Astigmatic

If you are thinking about having Lasik, IntraLasik, PRK, LASEK, Epi-Lasik, RLE, or P-IOL eye surgery, this is the forum to research your concerns or ask your questions.

Astigmatic

Postby Peter » Wed Jul 19, 2006 8:49 pm

Hi!

In about one month I will have a Lasek surgery on both my eyes. The correction in my contact lenses are SPH: L-0.5 and R-0.75 and CYL: -1.25 for both eyes. I do know that the astigmatism is the big issue of mine but on your home page you mention that if the CYL is more than half of the SPH it spells extra big trouble when having a LASEK op. I do not get why (would I be better off if I had a larger negative SPH?), and does this mean that I should consider having a LASIK surgery instead?
Moreover, my doctor has told me that my eyes are well suited for both LASIK and LASEK but I have chosen LASEK because of the more promising long term effects.

Best Regards,
Peter
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Postby LasikExpert » Wed Jul 19, 2006 10:43 pm

As a general rule, astigmatism (cylinder) is more difficult to correct than myopia or hyperopia (sphere). As a (very) general rule, when the cylinder is greater than half the sphere, the predictability of outcome is significantly less than if the cylinder is less than half the sphere.

Your situation is unique because you have a very small sphere. In fact, your sphere is so small that even your moderate amount of cylinder is about twice the sphere in either eye. The half-sphere rule does not as significantly apply in your case because your sphere is so very small. It applies, but it is not the same as someone who has 2.50 diopters of cylinder and 4.00 diopters of sphere.

The physics of astigmatic correction with laser eye surgery cause an automatic myopic (nearsighted) correction even if it is not desired. This coupling effect has a ratio of about 0.25 to 0.33 diopters of myopic correction for each diopter of astigmatic correction. You have just enough myopia that the coupling effect should not be a problem in your case, however if you require enhancement surgery the coupling effect may be more of a concern.

I concur with your conclusion that a surface ablation technique like LASEK would be a better consideration than Lasik or IntraLasik. Since most of your refractive error is cylinder, correcting this corneal irregularity is theoretically easier with a surface ablation technique. At least this is the opinion of several doctors.

A separate issue is the benefit v. risk of refractive surgery for you. With such a small amount of myopia and astigmatism, your net benefit from refractive surgery would be somewhat limited. You undoubtedly can function quite well without any corrective lenses. Although the correction you need is not great, you will be subject to the full risk of surgery (although relatively small). Put simply, you will have all the usual risk of refractive surgery, but a relatively small benefit. It is something to consider and discuss with your surgeon.
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Postby Peter » Thu Jul 20, 2006 9:16 pm

Thank you for the answer!

I am concerned about the risk involved but from what I have heard the risks involved increases with the magnitude of the correction. So that my correction would be less risky than average, isn't it so?

I have another question about something that has bothered me since I visited the clinic.
The clinic where I am going doesn't charge anything extra for using wavefront technique. However, they do NOT recommend it because they say that it's unnecessary. Other clinics, however, where you'll have to pay extra for wavefront they seem to recommend it. Maybe that isn't too surprising but I thought that wavefront always was AT LEAST as good as an ordinary correction and if it doesn't cost me anything extra, so why not? So the question is: can it actually be worse with wavefront than without, in my situation?

At the same time as they say that wavefront is overrated they argue that "aspheric profile" is a technique that has proved to improve the results relatively much. But this "aspheric profile" you don't mention at all on your homepage. So, I wonder, does it really make that much of a difference?

Kind regards,
Peter
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Postby LasikExpert » Thu Jul 20, 2006 9:54 pm

There is a middle range of correction consistently providing the highest probability of a good outcome that is about 2.00 to about 6.00 diopters myopic. This is the area that seems to have the best result overall. Much under 2.00 D myopic and there can be a raised concern of overcorrection into hyperopia (farsighted) vision. Much over 6.00 diopters and you start getting into regression concerns.

Don’t misunderstand. People with less than 2.00D and more than 6.00D can and do have excellent results. It is just that it seems this middle ground does the very best, overall.

Unless you have unusually elevated higher order aberrations (HOA), I think your doctor is absolutely correct that wavefront-guided ablation is not required. Your primary problem is an irregular cornea (astigmatism). That really is a topography problem. A topography oriented ablation would likely provide the desired result. Several recent studies are showing that for patients with irregular topography, a conventional topography-guided ablation may be best.

An aspheric profile relates to how flattened the cornea becomes after surgery. In your case there is really not going to be much flattening of the central portion of the cornea because you don’t need much myopic correction. The aspheric profile will be concentrated on resolving topography related problems - which is your primary concern.

My concern is a heightened probability of overcorrection into hyperopia. It sounds like your surgeon is taking steps to accommodate this concern. There have been anecdotal reports of wavefront-guided ablation overcorrecting when the myopia is very slight. This is another reason to more seriously consider conventional topography-guided ablation.
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Postby Peter » Sat Jul 22, 2006 10:55 pm

Ok, it seems to be a more complicated correction than I first thought.

But let's assume that everything goes fine, are my eyes then as good as the eyes of a person who has had 20/20 vision since birth, or will they always be a bit more sensitive or so even years after the surgical operation?

And what are my odds of having a successful correction? E.g. what is the chance that I end up being farsighted?
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Postby LasikExpert » Sun Jul 23, 2006 2:05 am

What I try to do is point out when a condition indicates an elevation of risk above the norm. In my opinion (which is based partly upon anecdotal information), your unique and individual situation indicates the risk of overcorrection with wavefront-guided ablation is elevated. The risk of overcorrection with conventional ablation is (again, in my opinion) is not elevated, but keep in mind I’m talking about above the norm. The norm always has some risk. Also keep in mind that I’m not a doctor who has examined your eyes. Your doctor’s advice will undoubtedly be better than mine.

If you choose a surface ablation technique like PRK, LASEK, or Epi-Lasik, your corneas will be as much the same as possible after surgery, but there will be a difference. As a general rule, all laser assisted refractive surgery techniques elevate higher order aberrations (HOA) somewhat. The elevation of HOA may not be noticeable and may not negatively affect your vision quality, but that change will exist.

A good way to predict if an elevation of HOA will be problematic is to have a wavefront diagnostic before surgery. If your natural HOA are already elevated above the norm, then surgery may put them over the top into problem territory. The probability of problems is not elevated above the norm if your natural HOA are normal or slightly below the norm. Although it may seem counterintuitive, if your HOA are very low, you actually have a somewhat greater risk of vision that seems problematic after surgery. This is because you have become accustomed to exceptionally high vision quality and even a small amount of increase in HOA may seem problematic. You surgeon can tell you your current HOA and how they compare to the norm.

Because we are talking about microsurgery on biological tissue, it is impossible for anyone to predict with total accuracy what will be your final outcome. Your surgeon would be the one who would be able to provide the highest level of prediction. What I endeavor to do is to inform you of potential problem areas that you need to discuss with your surgeon so you can make the most informed decision possible.
Glenn Hagele
Volunteer Executive Director
USAEyes

Lasik Info &
Lasik Doctor Certification

I am not a doctor.
LasikExpert
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