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LASIK vs. ASA: what’s the difference?

By Joel Hunter, MD | 5/15/17 7:58 AM
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Some of you may start this article with the pressing pre-requisite question, “what is ASA?” That’s an excellent question! We should answer it right here at the beginning. If you Google it, you’ll likely be informed about a Society of Anesthesiologists. I don’t know much about them, but I imagine they have important and sleepy meetings.

In this case, the ASA we’re talking about is Advanced Surface Ablation, a type of laser eye surgery. Its ancestor, PRK (Photorefractive Keratectomy), was the original laser eye surgery. Before LASIK came to exist, there was PRK. Over time, PRK had incremental improvements in comfort, speed of recovery, and quality of vision. After a decade or so of those incremental changes, refractive surgeons looked again to find that PRK had become a new and better procedure. In honor of this, and to distinguish it from the procedure it had redefined, they gave it the name Advanced Surface Ablation, or ASA.

In short, ASA is the modern and much-improved procedure that replaced PRK. For people who’ve researched PRK and want to know if it’s the best procedure for them, I’ll usually say “modern PRK” instead of ASA to avoid confusion. I do that for the same reason that if someone asked if I like my iPhone, I wouldn’t say, “You mean my sixth generation iPhone 6S?” There’s no reason to be overly formal about terminology when you could be helpful on the subject instead.

The reason for the brief explanation of ASA is because the procedure we have today is much different from PRK performed in the 90s. When we’re considering the matchup of LASIK vs. ASA, those differences in ASA as compared to the PRK procedure that came before it are relevant. Not to ruin this with a huge spoiler, but LASIK and ASA both yield the same excellent result, in part because we’ve come so far since the PRK of the late 90s.

The difference between LASIK and ASA involves how we get laser where it needs to go in order for vision correction to be permanent. The excimer laser is what changes the shape of the cornea to correct the vision. It’s accurate and effective because it treats only a tiny fraction of the surface in front of it. It doesn’t have any effect beyond that. This is great news for accuracy because it removes the need for calculating a target depth because the only depth possible is “whatever surface is facing the laser.” Yet, this also means a cornea isn’t ready for excimer laser at the beginning of the procedure.

There’s a layer of epithelium—clear skin on the eye’s surface—about 80 micrometers deep and always changing. With every blink, a couple of those epithelial cells wash away and a few newborn epithelial cells emerge to take their place. It’s a dynamic surface of living tissue, always depleting and renewing itself. That’s good news if you scratch your eye, because you can wait three days and, tada! You’re good as new! But that same renewability also means that laser eye surgery won’t work on the epithelium. The results would be fleeting. If you want to get out of glasses or contacts, the good news is that below the epithelial layer lies the stroma. The stroma is an amazing medium for excimer laser treatment. Resolute, durable, and unchanging, this layer has all the qualities we need to fix your vision for good.

So how do we get past the epithelium to the promised land of stroma beneath it? That’s where we find the difference between LASIK and ASA. In the case of ASA, the epithelium—which is accommodating in every way—is wiped away with a tiny sponge. Doing this is simple with modern techniques. It’s as if the epithelium were saying, “Do you need through here? Ah, very well, my apologies. I’ll clear out then.” (Quick note: this is one of the key differences with ASA today compared to PRK from yesteryear. The ability to remove the epithelium in a small, symmetrical way creates a result that’s minimally disruptive and quick-healing.) Once the epithelium is removed, it regrows over the next few days. During LASIK, we don’t remove the epithelium at all. We move it, along with an ultra-thin layer of stroma to gird it and make it sturdy. After laser treats the stroma underneath, we replace the layer, and the eye is none the wiser.

The difference between these two techniques is not found in the results. Studies have shown that LASIK and ASA provide identical vision correction. This makes sense because no matter which procedure you’re getting, the same laser is treating the same layer of the eye. The contrast exists in how long it takes to get those results.

Epithelium has to be smooth and pristine for good vision to be possible. Growing new epithelium happens very quickly with ASA—usually about three days. But it takes around 8 to 12 weeks for it to achieve the level of polish necessary for perfect vision. It doesn’t take long to see 20/20, which is usually achieved in the first week, but the vision keeps improving in sharpness and quality over the following weeks.

With LASIK, the layer of polished epithelium you started out with is moved and then put back into place. In this case, the wait time for great vision is measured in hours instead of weeks. There are few situations I approach with more trepidation than when a husband and wife are getting laser on the same day and one is getting ASA while the other gets LASIK. A short stretch down the road, all is well and everyone’s vision is wonderful. However, it’s important and sometimes challenging to paint that picture for them on day three when their surgery results are in different stages of recovery.

The reason the above situation arises is because people’s corneas are unique. Sometimes a procedure that’s risky for one person is the most conservative route for another, and vice versa. The discussion of LASIK vs. ASA could easily be posed as a discussion of why some eyes have different risk factors than others. If a cornea seems like it might be too weak for LASIK (even though most have a wide margin of strength) we don’t gamble on it. It may be because of old scars from injury or contact lenses, or because it’s too thin, or because our imaging shows the cornea looks suspicious for weakness.

If there’s any question about which procedure is best, it’s important to ask “should you?” instead of asking “could you?” In those cases, ASA is the safer option. Except for the microscopic amounts of stroma treated by the laser, we only need to remove a layer of epithelium for ASA. And by now you know that the epithelium is already in the habit of replacing itself anyway. There are also times when LASIK is a more conservative choice than ASA, like when healing speed or variability is an issue. Every eye is unique, so while preference matters and is a major factor, it’s always outranked by safety.

As a general guideline on LASIK vs. ASA, you can only make the best choice by getting a detailed evaluation of your eyes. The results of that evaluation need to be read by a refractive surgeon who knows what they mean—and who likes you enough to be honest about what’s best for your eyes. It may be that LASIK or ASA or the exasperating “wait a few years” is your best choice. What’s nice is that we now have enough technology to guide us to the right answer. You just have to know where to look—and not accidentally end up at the Society of Anesthesiologists.

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