Astigmatism
Posted , 4 users are following.
A recent "corneal wavefront" test showed these measurements for my cataract eye:
- WTR/ATR Astig -0.772 μm
- Oblique 2nd Astig -0.325 μm
- WTR/ATR 2nd Astig -0.557 μm
- Total astigmatism -1.30 D
I looked up "with the rule" and "against the rule", but I have some questions:
- Why are WTR and ATR apparently combined in the same measurement? Aren't they very different? Or maybe it's just one astigmatism, and it's not important to specify whether it's ATR or WTR?
- How does total astigmatism number relate to the others?
- How do the micrometers (μm) relate to the diopters?
- Will I need a toric lens? Which of the above numbers, if any, would determine that?
- Is this amount of astigmatism generally corrected with an IOL? With a Light Adjustable Lens?
Any answers or recommended reading on the subject would be appreciated.
0 likes, 9 replies
trilemma phil09
Edited
I am not familiar with the other factors, but the LAL can adjust out as much as two, and maybe three, D of astigmatism. Regular toric IOLs can do that also, but those will have lower resolution. I would be saddened if this were treated with a non-toric regular IOL.
phil09 trilemma
Edited
Thanks trilemma. My concern is that my primary astigmatism at -0.772 μm is too small, rather than too large for toric IOL correction. I see a lot of statements that toric lenses are recommended only when astigmatism exceeds 1.00 D or 0.75 D. My impression is that both enVista and LAL may be able to correct smaller amounts of astigmatism, but perhaps there would be some additional risk (lens rotation in the eye?) and relatively little benefit in getting the astigmatism corrected.
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I have been told that the secondary astigmatisms are not correctable by a toric lens, nor by LAL, and that LASIK or similar surgery is the only available correction for them. So, I'm focusing mainly on the primary astigmatism, which is listed at only -0.772μm.
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I did find a μm-to-diopters conversion calculator on the web, but it says that my -0.772 micron astigmatism is equal to about 1.3 million diopters 😃.
RonAKA phil09
Edited
I have never seen astigmatism reported in that manner. The only number that makes some sense to me is the Total Astigmatism of -1.30 D. On an eyeglass prescription that would be reported along with an angle (axis). The eyeglass numbers are an obvious over simplification of the astigmatism especially if it is irregular astigmatism. Since it is described as a corneal wavefront it would seem this is a cornea only astigmatism, and if so -1.30 D is certainly enough to correct with a toric lens for the best vision.
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In the optometry field I think the measurement conventions for astigmatism are a disaster. First, ophthalmologists use positive cylinder while optometrists use negative cylinder. To convert requires reversal of the cylinder sign, the addition or subtraction of 90 deg from the axis, and the subtraction (I think) of the cylinder from the sphere. A dog's breakfast! I use an on line conversion tool to do it.
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The other goofy part is the with the rule and against the rule making no intuitive sense. What rule??? The google answer to this is that with the rule means more common. With the rule is more common in children and as one ages the astigmatism is more likely to be against the rule. And, if I have the sign convention correct, in optometrist format against the rule is when the axis is around 90 degrees. With the rule angle is around 0 deg or 180 deg. And there is mixed astigmatism where the angle is oblique at 45 deg. I think viewing the topographical map of the cornea is the best way to visualize astigmatism. It shows how symmetrical it is and the axis orientations. It can be bow tie or hourglass shaped. Don't ask which one is with the rule and which is against the rule? Flip a coin!
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I have no idea how the um get converted to diopters. The bottom line is that none of this matters when you get an IOLMaster 700 measurement. All of this gets measured and automatically input into the IOL Power formula. The surgeon will likely use the Pentacam to get the topographical map to see what they are dealing with, and a second measurement of the cylinder and axis, but they are likely to go with the IOLMaster as I understand it.
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Is this amount enough to be corrected with a toric, then based on the -1.30 D, the answer is yes. Another quirky convention however is that there is a magnitude of the cylinder can be measured at the cornea plane, or the lens plane. The cornea plane is the one commonly referred to and I would assume where the -1.30 D is measured. A higher power is needed at the lens plane compared to the cornea plane (0.75 aprox ratio). If my assumptions are correct an IOL cylinder power of 1.30/0.75 or 1.75 D is needed at the lens plane. The enVista Toric is available with a 1.25 and 2.0 D (and more) for cylinder powers. You would appear to be right in the middle between using the 1.25 and 2.0. Some surgeons believe that you should under correct rather than over correct. Some say you just get it close as you can. When you over correct you flip the axis so for example if you were against the rule, you would become with the rule. Some think that is hard for the patient to adjust to, so they always under correct which leaves the axis the same. Simple stuff eh? But, don't worry. Experienced surgeons make those decisions many times a day.
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The LAL is said to be capable of correcting up to 2.0 D cylinder, so you would be just within that limit. And they can adjust down to 0.25 D steps so they should be able to hit the 1.75 D that you need.
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Keep in mind however that cylinder error has about 50% of the impact as sphere error. It is common for astigmatism to be off by 0.5 D which has an impact of 0.25 D which is at the noise level.
Dapperdan7 RonAKA
Posted
ron, so are you saying they can measure astigmatism at both cornea and lens but commonly refer to the cornea cylinder because the natural lens will be removed? is measuring both of them how they get the residual astigmatism after removal of cataract number?
RonAKA Dapperdan7
Posted
Measuring the astigmatism in the natural lens is somewhat pointless before cataract surgery because the natural lens is surgically removed during the surgery. Eyeglass prescriptions have the total astigmatism of the lens and cornea, but do not separate them. All that counts is the astigmatism in the cornea as that remains unless you get Lasik or PRK to correct it. When you use a toric lens you attempt to neutralize the astigmatism in the cornea by having the reverse of it in the IOL. But because of the location you need more cylinder in the IOL than you have error in the cornea.
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It is common to have some residual astigmatism even when a toric is used. Toric lenses come with quite large steps in cylinder and seldom will there be a perfect match. And, the effectiveness of the toric correction is dependent on how close the angle of the lens is to the needed angle. From Alcon: "the correction effect of the AcrySof toric is reduced by 10% for every 3 deg rotation off-axis. For the lens to lose its full effect, it would have to be off-axis by 30 deg."
phil09 RonAKA
Posted
Rotation is one of the many things I worry about. I am getting the cataract surgery on my near-vision eye only next year, and one surgeon told me that astigmatism is always less of a problem in a myopic eye. Also, I am anticipating a significant likelihood that I will need/want a LASIK correction for sphere after placement of my artificial lens, and astigmatism could also be addressed at that time if needed. I figure an IOL is subject to rotation, but my cornea is not.
So based on all that, should I consider a non-toric lens? Of course I will consult with my doctor, but the laymen's opinions I get here are very helpful in preparing for such discussions.
RonAKA phil09
Edited
Based on my experience I would not recommend getting a non toric lens if -1.3 D of cylinder is expected. I think you should take the best shot possible in getting both the sphere and cylinder corrected with an IOL. I would also target a little on the high side for SE, which I believe you are planning to do.
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Your thinking was the same as mine when I was getting my near eye done. My predicted astigmatism was 0.75 D and was irregular. The surgeon would not make a recommendation either way as to whether or not a toric would be of value. He went on to say that if I got a non toric I could always get Lasik to correct whatever I ended up with for cylinder. This made sense to me as the error was in my cornea and Lasik corrects the cornea. Seems like it is a root cause correction, not a cover up with IOL cylinder. I went for a non toric and it turned out Lasik was not a viable solution. This is why. He targeted my desired -1.50 D on a spherical equivalent basis which included the 50% contribution from my astigmatism and hit it almost exactly. The 0.75 D of astigmatism turned out to be a problem as I had drop shadows on letters. When I went to the Laser clincs I was essentially told that they could correct the astigmatism, but I would lose the near vision addition of 50% of the 0.75 D of cylinder. That would put me back down to near -1.0 D and not give me not enough reading vision to be eyeglasses free for near. And they further said they could reduce but not increase sphere myopia. I had boxed myself into a corner.
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So, my thoughts are that you consult with a Lasik specialist surgeon before you finalize your plan for cataract surgery and come up with a strategy. It could be as simple as targeting your outcome on a pure sphere basis using a non toric (not spherical equivalent) and then correct the astigmatism only after surgery. But, I would get the Lasik specialist to endorse that plan. My surgeon despite being a well recognized university teaching professor, obviously did not understand what could be done with Lasik, and misled me on what was possible.
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Now on hindsight I really wish I had opted for the toric lens, but I can't go there without an explant, which I am not willing to do.
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Hope that helps some.
phil09 RonAKA
Posted
Yes, very helpful, thank you.
I think I first need to better understand the corneal astigmatism measurement, whether from the wavefront scan or IOLmaster. The best course of action may be different if the correctible portion of the astigmatism is only 0.772, rather than the 1.30 total.
I gather you are thinking the risk of unwanted lens rotation during or after surgery is small, correct?
RonAKA phil09
Edited
There is often discussion of higher order aberrations. Possibly that secondary astigmatism is what that is. It is claimed that custom Lasik sometimes called wavefront guided, or topography guided, could correct higher order aberrations.
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I don't know how one can realistically convert that um number to diopters. I tried a converter and got the 1 million plus diopters that you got. It is obviously not valid. I would wait for the IOLMaster 700 measurements to get a realistic prediction of your sphere and cylinder outcomes. For sure I would get the IOLMaster 700 that includes the TK feature to measure both the anterior and posterior surfaces of the cornea. That will go some way to getting more accuracy from the power formulas and in particular the Barrett True-K that uses the TK numbers directly.
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One of the most important steps in the cataract surgery procedure is to ensure you get the 24 hour post surgery exam. At that time they will check the angular position of a toric, and if necessary they will go in and adjust it. The incision will still be open at that time. Overall I think the risk of toric lens rotation is very low if that inspection/adjustment is done.