If you’re wondering about the chances you’ll need to have an enhancement after your original LASIK procedure, you have a group of many thousands of friends who have asked the same question before LASIK. It’s one of the final questions people have when they’re making the decision on whether or not to “go for it” and get out of glasses for good. I think that's because the question represents a subconscious landmark in the decision-making process about LASIK. It means someone has theoretically passed from the stage of “do I want to get LASIK?” to the stage of “okay, I do want to get LASIK, but not twice.”
The answer to this question—much like the question “what are the odds I'll get mumps?”—varies based on the time in history or specific place you are asking it. If it is a while ago (i.e. the 1800s for mumps or the 1990s for LASIK) then your odds are significantly higher. If it is being asked in the year 2017, but in a high-risk place (i.e. for mumps, somewhere that collectively doesn't use vaccines because it is a third-world country or based their medical decisions on Facebook posts. For LASIK, a place that does buy-one-eye-get-one-eye-free LASIK) then again, your odds are significantly higher.
Good Diagnostics Mean Fewer Enhancements
Modern LASIK done with state-of-the-art equipment should have an extremely low enhancement rate. This is for two reasons. The first is that laser is really, really good at reshaping corneas now. The prescription can be changed in a way that leaves a much more natural shape to the cornea, which makes an unpredictable response to LASIK very rare. The second is that the current generation of imaging diagnostics used in analyzing LASIK candidacy are also very amazing. We have the ability to decide with much more specificity who is likely to do well with LASIK and who should pass on it.
In the same vein, modern diagnostics afford you the opportunity to have a lot more information about the chances that you'll need a LASIK enhancement. Your LASIK doctor should be able to give you a reasonable statistical estimate of the chances that you specifically would need an enhancement. There are patients for whom the odds of needing an enhancement are as low as 0.5%. There are others for whom the chances are much, much higher of needing two laser treatments to get to their best vision.
LASIK Discrepancy Curve
The reason for the discrepancy amongst LASIK candidates in who has an almost guaranteed immediate result and who has a high chance at needing a follow-up treatment a few months later is because of one factor we can't control. Namely, the way you heal. There is a bell curve for how eyes respond to LASIK that ranges from “overly compliant to laser” all the way to “overly complacent to laser.” Luckily, since it is a bell curve, most people are in the middle. But you can imagine if you're an outlier—say 10% more likely to have laser undercorrect your prescription—it’s going to have a lot more impact on your chances of needing an enhancement if your prescription is huge (10% off of a huge number is visually significant) vs. a tiny prescription (10% off on a tiny number has an unmeasurable effect and is visually insignificant).
The only way to get an idea of the odds that you specifically might need an enhancement after your laser procedure is to have a consult. There is more to figuring out an estimate of your chances of needing an enhancement, but the specifics are variable from person to person depending on what diagnostics reveal. Those specifics span much wider than would be possible or even remotely interesting to detail here. But if you've decided you may want to get LASIK, and have moved on to knowing that you don't want it twice, there are answers waiting for you.