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Corneal permeability increases after surface ablation, study shows
Long-term permeability changes should be considered when topical medications are prescribed in patients who have previously undergone PRK or LASEK.
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ROME – Eyes that have undergone refractive procedures using corneal surface ablation are likely to have a more permeable cornea and increased long-term penetration of topical medications, according to a study.
“We must be aware of these physiological changes and take them into account in case we prescribe eye drops, such as cortical steroids or glaucoma medications, in patients that have been previously treated with PRK or LASEK,” said Marie José Tassignon, MD, at the ESCRS Winter Refractive Surgery Meeting.
To evaluate the changes in corneal permeability after LASEK, Prof. Tassignon compared fluorophotometry results of post-LASEK eyes to the results of normal, untreated eyes. Fluorophotometry was performed using the OcuMetrics Fluorotron Master and fluorescein. The concentration of fluorescein – a water-soluble molecule of low molecular weight – was measured in the corneal tear film and in the anterior chamber.
Fluorescein was administered in the form of minitablets, a patent of the Ghent University pharmacology department in Belgium. The minitablets were positioned in the conjunctival sac of the patients and disintegrated over 6 hours, slowly releasing the substance. Prof. Tassignon pointed out that most studies on the permeability of the cornea are performed with fluorescein drops, which have shorter contact time with the corneal tear film and vary in concentration according to the amount of tears produced by the eye.
Barrier in normal eyes
Fluorescence in normal, untreated eyes was compared to that of the LASEK group at the corneal, lens and anterior chamber levels.
“There is normally a low degree of autofluorescence at the corneal level, mainly due to the epithelium, no fluorescence at the anterior chamber level and a higher autofluorescence in the lens,” she said.
In the same normal eye 4 hours after fluorescein release from a minitablet, a high concentration of fluorescein was detected at the corneal level, but no fluorescein had penetrated the anterior chamber, and the autofluorescence of the lens had remained unaltered. This demonstrated that in untreated eyes the cornea is an effective barrier against fluid penetration.
The study included six eyes treated with LASEK for a mean myopia error of –3.18 D. Surgery was performed using a 20% alcohol solution for detaching the epithelium and an InPro laser for the ablation. Postoperatively, a mixture of Trafloxal (ofloxacin, Tramedico), Voltaren (diclofenac sodium, Novartis Ophthalmics) and Deicol (dexamethasone 0.1%, chloramphenicol 0.4%, Viatris) was administered for 3 to 6 months, but the treatment was discontinued at least 4 weeks before the study started. The control group included six patients with a mean myopic error of –2.58 D. They received the same topical medications as the LASEK group in the right eye and no medications in the left eye.
The patients were subdivided into four groups. Group 1 was composed of three patients with 1 month of post-LASEK follow-up. Group 2 contained three LASEK patients with 6 months of follow-up. Group 3 contained the control eyes in which eye drops had been administered, and group 4 encompassed the control eyes that had not received the topical medications.
Fluorophotometry was performed in all eyes, measuring the concentration of fluorescein at all levels over a period of 8 hours after the minitablet had been positioned in the conjunctival sac, Prof. Tassignon said.
| Anterior segment fluorophotometry with/without fluorescein minitablet |
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In a normal, untreated eye (light green line) there is a low degree of autofluorescence at the corneal level, no fluorescence at the anterior chamber level and a higher autofluorescence in the lens. In the same eye, 4 hours after fluorescein release from a minitablet (dark green line), a high concentration of fluorescein was detected at the corneal level, but no fluorescein had penetrated into the anterior chamber, and the autofluorescence of the lens remained unaltered.  Source: Tassignon MJ |
Increased permeability
 Bio-erodible minitablets were used for fluorescein release prior to fluorophotometry. Image: Tassignon MJ |
A higher concentration of fluorescein in the corneal tear film was found in the control groups than in the LASEK groups.
“This was probably due to the fact that fluorescein in the LASEK eyes had already gone through the cornea and had penetrated into the anterior chamber,” Prof. Tassignon said.
When measuring fluorescein at the anterior chamber level, the concentration was significantly higher in the LASEK group, particularly at 1 month, with a slight reduction at 6 months. Analysis showed that the difference in fluorescein concentration was statistically significant between the LASEK groups and the control groups, but also between the control groups that had and had not received topical medications, she said.
“This demonstrates that there is an increased corneal permeability after LASEK at 1 and 6 months, though there is a tendency towards reduction of the permeability with time. The permeability is also increased by the use of topical medications,” Prof. Tassignon said.
She said that a longer follow-up is needed to determine the stability of these changes, although the duration of fluorophotometry tests is a limitation. At least 8 hours of consecutive measurements are necessary for each patient to obtain adequate nomograms.
“It would also be interesting to expand this study to LASIK and epi-LASIK groups and compare the results of the different techniques,” she said.
These preliminary conclusions, however, indicate that ophthalmologists need to consider the increased permeability of the cornea after surface ablation, she said.
For Your Information:
- Marie-José Tassignon, MD, can be reached at Universitair Ziekenhuis Antwerpen, Department of Ophthalmology, Wilrijkstraat 10, Edegem 2650, Belgium; +32-3-8213377; fax: +32-3-8251926; e-mail: oftalmo@uia.ua.ac.be. Prof. Tassignon has no direct financial interest in the products mentioned in this article, nor is she a paid consultant for any companies mentioned.
- Michela Cimberle is an OSN Correspondent based in Treviso, Italy.