 1.03 D, and +0.40 +/- 0.78 D for the control group. Mean haze in the low myopia MMC group was 0.06 and in the control group, 0.11; in the high myopia MMC group it was 0.1, and in the control group, 0.27 (all differences P<.05). No differences between baseline and postoperative BSCVA were noted. Early and late overcorrection were noted in the MMC group. Increased high order aberrations were evident at 1 month and 1 year in the MMC groups.

CONCLUSIONS

The use of mitomycin C 0.01% with LASEK significantly decreased subepithelial haze. However, refractive outcome was less predictable. High order aberrations increased after LASEK with MMC. Additional studies with longer follow-up are necessary to evaluate long-term effects, and ideal MMC concentration and exposure time.

Laser Epithelial Keratomileusis for Myopia

PURPOSE

To describe a new technique for excimer laser corneal surgery: laser epithelial keratomileusis (LASEK).

METHODS

We report 76 eyes with spherical equivalent refractive myopia ranging from -8.00 to -22.00 D (mean -11.00 +/- 3.00 D). Using ethanol applied over an 8.5-mm-diameter area for 20 seconds, the epithelium was detached and photorefractive keratectomy (PRK) was performed. The epithelium was reapplied to the ocular surface and covered with a therapeutic soft contact lens. Treatments were carried out with the Nidek EC-5000 excimer laser.

RESULTS

At a mean of 803 days after LASEK, mean spherical equivalent refraction was -1.80 +/- 2.40 D (range -9.27 to +2.00 D). Stability was reached in approximately 60 days. Fifty-nine percent of the epithelial flaps were easy to detach intact during surgery and 62.7% of patients reported no postoperative pain. Eighty-four percent of eyes had the therapeutic bandage lens removed by the fourth postoperative day. Twelve percent of eyes had incomplete epithelial flap removal. Epithelium can fall off the cornea during the healing phase. No significant subepithelial haze was seen in 95% of eyes.

CONCLUSION

With LASEK, it may be possible to treat a larger diameter ablation zone in thinner corneas without the problems of a LASIK stromal flap and with less postoperative pain than PRK, with minimal subepithelial haze.

Ascorbate Prophylaxis for Corneal Haze After Photorefractive Keratectomy

PURPOSE

To evaluate whether prophylactic systemic ascorbic acid influences the average level of haze and the incidence of late onset corneal haze after photorefractive keratectomy (PRK).

METHODS

Two consecutive groups of eyes treated with PRK for myopia with or without astigmatism were retrospectively compared. The patients had been treated similarly, with the exception that systemic ascorbate had been supplied orally in one of the groups. Haze intensity was quantified on a scale from 0 (clear cornea) to 4 (anterior chamber not visible). The diagnostic criterion for late onset corneal haze was a haze grade 2 or higher, occurring 4 to 12 months after surgery.

RESULTS

One week, 1, 3, 6, and 12 months after surgery, the group without ascorbate (314 eyes) showed haze of average levels 0.61, 0.51, 0.50, 0.32, 0.10, respectively, and the group with ascorbate (201 eyes) showed haze of average levels 0.38, 0.18, 0.16, 0.09, 0.06, respectively. Comparison of the respective values showed a statistically significant difference between the two groups (P<.01) at 1 week, 1, 3, and 6 months. Late onset corneal haze was observed in 11 eyes in the group without ascorbate, and none was observed in the group with ascorbate (P<.02).

CONCLUSION

This retrospective nonrandomized clinical study suggests that oral ascorbic acid supplementation may have a prophylactic effect against haze development after PRK. However, routine prophylactic use of ascorbate can be recommended only after a randomized, prospective clinical trial substantiates its efficacy.

Corneal optical coherence tomography before and after phototherapeutic keratectomy for recurrent epithelial erosions(2)

PURPOSE

To study the representation of corneal structures with optical coherence tomography (OCT) before and after excimer laser phototherapeutic keratectomy (PTK) for recurrent epithelial erosions.

SETTING

Departments of Ophthalmology, Vivantes Klinikum Neukölln, Berlin, and Medizinische Universität, Lübeck, Germany.

METHODS

This prospective study comprised 15 eyes of 14 patients with recurrent epithelial erosions. The central corneal and epithelial thickness as well as the wound-healing response in the anterior corneal stroma were assessed with slitlamp-adapted OCT before and after PTK.

RESULTS

After PTK, the symptoms improved in all patients without loss of best corrected, glare, or low-contrast visual acuity. The mean central corneal OCT thickness was 540 microm +/-28 (SD) preoperatively, 492 +/- 36 microm immediately after epithelial debridement and PTK, and 519 +/- 25 microm after 7 weeks (P <.01). The mean central epithelial OCT thickness changed from 70 +/- 13 microm preoperatively to 60 +/- 7 microm after 7 weeks (P >.01). Changes in the light-scattering properties in the anterior subepithelial stroma revealed a hyperreflective area with a mean thickness of 46 +/- 13 microm after 7 weeks.

CONCLUSIONS

Using noncontact corneal OCT, corneal and epithelial thickness changes and the wound-healing response in the anterior corneal stroma could be evaluated after PTK in patients with recurrent epithelial erosions.

Undersurface ablation of the flap for laser in situ keratomileusis retreatment

OBJECTIVE

To develop a novel technique, undersurface ablation of the flap (UAF), for laser in situ keratomileusis (LASIK) retreatment in eyes with insufficient posterior stroma.

DESIGN

Noncomparative, interventional case series.

PARTICIPANTS

From 30 eyes examined, 25 eyes with a spherical equivalent residual refraction between -0.75 and -3.25 diopters (D) and astigmatism between 0.0 and -1.5 D were prospectively included in the study. In these eyes, calculated postenhancement flap thickness was >150 micro m using micropachymetric optical coherence tomography (OCT), whereas with further ablation of the bed, posterior stromal thickness would have been <250 micro m. Primary LASIK procedures had been performed with the Hansatome microkeratome.

INTERVENTION

The flap was lifted and the eye deviated downward, so that the corneal visual axis mark aligned with the laser beam. Mirror pattern ablations with an optical zone of 5 mm were performed on the flap stroma using either the Summit Apex Plus excimer laser or the Technolas Keracor 217 spot-scanning excimer laser. New axis orientation for toric ablations was calculated with the formula: beta = 180 degrees - alpha.

MAIN OUTCOME MEASURES

Refraction, visual acuity, OCT pachymetry, tangential videokeratography, and patient satisfaction.

RESULTS

The average follow-up was 6.36 +/- 2.64 months (range, 3-12 months). Mean preenhancement spherical equivalent (-2.05 +/- 0.75 D) was reduced to -0.19 +/- 0.38 D at the last visit (P = 0.001). Mean cylinder decreased from -0.48 +/- 0.53 D before retreatment to -0.23 +/- 0.28 D at the last follow-up (P = 0.003). Best-corrected visual acuity worsened by 1 line in two eyes (8%), and no eye lost 2 or more lines. Satisfactory globe stabilization and stromal smoothness during ablation were more difficult to achieve than with conventional LASIK enhancements. The average central flap thickness before UAF, 187 +/- 13 micro m, decreased to 164 +/- 12 micro m after 1 month (P = 0.001). No keratectasia developed. Finally, 92% of cases were satisfied with surgery compared with 48% before UAF retreatment (P = 0.001).

CONCLUSIONS

UAF retreatment for low residual refractive errors after LASIK in eyes with sufficient flap stroma seems to be effective and may prevent future keratectasia.

200 Hz flying-spot technology of the LaserSight LSX excimer laser in the treatment of myopic astigmatism: six and 12 month outcomes of laser in situ keratomileusis and photorefractive keratectomy

PURPOSE

To evaluate safety, efficacy, predictability, and stability in the treatment of myopic astigmatism with laser in situ keratomileusis (LASIK) and photorefractive keratectomy (PRK) using the 200 Hz flying-spot technology of the LaserSight LSX excimer laser.

SETTING

SynsLaser Clinic, Tromsø, Norway.

METHODS

This retrospective study included 110 eyes treated with LASIK and 87 eyes treated with PRK that were available for evaluation at 6 and 12 months, respectively. The mean preoperative spherical equivalent (SE) was -5.35 diopters (D) +/- 2.50 (SD) (range -1.13 to -11.88 D) in the LASIK eyes and -4.72 +/- 2.82 D (range -1.00 to -15.50 D) in the PRK eyes. The treated cylinder was 4.00 D in both groups. Eleven (8.5%) LASIK eyes and 8 (7.4%) PRK eyes had secondary surgical procedures before 6 and 12 months, respectively, and were excluded when the 6 and 12 month outcomes were analyzed.

RESULTS

None of the eyes lost 2 or more lines of best spectacle-corrected visual acuity. Seventy-seven percent of the LASIK eyes and 78% of the PRK eyes achieved an uncorrected visual acuity of 20/20 or better; 98% in both groups achieved 20/40 or better. The SE was within +/-0.5 D of the desired refraction in 83% of the LASIK eyes and 77% of the PRK eyes; it was within +/-1.0 D in 97% and 98%, respectively. The cylinder correction had a mean magnitude of error of 0.04 +/- 0.31 D (range -0.96 to +0.85 D) in the LASIK eyes and 0.02 +/- 0.37 D (range -1.44 to +0.72 D) in the PRK eyes. Refractive stability was achieved at 1 month and beyond in the LASIK eyes and at 3 months and beyond in the PRK eyes.

CONCLUSION

The outcomes of this study are comparable to those achieved with lasers that use small-beam technology with a lower frequency, as well as with other types of delivery systems. They suggest that the 200 Hz technology used in the LaserSight LSX excimer laser is safe, effective, and predictable and that with LASIK and PRK the results are stable when treating low to moderate myopia and astigmatism up to 4.0 D.

Correlation between ultraviolet radiation level and the incidence of late-onset corneal haze after photorefractive keratectomy

PURPOSE

To investigate the correlation between environmental changes in ultraviolet (UV) radiation levels and the incidence of late-onset cornea haze (LOCH) after photorefractive keratectomy (PRK).

SETTING

SynsLaser Clinic, Tromsø, Norway.

METHODS

The study comprised 404 eyes that had myopic PRK and photoastigmatic refractive keratectomy from February 1996 through July 1998. The high latitude (70 degrees N) of the observation site provided "natural laboratory" conditions to look at the occurrence of LOCH with high and low UV-radiation levels, which occurred during summers and winters, respectively. The diagnostic criterion for LOCH was acute haze of grade > or =2 occurring between 4 and 12 months postoperatively.

RESULTS

The follow-up ranged from 12 to 41 months. Of the 314 eyes that met the inclusion criteria, 11 developed LOCH when the environmental UV-radiation level was high. No eye developed LOCH when the level was low. The correlation between a high level of environmental UV radiation and the occurrence of LOCH was statistically significant (P =.001).

CONCLUSION

Environments with high UV-radiation levels may increase the risk of LOCH after PRK in eyes with moderate to high myopia. Use of UV-protective eyewear should be encouraged during the first year after PRK.

Optical coherence tomography evaluation of the corneal cap and stromal bed features after laser in situ keratomileusis for high myopia and astigmatism

OBJECTIVE

To study the corneal microstructure by optical coherence tomography (OCT) after laser in situ keratomileusis (LASIK) for high myopia with and without astigmatism.

DESIGN

Nonrandomized self-controlled comparative trial.

PARTICIPANTS

Sixty-three consecutive LASIK eyes with spherical equivalent refraction between -6.0 and -17.0 diopters (D) and astigmatism between 0.0 and -5.0 D were prospectively recruited for examination.

INTERVENTION

LASIK was performed with the Chiron Hansatome microkeratome (160-microm fixed plate) and Summit Apex Plus excimer laser using a 5.5/6.0/6.5-mm multizone pattern. Proper preoperative calculations were performed to ensure stromal beds thicker than 250 microm.

MAIN OUTCOME MEASURES

OCT imaging and measurement of corneal thickness was performed preoperatively. In addition, corneal cap and stromal bed thickness measurements were performed 1 day, 1 month, and 3 months postoperatively.

RESULTS

The average central corneal pachymetry was 538.9 +/- 26.2 microm preoperatively. Mean corneal cap thickness measured 124.8 +/- 18.5 microm 1-day postoperatively. Mean stromal bed thickness was 295.2 +/- 37.1 microm on the first postoperative day. Compared with the 1-day postoperative examination, the average stromal bed thickness increased significantly by 5.9 microm (P = 0.001) and 7.2 microm (P = 0.001) at the 1-month and 3-month postoperative examinations, respectively. Mean difference between actual (118.7 +/- 27.8 microm) and predicted (104.1 +/- 20.8 microm) central ablation depths was 14.6 +/- 16.7 microm (P = 0.0001). A weak but statistically significant positive association was found between preoperative refraction and the difference between expected and real ablation depth values (R = 0.26; P = 0.042). Posterior stromal beds were more than 250-microm thick in 58 eyes (89.9%) 1 day postoperatively. This safety requirement improved at the 1-month postoperative examination, when the partial regression accounted for slightly thicker stromal beds and only two cases (3.2%) exhibited posterior stromal tissue thinner than 250 microm. These two cases were seen only for corrections exceeding 12 D (P = 0.04).

CONCLUSIONS

OCT appears to be a useful tool for the evaluation of both the qualitative and quantitative anatomic outcome of LASIK. Corrections of higher degrees of ametropia run a higher risk of producing a thinner than expected central cornea. Particularly, corrections greater than 12 D may lead eventually to stromal beds thinner than 250 microm, despite proper preoperative calculations. Because corneal flaps are usually thinner than expected with the microkeratome used herein, adequate posterior corneal stroma is preserved in most instances.

Corneal epithelial alterations resulting from use of chlorine-disinfected contact tonometer after myopic photorefractive keratectomy

OBJECTIVE

This study aimed to describe a previously unreported complication associated with the use of chlorine-disinfected applanation tonometer heads for intraocular pressure measurement after excimer laser photorefractive keratectomy.

DESIGN

Two retrospective case reports.

PARTICIPANTS

Two patients underwent, respectively, a 7-diopter and a 4-diopter myopic excimer laser correction in their first eye 2 weeks apart. Complete epithelial closure of the ablated area was observed by biomicroscopy in the first-week examination.

INTERVENTION

Four weeks after photorefractive keratectomy, a complete ophthalmic examination was performed. Goldmann applanation tonometry was performed bilaterally after thoroughly rinsing and drying the tonometer biprism, which had been immersed regularly in a chlorine 5000-parts per million solution.

MAIN OUTCOME MEASURES

Slit-lamp examination and corneal topographic surface regularity were measured.

RESULTS

A few minutes after applanation tonometry, both patients reported ocular discomfort in the excimer laser-treated eyes, whereas the untreated fellow eyes were painless. Punctate corneal lesions and superficial epithelial cell clumping were present in the first patient's treated eye, predominantly in the inferior aspect of the applanated cornea. Visual inspection showed a normal tonometer tip. In the second patient's treated cornea, a focal epithelial defect was identified biomicroscopically, which corresponded to the steeper region within the ablation zone on the videokeratograph. In this case, crystal deposits were found on the tonometer tip. The epithelial alterations resolved without sequelae in both cases.

CONCLUSIONS

Disinfecting solutions of chlorine can cause crystal deposit formation on the tonometer head. Applanation tonometry after repeated disinfection with chlorine solutions appears to have the potential for disrupting the epithelial layer of the healing cornea. Covered contact tonometry or noncontact tonometry should be evaluated as alternative methods to chemically disinfected contact tonometry for intraocular pressure measurement after excimer laser surgery, especially during the first postoperative month.

Reproducibility of digital image analysis for measuring corneal haze after myopic photorefractive keratectomy

PURPOSE

To evaluate the usefulness of digital image analysis for quantifying corneal haze by determining the reproducibility of its measurements at the corneal plane.

METHODS

In a prospective study, 20 randomly selected eyes that had undergone myopic photorefractive keratectomy were photographed focusing the slit beam on their anterior corneal surface. Each photograph was examined using computer image analysis techniques that detect the edge of the reticular pattern of the image. Quantification of the difference between two areas, treated and adjacent untreated cornea, each containing 3,750 pixels with a resolution of 256 gray levels, was performed. Intra-analyzer variation was determined by evaluating the photographs obtained by two analyzers under standard conditions on four separate visits. Interanalyzer variation was calculated using one measurement and the mean of the four measurements.

RESULTS

The pooled standard deviation of the measurements for the analyzers was 0.63 and 0.62 gray levels (coefficient of variation, 4.1% and 3.3%). An association between less severe haze measurements and higher reproducibility scores was found (r = .42; P = .007). The mean interanalyzer variation was smaller for the average of four measurements, 0.55 +/- 0.37 gray levels, than for one measurement, 0.94 +/- 0.73 gray levels (P = .014).

CONCLUSIONS

Good reproducibility for haze measurements by digital image analysis of the differences between the treated and adjacent untreated corneal areas was obtained. When the average of four measurements was used instead of a single measurement, interanalyzer reproducibility increased significantly. This new technique may be used to quantify and analyze corneal haze after myopic photorefractive keratectomy.