Specular microscopic evaluation of the corneal epithelium after excimer laser photorefractive keratectomy

Specular microscopic study of X-ray-irradiated rabbit cornea

PURPOSE

To investigate the influence of ionizing radiation on the corneal epithelium and endothelium of rabbit eyes.

METHODS

Five healthy mature albino rabbits were unilaterally irradiated with 20 Gy of X-rays (250 kV, 12 mA). Slit-lamp biomicroscopic observation and specular microscopic examination of the corneal epithelium and endothelium were carried out before and 1, 4, 8, 12, 16, 20, 24, and 36 weeks after irradiation. We evaluated mean area of the superficial corneal epithelial cells, mean area and the percentage of hexagonal cells of the corneal endothelial cells, and corneal thickness. The statistical difference between the irradiated and control eyes was assessed using paired t-test.

RESULTS

All animals developed cataract within 24 weeks. Slit-lamp biomicroscopy showed no apparent corneal abnormalities over the 36-week follow-up period. Specular microscopy revealed a significant enlargement of the superficial corneal epithelial cells from 4 to 12 weeks after irradiation (P<0.01), which disappeared at 16 weeks post-irradiation. Specular microscopy of the corneal endothelium showed enlargement and morphological alterations of the cells beginning 8 weeks after irradiation (P<0.05). These changes persisted throughout the study period. There were no statistically significant changes in corneal thickness.

CONCLUSION

After X-ray radiation of 20 Gy, transient damage occurred in the corneal epithelium, while delayed and irreversible changes were seen in the endothelium.

Changes in corneal epithelial barrier function after excimer laser photorefractive keratectomy

PURPOSE

To use fluorophotometry to measure corneal epithelial barrier function after excimer laser photorefractive keratectomy (PRK).

SETTING

Seoul National University Hospital, Seoul, Korea.

METHODS

Twenty-five eyes of 21 patients (13 women, 8 men) had PRK to correct myopia. Corneal epithelial healing time was measured and corneal epithelial permeability to sodium fluorescein evaluated by fluorophotometry 1, 2, and 3 weeks after surgery.

RESULTS

Epithelial permeability showed a statistically significant increase 1 week after surgery and returned to its preoperative level 1 week later. Comparative studies according to epithelial healing day and corrected diopter showed results that were not statistically significant (P > .05).

CONCLUSION

These results suggest that PRK delays complete reconstruction of corneal epithelial barrier function. In humans, the corneal epithelium regained its normal barrier function 2 weeks after PRK. Thus, at least during these weeks, care should be taken to minimize further epithelial trauma.

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

PURPOSE

To investigate endothelial cell loss in pairs of fresh human autopsy globes following high-diopter myopic photorefractive keratectomy (PRK) or laser in situ keratomileusis (LASIK).

SETTING

Center for Research on Ocular Therapeutics and Biodevices and Magill Laser Center for Vision Correction, Storm Eye Institute, Charleston, South Carolina, USA.

METHODS

In the first part of the study, 12 globes had either -10 diopters (D) multizone surface PRK or -10 D single-zone LASIK. In the second part, three groups of 5 globes each had -15 D, -20 D, or -25 D multizone-blend LASIK procedures. Fellow globes in both groups were used as untreated controls. Corneoscleral buttons were excised from all globes. Following 7 days in corneal organ culture, the endothelial surface was stained with two vital dyes: calcein-AM and ethidium homodimer. Fluorescence microscopy was used to obtain endothelial cell counts.

RESULTS

The mean dead cells per square millimeter (cells/mm2) were 0.94 in the -10 D PRK treated corneas compared with 0.91 in the fellow untreated eyes (P = 0.06(. The mean dead cells/mm2 in the -10 D single-zone LASIK-treated corneas and in the fellow untreated eyes were 0.61 (P = 0.88). The mean dead cells/mm2 in the -15 D, -20 D, and -25 D multizone-blend LASIK-treated corneas were 3.08, 2.33, and 5.55, respectively, compared with 3.49, 1.92, and 5.01 in the fellow untreated eyes (P = 0.276, P = 0.339, and P = 0.427, respectively). Dead cell counts for treated and control paired corneas were highly correlated in all treatment groups.

CONCLUSIONS

No significant endothelial cell loss occurred after -10 D PRK or LASIK corrections up to -25 D. Although this study has limitations that prevent direct extrapolation to the clinical situation, it does afford a comparable clinical correlate for endothelial cell toxicity following a typical excimer laser ablations.

Management and Treatment of Central Steep Islands after Excimer Laser Photorefractive Keratectomy

BACKGROUND

Permanent central steep islands are an undesirable phenomenon that cause distorted images and a significant reduction in visual acuity. We describe treatment of central steep islands with repeat excimer laser photoablation in the central cornea.

METHODS

Three patients with preoperative refractions of -7.50 -2.50 x 170 degrees D (right eye), -8.00 -2.25 x 10 degrees D (right eye) and -6.00 -1.50 x 90 degrees (right eye) developed central steep islands which persisted more than 12 months. All patients lost more than two lines of spectacle-corrected visual acuity and complained of visual disturbances. We retreated the central steep islands with a VISX 20/20 excimer laser PRK ablation that matched the size of the central island measured on videokeratography.

RESULTS

Symptomatic glare and distortion were significantly reduced and the central steep islands were resolved. Several weeks after reablation, spectacle-corrected visual acuity improved to the preoperative level.

CONCLUSION

Central steep island, an infrequent complication of excimer laser photorefractive keratectomy, can be safely removed with a repeat laser ablation that matches the central circular steep area.

Corneal epithelial recovery following photorefractive keratectomy

AIMS

To further understand the morphological and functional recovery of corneal epithelium following excimer laser photorefractive keratectomy (PRK).

METHODS

The right eyes (group 1) of 15 male, New Zealand white rabbits weighing 2-3 kg underwent PRK. The left eye of each rabbit (group 2) underwent simple mechanical de-epithelialisation and were examined as treated controls. Both eyes of another eight rabbits (group 3) served as untreated controls. All eyes underwent a corneal epithelial permeability study by fluorophotometry at 2, 4, and 8 weeks after surgery. Five animals in groups 1 and 2 were sacrificed at 9, 10, and 12 weeks after surgery. The animals in group 3 were sacrificed at the end of the 12 week experimental period. Both eyes of each sacrificed animal were enucleated immediately and processed for both haematoxylin and eosin stain and electron microscopic study. The electron micrograph was magnified to 14,000x and the extent of hemidesmosome formation was quantified and analysed.

RESULTS

The corneal epithelial barrier to sodium fluorescein was subnormal and returned to a normal barrier state 4 weeks after PRK in group 1 whereas it was normal in group 2 throughout the examination period. The extent of hemidesmosome formation was abundant yet subnormal in both groups 1 and 2 up to 12 weeks, when compared with that in group 3.

CONCLUSION

The corneal epithelium regained its functional barrier 4 weeks after PRK in rabbits while the extent of hemidesmosome formation was still subnormal 12 weeks after mechanical de-epithelialisation, with or without PRK.

Tracker-Assisted Photorefractive Keratectomy for Myopia of -1 to -6 Diopters

BACKGROUND

The Autonomous Technologies T-PRK (Tracker-assisted Photorefractive Keratectomy) excimer laser system uses a small beam scanner that allows flexibility in the ablation pattern that is applied to the cornea and incorporates a sophisticated LADARVision eye tracker that is capable of following saccadic movements. This paper describes the first clinical results on sighted eyes for the correction of low myopia.

METHODS

Forty-two normal sighted eyes of 42 patients were treated for spherical myopia between -1.00 diopters (D) and -6.00 D with 6 mm ablations. Visual acuity, refractive error, contrast sensitivity (with and without glare), corneal haze, endothelial cell density, and patient satisfaction were measured.

RESULTS

Mean manifest refraction was -0.39 D +/- 0.68 D at 1 month with regression to -0.94 D at 3 months and -1.05 D at 6 months. At 6 months, 5 (20%) eyes were +/- 0.50 D and 14 (56%) eyes were +/- 1.00 D. Consistent with this undercorrection and regression, uncorrected visual acuity (UCVA) of 20/20 and 20/40 or better was achieved by 10 (40%) and 34 (85%) eyes at 3 months and 16 (40%) and 17 (68%) eyes at 6 months. None of the eyes lost 2 or more lines of spectacle corrected visual acuity. Corneal haze was graded as 1/2 trace or less in 89% to 100% of eyes at all intervals. There was no loss of endothelial cells (mean +/- SD cell density centrally: preop 3115 +/- 322 and 6 months 3220 +/- 333) and contrast sensitivity recovered to baseline levels at 3 months.

CONCLUSIONS

The Autonomous Technologies T-PRK excimer laser system is safe and effective for the reduction or correction of myopia from -1.00 D or -6.00 D. The refractive results may be improved by adjusting the calibration to reduce the undercorrection and by instituting use of topical corticosteroids on an individual basis for those who regress.

Corneal epithelial permeability after excimer laser photorefractive keratectomy

PURPOSE

To evaluate the effect of excimer laser photorefractive keratectomoy (PRK) on the corneal epithelial barrier function.

METHOD

Corneal uptake of 5,6 carboxyfluorescein (CF) was measured by Bernal and Ubels' method after excimer laser PRK in New Zealand white rabbits (N = 40). One cornea in each rabbit was treated, and the fellow cornea was used as a control. In both eyes, the central 7.0 mm of the corneal epithelium was removed. Myopic PRK treatments were performed at 37.75 microns (3.3 diopters [D]) and at 52.50 microns (5.0 D). The animals were euthanized and the eyes placed in CF for 5 minutes at 3 days, and at 1, 2, and 4 weeks following PRK. The corneas were then excised and dialyzed in balanced salt solution. The CF concentration in the dialysate was measured by fluorometry. Four corneas were also prepared for transmission electron microscopy using fixative containing ruthenium red.

RESULTS

Three days after PRK, CF uptake increased in all study eyes compared with normal eyes (n = 5). One week after PRK, the control corneas showed a decreased CF uptake while the study corneas still had an increased CF uptake (P < .05). Two weeks after PRK, CF uptake in corneas with a 5.0 D ablation remained increased but decreased in corneas with a 3.3 D ablation (P < .05). Four weeks after PRK, CF uptake returned to normal in all corneas. The ruthenium red penetrated into the deeper layers of the corneal epithelium 1 week after PRK; only the superficial cell layer was stained 4 weeks after PRK.

CONCLUSIONS

Excimer laser PRK affected the corneal epithelial barrier function at 1 and 2 weeks postoperatively even though the corneal epithelium covered the ablated area. Deeper laser ablations showed higher corneal CF uptake for longer periods than shallower ablations.