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Why is there less of a chance for LASIK if I’m farsighted?

By Joel Hunter, MD | 2/2/18 7:00 AM
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I wish it was a myth that the chance of LASIK candidacy is lower for folks who are farsighted. I wish I could say, “Good news! LASIK is almost always amazing for people who are farsighted!” But I can’t say that. It isn’t a myth. It’s just science that determines the decreased chances for LASIK if you’re farsighted. And as I learned during my demoralizing attempts to build a hoverboard as a child, science doesn’t care about my hopes and dreams.

We know the reason it’s harder to be a candidate for LASIK if you’re hyperopic (if we’re talking science, it seems like we should use the official language for farsightedness). LASIK works by changing the shape of the clear, domed window on the front of your eye called the cornea. The excimer laser used in LASIK is able to sculpt that cornea with extreme precision without damaging tissue that we don’t want to sculpt. The keyword, however, when we’re describing the difference with hyperopic LASIK isn’t precision; the keyword is sculpt.

Myopic vs. Hypertropic LASIK

The laser (and any other technology, for that matter) can’t change the shape of the cornea by adding anything to the shape. It can only sculpt — that is subtract — to make the shape into what you’d need to have clear vision. With myopic LASIK, that’s easy to describe: the cornea is too steep to see clearly so it is made more flat. That’s a situation set up perfectly for subtraction. With hyperopic LASIK, we’ve got the opposite issue. The cornea is too flat to see clearly and must be made steeper. But we can’t add to the hilltop. How would we make it steeper?

The answer is in sculpting down the sides of the hill to make everything more steep. Part of why LASIK used to be a non-starter altogether for hyperopes is that the algorithms for driving the laser weren’t sophisticated enough to treat only the periphery and leave the center alone. It could treat hilltops, but didn’t do a good job of treating anywhere else. Then LASIK got better and could be done on hyperopes, but not very well. The regression effect (“my LASIK wore off”) was really terrible because the laser could basically sculpt a moat around the center of the cornea. Now, with modern laser, there are very good hyperopic treatments that are able to smooth down the periphery and steepen the cornea much more naturally. At last, people who are hyperopic have a LASIK option.

What are my chances?

The chances of being a hyperopic candidate, however, are still much lower than if you are myopic. That’s because the way the prescription is fixed has become unimaginably more precise, but relies on the same principle of sculpting that it always did. We still can only smooth down the sides of the hill. As hyperopic prescriptions get higher (usually around +2.00 or above) the amount of sculpting in the periphery necessary becomes problematic. The hilltop — the center of the cornea — remains untouched but the more we steepen the hill, the smaller the hilltop becomes.

That center of the cornea is the optical zone you’re looking through for clear vision, so as you’d assume, a large optical zone matters. If the “treated” optical zone (even though laser never touched it) is made too small, then you’ll end up with problems we used to see in the old days of LASIK. The chances increase of night vision trouble, lack of clarity, halos, etc. So while it is a near certainty of having 20/20 distance vision and reading vision without glasses, the cost to the quality of your vision isn’t worth it. There is such a thing as a bad 20/20.

With the imaging diagnostics available today, and our understanding of how corneal topography influences vision, we can make the right choice ahead of time. We don’t have to go into LASIK wondering how it will turn out. That’s great news for candidates and non-candidates alike, because having eyes that see clearly without glasses is amazing, but having eyes that are able to see clearly with or without glasses is even more important. There are cases where hyperopic LASIK is fantastic, and you get to have both: clear vision and no glasses. But when you’re farsighted and LASIK isn’t an option, you can take comfort in knowing you’re probably going to have options that are even better to get out of glasses — either in the future or right now.

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These articles are brought to you by Hunter Vision. We help people in Orlando discover life after glasses and contacts.
 
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