Objective assessment of transient corneal haze and its relation to visual performance after photorefractive keratectomy

A new method for in vivo assessment of corneal transparency using spectral-domain OCT

Corneal transparency is essential to provide a clear view into and out of the eye, yet clinical means to assess such transparency are extremely limited and usually involve a subjective grading of visible opacities by means of slit-lamp biomicroscopy. Here, we describe an automated algorithm allowing extraction of quantitative corneal transparency parameters with standard clinical spectral-domain optical coherence tomography (SD-OCT). Our algorithm employs a novel pre-processing procedure to standardize SD-OCT image analysis and to numerically correct common instrumental artifacts before extracting mean intensity stromal-depth (z) profiles over a 6-mm-wide corneal area. The z-profiles are analyzed using our previously developed objective method that derives quantitative transparency parameters directly related to the physics of light propagation in tissues. Tissular heterogeneity is quantified by the Birge ratio Br and the photon mean-free path (ls) is determined for homogeneous tissues (i.e., Br~1). SD-OCT images of 83 normal corneas (ages 22-50 years) from a standard SD-OCT device (RTVue-XR Avanti, Optovue Inc.) were processed to establish a normative dataset of transparency values. After confirming stromal homogeneity (Br <10), we measured a median ls of 570 μm (interdecile range: 270-2400 μm). By also considering corneal thicknesses, this may be translated into a median fraction of transmitted (coherent) light Tcoh(stroma) of 51% (interdecile range: 22-83%). Excluding images with central saturation artifact raised our median Tcoh(stroma) to 73% (interdecile range: 34-84%). These transparency values are slightly lower than those previously reported, which we attribute to the detection configuration of SD-OCT with a relatively small and selective acceptance angle. No statistically significant correlation between transparency and age or thickness was found. In conclusion, our algorithm provides robust and quantitative measurements of corneal transparency from standard SD-OCT images with sufficient quality (such as 'Line' and 'CrossLine' B-scan modes without central saturation artifact) and addresses the demand for such an objective means in the clinical setting.

Corneal Confocal Analysis after Scleral Lens Wear in Keratoconus Patients: A Pilot Study

SIGNIFICANCE

Scleral lenses rely on the scleroconjunctival region without bearing the cornea, which could improve the symptoms and modify the corneal nerve plexus morphology.

PURPOSE

This study aimed to evaluate symptoms and changes in corneal nerve plexus morphology and density of Langerhans cells in keratoconus patients with and without intrastromal corneal ring before and after scleral lens wear.

METHODS

Sixteen scleral lens wearers with keratoconus were recruited for this short-term experimental pilot study. Subjects were divided into two groups: keratoconus group and intrastromal corneal ring segment group. All subjects were examined in two visits: baseline (before scleral lens wear) and after 6 months of scleral lens wear, after lens removal. The Schirmer I test, the Ocular Surface Disease Index, tear breakup time, and in vivo confocal microscopy were evaluated.

RESULTS

The mean age was 42.33 ± 11.27 years. A significant decrease in tear breakup time was found in the total group ( P = .01, Wilcoxon) compared with baseline. Ocular Surface Disease Index score had decreased after 6 months of scleral lens wear in the keratoconus with intrastromal corneal ring segment subgroup ( P = .03, Wilcoxon) and in the total group ( P = .001, Wilcoxon). No statistical changes in nerve density, tortuosity, and ramification were found for either the total group or the subgroup. However, the mean nerve length was higher in all groups, especially in the keratoconus subgroup ( P = .03, Wilcoxon) after 6 months of scleral lens wear. Regarding optical density, the total group showed a significant increase after 6 months of wearing ( P = .02, Wilcoxon). Finally, Langerhans cell density was not statistically different in any group.

CONCLUSIONS

These results suggest that scleral lens wearing improves the symptoms and increased the mean length nerves after 6 months of wear use in keratoconus patients.

Effect of Laser-assisted Subepithelial Keratectomy with Mitomycin C on Corneal Optical Density Measured with Confocal Microscopy

SIGNIFICANCE

The development of confocal microscopy allows one to obtain high-resolution corneal images like its optical density. Some studies have evaluated the optical density with Scheimpflug cameras in the early post-operative period after photorefractive keratectomy, but no studies have evaluated the long-term evolution of optical density after surface ablation when mitomycin C is used.

PURPOSE

This work aimed to study the changes in corneal optical density measured with confocal microscopy in eyes treated with laser-assisted subepithelial keratectomy (LASEK) and intraoperative mitomycin C (MMC) to correct myopia.

METHODS

A study of 24 consecutive myopic eyes that underwent LASEK with 0.02% MMC and a control group of 24 healthy nontreated eyes was performed. Optical density was measured using the images by the confocal microscopy of the Heidelberg Retina Tomograph II with the Rostock Cornea Module. An analysis of confocal microscopy images was performed using the ImageJ software to obtain the optical density, in gray-scale units (GSU). The optical density of the stromal bed was evaluated 3 months, 15 months, and 3 years after surgery and was compared with the optical density at the equivalent depth of the stroma in controls.

RESULTS

The mean values of optical density for the LASEK group were 81.7 ± 9.7, 78.6 ± 11.7, and 73.6 ± 18.7 GSU at 3 months, 15 months, and 3 years, respectively, and it was 61.8 ± 8.2 GSU for the control group. A statistically higher optical density 3 and 15 months after LASEK with MMC was found compared with controls (P < .001). No significant difference was found in optical density at 3 years post-operatively.

CONCLUSIONS

Our study suggests that, after LASEK with MMC, the anterior corneal stroma has a higher optical density at 3 and 15 months post-operatively, which gradually returns to normal values 3 years after surgery.

Anterior ophthalmic imaging

Improvements in imaging chips and computer processing power have brought major advances in imaging of the anterior eye. Digitally captured images can be visualised immediately and can be stored and retrieved easily. Anterior ocular imaging techniques using slitlamp biomicroscopy, corneal topography, confocal microscopy, optical coherence tomography (OCT), ultrasonic biomicroscopy, computerised tomography (CT) and magnetic resonance imaging (MRI) are reviewed. Conventional photographic imaging can be used to quantify corneal topography, corneal thickness and transparency, anterior chamber depth and lateral angle and crystalline lens position, curvature, thickness and transparency. Additionally, the effects of tumours, foreign bodies and trauma can be localised, the corneal layers can be examined and the tear film thickness assessed.

Assessment of Corneal Haze After PRK and the Effect of Sutureless Amniotic Membrane Graft by Corneal Densitometry

PURPOSE

To assess whether densitometry analysis appropriately monitors the development of haze in myopic patients after photorefractive keratectomy (PRK) when compared to subjective slit-lamp haze grade examinations, and whether sutureless cryo-preserved amniotic membrane reduced postoperative haze development when compared to the standard bandage contact lens.

METHODS

In this retrospective cohort at the Center for Refractive Surgery, Walter Reed National Military Medical Center, a secondary analysis of prospectively collected data was performed. In the prospective study, participants underwent PRK for myopia. Postoperatively, a standard bandage contact lens was applied to the dominant eye and a sutureless cryo-preserved amniotic membrane graft to the nondominant eye. Participants were evaluated at 1, 3, and 6 months postoperatively for haze formation and corneal densitometry using slit-lamp biomicroscopy and Scheimpflug imaging, respectively.

RESULTS

Densitometry measurements at 6 months postoperatively were positively and significantly associated with the presence or absence of haze as assessed by slit-lamp examination in 39 patients (78 eyes; age range: 21 to 44 years). Eyes with increased densitometry measurements had 2.3 to 3.4 times the odds (P ⩽ .014) of having clinical haze on slit-lamp examination. Eyes with the amniotic membrane graft showed a positive correlation with increased corneal densitometry throughout most layers of the cornea.

CONCLUSIONS

Densitometry analysis appears to be a useful tool to supplement slit-lamp examination in monitoring haze development after PRK. The amniotic membrane failed to show a reduction in corneal densitometry in myopic eyes after PRK. [J Refract Surg. 2020;36(5):293-299.].

Quantitative measures of corneal transparency, derived from objective analysis of depth-resolved corneal images, demonstrated with full-field optical coherence tomographic microscopy

Loss of corneal transparency, as occurs with various pathologies, infections, immune reactions, trauma, aging, and surgery, is a major cause of visual handicap worldwide. However, current means to assess corneal transparency are extremely limited and clinical and eye-bank practice usually involve a subjective and qualitative observation of opacities, sometimes with comparison against an arbitrary grading scale, by means of slit-lamp biomicroscopy. Here, we describe a novel objective optical data analysis-based method that enables quantifiable and standardized characterization of corneal transparency from depth-resolved corneal images, addressing the demand for such a means in both the laboratory and clinical ophthalmology setting. Our approach is based on a mathematical analysis of the acquired optical data with respect to the light attenuation from scattering processes in the corneal stroma. Applicable to any depth-resolved corneal imaging modality, it has been validated by means of full-field optical coherence tomographic microscopy (FF-OCT or FF-OCM). Specifically, our results on ex-vivo corneal specimens illustrate that 1) in homogeneous tissues, characterized by an exponential light attenuation with stromal depth (z), the computation of the scattering mean-free path (ls) from the rate of exponential decay allows quantification of the degree of transparency; 2) in heterogeneous tissues, identified by significant deviations from the normal exponential z -profile, a measure of exponential-decay model inadequacy (e.g., by computation of the Birge ratio) allows the estimation of severity of stromal heterogeneity, and the associated depth-dependent variations around the average ls enables precise localization of the pathology.

Corneal Surface Ablation Laser Refractive Surgery for the Correction of Myopia: A Network Meta-analysis

PURPOSE

To systematically compare the efficacy, predictability, safety, postoperative haze, pain scores, and epithelial healing time of four corneal surface ablation procedures.

METHODS

PubMed, Embase, Cochrane Library, and the U.S. trial registry were searched up to June 2018. Randomized controlled trials were selected. Efficacy (uncorrected distance visual acuity of 20/20 or better), predictability (refractive spherical equivalent within ±0.50 diopters [D] of the target), and safety (loss of two or more lines of spectacle corrected distance visual acuity) were set as primary outcome measures. Haze, pain scores, and epithelial healing time were set as secondary outcome measures.

RESULTS

Eighteen studies involving 1,423 eyes were included. According to the Grading of Recommendations Assessment, Development, and Evaluation, the quality of outcomes were moderate to high (70.6%). There were no differences in efficacy, predictability, safety, haze, day 1 pain, and epithelial healing time between treatments. Epithelial laser in situ keratomileusis (epi-LASIK) had statistically significantly higher pain scores on day 3 compared to photorefractive keratectomy (PRK) (weighted mean differences [WMD] = 2.2, 95% credible intervals [CrI] = 0.19 to 4.01) and transepithelial PRK (T-PRK) (WMD = 2.7, 95% CrI = 0.51 to 4.84). The surface under the cumulative ranking curve ranking results (best to worst) showed laser epithelial keratomileusis (LASEK) ranked highest for efficacy, predictability, safety, and day 1 pain scores. Epi-LASIK ranked best for grade 1 haze scores. T-PRK ranked best for haze of 0.5 or higher, haze scores day 3 pain scores, and epithelial healing time.

CONCLUSIONS

Surface laser refractive surgeries are comparable in terms of efficacy, predictability, safety, and postoperative haze except for day 3 pain scores, with epi-LASIK being more painful compared to PRK and T-PRK. [J Refract Surg. 2018;34(11):726-735.].

Corneal backward scattering and higher-order aberrations in children with vernal keratoconjunctivitis and normal topography

PURPOSE

To investigate the corneal backward scattering and higher-order aberrations (HOAs) in children with vernal keratoconjunctivitis (VKC) and normal topography.

METHODS

Thirty-six eyes of 22 patients with VKC and 54 eyes of 34 normal subjects were included. All participants had clear cornea, absence of dry eyes and a normal corneal tomography. Scheimpflug imaging was used to measure corneal backward scattering in zones centred on the corneal apex (central 2-mm zone and paracentral 2- to 6-mm zone), and HOAs were compared between VKC and normal control.

RESULTS

The mean age of participants was 12.0 ± 4.1 years in VKC group and 11.2 ± 4.1 years in control group (p = 0.339). There was no significant intergroup difference in mean keratometry, astigmatism and apex pachymetry (p ≥ 0.076). Total corneal backscatter was higher in the VKC group compared to the control group (p ≤ 0.012). Anterior and posterior cornea displayed a higher level of backward scattering in the VKC group (p < 0.001 for anterior; p ≤ 0.048 for posterior). Patients with VKC exhibited higher total HOAs and coma (p ≤ 0.036). There were significant correlations between total anterior HOAs and backward scattering measured at the central (r = 0.500; p = 0.032) and paracentral zones (r = 0.470; p = 0.024) for VKC.

CONCLUSION

The current study showed optical quality changes in patients with clear corneas and quiescent VKC. An increase in corneal backward scattering and HOAs was noted in patients with VKC as compared to normal patients.

Comparison of optical quality after implantable collamer lens implantation and wavefront-guided laser in situ keratomileusis

AIM

To compare the optical quality after implantation of implantable collamer lens (ICL) and wavefront-guided laser in situ keratomileusis (WG-LASIK).

METHODS

The study included 40 eyes of 22 patients with myopia who accepted ICL implantation and 40 eyes of 20 patients with myopia who received WG-LASIK. Before surgery and three months after surgery, the objective scattering index (OSI), the values of modulation transfer function (MTF) cutoff frequency, Strehl ratio, and the Optical Quality Analysis System (OQAS) values (OVs) were accessed. The higher order aberrations (HOAs) data including coma, trefoil, spherical, 2^nd astigmatism and tetrafoil were also obtained. For patients with pupil size <6 mm, HOAs data were analyzed for 4 mm-pupil diameter. For patients with pupil size ≥6 mm, HOAs data were calculated for 6 mm-pupil diameter. Visual acuity, refraction, pupil size and intraocular pressures were also recorded.

RESULTS

In both ICL and WG-LASIK group, significant improvements in visual acuities were found postoperatively, with a significant reduction in spherical equivalent (P< 0.001). After the ICL implantation, the OSI decreased slightly from 2.34±1.92 to 2.24±1.18 with no statistical significance (P=0.62). While in WG-LASIK group, the OSI significantly increased from 0.68±0.43 preoperatively to 0.91±0.53 postoperatively (Wilcoxon signed ranks test, P=0.000). None of the mean MTF cutoff frequency, Strehl ratio, OVs showed statistically significant changes in both ICL and WG-LASIK groups. In the ICL group, there were no statistical differences in the total HOAs for either 4 mm-pupil or 6 mm-pupil. In the WG-LASIK group, the HOA parameters increased significantly at 4 mm-pupil. The total ocular HOAs, coma, spherical and 2^nd astigmatism were 0.12±0.06, 0.06±0.03, 0.00±0.03, 0.02±0.01, respectively. After the operation, these values were increased into 0.16±0.07, 0.08±0.05, -0.04±0.04, 0.03±0.01 respectively (Wilcoxon signed ranks test, all P<0.05). At 6 mm-pupil, the induction of total HOAs was not statistically significant in the WG-LASIK group.

CONCLUSION

ICL implantation has a less disturbance to optical quality than WG-LASIK. The OQAS is a valuable complementary measurement to the wavefront aberrometers in evaluating the optical quality.

Correlation between practice location as a surrogate for UV exposure and practice patterns to prevent corneal haze after photorefractive keratectomy (PRK)

Background

PRK is a refractive surgery that reshapes the corneal surface by excimer laser photoablation to correct refractive errors. The effect of increased ultraviolet (UV) exposure on promoting post-PRK corneal haze has been reported in the literature; however, information is lacking regarding the effect of ambient UV exposure on physician practice patterns. The aim of this study was to evaluate the effect of ophthalmologists’ practice location on their reported practice patterns to prevent post-PRK corneal haze.

Methods

A cross-sectional observational study was conducted through an online survey sent to ophthalmologists performing PRK. The survey recorded the primary city of practice from which the two independent variables, latitude and average annual sunshine days, were determined. It also measured the frequency of use of postoperative preventive interventions (dependent variables) which are as follows: intraoperative Mitomycin-C, oral vitamin C, sunglasses, topical corticosteroids, topical cyclosporine, oral tetracyclines and amniotic membrane graft.

Results

Fifty-one ophthalmologists completed the survey. Practice locations’ mean latitude was 36.4 degrees north, and average sunshine days annually accounted for 60% of year days. There was no significant relation between latitude/average annual sunshine days and usual post-PRK prophylactic treatments (P > 0.05). The commonest protective maneuvers were sunglasses (78%), prolonged topical corticosteroids (57%), Mitomycin-C (39%) and oral vitamin C (37%).

Conclusion

We found no significant difference in ophthalmologists’ practice patterns to prevent post-PRK corneal haze in relation to practice location latitude and average sunshine days. Moreover, the results demonstrated that the most widely used postoperative measures to prevent post-PRK haze are sunglasses, Mitomycin-C, topical corticosteroids, and oral Vitamin C.

Measuring Corneal Haze by Using Scheimpflug Photography and Confocal Microscopy

PURPOSE

We compared corneal backscatter estimated from a Scheimpflug camera with backscatter estimated from a clinical confocal microscope across a wide range of corneal haze.

METHODS

A total of 59 corneas from 35 patients with a range of severity of Fuchs' endothelial corneal dystrophy and 15 corneas from 9 normal participants were examined using a Scheimpflug camera (Pentacam) and a confocal microscope (ConfoScan 4). The mean image brightness from the anterior 120 μm, midcornea, and posterior 60 μm of the cornea across the central 2 mm recorded by the Scheimpflug camera and analogous regions from the confocal microscope were measured and standardized. Differences between instruments and correlations between backscatter and disease severity were determined by using generalized estimating equation models.

RESULTS

Backscatter measured by the two instruments in the anterior and midcornea were correlated (r = 0.67 and 0.43, respectively, P < 0.001), although in the posterior cornea they were not correlated (r = 0.13, P = 0.66). Measured with the Scheimpflug camera, mean backscatter from the anterior and midcornea were greater, whereas backscatter from the posterior cornea was lower (P < 0.001) than that measured by the confocal microscope. Backscatter from the anterior cornea was correlated with disease severity for both instruments (Scheimpflug, r = 0.55, P < 0.001; confocal, r = 0.49, P = 0.003).

CONCLUSIONS

The Scheimpflug camera and confocal microscope should not be used interchangeably to measure corneal haze. The ability to detect changes in backscatter with disease severity is superior with the Scheimpflug camera. However, the confocal microscope provides higher resolution of corneal structure.

Comparison of optical quality parameters and ocular aberrations after wavefront-guided laser in-situ keratomileusis versus wavefront-guided laser epithelial keratomileusis for myopia

BACKGROUND

To compare optical quality, ocular scattering, and higher-order aberrations (HOAs) after laser in-situ keratomileusis (LASIK) versus laser epithelial keratomileusis (LASEK).

METHODS

A total of 47 eyes from 47 participants who had undergone LASIK (group I) or LASEK (group II) procedure at least 6 months prior were enrolled. Ocular aberrations and modulation transfer function (MTF) values measured using iTrace, a ray-tracing type aberrometer, were compared to MTF (modulation transfer function) cut-off values, Strehl ratio, and objective indices of scattering obtained using the Objective Quality Analysis System II (OQAS II).

RESULTS

There was no significant correlation between the postoperative optical quality parameters and the HOAs between both groups. In group I, the MTF cut-off value was significantly correlated with cylinder refraction (p = 0.037), and the objective scattering index (OSI) was positively correlated with spherical equivalent (p = 0.023). In group II, there was a statistically significant correlation between the OSI and achieved refractive correction (p = 0.001). Regression analysis showed that the OSI was the most significant predictor of MTF cut-off values after refractive surgery. Additionally, MTF values measured by OQAS were significantly lower than those measured by iTrace without correlation.

CONCLUSION

Optical quality after refractive surgery may be influenced by not only ocular aberrations but also by scattering. Even though the accuracies of the machines used in this study to measure optical quality have yet to be proven, this study showed limited correlation among the values measured using the two different machines after refractive surgery. Therefore, for more generalized evaluation of visual function after refractive surgery, more advanced optical devices still need to be developed.

Wavefront-optimized excimer laser in situ keratomileusis for myopia and myopic astigmatism: refractive outcomes and corneal densitometry

PURPOSE

To determine the refractive outcomes of wavefront-optimized laser in situ keratomileusis (LASIK) treatments, in particular to measure corneal densitometry after LASIK using the densitometry function of the Pentacam Scheimpflug system.

SETTING

Division of Ophthalmology and Visual Sciences, University of Nottingham, United Kingdom.

DESIGN

Cohort study.

METHODS

Changes in postoperative visual acuity, refraction, and contrast sensitivity were evaluated after wavefront-optimized laser treatment. Corneal densitometry was evaluated with the Scheimpflug system before and after LASIK.

RESULTS

One year postoperatively, the uncorrected distance visual acuity was 6/6 or better in 92% of eyes and 6/9 or better in all eyes. Eighty-six percent of eyes had no change in the corrected distance visual acuity (CDVA); 4% gained 1 or more lines. Wavefront-optimized LASIK was stable over 1 year postoperatively. Eighty-nine percent of eyes were within ±0.50 diopter (D) and 100% were within ±1.00 D of the intended correction 1 year postoperatively. Contrast sensitivity showed a nonsignificant improvement (1.55 ± 0.10 [SD] preoperatively to 1.57 ± 0.09 12 months postoperatively) (P > .05). There was a nonsignificant increase in corneal densitometry 1 year postoperatively (from 12.72 ± 2.43 to 13.04 ± 2.58) (P > .05). No correlation was found between corneal densitometry and contrast sensitivity or CDVA.

CONCLUSIONS

Wavefront-optimized LASIK gave excellent refractive and visual outcomes and did not seem to affect corneal densitometry significantly 1 year postoperatively. However, larger studies may show a masked effect on corneal densitometry.

FINANCIAL DISCLOSURE

No author has a financial or proprietary interest in any material or method mentioned.

Biosynthetic corneal substitute implantation in dogs

PURPOSE

To assess integration of a biosynthetic corneal implant in dogs.

METHODS

Three normal adult laboratory Beagles underwent ophthalmic examinations, including slit-lamp biomicroscopy, indirect ophthalmoscopy, applanation tonometry, and Cochet-Bonnet aesthesiometry. Biosynthetic corneas fabricated from glutaraldehyde crosslinked collagen and copolymers of collagen and poly(N-isopropylacrylamide-co-acrylic acid-co-acryloxysuccinimide, denoted as TERP) were implanted into dogs by a modified epikeratoplasty technique. Ophthalmic examinations and aesthesiometry were performed daily for 5 days and then weekly thereafter for 16 weeks. Corneal samples underwent histopathological and transmission electron microscopy examination at 16 weeks.

RESULTS

Implants were epithelialized by 7 days. Intraocular pressure was within normal range throughout the study. Aesthesiometry values dropped from an average of 3.67 cm preoperatively to less than 1 mm for all dogs for the first postoperative weeks. By week 16, the average Cochet-Bonnet value was 1.67 cm, demonstrating partial recovery of functional innervation of the implant. No inflammation or rejection of the implant occurred, and minimal haze formation was noted. Light microscopy revealed thickened but normal epithelium over the implant with fibroblast migration into the scaffold. On transmission electron microscopy, the basement membrane was irregular but present and adhesion complexes were noted.

CONCLUSION

Biosynthetic corneal implantation is well tolerated in dogs, and the collagen-polymer hybrid construct holds promise for clinical application in animals and humans.

Comparison of custom ablation and conventional laser in situ keratomileusis for myopia and myopic astigmatism using the Alcon excimer laser

PURPOSE

To compare the refractive outcomes, higher order aberrations, and contrast sensitivity after laser in situ keratomileusis (LASIK) using wavefront ablation and conventional ablation.

SETTING

Private practice in Córdoba, Spain and a free-standing outpatient surgery center.

METHODS

This was a prospective, nonrandomized, observational case series comparing outcomes of 239 eyes that underwent LASIK for myopia and myopic astigmatism with either wavefront or conventional ablation using the LADARVISION excimer laser. Manifest refractive sphere ranged from 0.50 D to -8.00 D with astigmatism up to -4.00 D. Eighty-nine eyes underwent conventional LASIK (conventional group), and 150 eyes underwent custom ablation (custom group). Refractive outcomes, ocular higher order root mean square (HOA-RMS), and contrast sensitivity were tested for statistically significant differences between groups. A P-value less than 0.05 was considered statistically significant. Six month postoperative data are reported here.

RESULTS

Postoperatively, the mean SE was -0.03 D +/- 0.19 D for the custom group, and -0.14 D +/- 0.35 D for the conventional group (P = 0.003). Ninety-nine percent of the eyes in the custom group, and 92% of the eyes in the conventional group were within 0.50 D of the intended correction (P > 0.05). The HOA-RMS was 0.16 mum lower in the custom group (P < 0.001). Contrast sensitivity was statistically significantly better at 3 cycles per degree (cpd) (P < 0.001) and 6 cpd (P = 0.009) in the custom group.

CONCLUSION

There was a statistically significant lower induction of HOA-RMS and better predictability and contrast sensitivity in eyes that underwent custom ablation with the LADARVISION excimer laser.

Changes in contrast sensitivity function and ocular higher order aberration by conventional myopic photorefractive keratectomy

PURPOSE

To evaluate the relation between induced changes in ocular higher order aberrations and changes in the contrast sensitivity function in patients undergoing excimer laser photorefractive keratectomy (PRK).

METHODS

Myopic PRK using excimer laser was performed in 31 patients (56 eyes). The preoperative refractive error was -6.2 +/- 2.9 diopters. Before and 1 month after surgery, we measured the ocular higher order aberrations for a 4-mm pupil, and three indices of contrast sensitivity function. From the data collected, the area under the log contrast sensitivity function (AULCSF) was calculated.

RESULTS

PRK significantly reduced AULCSF (P = 0.004), low-contrast visual acuity (P = 0.004), and letter-contrast sensitivity (P = 0.013). Coma-like (P < 0.001) and spherical-like (P < 0.001) aberrations were significantly increased by surgery. The change in AULCSF by surgery significantly correlated with the change in coma-like (r = -0.468, P < 0.001) and spherical-like (r = -0.291, P = 0.033) aberrations. The change in low-contrast visual acuity by PRK significantly correlated with the change in coma-like aberration (r = 0.599, P < 0.007), but not with change in spherical-like aberrations (r = 0.136, P = 0.326). There were significant correlations between changes in letter-contrast sensitivity and changes in coma-like (r = -0.450, P < 0.001) and spherical-like (r = -0.255, P = 0.048) aberrations.

CONCLUSIONS

PRK significantly increases ocular higher order aberrations, which compromise contrast sensitivity function after surgery.

Wound healing following refractive surgery in hens

The wound-healing response is critical to the outcome of refractive surgery and studying wound healing contributes to an understanding of the pathophysiology of other corneal injuries. Animal models allow research to be conducted with sufficient samples and under controlled parameters. We studied the hen to determine the healing process from clinical, biophysical, and biological standpoints after photorefractive keratectomy (PRK). PRK (-6.0 diopters) was performed in hen eyes. At 3, 6, 12, 24, 48, and 72 h and 5, 7, 15, 30, and 60 days postoperatively, we studied the clinical follow-up, objective measurements of light transmission (direct transmittance), apoptosis by TUNEL assay, proliferation by immunocytochemical analysis of 5-bromo-2'-deoxyuridine, and expression of alpha smooth muscle actin (SMA) in myofibroblasts in the corneas. Hen corneas reepithelialize quickly. Haze developed from 5 to 60 days after surgery and was correlated with the appearance and finalization of the expression of SMA. The direct transmittance of light was low during the first 15 days and improved at 30 and 60 days. TUNEL-positive cells were observed 3 h after surgery and the numbers decreased thereafter. Epithelial proliferation began at 12 h and was greater at 48 h, while stromal cell proliferation began at 24 h and was greater at 72 h. The hen cornea is anatomically similar to the human cornea, and the manner in which it heals is a good model for studying different surgical techniques and pharmacologic assays.

Agreement and Repeatability of Central Thickness Measurement in Normal Corneas Using Ultrasound Pachymetry and the OCULUS Pentacam

PURPOSE

To compare the accuracy and repeatability of the OCULUS Pentacam (a new Scheimpflug-based imaging system) with ultrasound pachymetry in the measurement of central corneal thickness (CCT).

METHODS

CCT was measured in 21 subjects (21 normal corneas) on 2 separate occasions by the same examiner, using an Allergan-Humphrey 850 ultrasonic pachymeter and an OCULUS Pentacam instrument.

RESULTS

Mean values of CCT for both visits for each instrument were 534 +/- 47 microm and 528 +/- 45 microm using the ultrasonic pachymeter and the Pentacam, respectively. Plots of differences against means displayed relatively good agreement (limits of agreement were -13.0 to +26.6 microm). The repeatability (limits of agreement) of the ultrasound pachymeter was -18.3 to +17.7 microm, while for the OCULUS Pentacam it was -24.1 to +21.1 microm.

CONCLUSIONS

Our data showed that the Pentacam instrument provided measurements that were slightly but systematically lower than the measurements provided by ultrasonic pachymetry, which is currently the clinical gold standard method. The results, coupled with a unique ability to image and analyze the anterior chamber in vivo, make the OCULUS Pentacam a promising new instrument for anterior eye evaluation.

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.

Influence of wavefront aberration and corneal subepithelial haze on low-contrast visual acuity after photorefractive keratectomy

PURPOSE

To assess low-contrast visual acuity (LCVA) after photorefractive keratectomy in relation to ocular higher-order wavefront aberration and corneal subepithelial haze.

DESIGN

Prospective, cross-sectional analysis.

METHODS

Photorefractive keratectomy was performed in 51 eyes of 27 subjects with myopic refractive error of -2.0 to -10.5 diopters. Ocular higher-order wavefront aberrations for a 4-mm pupil were measured using Topcon Hartmann-Shack wavefront aberrometer, and the extent of corneal subepithelial haze was quantified with Nidek TSPC-3 hazemeter before and 1 month after photorefractive keratectomy. Low-contrast visual acuity was recorded with Vector Vision CSV-1000LanC10% chart. Total higher-order, third-order (coma-like), and fourth-order (spherical-like) aberrations of the eye were determined. The influence of wavefront aberration and corneal subepithelial haze on LCVA was analyzed.

RESULTS

Total higher-order, third-order, and fourth-order aberrations significantly increased by surgery (P < .001, Wilcoxon signed rank test). Photorefractive keratectomy induced a significant increase in corneal haze (P < .01), but no case presented severe corneal haze (grade 3 or greater by Fantes grading). By surgery, LCVA was reduced significantly (P < .001). The logarithm of the minimal angle of resolution LCVA showed a significant correlation with total higher-order aberration (Spearman rank correlation coefficient, r(s) = 0.642, P < .0001). Both third-order (r(s) = 0.618, P < .0001) and fourth-order aberrations (r(s) = 0.552, P < .0001) also significantly correlated with logarithm of the minimal angle of resolution LCVA. There was no correlation between the degree of corneal haze and logarithm of the minimal angle of resolution LCVA (r(s) = 0.094, P = .523).

CONCLUSIONS

In eyes with mild to moderate corneal haze after photorefractive keratectomy, deterioration of LCVA is mainly attributable to increases in wavefront aberration, and not to corneal haze.

Glare sensitivity and optical side effects 1 year after photorefractive keratectomy and laser in situ keratomileusis

PURPOSE

To compare the outcome of low-contrast visual acuity and glare sensitivity after photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK).

SETTING

Department of Ophthalmology, Cantonal Hospital of Lucerne, Lucerne, Switzerland.

METHODS

In this prospective study, patients selected PRK or LASIK after the advantages and disadvantages of both had been described. Snellen visual acuity and disability glare were measured with the Berkeley glare test preoperatively and 1 year postoperatively. At the 1-year follow-up, haze was graded and patients had to assess their quality of vision subjectively.

RESULTS

One-year follow-up of 58 patients in the PRK group and 64 patients in the LASIK group was achieved. In both groups, the mean uncorrected visual acuity was 20/32 (P =.63) and the mean best corrected visual acuity, 20/20 with no statistically significant difference (P =.20). There were no preoperative or postoperative differences between the 2 groups in low-contrast visual acuity under 4 glare conditions. At 1 year, LASIK eyes had significantly lower postoperative haze scores than PRK eyes (P =.0013). The number of eyes with visually moderate and disturbing halos or disturbances in night vision did not differ considerably between the groups (P =.88).

CONCLUSIONS

Efficacy outcomes were generally similar in the PRK and LASIK groups. Both achieved good objective and subjective results after treatment with a second-generation excimer laser.

Late-onset corneal haze and myopic regression after photorefractive keratectomy (PRK)

OBJECTIVE

To report the incidence and clinical course of a series of patients who developed both delayed-onset, clinically significant progressive haze and myopic regression after photorefractive keratectomy (PRK).

METHODS

In this retrospective case series, the charts of 542 consecutive patients who had undergone PRK with the VISX Star Excimer or Nidek EC-5000 laser between July 1996 and October 1998 and who had a minimum of 6 months of follow-up were reviewed. Ten eyes of 8 patients developed progressive haze to greater than 1+ and myopic regression equal to or more than -1 D 3 months or more after PRK. The historical and clinical features were reviewed.

RESULTS

The incidence of combined progressive haze and myopic regression was 1.8%. The average age was 40.5 years. Three of the 8 patients were female. The median spherical equivalent (SE) attempted correction was -6.69 D (range -4.00 to -12.25 D). Five patients who underwent bilateral PRK had unilateral involvement. The mean SE regression was -2.01 +/- 0.79 D (range -1.00 to -3.00 D). Regression plateaued at a mean of 9.8 months. Haze ranging up to 4+ peaked at a mean of 7.4 months. Topical steroid treatment and/or epithelial scraping was attempted in 3 eyes but was ineffective.

CONCLUSIONS

Combined delayed-onset progressive haze and myopic regression can occur after PRK. In such cases, the amount of haze appears to correlate with the magnitude of attempted initial correction (r = 0.639, P = 0.046) although not with the magnitude of subsequent regression. Patients may need at least 10 months of follow-up to achieve a stable refraction and level of haze. These observations suggest a need for improved understanding of corneal wound healing following PRK and of biologic factors that may contribute to variability in outcomes.

Quantification of simulated corneal haze by measuring back-scattered light

The Scheimpflug anterior eye segment analysis system EAS-1000 was modified to specifically quantitate the degree of corneal haze. In order to confirm the basic measurement performance of the new system, TSPC-3 hazemeter, in vitro experiments were conducted. Using white latex microsphere solutions, tests were performed for the linearity of the measurements, the influence of slit lamp illuminance and the influence of the length of the light path through the latex solution. The validity of Beer's law was confirmed, indicating that a sufficiently wide range of latex concentrations (haze intensities) can be covered by the hazemeter. The scattering intensity measured with the hazemeter showed a significant linear correlation with the latex concentration. The obtained scattering intensity was directly proportional to the illuminance of the slit lamp. The ratio of integrated scattering values was proportional to that of the light path length. It was shown that the TSPC-3 hazemeter possesses sufficient capability for a quantitative evaluation of corneal haze.