This article is more than 5 years old. Information may no longer be current.
Epithelium plays crucial role in refractive outcomes, surgeon says
All refractive surgery procedures have potential epithelial-related complications, but there are strategies to prevent or manage them.
ALICANTE, Spain – The epithelium plays a critical role in modulating corneal healing after refractive surgery procedures, not only at a superficial level but also deeper in the underlying stroma, according to one surgeon speaking here.
Corneal wound healing after laser surgery is a complex process in which both structure and function must be restored, Harminder S. Dua, MD, PhD, of Nottingham University, United Kingdom, said at Alicante Refractiva International. In some cases, something interferes negatively with this process. The stimulus can be degradation of the epithelium such as in LASEK, an epithelial free cap in epi-LASIK, the stromal cut as in some cases of LASIK, or the laser ablation itself.
“A chain reaction which involves cellular necrosis or apoptosis, cell activation, migration and proliferation occurs and may eventually lead to delayed healing, scarring and regression,” Dr. Dua said. “In the normal healing process, epithelial reconstruction is not as straightforward as it may appear on the surface.
“A circular epithelial wound does not heal by a process of regular, concentric circles of increasingly smaller diameter that eventually close in the center,” he said. “What really happens is that you have three or four convex epithelial sheets moving in from the limbus to the center of the cornea. They meet, form all sorts of different geometric shapes and, where they come into contact, they form contact lines that are described as pseudodendrites or dendritiform figures on the surface of the cornea.
“When you see these lines at the end of surface ablation, all you have to do is give lubrication drops and they will eventually disappear. They are normal healing patterns of the corneal epithelium that occur after any kind of injury,” Dr. Dua said.
When performing LASEK, the alcohol kills 30% to 70% of the epithelial cells, and short-term outcomes will change according to flap viability, he said. In all cases, a new epithelium grows underneath the epithelial flap. The two layers of epithelium compete with each other, and while the remodeling takes place, the surface will continue to be irregular. Until the new epithelium takes over and the damaged, more superficial layer falls off, vision will remain poor.
“If you have a damaged epithelial flap, which is no longer viable, full recovery of vision will eventually occur but will take longer. If the flap is healthy, recovery is faster because the healthy flap stops the underlying keratocytes activation. There is very little haze, and the patient’s vision improves within 3 days after the procedure,” Dr. Dua said.
|
Image: Dua H |
A circular epithelial wound does not heal by a process of regular, concentric circles of increasingly smaller diameter that eventually close in the center. Three or four convex epithelial sheets move in from the limbus to the center of the cornea. They meet, form geometric shapes and contact lines that are described as pseudodendrites or dendritiform figures on the surface of the cornea. Beware of dry eyes
Corneal nerves are also involved in the healing process and functional recovery after refractive surgery. Studies have shown that although corneal sensation is fully recovered after a variable amount of time, the corneal innervation pattern, as seen by confocal microscopy, never goes back to the original configuration.
“From this we could draw two different conclusions. On one hand, there might be a sufficient number of nerves in the cornea to make the damage of some of them inconsequential in terms of corneal sensitivity. On the other hand, it might be that our current level of ability to test corneal sensitivity is very poor and we cannot detect the drop in corneal sensations,” Dr. Dua said.
This second hypothesis would explain why dry eye syndrome and persistent punctate keratitis occur in some cases after refractive surgery, both LASIK and surface ablation.
“What I definitely recommend is that in cases of preoperative punctate keratitis, surgeons should not do any kind of laser ablation. Also the use of punctum plugs does not resolve the problem at the source. I’d rather offer phakic implants to these patients than attempt a laser ablation, which would only worsen the pre-existing punctate erosion,” Dr. Dua said.
Break the connection with the limbus to prevent ingrowth
Epithelial ingrowth may occur also under a LASIK flap and lead in some cases to large microcysts, fingerprint dystrophy or flap melt.
It is important for a surgeon to learn how to distinguish between patches of dead cells that can be left there and will be spontaneously reabsorbed by the cornea and live cells that continue to grow and infiltrate within the flap interface.
“Remember that cell migration is always centripetal. Although in vitro epithelial proliferation may occur from the surface downward, in real life, most of the epithelial ingrowth occurs from the periphery inward, starting from the limbus. Knowing this will help you detect ‘live’ cellular movements and adopt effective methods of treating this complication,” Dr. Dua said.
If ingrowing cells originate from the limbus and the surgeon breaks the connection of the ingrowing patch with the source of cellular proliferation, these cells will behave like a plant that has been cut away from its roots: They will soon die and dissolve within the stroma.
“You must break the connection with the limbus,” he said.
The risk of ingrowth is even greater after re-treatment because to re-lift the flap, surgeons usually make a small opening and peel the flap off from there. This maneuver often leads to a lot of epithelial tearing and abrasion, which triggers off the chain of healing processes and consequent epithelial ingrowth.
To avoid this, Dr. Dua suggests cutting along the previous LASIK edge with a very fine needle and then lifting the flap.
|
|
|
The best procedure for each patient
Dr. Dua said LASEK is his first choice in patients with less than 3 D of correction.
“If used correctly, in most cases, alcohol allows about 70% of cells to survive, and this makes a good flap with a nice hinge, which is likely to encourage a fast and uneventful recovery,” he said.
Dr. Dua said he always repositions the epithelial flap after the procedure, puts a contact lens on top and waits until the day after. At that point, if he sees that the flap is unstable, he brushes it away.
“That’s why I don’t use LASEK for more than 3 D. The more you ablate, the more chances you have of scarring and regression if the flap falls off,” he said.
Epi-LASIK has, in this sense, a much higher risk of losing the flap.
“It is better for the quality of flaps, but there is a high percentage of tearing as you reach the hinge level, and then what you really do is PRK. I don’t believe that epi-LASIK has any true advantage over LASEK,” he said.
Between 3 D and 10 D, whenever pupil diameter, corneal thickness and conditions are favorable, Dr. Dua said he uses LASIK.
But in myopic eyes from 5 D and more than 10 D, he said he considers phakic implants a viable option.
For more information:
- Harminder S. Dua, MD, PhD, can be reached at the Department of Ophthalmology and Visual Science, Nottingham University Hospital, Nottingham NG7 2UH, United Kingdom; e-mail: harminder.dua@nottingham.ac.uk.
- Michela Cimberle is an OSN Correspondent based in Treviso, Italy, who covers all aspects of ophthalmology. She focuses geographically on Europe.