Keratocyte loss after corneal deepithelialization in primates and rabbits

Photorefractive Keratectomy: Technical Evolution, Refractive Outcomes, Corneal Wound Healing Response, and Complications

Photorefractive keratectomy (PRK) was the first excimer laser procedure developed to treat refractive errors. The safety and efficacy of PRK established it as one of the most performed corneal refractive procedures worldwide. With the introduction of laser in situ keratomileusis (LASIK), and more recently keratorefractive lenticule extraction (KLEx) procedures, many corneal surgeons favor these newer corneal procedures as the first choice due to faster visual rehabilitation and less discomfort during the early postoperative period. Importantly, however, PRK remains a viable alternative for most corneal refractive candidates and there are many situations in which PRK remains the refractive procedure of choice. This review addresses the technical evolution of PRK-mechanical epithelial debridement versus alcohol-assisted epithelial removal versus excimer laser-assisted epithelial debridement (transepithelial) PRK-and reports the PRK refractive outcomes compared to other keratorefractive laser procedures. The corneal wound response associated with each PRK technique and the indications, limitations, and complications of PRK are reviewed to aid refractive surgeons to best position PRK in their overall practice. [J Refract Surg. 2024;40(10):e754-e767.].

Temporal evolution of the biological response to laser-induced refractive index change (LIRIC) in rabbit corneas

Laser-induced refractive index change (LIRIC) is a new, non-incisional, non-ablative, femtosecond photo-modification technique being developed for vision correction in humans. Prior, exvivo studies showed intra-tissue refractive index change to induce minimal cell death, restricted to the laser focal zone in the corneal stroma, and with no observable damage to the epithelium or endothelium. Here, we used live rabbits to ascertain longer-term consequences of LIRIC in vivo. Specifically, we assessed cell death, fibrosis, corneal nerve distribution, endothelial cell density, and corneal structure for up to 3 months after LIRIC. A +2.5 D gradient-index LIRIC Fresnel lens was inscribed inside 20 applanated corneas of Dutch Belted rabbits, over a circular region of the mid-stroma measuring 4.5 mm in diameter. Twelve additional rabbit eyes were used as applanation-only controls to differentiate the effects of laser treatment and suction applanation on biological and structural parameters. In vivo optical measurements were performed pre-operatively, then immediately, 2, 4, and 12 weeks after the procedure, to measure endothelial cell density and changes in corneal structure. Groups of four rabbits were sacrificed at 4 hours, 2, 4, and 12 weeks after LIRIC for histological determinations; the TUNEL assay was used to evaluate cell death, H&E staining was used to assess inflammatory infiltration, and immunostaining for α-smooth muscle actin (α-SMA) and βIII tubulin (Tuj-1) was performed to assess myofibroblast differentiation and corneal nerve distribution, respectively. Consistent with prior ex vivo data, only minimal cell death was observed in the laser focal zone, with TUNEL-positive cells restricted to the stromal region of refractive index change 4 h after LIRIC. No TUNEL-positive cells were evident anywhere in the cornea 2, 4, or 12 weeks after LIRIC. Applanation-only corneas were completely TUNEL-negative. Neither LIRIC-treated nor applanation-only eyes exhibited α-SMA-positive staining or altered corneal nerve distributions at any of the time points examined. In vivo confocal imaging revealed normal endothelial cell densities in all eyes (whether LIRIC-treated or applanation-only) at all time points. Optical coherence tomography showed suction applanation to cause a temporary decrease in central corneal thickness, which returned to normal within 4 h. Corneas into which LIRIC Fresnel lenses were written while applanated did not undergo major structural or shape changes beyond the temporary thinning already described for suction applanation. The present findings suggest that LIRIC patterns, which generated a clinically-relevant refractive correction in the mid-stromal region of live rabbit corneas, induced little-to-no disruption to corneal structure and biology for 3 months after the procedure. This affirms the relative safety of LIRIC and predicts that compared to traditional laser vision correction surgeries, common post-operative complications such as dry eye, haze, or patient discomfort may be entirely avoided.

The Effectiveness of Corneal Epithelial Debridement Performed with an Alcohol-Impregnated Sponge

OBJECTIVE

To evaluate the clinical outcomes of patients who underwent corneal surgery with debridement of epithelium by localised application of an alcohol-impregnated sponge instead of the usual alcohol reservoir method.

DESIGN

This study retrospectively included 52 eyes of 27 patients who underwent LASEK (laser-assisted subepithelial keratectomy) and CXL (cross-linking) surgery after application of this alcohol-assisted debridement method between January 2019 and April 2019.

PARTICIPANTS

Twenty-four patients who underwent LASEK and three patients who underwent CXL surgery were included in this study, which comprised a total of 52 eye operations.

METHODS

Epithelial debridement was performed after placing a 20% ethanol-impregnated sponge on the cornea for 20 seconds.

RESULTS

The mean pachymetry value was 509.40 ± 30.37 µm, the mean preoperative spherical equivalent was - 2.14 ± 1.02 dioptres, and the highest myopia and astigmatism values were - 4.25 dioptres and - 2.75 dioptres, respectively. It was seen that the corneal epithelium was completely and easily debrided in a size identical to that of the applied sponge.

CONCLUSIONS

Complete debridement of the corneal epithelium can be performed quickly, safely, and easily with this specially prepared sponge.

Corneal wound healing

The corneal wound healing response is typically initiated by injuries to the epithelium and/or endothelium that may also involve the stroma. However, it can also be triggered by immune or infectious processes that enter the stroma via the limbal blood vessels. For mild injuries or infections, such as epithelial abrasions or mild controlled microbial infections, limited keratocyte apoptosis occurs and the epithelium or endothelium regenerates, the epithelial basement membrane (EBM) and/or Descemet's basement membrane (DBM) is repaired, and keratocyte- or fibrocyte-derived myofibroblast precursors either undergo apoptosis or revert to the parent cell types. For more severe injuries with extensive damage to EBM and/or DBM, delayed regeneration of the basement membranes leads to ongoing penetration of the pro-fibrotic cytokines transforming growth factor (TGF) β1, TGFβ2 and platelet-derived growth factor (PDGF) that drive the development of mature alpha-smooth muscle actin (SMA)+ myofibroblasts that secrete large amounts of disordered extracellular matrix (ECM) components to produce scarring stromal fibrosis. Fibrosis is dynamic with ongoing mitosis and development of SMA + myofibroblasts and continued autocrine-or paracrine interleukin (IL)-1-mediated apoptosis of myofibroblasts and their precursors. Eventual repair of the EBM and/or DBM can lead to at least partial resolution of scarring fibrosis.

Inhibition of Human Corneal Myofibroblast Formation

Purpose

Transforming growth factor-beta (TGF-β) isoform 1 (T1) is involved in corneal fibrotic wound healing by stimulating myofibroblast transformation and altering fibrotic gene expression. In this study, two specific inhibitors were used to dissect the relationship between myofibroblast generation and the TGF-β/Smad- or TGF-β/p38-signaling pathway in human corneal fibroblasts (HCF).

Methods

In HCF, Trx-SARA (Smad-pathway inhibitor) was used to block the TGF-β/Smad-signaling pathway, and the p38 inhibitor (p38inh, SB202190) was used to inhibit p38MAPK, thus blocking the TGF-β/p38-signaling pathway. HCF ± Trx-SARA or Trx-GA (SARA control) were serum starved overnight in Eagle's minimum essential medium (EMEM) ± p38inh, grown in EMEM ± T1 ± p38inh for 24 hours, and then processed for indirect-immunofluorescence, Western blot, or quantitative real-time polymerase chain reaction to examine α-smooth muscle actin (αSMA) and other fibrotic genes, such as fibronectin, thrombospondin1, and type III collagen. In addition, the morphology and the effect of p38inh on myofibroblast phenotype after myofibroblast formation were examined.

Results

We observed that Trx-SARA had little effect on αSMA expression, indicating that blocking the Smad pathway did not significantly inhibit myofibroblast formation. However, p38inh did significantly inhibit αSMA and other fibrotic genes, thus efficiently preventing the transition of HCFs to myofibroblasts. In addition, morphology changed and αSMA decreased in myofibroblasts exposed to p38inh medium, as compared with controls.

Conclusions

HCF transition to myofibroblasts was mainly through the p38 pathway. Therefore, blocking the p38 pathway may be a potential therapeutic tool for human corneal fibrosis prevention/treatment, because it controls myofibroblast formation in human corneal cells, while leaving other functions of T1 unaffected.

The Effect of Topical Substance-P Plus Insulin-like Growth Factor-1 (IGF-1) on Epithelial Healing After Photorefractive Keratectomy in Rabbits

Purpose

To determine whether topical Substance-P (SP) plus insulin-like growth factor-1 (IGF-1) can improve corneal healing after photorefractive surface ablation in a rabbit.

Methods

After a 9.0-mm corneal de-epithelialization using a combination of chemical (18% alcohol) and mechanical debridement, excimer photorefractive surface ablation was performed bilaterally in eight rabbits (16 eyes) with an 8.0-mm ablation zone and 70-μm depth. The right eye was treated with SP (250 μg/mL) and IGF-1 (25 ng/mL) in hyaluronic acid, one drop twice a day, and the other eye treated with only hyaluronic acid. The epithelial healing process was documented photographically twice a day until healing was complete. Six rabbits were sacrificed 6 weeks after photorefractive keratectomy (PRK) and corneas examined histologically.

Results

Seven of eight rabbit eyes treated with SP/IGF-1 healed in a shorter time than the untreated eye. For rabbit #6, both eyes healed at the same time. The average healing time (total time until wound closure) for the treated eyes was 99 hours, while the average healing time for the untreated eyes was 170 hours (P = 0.0490). A persistent epithelial defect was found in two of the nontreated eyes but none in the treated eyes. Corneal pathology showed some degree of epithelial separation in the central corneal wound in three out of six nontreated eyes and in just the treated eye of rabbit #6.

Conclusion

Topical SP plus IGF-1 increases the epithelial healing rate after PRK. There may have been beneficial effects upon cell adhesion as well.

Translational Relevance

Better and faster healing.

Contrasting cellular damage after Blue-IRIS and Femto-LASIK in cat cornea

Blue-intra-tissue refractive index shaping (Blue-IRIS) is a new approach to laser refractive correction of optical aberrations in the eye, which alters the refractive index of the cornea rather than changing its shape. Before it can be implemented in humans, it is critical to establish whether and to what extent, Blue-IRIS damages the cornea. Here, we contrasted the impact of -1.5 D cylinder refractive corrections inscribed using either Blue-IRIS or femtosecond laser in-situ keratomileusis (femto-LASIK) on corneal cell viability. Blue-IRIS was used to write a -1.5 D cylinder gradient index (GRIN) lens over a 2.5 mm by 2.5 mm area into the mid-stromal region of the cornea in six freshly-enucleated feline eyes. The same correction (-1.5 D cylinder) was inscribed into another four cat eyes using femto-LASIK. Six hours later, all corneas were processed for histology and stained for terminal deoxynucleotidyl transferase-mediated dUTP-digoxigenin nick end labeling (TUNEL) and p-γ-H2AX to label damaged cells. In Blue-IRIS-treated corneas, no tissue was removed and TUNEL-stained cells were confined to the laser focal zone in the stroma. In femto-LASIK, photoablation removed 14 μm of anterior stroma, but in addition, TUNEL-positive cells clustered across the femto-flap, the epithelium at the flap edges and the stroma below the ablation zone. Keratocytes positive for p-γ-H2AX were seen adjacent to all Blue-IRIS focal zones, but were completely absent from femto-LASIK-treated corneas. Unlike femto-LASIK, Blue-IRIS attains refractive correction in the cornea without tissue removal and only causes minimal, localized keratocyte death within the laser focal zones. In addition, Blue-IRIS induced DNA modifications associated with phosphorylation of γ-H2AX in keratocytes adjacent to the laser focal zones. We posit that this p-γ-H2AX response is related to alterations in chromatin structure caused by localized changes in osmolarity, a possible mechanism for the induced refractive index changes.

Ultraviolet Radiation-Induced Corneal Degeneration in 129 Mice

Ultraviolet radiation (UVR) is a risk factor for the development of ocular disease in humans, including acute photokeratitis, chronic corneal spheroidal degeneration, and cataract formation. This report describes the ocular lesions seen in 21 mice chronically exposed to UVR as part of a skin carcinogenicity study. All globes were affected to varying degrees. The primary lesion, not previously reported in UVR-exposed mice, was marked loss of keratocytes relative to age-matched controls. Secondary lesions included corneal stromal thinning, keratoconus, corneal vascularization and fibrosis, keratitis, globe rupture, and phthisis bulbi. In addition, more than 90% of UVR-exposed and unexposed lenses had evidence of cataract formation; this is the first report of the occurrence of spontaneous cataracts in 129 mice. In a subsequent study, apoptotic cells were identified histologically and by cleaved caspase 3 immunoreactivity in the corneal epithelium and, less commonly, in the corneal stroma after acute UVR exposure. Based on this finding, we propose that the loss of keratocytes observed in the chronic study was due to UVR-induced apoptosis.

Long-Term Dehydrated Donor Lamella Survival in Anterior Keratoplasty: Keratocyte Migration and Repopulation of Corneal Stroma

PURPOSE

The aim of this study was to investigate the ability of host keratocytes to colonize the donor lamella transplanted without viable cells (dehydrated) in Descemetic (deep anterior lamellar keratoplasty) and in pre-Descemetic keratoplasty (excimer laser-assisted lamellar keratoplasty).

METHOD

A total of 17 eyes (8 deep anterior lamellar keratoplasties and 9 excimer laser-assisted lamellar keratoplasties) were included in this observational retrospective study; patients underwent ophthalmic examinations, and histological staining was performed ex vivo on the graft in cases of failure.

RESULTS

In Descemetic keratoplasty, the long-term survival of the graft is compromised with the central corneal thickness decreasing; corneal pachymetry and in vivo and ex vivo keratocyte densities are significantly reduced (pachymetric reduction of -86 μm in the apex and -87 μm in the thinnest point; density cell reduction of 72% at a depth of 100 μm, 62% at a depth of 250 μm, and -66% at a depth of 400 μm). In pre-Descemetic keratoplasty, clinical complications, reduction of central thickness, or alterations of keratocyte density were not observed.

CONCLUSIONS

In Descemetic keratoplasty, the migration of the host peripheral keratocytes does not seem enough to repopulate the donor graft, whereas in pre-Descemetic keratoplasty, long-term survival of the graft is good. Keratocyte repopulation was observed only by extensive contact between the donor and host parenchyma.

Keratocyte density after laser-assisted subepithelial keratectomy with mitomycin C

PURPOSE

To study the effects of laser-assisted subepithelial keratectomy (LASEK) with mitomycin C (MMC) on the keratocyte population.

DESIGN

Prospective, nonrandomized, interventional, comparative case series.

METHODS

Fifty-six eyes treated at Vissum Santa Hortensia, Madrid, Spain, were included in the study. We compared 28 eyes treated with LASEK with intraoperative 0.02% MMC versus 28 non-treated eyes. Keratocyte density was measured 3 months after the surgery in the anterior, mid, and posterior stroma and was compared with the corresponding layers in the control eyes. The anterior layer in the LASEK group was compared with 2 layers in the control group: the most anterior stromal layer and the 80 μm-deep layer, because that was the mean ablation depth performed in eyes that underwent LASEK.

RESULTS

We found a statistically significantly lower keratocyte population in the most anterior stromal layer after LASEK with MMC compared with both the most anterior stromal layer and the 80 μm-deep layer in controls. On the contrary, the treated group showed a significantly higher keratocyte density in both the mid stroma and the deep stroma. The comparison between the average densities through the entire cornea showed a significantly higher keratocyte population in the LASEK with MMC group.

CONCLUSIONS

LASEK with MMC seems to cause a decrease in the anterior stromal cells 3 months after the surgery compared with nonoperated corneas. There seems to be a compensating proliferation of keratocytes in the deeper corneal layers, suggesting that the ability of keratocytes to repopulate the cornea is maintained after the surgical procedure.

Early keratocyte apoptosis after epithelial scrape injury in the human cornea

Animal studies in mice, rats, rabbits, pigs and hens demonstrated that anterior keratocytes undergo programmed cell death or apoptosis after corneal epithelial injury. Many other wound healing changes subsequently follow the keratocyte apoptosis response. This study evaluated early keratocyte apoptosis after corneal epithelial scrape injury in human eyes scheduled for enucleation for malignancy. Two eyes had corneal epithelial scrape 1 h prior to the enucleation and another eye served as a control and had no corneal scrape prior to enucleation. One additional eye was enucleated, washed with balanced salt solution, and then had the corneal epithelium scraped 1 h prior to processing for analysis. Apoptosis was identified by terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) assay and confirmed by transmission electron microscopy (TEM). Anterior keratocyte apoptosis was detected in the three corneas that had epithelial scrape injury, but not in the control unwounded cornea. This study confirmed that keratocyte apoptosis is also an early response to corneal epithelial injury in humans and showed that tears are not essential for keratocyte apoptosis to occur in response to epithelial injury.

The effect of topical cyclosporine A treatment on corneal thickness in patients with trachomatous dry eye

PURPOSE

The purpose of the present study was to evaluate the effect of topical cyclosporine A (CsA) treatment on corneal thickness (CT) in patients with trachomatous dry eye.

METHODS

Sixty-four patients with trachomatous dry eye with a Schirmer test showing 5 mm or less and a tear film break-up time (TFBUT) of five seconds or less were included. Thirty-two patients were treated with twice daily application of CsA (0.05% ophthalmic emulsion) plus non-preserved artificial tears, while the remaining 32 patients serving as controls received only non-preserved artificial tears. CT was measured using ultrasonic pachymetry at five locations of the central (CCT) and mid-peripheral cornea, at baseline and after one, three and six months of treatment.

RESULTS

At the sixth month of treatment, CT measurements were significantly changed in both groups, compared to baseline. In the CsA treatment group, the mean CCT before and after six months of treatment were 517.4 +/- 36.2 and 546.5 +/- 32.4 microm, respectively (p < 0.001); yielding an average CCT increase of 29.1 +/- 8.0 microm (5.62 per cent) from baseline. In the control group, corresponding figures were 520.2 +/- 34.2 and 526.0 +/- 35.4 microm, respectively (p < 0.01), with an average increase of 5.8 +/- 3.1 microm (1.11 per cent).

CONCLUSIONS

In the present study, the CsA treatment group exhibited significantly greater increases in CT compared to controls. Such an increase may indicate an improvement in the integrity of the ocular surface and resolution of the underlying inflammation as a consequence of topical CsA treatment.

Safety of UVA-Riboflavin Cross-Linking of the Cornea

PURPOSE

To study potential damage to ocular tissue during corneal collagen cross-linking (X-linking) by means of the riboflavin/UVA (370 nm) approach.

METHODS

Comparison of the currently used technique with officially accepted guidelines regarding direct UV damage and the damage created by the induced free radicals (photochemical damage).

RESULTS

The currently used UVA radiant exposure of 5.4 mJ/cm and the corresponding irradiance of 3 mW/cm2 is below the known damage thresholds of UVA for the corneal endothelium, lens, and retina. Regarding the photochemical damage caused by the free radicals, the damage thresholds for keratocytes and endothelial cells are 0.45 and 0.35 mW/cm, respectively. In a 400-microm-thick cornea saturated with riboflavin, the irradiance at the endothelial level was 0.18 mW/cm, which is a factor of 2 smaller than the damage threshold.

CONCLUSIONS

After corneal X-linking, the stroma is depopulated of keratocytes approximately 300 microm deep. Repopulation of this area takes up to 6 months. As long as the cornea treated has a minimum thickness of 400 microm (as recommended), the corneal endothelium will not experience damage, nor will deeper structures such as lens and retina. The light source should provide a homogenous irradiance, avoiding hot spots.

Distribution of Precursors in Human Corneal Stromal Cells and Endothelial Cells

PURPOSE

We identified original tissue-committed precursors with limited self-renewal capacity from human corneal stromal (HCS) cells and human corneal endothelial (HCE) cells, then tried to determine the distribution and proliferative capacity of the precursors.

DESIGN

Experimental study.

PARTICIPANTS

Eighteen human corneas from donors 56 to 68 years old.

METHODS

Human corneal stromal cells were divided into groups based on distance from the center of the cornea: <6 mm (central), 6 to 8 mm (paracentral), and 8 to 10 mm (peripheral). Human corneal endothelial cells were separated into 2 groups: <7.5 mm (central) and 7.5 to 10 mm (peripheral) from the center. Each group was subjected to the sphere-forming assay using serum-free medium containing growth factors in floating culture. Sphere numbers and the proliferative capacity of spheres in adherent culture were compared among the groups.

MAIN OUTCOME MEASURES

Density and proliferative capacity of precursors from each area of HCS and HCE cells.

RESULTS

Primary spheres were isolated from all groups of HCS and HCE cells. The rate of primary sphere formation from peripheral HCS cells was higher than those of the other 2 groups, being 1.5-fold greater than in the paracentral cornea and 4-fold greater than in the central cornea. The rate of primary sphere formation by peripheral HCE cells was significantly higher than that by central HCE cells, being 4-fold greater than in the central cornea. There were no differences in the proliferative capacity of HCS and HCE cell spheres from the different areas after adherent culture.

CONCLUSIONS

All HCS and HCE cells contain a significant number of precursors, but the peripheral cells have a density of precursors higher than that of the central cells. Precursors from each area do not show differences of proliferative capacity. Our findings may in part explain changes after excimer laser treatment and may have implications for corneal transplantation procedures.

Corneal stroma regeneration in felines after supradescemetic keratoprosthesis implantation

PURPOSE

To show corneal regeneration in 3 cats that underwent lamellar keratectomy (90%) depth during supradescemetic keratoprosthetic implantation.

METHODS

Three 2-year-old cats that underwent spontaneous keratoprosthesis extrusion between 15 and 150 days after implanting a supradescemetic prosthesis into their right eyes were studied. Corneal structures and stroma thickness were evaluated by slit-lamp photographs, pachymetry, and confocal microscopy. Regenerated corneal epithelial cells, stroma matrix, and keratocyte morphology were studied with histology and transmission electron microscopy. Epithelial and stromal cell immunocharacterization was performed.

RESULTS

Corneas progressively regained normal thickness and improved clarity within 40 to 60 days. Slit-lamp photographs and pachymetry showed gains in stromal thickness until 600 microm or more. In vivo confocal microscopy showed the restoration of normal epithelium and stroma in all cats. Corneal nerves were seen in the regenerated stroma of 2 cats. Immunostaining showed absent alpha-smooth muscle actin (SMA) expression and a keratin K3-expressing epithelium. Electron microscopy showed regeneration of normal epithelium with a well-formed basement membrane, organized corneal lamellae, and the presence of normal keratocytes.

CONCLUSION

Felines are capable of regenerating corneal structures including epithelium and reinnervated stroma matrix after deep lamellar keratectomy. The use of feline models in corneal keratoprosthesis is therefore questionable.

Assessment of stromal keratocytes and tear film inflammatory mediators during extended wear of contact lenses

PURPOSE

To monitor quantitative changes in stromal keratocyte density and the level of tear film inflammatory mediators following extended contact lens wear.

METHODS

Twenty-two subjects aged 32 +/- 11 years participated in this cross-sectional study. Eleven subjects had worn silicone hydrogel (Si-H) lenses on a 30-day continuous wear basis for 12 months. Eleven subjects had worn rigid gas permeable lenses on the same basis for 12 months. Eleven age-matched control subjects were also recruited. Ultrasound pachometry, confocal microscopy, and tear fluid sample collection were performed on all subjects. Tear samples were assayed for epidermal growth factor (EGF), hepatocyte growth factor (HGF) and interleukin (IL)-8.

RESULTS

Corneal thickness was similar for all subject groups. Total keratocyte density was not different between the 3 groups; however, keratocyte density was lower for rigid lens wearers in the anterior to mid stroma and lower for Si-H lens wearers in the posterior stroma compared with control subjects. Rigid lens wearers exhibited an irregular keratocyte distribution across the corneal stroma. EGF concentration and rate of release was greater in the tears collected from the rigid lens wearers and Si-H lens wearers, and IL-8 concentration was higher in the samples collected from the rigid lens wearers compared with the samples collected from the control subjects.

CONCLUSIONS

Mechanical stimulation of the corneal surface due to the physical presence of a contact lens and the consequent release of inflammatory mediators may account for a loss or redistribution of keratocytes.

Ceramide-induced apoptosis in rabbit corneal fibroblasts

PURPOSE

To evaluate the effect of various ceramides on the apoptosis of corneal fibroblasts and to determine the pathway on which they act.

METHOD

Corneal fibroblasts isolated and cultured from New Zealand white rabbits were exposed to various concentrations of ceramide types II and VI and phytoceramide types II and VI, and their apoptotic response was evaluated using an LDH assay and Hoechst and Annexin V staining. Corneal fibroblasts were preincubated with various concentrations of the CPP32-like protease inhibitor Z-VAD-FMK, the caspase-8 inhibitor IETD-CHO, and the caspase-9 inhibitor Z-LEHD-FMK before treatment with ceramide, and apoptotic response was assayed by LDH assay. In addition, cells treated with ceramide or phytoceramide were stained with an antibody to cytochrome c.

RESULTS

At concentrations of 20 microM and higher, all 4 ceramides increased fibroblast apoptotic response significantly after 12 hours. Hoechst staining showed shrinkage of the cytoplasm, formation of apoptotic bodies, and nuclear fragmentation after ceramide exposure, and Annexin V staining showed small vesicles around the cell membrane. The CPP32-like protease inhibitor reduced the apoptotic response to all 4 ceramides. The specific caspase-8 inhibitor reduced the apoptotic response to ceramide type VI and phytoceramide types II and VI, whereas the specific caspase-9 inhibitor significantly reduced the apoptotic response to phytoceramide types II and VI. Following exposure to ceramides, corneal fibroblasts stained positively with antibody to cytochrome c.

CONCLUSION

Ceramide induced apoptosis in cultured corneal fibroblasts. This apoptosis involved the caspase cascade and the mitochondrial pathway.

Keratocyte loss in corneal infection through apoptosis: a histologic study of 59 cases

BACKGROUND

Keratocyte loss by apoptosis following epithelial debridement is a well-recognized entity. In a study of corneal buttons obtained from patients of corneal ulcer undergoing therapeutic keratoplasty, we observed loss of keratocytes in the normal appearing corneal stroma, surrounding the zone of inflammation. Based on these observations, we hypothesized that the cell loss in the inflammatory free zone of corneal stroma is by apoptosis that could possibly be a non-specific host response, independent of the nature of infectious agent.

METHODS

To test our hypothesis, in this study, we performed Terminal deoxyribonucleotidyl transferase-mediated d-Uridine 5" triphosphate Nick End Labelling (TUNEL) staining on 59 corneal buttons from patients diagnosed as bacterial, fungal, viral and Acanthamoeba keratitis. The corneal sections were reviewed for morphologic changes in the epithelium, stroma, type, degree and depth of inflammation, loss of keratocytes in the surrounding stroma (posterior or peripheral). TUNEL positivity was evaluated in the corneal sections, both in the zone of inflammation as well as the surrounding stroma. A correlation was attempted between the keratocyte loss, histologic, microbiologic and clinical features.

RESULTS

The corneal tissues were from 59 patients aged between 16 years and 85 years (mean 46 years) and included fungal (22), viral (15), bacterial (14) and Acanthamoeba (8) keratitis. The morphological changes in corneal tissues noted were: epithelial ulceration (52, 88.1%), destruction of Bowman's layer (58, 99%), mild to moderate (28; 47.5%) to severe inflammation (31; 52.5%). Morphologic evidence of disappearance or reduced number of keratocytic nuclei in the corneal stroma was noted in 49 (83%) cases; while the TUNEL positive brown cells were identified in all cases 53/54 (98%), including cases of fungal (19), bacterial (14), viral (13), and Acanthamoeba keratitis. TUNEL staining was located mostly in the deeper stroma and in few cases the peripheral stroma. TUNEL positivity was also noted with the polymorphonuclear infiltrates and in few epithelial cells (10 of 59, 17%) cases, more with viral infections (6/10; 60%).

CONCLUSIONS

We report apoptotic cell death of keratocytes in the corneal stroma in infectious keratitis, a phenomenon independent of type of infectious agent. The inflammatory cells in the zone of inflammation also show evidence of apoptotic cell death. It could be speculated that the infective process possibly triggers keratocyte loss of the surrounding stroma by apoptosis, which could possibly be a protective phenomenon. It also suggests that necrotic cell death and apoptotic cell deaths could occur simultaneously in infective conditions of the cornea.

Analysis of the efficacy, predictability, and safety of LASEK for myopia and myopic astigmatism using the Technolas 217 excimer laser

PURPOSE

To evaluate the efficacy, predictability, and safety of laser-assisted subepithelial keratectomy (LASEK) for the treatment of myopia and myopic astigmatism.

SETTING

Stanford University Eye Laser Center, Stanford, California, USA.

METHODS

This retrospective analysis comprised 102 eyes that had LASEK for myopia using the Bausch & Lomb Technolas 217 excimer laser. Primary outcome variables including uncorrected visual acuity (UCVA), best spectacle- corrected visual acuity (BSCVA), manifest refraction, and complications were evaluated at 3, 6, and 12 months. Vector analysis was performed on eyes that received astigmatic correction.

RESULTS

The mean spherical equivalent was -7.03 diopters (D) +/- 2.61 (SD) preoperatively, +0.19 +/- 0.64 D at 3 months, +0.23 +/- 0.82 D at 6 months, and +0.03 +/- 0.63 D at 12 months (P<.001). At 3, 6, and 12 months, the UCVA was 20/20 or better in 66%, 67%, and 83% of eyes, respectively, and 20/40 or better in 98%, 99%, and 100%; 74%, 70%, and 83%, respectively, were within +/-0.5 D of emmetropia, and 89%, 86%, and 97%, respectively, were within +/-1.0 D. No eye lost more than 2 lines of BSCVA. At 3, 6, and 12 months, 10.0%, 8.7%, and 0% of eyes, respectively, had trace corneal haze. Vector analysis found a success rate of approximately 78% to 80% in achieving the astigmatic surgical correction at the 3 postoperative visits.

CONCLUSIONS

Laser-assisted subepithelial keratectomy was an effective, predictable, and safe procedure for the treatment of myopia and myopic astigmatism. Further studies are needed to determine the role of LASEK in the refractive surgery spectrum.

Decreased keratocyte death after laser-assisted subepithelial keratectomy and photorefractive keratectomy in rabbits

PURPOSE

To compare keratocyte loss in the corneal stroma after laser-assisted subepithelial keratectomy (LASEK) and photorefractive keratectomy (PRK) in rabbits.

SETTING

Department of Ophthalmology, University of Essen, Essen, and the Institute of Anatomy, University of Bochum, Bochum, Germany.

METHODS

Laser-assisted subepithelial keratectomy and PRK were performed in rabbits and studied 1, 3, 10, and 20 days after surgery. Excimer photoablation was done unilaterally with a 6.0 mm ablation zone and an 80 microm depth, equivalent to -6.0 diopters. Keratocyte death was analyzed using DNA fragmentation-detecting terminal deoxynucleotidyl transferase deoxy-UTR-nick end labeling (TUNEL) assay and transmission electron microscopy.

RESULTS

Numerous TUNEL-positive keratocytes occurred 1 day after PRK; the number decreased significantly after 3 days. After LASEK, significantly fewer TUNEL-positive keratocytes were noted at the early time points (P<.001 at 1 day; P< or =.05 at 3 days). At 10 days, the number of TUNEL-positive keratocytes decreased in both groups but remained significantly higher after PRK than after LASEK (P<.001). Twenty days after both procedures, no significant signs of keratocyte death were found in the corneal stroma. Transmission electron microscopy revealed few apoptotic keratocytes after LASEK. After PRK, apoptotic keratocytes, characterized by chromatin condensation, apoptotic bodies, and cell shrinkage, were scattered in the stroma. The ultrastructural findings confirmed the results obtained with the TUNEL assay.

CONCLUSIONS

Laser-assisted subepithelial keratectomy induced significantly less apoptotic keratocyte death than PRK and promoted wound healing in the acute phase after photoablation. This procedure may offer the possibility of treating higher myopia with a decreased risk for developing wound healing-related complications known to occur after PRK.

Mechanical damage to corneal stromal cells by epithelial scraping

OBJECTIVE

Corneal epithelial scraping, a common clinical procedure, triggers a loss of underlying keratocytes. This study was conducted to examine whether the physical impact of epithelial scrape injury plays any role in the death of these cells.

METHODS

Epithelial debridement was carried out on the cornea of a freshly killed mouse either by mechanical scraping with a blunt spatula, as in the clinical scraping, or lifting by repeated touching with a gelatin-coated slide. Subsequently, nuclei, nuclear envelope, microtubules, microfilaments, and plasma membranes were examined with specific probes. Some corneas were fixed with alcohol before scrape injury. Fate of keratocytes after epithelial injury was investigated with isolated eyes ex vivo and living mice in vivo. Some of the procedures were performed with human donor corneas.

RESULTS

Within seconds of, or possibly simultaneous to, mechanical epithelial scraping, nuclei of underlying keratocytes became grossly deformed to assume extremely stretched morphology, accompanied by destruction of microfilaments and microtubules as well as compromised plasma membranes. These cells deteriorated gradually over several hours both ex vivo and in vivo. Nuclear deformation was observed even when the cornea was fixed with alcohol before epithelial scraping. When the epithelium was removed by gentle lifting, nuclei remained mostly intact. Similar results were obtained with human donor corneas.

CONCLUSION

Mechanical epithelial scraping can cause immediate damage to underlying anterior keratocytes by a physical impact, which appears to lead to degeneration of these cells.

Sodium hyaluronate (hyaluronic acid) promotes migration of human corneal epithelial cells in vitro

PURPOSE

Sodium hyaluronate (hyaluronic acid) is known to promote corneal epithelial wound healing in vivo and in vitro, in animal experiments. Sodium hyaluronate is the ligand for CD44, a cell surface adhesion molecule which has been found on normal human corneal epithelial cells. The purpose of this study was to investigate the effect of sodium hyaluronate on human corneal epithelial cell migration, proliferation, and CD44 receptor expression.

METHODS

Human corneal epithelial cell cultures were established from 32 donor corneoscleral rims and maintained separately in three different culture conditions: (1) culture medium only, (2) sodium hyaluronate enriched (0.6 mg/ml) medium, and (3) hydroxypropylmethylcellulose enriched (2.5 mg/ml) medium. The total area of migrating epithelial cell sheets in each case was measured by planimetry on days 4, 8, 12, and 16. Cytospin preparations of cells cultured in the different culture conditions were examined immunohistochemically for proliferation and CD44 receptor expression using antibodies directed against Ki67 and CD44 respectively.

RESULTS

Cells cultured in the presence of sodium hyaluronate showed significantly increased migration at days 12 and 16 (Friedmen test: p = 0.0012, day 16; p = <0.001, day 12) compared with cells cultured in the other media. There was no difference in cell proliferation (Ki67) or CD44 expression on cells cultured in the different culture conditions.

CONCLUSIONS

Sodium hyaluronate promotes migration but not proliferation or CD44 expression on human corneal epithelial cells in vitro. The beneficial effect of sodium hyaluronate in corneal wound healing is likely to be related to rapid migration of cells leading to rapid wound closure. This may be facilitated by the adhesion between CD44 on the cells and hyaluronic acid, which coats the surface of the denuded cornea.

Keratocyte apoptosis after corneal collagen cross-linking using riboflavin/UVA treatment

PURPOSE

Combined riboflavin/UVA treatment inducing collagen cross-links in the cornea has been shown to increase the biomechanical rigidity of the cornea and has been used successfully in the treatment of progressive keratoconus. The current study was undertaken to investigate the possible cytotoxic effect of combined riboflavin/UVA treatment on corneal keratocytes in vivo.

METHODS

Thirty-four New Zealand white rabbits were treated with 0.1% riboflavin solution and surface UVA irradiances ranging from 0.75 to 4 mW/cm2 (1.35- 7.2 J/cm2) for 30 minutes. The animals were euthanized either 4 (n = 6) or 24 (n = 28) hours postoperatively. Four additional control eyes underwent epithelial debridement alone. The corneas of the enucleated eyes were evaluated in routine histologic sections. In addition, the TUNEL technique and transmission electron microscopy were used for the detection of keratocyte apoptosis.

RESULTS

In the control eyes with corneal epithelial debridement only, apoptotic keratocytes were found in the anterior 50 microm of the corneal stroma 4 hours postoperatively. However, riboflavin/UVA-induced apoptosis was only visible in the rabbit eyes enucleated 24 hours postoperatively. In these eyes, we found apoptosis of keratocytes down to a variable stromal depth depending on the applied UVA irradiance. A cytotoxic UVA irradiance for keratocytes in the range of 0.5-0.7 mW/cm2 could be deduced.

CONCLUSIONS

Riboflavin/UVA treatment leads to a dose-dependent keratocyte damage that can be expected in human corneas down to a depth of 300 microm using a surface UVA dose of 5.4 J/cm2. Future studies should be done to examine the keratocyte repopulation and exclude possible adverse sequelae of keratocyte loss like stromal scarring or thinning.

Keratocyte cytotoxicity of riboflavin/UVA-treatment in vitro

PURPOSE

Collagen crosslinking using ultraviolet- A (UVA) -irradiation combined with the photosensitizer riboflavin is a new technique for treating progressive keratoconus. It has been shown to increase effectively the biomechanical strength of the cornea and to stop or even reverse the progression of keratoconus. As part of a safety evaluation, the present study was undertaken to investigate in vitro the possible cytotoxic effect of combined riboflavin/UVA-treatment on corneal keratocytes and to compare it to UVA-irradiation alone.

METHODS

Cell cultures established from porcine keratocytes were treated with 0.025% riboflavin solution and various UVA (370 nm)-irradiances ranging from 0.4 to 1.0 mW/cm2 and with UVA alone between 2 and 9 mW/cm2 for 30 min. The cell cultures were evaluated for cell death 24 h after irradiation using trypan-blue and Yopro-fluorescence staining.

RESULTS

An abrupt cytotoxic irradiance level was found at 0.5 mW/cm2 for keratocytes after UVA-irradiation combined with the photosensitizer riboflavin, which is 10-fold lower than the cytotoxic irradiance of 5 mW/cm2 after UVA-irradiation alone.

CONCLUSIONS

A cytotoxic effect of combined riboflavin/UVA-treatment on keratocytes is to be expected at 0.5 mW/cm2, which is reached in the clinical setting in human corneas down to a depth of 300 microm using the standard surface UVA-irradiance of 3 mW/cm2.

Lacrimal gland as the major source of mouse tear factors that are cytotoxic to corneal keratocytes

We have previously shown that mouse tears are cytotoxic to the corneal keratocytes. Since tear components are derived from both lacrimal tissues and ocular surface epithelium, we sought to determine the source of the cytotoxic factors in the mouse tear fluid. Cytotoxicity to keratocytes was assessed by an ex vivo assay using an isolated eye; after treatment with test samples, segmentation and disappearance of stromal nuclei were determined by DAPI nuclear staining. Following biological tissues and fluids were examined either directly or after preincubation at 37 degrees C for 2-15 hr: extraorbital lacrimal gland (ELG), intraorbital lacrimal gland (ILG), Harderian gland, Meibomian gland, corneal epithelium, bulbar conjunctiva, palpebral conjunctiva, serum, aqueous humor, and lacrimal fluid collected from a secretory duct of ELG. Under the ex vivo assay conditions, ELG and ILG, with or without preincubation, exhibited a cytotoxic effect comparable to that of diluted tears. Lacrimal fluid collected from an ELG duct was similarly effective. These specimens triggered nuclear segmentation that is typical of apoptotic nuclei. All other specimens showed no effect on stromal nuclei under the identical conditions. In some animals, ELG was surgically removed and the tear cytotoxicity was examined in vivo. The tear cytotoxicity in these animals was lost after the surgery, indicating an involvement of ELG, but it was restored 4-24hr afterward, suggesting a compensatory role of ILG.These results suggest that ELG and ILG are the major sources of tear factors that are cytotoxic to the keratocytes in the mouse.

Epi-LASEK for the correction of myopia and myopic astigmatism

PURPOSE

To analyze patient results after laser-assisted subepithelial keratectomy (epi-LASEK) for myopia and myopic astigmatism.

SETTING

Private practice setting, Columbus, Ohio, USA.

METHODS

Three hundred forty-three eyes of 188 patients with myopia or myopic astigmatism were prospectively evaluated after having epi-LASEK by a single surgeon using the VISX Star S2 excimer laser. Uncorrected visual acuity (UCVA), manifest refraction, postoperative pain, time to epithelial healing, and postoperative haze were recorded. The patients were followed for up to 6 months.

RESULTS

The mean preoperative sphere and cylinder were -5.42 diopters (D) +/- 2.62 (SD) (range -1.0 to -14.0 D) and 0.87 +/- 0.75 D (range 0 to 4.75 D), respectively. At 1 week, the mean UCVA was 20/30. At 6 months, it was 20/40 or better in 98% of patients. At 3 months, 78% and 92% of patients were within +/-0.5 D and +/-1.0 D, respectively, of the intended correction. This improved to 85% and 94%, respectively, at 6 months. The mean time to epithelial healing was 4.76 days (range 3 to 9 days). Most patients (87%) reported no postoperative pain. In the first 3 months, haze was noted in 1.6% of patients.

CONCLUSIONS

Epi-LASEK appeared to be a safe and effective treatment for the correction of myopia and myopic astigmatism. Most patients achieved postoperative visual acuities comparable to those with laser in situ keratomileusis and photorefractive keratectomy. There was a low incidence of haze and pain postoperatively.

Corneal thickness in trachomatous dry eye

PURPOSE

To investigate the effect of severe desiccation on corneal thickness in scarring trachoma by comparing the thickness of normal and trachomatous dry eye corneas.

METHODS

Ultrasonic pachymetry was used to measure the corneal thickness at nine points in the central and peripheral cornea (superior, superonasal, nasal, inferonasal, inferior, inferotemporal, temporal, superotemporal) in 45 eyes of 27 patients with trachomatous dry eye and 54 eyes of 31 normal subjects.

RESULTS

The average thickness of the nine sites in the central and midperipheral cornea was significantly less in trachomatous dry eyes than normal eyes. The superior cornea was the thickest area in both groups, measuring 574.03+/-31.62 microm in trachomatous dry eyes and 611.33+/-34.99 microm in normal eyes (p<0.001). The centre of the cornea was the thinnest, measuring 510.43+/-32.12 microm in trachomatous dry eyes and 546.27+/-36.20 microm in normal eyes (p<0.001).

CONCLUSIONS

The thickness of the central and midperipheral cornea was significantly reduced in patients with trachomatous dry eye. The chronic state of severe desiccation, tear film instability and increased immune activation in trachomatous dry eye may contribute to this thinning.

Flap haze after epithelial debridement and flap hydration for treatment of post-laser in situ keratomileusis striae

PURPOSE

To report a case of corneal haze after attempts to eliminate post-laser in situ keratomileusis (LASIK) lamellar striae.

METHODS

Case report.

RESULTS

A 24-year-old woman with visually significant flap striae 2 months after LASIK underwent manual epithelial debridement and flap hydration, refloating, and stretching to eliminate the striae. Three weeks after this intervention, she developed visually significant haze that was confined to the stroma of the flap. The haze slowly improved with use of a topical steroid.

CONCLUSION

Stromal haze can develop after treatment of flap striae with epithelial debridement and hypo-osmolar irrigation. We speculate that these maneuvers may have induced cell death of anterior keratocytes and led to haze formation, as can occur after simple epithelial debridement and epithelial scrape-photorefractive keratectomy.

The epithelial flap for photorefractive keratectomy

BACKGROUND/AIMS

Epithelial debridement for photorefractive keratectomy (PRK) is associated with pain, slower visual recovery, and may be aetiological in haze production. The aim of this study was to assess the clinical results of a new technique involving raising and replacing of an epithelial flap in photorefractive keratectomy.

METHODS

A prospective, non-randomised, comparative, paired eye trial was performed in 72 eyes of 36 patients who underwent PRK with a Nidek EC-5000 excimer laser. For epithelial debridement before PRK, the eyes were divided into two groups. The first eye of each patient was treated with 20% ethanol debridement and the second eye with an epithelial flap which was replaced after treatment. PRK was carried out with the same laser and nomogram in both groups by the same surgeon. Visual and refractive outcome of PRK treatment was compared in both groups.

RESULTS

The mean (SD) preoperative mean spherical equivalent (MSE) was -3.61 (1.38) dioptres (D) (range -1.00 D to -7.88 D) with no significant difference between the two groups. After a mean follow up period of 62.6 weeks (range 52-70) the final MSE was +0.07 (0.61) D (range -5.50 D to +4.50 D) in the debridement group and -0.24 (0.43) D in the epithelial flap group. There was no statistically significant difference between the two groups in postoperative MSE. The best corrected visual acuity was better in the epithelial flap group at all visits; this difference was statistically significant (p<0.05). The corneal haze was less in the epithelial flap group and this difference was also statistically significant (p<0.05).

CONCLUSIONS

Managing the corneal epithelium as a hinged flap with 20% ethanol is a safe technique with faster visual rehabilitation and reduced haze compared with debridement of the epithelium with alcohol. Further studies need to be performed to compare pain levels postoperatively with the epithelial flap and epithelial debridement.

Reduction in corneal haze and apoptosis by amniotic membrane matrix in excimer laser photoablation in rabbits

PURPOSE

To determine whether preserved human amniotic membrane can reduce corneal haze and keratocyte apoptosis induced by excimer laser photoablation in rabbit corneas.

METHODS

Excimer photoablation was performed bilaterally in 30 rabbits with a 6.0 mm ablation zone and 120 microm depth using the VISX Star laser with the phototherapeutic keratectomy (PTK) mode. One eye was randomly covered by preserved human amniotic membrane secured with 4 interrupted 10-0 nylon sutures, and the other eye served as the control. The amniotic membranes were removed at 1 week, and corneal haze was graded with slitlamp biomicroscopy by 3 masked corneal specialists biweekly for the ensuing 12 weeks until the rabbits were killed. Another 18 rabbits were divided into 4 subgroups and received PTK alone, PTK with membrane, PTK with sham sutures, or PTK with tarsorrhaphy. All eyes were studied histologically, and 3 eyes in each group were studied by in situ terminal deoxynucleotidyl transferase deoxy-UTP-nick end labeling assay at 1, 3, and 7 days and 12 weeks, respectively.

RESULTS

A consistent grading of differences in corneal haze scoring between the control corneas and the amniotic-membrane-covered corneas was noted among the 3 masked observers. Organized reticular post-PTK corneal haze peaked at 7 weeks in both groups, and the corneal haze score in the amniotic-membrane-covered group was significantly less than in the control group from 7 to 12 weeks (all P < .001). Compared to the control corneas, the amniotic-membrane-covered corneas had less inflammatory response at 1 and 3 days, less keratocyte apoptosis in the ablated anterior corneal stroma at 1, 3, and 7 days (P < .001), and less stromal fibroblast cellularity and epithelial hyperplasia at 12 weeks.

CONCLUSIONS

Amniotic membrane matrix introduced at an early stage of the corneal wound healing process effectively reduced corneal haze induced by excimer laser photoablation in rabbits. Studies linking suppression of apoptosis in the acute wound-healing process with reduction of subsequent corneal scarring may have useful clinical applications.

Effects of contact lens wear on corneal ultrastructure

Over the past 30 years, a number of studies employing the techniques of light and electron microscopy have been conducted on the corneas of human and animal eyes in an effort to more fully understand the histopathological basis of clinically-observed contact lens-related phenomena. This paper reviews studies that have sought to explain such effects, including epithelial abrasion, punctate staining and thinning; stromal oedema and thinning, and endothelial polymegethism. It is demonstrated that an understanding of contact lens-induced changes to corneal ultrastructure can have a significant impact on the clinical appreciation of the severity or otherwise of various conditions as observed with the slit-lamp biomicroscope.

Corneal haze after photorefractive keratectomy using different epithelial removal techniques: mechanical debridement versus laser scrape

PURPOSE

To determine differences of corneal wound healing and haze after photorefractive keratectomy (PRK) using either mechanical epithelial debridement or laser-scrape epithelial removal in human subjects.

DESIGN

A 6-month randomized, masked, prospective, paired-eye clinical study.

PARTICIPANTS

Twenty eyes in 10 myopic patients treated between March 1999 and May 1999.

INTERVENTION

Photorefractive keratectomy treatments with two different epithelial removal techniques. Continuous z-scan of confocal image, termed confocal microscopy through focusing (CMTF), was performed before surgery and at 3 weeks, 6 weeks, 3 months, and 6 months after surgery.

MAIN OUTCOMES MEASURES

Epithelial and stromal thickness measurement, achieved stromal ablation depth, and objective assessment of corneal light-backscattering (corneal haze) were obtained from digital image analysis of the CMTF scans. Manifest refraction was also measured. Student's paired t test or two-way repeated-measures analysis of variance after rank transformation were performed to evaluate statistical differences between groups.

RESULTS

Comparison of the mean posttreatment spherical equivalent between the two techniques showed no statistically significant difference. In preoperative corneas, mean epithelial thickness was 50.08+/-3.70 microm in the mechanical debridement group and 50.49+/-4.01 microm in laser-scrape group (not significant). For both groups, the epithelium was significantly thinner at 3 weeks, but returned to preoperative values by 6 months, with no difference between groups. Planned stromal ablation depth by PRK was 59.38+/-11.48 microm (39-73 microm; n = 8) in the mechanical group and 57.75 +/- 7.21 microm (48-70 microm; n = 8) in the laser-scrape group. Achieved stromal ablation depth was not significantly different between the two groups. Most importantly, in both groups CMTF-measured corneal haze increased significantly after surgery, peaked at 3 months, and then decreased at 6 months, with no significant difference between groups. (Power = 0.96).

CONCLUSIONS

There is no significant difference in the corneal wound healing response between mechanical epithelial debridement versus laser-scrape technique in human myopic eyes undergoing PRK.

Kinetics of keratocyte proliferation in response to epithelial debridement

Over the past 40 years, several groups have shown that epithelial debridement results in the death of keratocytes subjacent to the wound area. More recently this cell death has been shown to involve apoptosis. The purpose of this project was to examine the proliferative response of the normally quiescent keratocytes to repopulate the apoptotic area. Three mm wounds were made in the central cornea of adult rats and allowed to heal 4 hr to 14 days. Cryostat sections were stained with propidium iodide to mark the nuclei of all cells. Actively proliferating cells were identified with anti-Ki67, a marker of the late G1-M phase of the cell cycle. Anti-alpha-smooth muscle actin was used to determine if myofibroblasts were present. In unwounded corneas, keratocytes were uniformly spread throughout the stroma, and less than one proliferating cell per mm was observed. By 4 hr after wounding, the anterior one-half to three-fourths of the stroma subjacent to the wound was devoid of cells. No increase in Ki67-expressing cells was observed in the stroma until 24 hr after wounding (3.9 +/- 0.5 and 6. 3 +/- 0.5 mm(-1)in the wound center and edge, respectively). The number of Ki67-expressing cells steadily increased, peaking 44 hr after debridement (41.2 +/- 1.7 and 39.6 +/- 1.0). These cells were confined to a narrow zone adjacent to the area of cell death. No change in the number of cells expressing Ki67 was observed in the keratocytes distal to the original debridement. Ki67 levels did not return to control levels until 7 days after wounding. No alpha-smooth muscle actin was detected at any time point. This study indicates that epithelial debridement stimulates a synchronous increase in keratocyte proliferation. This stimulation is specific for cells immediately adjacent to the area of cell death. This activation does not involve the transformation of the stromal cells to a myofibroblast phenotype.

The influence of corneal stromal matrix proteins on the migration of human corneal fibroblasts

Motivated by the alterations seen in the corneal matrix composition after photorefractive keratectomy and the migration of corneal keratocytes seen following this procedure, the locomotor response of corneal stromal fibroblasts to various extracellular matrix proteins was determined. In addition, the involvement of integrin mediated attachment to the matrix proteins was investigated. Quantitative invasion assays were performed using collagen gels, supplemented with either fibronectin, tenascin, collagen type V, collagen type VI, chondroitin sulfate or keratan sulfate. The ultrastructure of the gels was visualized by scanning electron microscopy and related to the migration results. The extent of alpha(1)beta(1), alpha(2)beta(1), alpha(3)beta(1)and alpha(5)beta(1)integrin mediated attachment to the matrix proteins was evaluated using blocking antibodies. Fibronectin increased corneal fibroblast migration significantly, and served as an excellent substrate for cellular attachment, mediated by the alpha(5)beta(1)integrin. Addition of tenascin to the fibronectin-containing gels disrupted these effects, while attachment to this matrix also involved the integrins alpha(2)beta(1)and alpha(3)beta(1). Chondroitin sulfate and collagen types V and VI primarily altered the structure of the collagen matrix, resulting in an inhibition of migration by the collagens and an increase by chondroitin sulfate. They all served as poor substrates for attachment. Thus, the migratory activity of corneal fibroblasts in vitro is influenced by the composition of the surrounding extracellular matrix, either by integrin mediated cell-matrix interactions or through matrix-matrix interactions. This study provides evidence that the provisional matrix deposited in a corneal stromal wound may facilitate the entry of migrating corneal fibroblasts.

Wound healing after photorefractive keratectomy

For more than 15 years, the excimer laser has been used as a surgical instrument on the cornea. Photorefractive keratectomy (PRK) followed radial keratotomy as researchers sought a more precise technique. In PRK, precision turned out to depend on surgical technique as well as the wound-healing process, with the 2 factors interdependent. The PRK technique has evolved toward a large diameter, flat ablation curvatures, and an even surface. The role of such factors as cytokines and interleukins has become more clear in the past 10 years. However, understanding the wound-healing process becomes more complicated with increasing know edge. Learning the contributing factors and performing trials with new drugs and antibodies to modulate wound healing have shown positive results on the experimental level. Patient selection based on the concentration of epidermal growth factor in tears may be another way to increase PRK s precision. The PRK technique has taught much about wound healing. For the technique to be competitive, increased precision, particularly in eyes with high myopia, is needed. Two other factors are imperative: controlling postoperative pain and decreasing visual rehabilitation time.

Soluble Fas and Fas ligand in human tear fluid after photorefractive keratectomy

BACKGROUND/AIMS

The Fas-Fas ligand system is thought to be involved in stromal cell apoptosis after corneal wounding. The aim was to measure changes in human tear fluid levels of soluble Fas (sFas) and Fas ligand (sFasL) following myopic photorefractive keratectomy (PRK).

METHODS

Tear samples of 59 patients were collected preoperatively, and 1 or 2 days after PRK. Tear fluid sFas or sFasL concentrations were determined using sandwich ELISAs. Subsequently, tear flow corrected concentrations (releases) were calculated to compensate for the postoperative tear hypersecretion.

RESULTS

The preoperative tear fluid flows (TFF) were 6.4 (1.7) microl/min (mean (SEM)) in sFas group (n = 18), and 7.5 (1.5) microl/min in sFasL group (n = 39). Postoperatively TFFs increased to 37.9 (10.9) microl/min (p = 0.003) and 58.3 (7.0) microl/min (p = 0.000), respectively. The mean preoperative sFas concentration (24.4 (11.6) U/ml) decreased to 9.7 (4.1) U/ml (p = 0.001) postoperatively, and the mean sFasL concentration (299.1 (28.8) ng/l) to 118.7 (15.9) ng/l (p = 0.000). However, the release of both substances increased significantly: sFas from 87.3 (29.4) mU/min to 229.4 (82.9) mU/min (p = 0.002) and sFasL from 1620.6 (226.4) fg/min to 4777.1 (596.1) fg/min (p = 0.000).

CONCLUSIONS

Both sFas and sFasL are normal constituents of human tears. Despite a decrease in concentrations related to reflex tears, the release of sFas and sFasL increases significantly after excimer laser photorefractive keratectomy, which suggests that they are involved in corneal healing after PRK in humans.

Keratocyte apoptosis associated with keratoconus

Keratoconus is an ectatic corneal dystrophy associated with stromal thinning and disruption of Bowman's layer. The purpose of this study was to explore a possible association between keratocyte apoptosis and keratoconus. Keratocyte apoptosis was evaluated in corneas of patients with keratoconus, corneas of patients with stromal dystrophies, and normal donor corneas using the transferase-mediated dUTP-digoxigenin nick and labeling (TUNEL) assay. Keratocyte apoptosis was also studied in keratoconus and normal corneas using transmission electron microscopy. TUNEL-stained keratocytes were detected in 60% of corneas with keratoconus, but only 35% of corneas with stromal dystrophies (P =0.03). The number of TUNEL-positive keratocytes detected in the keratoconus, stromal dystrophy, and normal corneas was 7+/-1 (mean+/-standard error, range 0-20), 2+/-0. 8 (range 0-9), and 0+/-0 (range 0-0) TUNEL-positive cells per section, respectively. The differences between the keratoconus and the stromal dystrophy (P =0.0097) or the normal cornea (P =0.01) groups were statistically significant. The difference between the stromal dystrophy and normal cornea groups was not statistically significant (P =0.45). The stromal dystrophy group was included to account for surgery-associated keratocyte apoptosis. No TUNEL-stained keratocytes were detected in normal corneas. Cell morphologic changes consistent with apoptosis were detected by transmission electron microscopy (TEM) in keratocytes of keratoconus corneas, but not in keratocytes in normal corneas. Chronic keratocyte apoptosis associated with ongoing epithelial injury may link risk factors associated with keratoconus such as chronic eye rubbing, contact lens wear, or atopic eye disease. Similarly, increases that have been detected in several different degradative enzymes in keratoconus corneas could be associated with chronic keratocyte apoptosis and less than perfect control of release of intracellular contents.

Comparison of mechanical and transepithelial debridement during photorefractive keratectomy

OBJECTIVE

Comparison of clinical results from mechanical and transepithelial debridement during photorefractive keratectomy.

DESIGN

Randomized, prospective study.

PARTICIPANTS

Two hundred seventy-eight eyes in 173 patients between November 1995 and June 1997.

INTERVENTION

Photorefractive keratectomy treatments with a Summit Omnimed excimer laser (6-mm). Clinical results were measured at 1, 3, and 6 months.

MAIN OUTCOMES MEASURES

Uncorrected visual acuity (UCVA), best spectacle-corrected visual acuity (BCVA), mean spherical equivalent (MSE), astigmatism, corneal haze, and subjective vision (day, night, glare, and halo).

RESULTS

At all postoperative intervals, no significant difference was present between mean values of MSE, haze, or any subjective parameters. For mean UCVA, BCVA, and astigmatism values, a significant difference (P < 0.05) was present only at 6 months. At all postoperative intervals, mechanical values tended to be superior to transepithelial valves.

CONCLUSION

Clinical results were not statistically different between the two techniques; however, the mechanical technique tended to have superior values for almost all tested parameters.

Differences in Keratocyte Apoptosis Following Transepithelial and Laser-scrape Photorefractive Keratectomy in Rabbits

BACKGROUND

Anterior stromal keratocyte cells undergo programmed cell death (apoptosis) in response to corneal epithelial injury. Keratocyte apoptosis may be an initiator of the corneal wound healing response that includes keratocyte proliferation and activation, as well as changes to the overlying epithelium, occurring following refractive surgical procedures such as photorefractive keratectomy (PRK). This study compared the effect of laser-scrape and transepithelial PRK on keratocyte apoptosis.

METHODS

Photorefractive keratectomy was performed in both eyes of 10 New Zealand white rabbits using the Summit Apex excimer laser. Surgery was performed using transepithelial PRK in one eye and laser-scrape PRK in the other. The central cornea was analyzed at 4 hours after surgery using a quantitative TUNEL assay that detects fragmented DNA characteristic of apoptosis. Hepatocyte growth factor (HGF) production by keratocytes was detected by immunocytochemistry.

RESULTS

Average apoptotic cells per 400X microscopic field determined by 2 independent masked observers were 0.9 +/- 0.5 (scanning electron microscopy) and 0.2 +/- 0.2 in the transepithelial PRK group compared with 5.1 +/- 2.9 and 4.1 +/- 3.2 in the laser-scrape group. The difference between the two groups was statistically significant for both observers (P < .05, ANOVA). HGF was detected within keratocytes throughout the corneal stroma. Less HGF was detected in the anterior stroma in the laser-scrape group at 4 hours after surgery, consistent with more anterior keratocyte apoptosis in this group.

CONCLUSIONS

Transepithelial PRK induced less anterior keratocyte apoptosis in rabbits than laser-scrape PRK. This suggests that transepithelial PRK could be useful in preventing or minimizing refractive regression and subepithelial scarring.

A prospective evaluation of alcohol-assisted versus mechanical epithelial removal before photorefractive keratectomy

OBJECTIVE

The purpose of the study is to compare alcohol-assisted versus mechanical debridement of the corneal epithelium before photorefractive keratectomy (PRK) for low-to-moderate myopia.

DESIGN

A prospective study was performed on a group of consecutive patients operated on at the Massachusetts Eye and Ear Infirmary from February to April 1996 and followed for 6 months.

PARTICIPANTS

Eighty patients (eyes) were divided in 2 groups: 40 alcohol and 40 mechanical.

INTERVENTION

The patients underwent PRK for myopia (-1.5 to -7.5 diopters) with a Summit Apex excimer laser. The corneal epithelium was removed either with 20% ethanol or with a scalpel blade.

MAIN OUTCOME MEASURES

The two groups were compared for epithelial removal time, epithelial defect size at the end of surgery, and rate of re-epithelialization. Uncorrected visual acuity (UCVA), refractive outcome, best-corrected visual acuity (BCVA), and subjective haze were measured at 4 days and at 1, 3, and 6 months. In an additional short-term study, 40 patients (20 alcohol, 20 mechanical) had intraoperative pachymetry performed.

RESULTS

Alcohol-assisted de-epithelialization was faster than mechanical debridement (107 [+/-20.6 standard deviation] versus 141 [+/-30.5] seconds [P < 0.0001]) and led to a more circumscribed and reproducible epithelial defect at the end of surgery (87,739 [+/-11,852] versus 103,518 [+/-33,942] square pixels [t test, P = 0.04; f test, P = 0.001]). At 4 days, 95% of the alcohol-treated patients had healed compared with 78% of the mechanically scraped patients (Fisher's exact test, P = 0.04). The alcohol group had a better UCVA at 4 days (logarithm of the minimum angle of resolution UCVA 0.36 [+/-0.22] versus 0.51 [+/-0.26]) and at 1 month (0.14 [+/-0.17] versus 0.22 [+/-0.16] [Mann-Whitney U test, P = 0.02 and P = 0.03]) but equalized at 3 months (0.10 [+/-0.14] versus 0.13 [+/-0.16]) and at 6 months (0.11 [+/-0.15] versus 0.14 [+/-0.13] [Mann-Whitney U test, P = 0.23 and P = 0.34]). There was a trend toward less subjective haze in the alcohol-treated patients over the course of the study (area under the curve, 71.9 [+/-35.3] versus 87.9 [+/-33.8] [Mann-Whitney U test, P = 0.07]). The difference from target was equivalent in both groups at 6 months (-0.22 [+/-0.58] diopter in the alcohol group and -0.43 [+/-0.52] diopter in the mechanical group [t test, P = 0.14; f test, P = 0.57]). There were no differences in intraoperative pachymetry, corneal uniformity index as calculated from the corneal topography, and loss of BCVA between the two groups.

CONCLUSIONS

Twenty percent ethanol is a simple, safe, and effective alternative to mechanical scraping before PRK and appears to be associated with a quicker visual rehabilitation.

Apoptosis in the cornea: further characterization of Fas/Fas ligand system

This study was performed to further characterize expression and function of the Fas/Fas ligand system in the cornea. Specifically, these experiments examined (1) the effect of genetic inactivation of Fas or Fas ligand genes on keratocyte apoptosis in response to corneal epithelial wounding, (2) whether cultured human corneal epithelial and endothelial cells are competent to undergo apoptosis in response to Fas activation, (3) expression of membrane bound and soluble Fas and Fas ligand in corneal cells, and (4) the effect of IL-1 on expression of Fas and Fas ligand in corneal fibroblasts. Keratocyte apoptosis in response to corneal epithelial scrape detected by TUNEL assay and transmission electron microscopy was significantly decreased, but not eliminated, in Fas ligand -/- mice compared with control +/+ mice. There was also a decrease in Fas -/- mice that did not reach statistical significance. Thus, while the Fas/Fas ligand system is likely involved in regulating keratocyte apoptosis in response to epithelial wounding, systems with redundant function probably also modulate this response. Activation of the Fas receptor triggered death with ultrastructural changes characteristic of apoptosis in corneal epithelial and endothelial cells in culture. Since these cell types express both Fas and Fas ligand in vivo, systems must be in place to prevent uncontrolled activation via autocrine ligand-receptor interaction. Messenger RNAs coding for both membrane bound and soluble splicing variants of Fas were expressed in each corneal cell type, suggesting that soluble Fas production could be one mechanism to antagonize membrane bound Fas activation. Soluble Fas ligand protein was expressed in wounded ex vivo corneal epithelium, providing a mechanism for Fas ligand from epithelium to mediate keratocyte apoptosis. IL-1, however, also stimulated corneal fibroblasts to express Fas ligand mRNA and protein. Therefore, an alternative mode for epithelial injury to trigger keratocyte apoptosis may be by release of IL-1, subsequent induction of Fas ligand in keratocytes, and apoptosis mediated by autocrine mechanisms.

Alcohol versus mechanical epithelial debridement: effect on underlying cornea before excimer laser surgery

PURPOSE

To determine the effects of 70% isopropyl alcohol used for corneal debridement on surface smoothness, stromal keratocytes, and ease of epithelial removal.

SETTING

Cornea Research Laboratory, University of Rochester, and Excimer Laser Laboratory, Genesee Valley Eye Institute, Rochester, New York, USA.

METHODS

Rabbit corneas were de-epithelialized mechanically or with 70% alcohol. The rabbits were split into groups and evaluated immediately or after a 50 microns deep excimer laser phototherapeutic keratectomy. All tissue was evaluated and compared in terms of surface smoothness parameters, loss of keratocytes, and inflammatory response to de-epithelialization.

RESULTS

Computerized laser interferometric microscopy showed no between-group difference in the surface smoothness parameters. There was a marked absence of keratocytes in the superficial 25% of the corneal stroma. The loss of keratocytes was significantly higher (P < .001) in corneas treated with isopropyl alcohol. The inflammatory response 24 hours after epithelial removal was significantly higher (P < .001) in the corneas treated with alcohol.

CONCLUSION

The use of 70% isopropyl alcohol applied for 2 minutes for epithelial removal did not enhance the quality of the subsequent excimer laser procedure. In contrast, isopropyl alcohol increased the inflammatory response, and it may have damaging effects on keratocytes. We would not advocate the use of 70% isopropyl alcohol as administered in our study to remove corneal epithelium before excimer laser surgery.

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

PURPOSE

Corneal wound healing after excimer laser photorefractive keratectomy (PRK) passes through a series of characteristic stages which have earlier been defined by means of histological, histochemical, and biochemical approaches. We investigated the potential of confocal microscopy to verify morphological changes in human corneas in vivo after PRK.

METHODS

Ten corneas of eight patients that had earlier undergone PRK were examined at different postoperative time points (7 days-34 months). One of the PRK patients was examined sequentially three times. Three additional corneas, which had earlier undergone corneal grafting surgery and then were subjected to excimer laser photoastigmatic keratectomy (PARK), were studied as well. Seven healthy untreated corneas served as controls to define the normal morphology of human cornea. A tandem scanning confocal microscope (TSCM) was used to generate real-time images of the corneas on an S-VHS videotape. The images were either digitized and further processed or the individual video frames were produced with a video printer.

RESULTS

Seven days post-PRK in vivo confocal microscopy revealed the presence of morphologically immature surface epithelial cells. Delicate nerves, activated keratocytes and deposition of extracellular light-reflecting scar tissue were perceived. The epithelium appeared normal one month post-PRK. Ongoing activation of the anterior stromal keratocytes along with extracellular scar tissue were detected. We also observed increasing numbers of regenerating subepithelial nerve leashes with somewhat twisted pattern. Highly reflective, presumably activated keratocytes were no longer detected 6-7 months post-PRK. Hypercellularity with scar tissue could still be found up to 30 months post-PRK. Only one cornea examined 34 months post-PRK showed normal keratocyte morphology and recovery of the anterior stroma. However, the morphology of subepithelial nerves was still somewhat abnormal. The two corneal grafts examined 11 or 32 months post-PARK exhibited a normal-appearing epithelium but considerable stromal hypercellularity and extracellular scar deposition. The subepithelial nerves were poorly regenerated in one eye and fairly well detectable in the other. The third graft examined 15 months post-PARK revealed the presence of enlarged surface epithelial cells and dense stromal scarring but no nerves.

CONCLUSION

TSCM clinically confirms the earlier histological data on healing of excimer laser wounds. It offers a distinct improvement in the assessment of excimer laser-treated corneas, as it enables cellular details and nerves to be perceived in vivo. In addition the thickness of the stromal scar can be be measured for e.g. planning of phototherapeutic keratectomy.

Confocal microscopic characterization of initial corneal changes of surfactant-induced eye irritation in the rabbit

We have previously demonstrated with slightly and severely irritating surfactants that the new technology of noninvasive, in vivo confocal microscopy (CM) can be a useful approach to a better understanding of the pathobiology of ocular irritation in situ. In this study, in vivo CM was used to qualitatively and quantitatively characterize the initial microscopic corneal changes occurring with surfactants of slight, mild, moderate, and severe irritation. Surfactants were directly applied to the corneas of rabbits (6/group) at a dose of 10 microl. Eyes and eyelids were examined macroscopically and scored for irritation beginning at 3 hr after dosing and periodically through Day 35. Concurrently, the corneas were evaluated by in vivo CM; 3D data sets extending from the surface epithelium to the endothelium were assessed for surface epithelial cell size, epithelial layer thickness, total corneal thickness, and depth of keratocyte necrosis. The average macroscopic scores at 3 hr for the slight, mild, moderate, and severe irritants were 6.0, 39.3, 48.5, and 68.7, respectively, of a possible 110. At 3 hr, in vivo CM revealed corneal injury with the slight irritant limited to the epithelium, resulting in reductions in epithelial cell size and thickness to 59.0 and 82.4% of controls (p < 0.001 and p < 0.01, respectively). These parameters returned to normal by Day 3. For the mild irritant, at 3 hr the epithelium was absent, corneal thickness was increased to 157.6% of controls (p < 0.001), and necrosis of keratocytes extended to an average depth of 4.3 microm (0.8% of the corneal thickness); these parameters were essentially normal by Day 14. For the moderate irritant, at 3 hr the epithelium was markedly attenuated, corneal thickness was increased to 155.8% of controls (p < 0.001), and keratocyte necrosis extended to an average depth of 19.0 microm (3.6% of corneal thickness; statistically greater than with the mild irritant, p < 0.001); these parameters were essentially normal by Day 14. For the severe irritant, at 3 hr the epithelium was significantly thinned, corneal thickness was increased to 165.9% of controls (p < 0.001), and keratocyte necrosis occurred to an average depth of 391.1 microm (70.1% of corneal thickness). These findings demonstrate that significant differences in area and depth of injury occur with surfactants of differing irritancy. The data suggest that differences at 3 hr can be used to distinguish different levels of ocular irritation. Data such as these will be important in the development and evaluation of future mechanistically based in vitro alternatives for ocular irritancy testing.

Molecular Cell Biology for the Refractive Corneal Surgeon: Programmed Cell Death and Wound Healing

BACKGROUND

Variability of outcome following refractive surgical procedures is affected by corneal wound healing. Interactions between the corneal epithelium and stromal keratocytes affect both stromal remodeling and healing of the epithelium. These processes contribute to regression of initial effect, surface irregularity, and stromal scarring that occur following excimer laser photorefractive keratectomy (PRK). I review recent discoveries related to stromal-epithelial molecular interactions that provide insights into the cellular responses to refractive surgical procedures.

RESULTS

Injury to the corneal epithelium stimulates programmed cell death (apoptosis) of the underlying anterior stromal keratocytes. I hypothesize that apoptosis of the keratocytes occurring immediately after epithelial injury associated with refractive surgical procedures initiates the subsequent wound healing response. Activated keratocytes subsequently repopulate the anterior corneal stroma where they produce collagen and other components associated with stromal remodeling. In addition, secretion of hepatocyte growth factor and keratinocyte growth factor by keratocytes increases after corneal epithelial wounding and these growth factors stimulate proliferation and inhibit differentiation of epithelial cells, effects which could promote epithelial hyperplasia associated with regression after photorefractive keratectomy.

CONCLUSION

Corneal stromal-epithelial interactions help explain the different results that occur following excimer laser photorefractive keratectomy and laser in situ keratomileusis.