Evaluating Epithelium strength/replacment in LASEK and epiLASEK vs. PRK
Timothy Asks:
I have seen on the web articles or statements that no doctor can necessarily guarantee that a LASEK (as opposed to PRK) procedure will, in fact, occur. This is because (as I have read) a doctor cannot determine by examination whether a person’s epithelium would be strong enough to withstand the LASEK procedure, and as such, may not be able to be replaced. This would mean that the LASEK procedure effectively became a PRK procedure?? If this is true, how often would you say that this occurs in your practice and what do you think this mean as a practical matter??
Emil Chynn replies:
I performed a study in 2005-2006 regarding removing the epithelium vs. replacing the epithelium in LASEK and epi-LASEK.
The genesis of this study, by myself and a half-dozen other prominent refractive surgeons in the US, was we all had cases where we could not replace the epithelium afterwards (because of a tear or something like that), and in those cases had to remove rather than replace the epithelium.
Surprisingly, when we saw the patients 1 day and 4 days postop, we were expecting the eyes which had the epithelium removed rather than replaced to be more uncomfortable, and to have worse vision, and to heal more slowly.
In fact, we observed the opposite: that in the eyes with the epithelium removed rather than replaced, the recovery time was quicker, discomfort less, and vision better.
So, we surgeons all did our own, larger, formalized studies where we intentionally removed the epithelium in 1 eye, vs. replacing it in the fellow eye, and formally assessed endpoints such as uncorrected visual acuity (UCVA), time to full re-epithelialization, and subjective pain scores. Again, in the larger, formal studies, the eyes in which the epithelium was removed and not replaced actually did better than the fellow eyes where the epithelium was replaced.
These studies demonstrate how scientific progress is achieved in the real world, as opposed to in the laboratory: namely, by astute clinicians observing differences in their own patients, then formalizing studies to test this clinical observation.
The mechanistic explanation as to why this is true is as follows. The epithelium that is replaced after LASEK and epiLASEK is not 100% normal, and further radioisotope labling studies have shown that it is replaced over a period of 3-4 weeks postop, anyway. So, by removing and not replacing the epithelium, we allow brand new, perfectly normal epithelium to grow in from the limbus in the 1st place, in which case you have a brand new epithelial covering in 1 week, rather than in 3-4 weeks.
Importantly, there is still a significant clinical benefit to LASEK and epiLASEK over PRK in terms of a marked improvement in postop discomfort, rapid healing, better vision, and decreased chances of scarring.
Again, the mechanistic explanation is as follows. In a PRK (which I last performed in 1999, to tell you how outdated that procedure is), you use an Amoilis brush or spatula to physically scrape the epithelium off. This crushes the epithelial cells, which, as they are crushed and dying, release chemical mediators called cytokines, which are the cells’ signal to other cells that they have been damaged and killed, which causes an inflammatory cascade to commence, leading to pain, inflammation, delayed healing, and scarring.
EM (electron microscopy) pictures have been taken of the corneal surface after PRK vs. LASEK and epiLASEK (I should probably have my webmasters Raf, Roman, and Alan google these pics to post on our website), and the images are striking–you can see crushed cells all over the place after PRK, whereas there are no crushed or damaged cells, just a smooth interface on a microscopic level after LASEK and epiLASEK.
In summary, because of the clean epithelial separation on a microscopic level after LASEK and epiLASEK, you don’t get cellular distruction like you do after PRK, so the healing is quicker, vision better, and there is no pain, as compared to PRK.
When I was doing PRKs in the 1990s, my patients were very uncomfortable, and had to miss 1-2 weeks of work, which is why laser vision correction performed by PRK never took off in popularity. These days, when I am performing LASEK and epiLASEK, patients need to only take 1-3 days off from work, and there is really no pain, so that’s why these newer procedures are really taking off in popularity.
