I finally decided to write a story. Its’s not a traditional one as others on this forum, but I hope you’ll like it.
In year 1996, Beck Weathers was preparing to climb Mt. Everest after having radial keratotomy (RK) to treat myopia. Having arrived into the Everest Base Camp (~5370 m AMSL) he noticed difficulty in his near vision. Beeing 50 and trained physician he knew that presbyopia was nothing new, but having put his glasses on, he realised something else. He felt like this weren’t his glasses. Even his distance vision wasn’t as good as usually, specialy in the dark, but nobody knew about this untill the day of final ascent. At a bit over 8000 meters, right in the death zone, he was forced to stop his ascent. He could only hope that his vision will improve, but it didn’t. Having never summited Mt. Everest, he sufferd severe frostbites and was left to die, but he survived. The rest is history.
To understand why this happend to him and to prevent this from happening ever again, a group of ophthalmologists started researching the effects of high altitude on human eye after refractive surgeries. This was in early 2000s, so most dominant procedure was radial keratotomy. The idea behind RK is that by making radial incisions in the cornea, pressure from the inside would change shape of the cornea to correct myopia and astigmatism. It was accidently discovered method and it worked for some time untill the Evereste disaster in 1996.
In year 1999, a mountaineer, a bit younger then Weathers summited Mt. Everest after experiencing similar visual issues. He had undergone RK in his 30s (-5.25, -3.75). In early 40s, while climbing above 4000 meters he experienced problems with his near vision, but as soon as he descended lower, it impoved. In 1999, his cycloplegic reftaction was +3.00 and +2.75 with some astigmatism. Before his ascent to Mt. Everest he got reading glasses and prescription glacier glasses. He successfuly summited Everest without prescription glacier glasses.
There are many explanations for this effect. Firstly, as we climb higher: 1) air pressure decreases and 2) oxygen level decreases. This causes thickening in cornea due to cornela edema. For normal corena, it’s no problem, but incisions made during RK make cornea more susceptible to changes. Central parts of the cornea start flattening causing hyperopic shift, which can or can’t be compensated by accomodation depending on the age. he main question which rises from this is wheather people who underwent modern refractive surgery procedures like LASIK and PRK can also suffer similarly. The main principle behind both LASIK and PRK is reshaping corean by removing tissue. Let’s assume the person is myopic (so we can compare to RK). We’re removing tissure from the central part of the corena to make it flatter. We can do it either right below epithelium (PRK) or in stoma under the flap (LASIK). In either cases, stresses on the corneal tissue are distributed the same as if person hadn’t undergone the procedure. The only difference is that cornea is slightly thinner (PRK) or is thinner and has a flap (LASIK). In case of RK, stresses on the tissue can’t be spread normally. Hence, we can conclude that LASIK and PRK don’t cause the same problem as RK. LASIK flap is held in place by adhesion forces and by the scar tissue surrounding it. Depending on the method of creation, flap created using femtosecond laser will always have more precise thickness and sharper edges which will help keep the flap in place. Compared to flap created using manual microkeratome, which will be cut “flat”, so it’ll have thicker edges compared to center and will havenot so sharp edges and scar tissue won’t keep it in place as tightly. Hence, we prefer femtosecond laser for flap creation. PRK doesn’t have a flap at all, but there are differenet methods of removing epithelium, but even the most advanced PRK today ends up by manually removing the last bits of tissue left behind. You can go even one step further and not remove epithelium at all:1) push it to the side or 2) make a very thin flap. Non of this alterantives have been so much more efficient than classical PRK to keep using them. Compared to LASIK, PRK correction is much shallower and is suceptible to changes due to tissue growth. The more tissue you remove, the more it wants to grow. Non of the current resaerches I’m aware of have shown any significant refractive changes in post-LASIK and post-PRK patients who were exposed to high altitude. Both of them are also certified for commercial pilots by EASA, FAA and some other agencies. Astronauts can undergo this procedures as well, just as military pilots in many countries. One interesting fact is that LASIK flap is safe even in case of ejection from fighet jets.
I will conclude with this: refractive surgeries were initialy created to reduce dependency on glasses and/or contact lenses. They were never designed to provide what today’s patients want-vision better than normal. But thanks to precise engineering of the devices, we can give them exactly what they want: perfect vision. Yes, things sometimes go wrong and they can go wrong a lot, but mostly it comes up just as expected. One of the most common procedures done today are cataract suregries. People have 2 eyes (at least most of them) and they live longer today than they did before, hence more of them get cataracts, which need to be removed. The procedure is safe and precise, but still we sometimes encounter refractive suprises. Most of them hapoen due to previous refractive surgeries and unknow data about them, but this is whole new subject.
https://vision-and-spex.com/effects-of-high-altitude-and-hypoxia-after-refract-t2668.html