A good consultation, and a new recommendation - enVista monovision

An interesting quotation from Dr. Hill reported in Review of Ophthalmology:

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• What about eyes with prior refractive surgery? For these eyes, Dr. Hill recommends using the ASCRS post-refractive online calculator, created with Li Wang, MD, Ph.D. and Douglas Koch, MD, at iolcalc.ascrs.org. “Many different measurement devices can be used with these eyes, but the Lenstar, the IOLMaster and the Zeiss Atlas topographer have the greatest overall utility,” he says. “We do this type of calculation every day and get the best results using the Barrett True K method.”

Prior refractive surgery is a special case and needs extra care. I think one would be advised to go with the LAL or at least a surgeon that is experienced in dealing with prior refractive surgery eyes to get the best accuracy.

I have not looked at it for a while but there is a video that goes into a lot of detail on the Hill formula. I don't think it considers prior refractive surgery though. If those factors were to be included in the AI database I think it would be a significant step up in working with the more difficult eyes to measure. But then again perhaps the issues are more in the variability of the eye measurements, than in the details of the formula. Google this - Introducing Hill-RBF 3.0: Improving Refractive Outcomes

The best way to avoid the potential "thumb on the scale" issue is to ask to see the IOL calculation sheet, and then agree with the surgeon what power of lens is to be used. You will see the predicted outcome of the potential lenses powers that could be used.

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The other thing to consider is that studies have found some IOL Calculation formulas are more accurate when targeting intentional myopia. This study from 2021 found that the Barrett Universal II was the most accurate, and the Hill-RBF 2.0 was second best for accuracy. Since that time the Hill-RBF formula has been updated to version 3.0, and based on other studies is probably more accurate than the Barrett formulas.

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So the bottom line is to ask for a copy of the IOL Calculation sheet and agree ahead of time what power of lens is to be used, so there is no misunderstanding.

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Google this for the study:

OPTIMIZING OUTCOMES WHEN THE TARGET IS LOW MYOPIA

BY ANDREW M.J. TURNBULL, BM, PGDIPCRS, FRCOPHTH; WARREN E. HILL, MD; AND GRAHAM D. BARRETT, MB BCH SAF, FRACO, FRACS PDF

Thanks, RonAKA. The conclusion I take away from this study is that there's a roughly 25% chance of an error greater than 0.50 D, when targeting reading vision, as I plan to do. The formulas studied range from about 15% to 35% probability of such an error.

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And eyes like mine, having had prior refractive surgery, are not even included in the study, presumably because they are even less predictable. Perhaps in my case, there's a 25% chance of prediction error greater than 0.75 D. I hope my doctor does a good job, but will want to be prepared for a hefty refractive error.

As Lynda says, there is no evidence one is better than the other. If the surgeon uses the blade then that is what they will be experienced in doing, and there is no concern. Quite frankly I think the laser incision is just a method used to "pad the bill".

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Probably much more of concern is whether or not the surgeon compensates for the astigmatism induced by the incision. Some IOL Calculation formulas have a provision to include that effect. And a further question would be whether they select the incision location to reduce the total astigmatism, or in other words do they use it to offset some of the astigmatism in the cornea that is expected. There may be some practical considerations on to what extent that can be done as the incision also has to be located so the surgeon can easily extract the natural lens and insert the IOL. If you watch a video of a cataract surgery you will see what I mean.

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In my first eye my astigmatism came out a bit higher than expected and I attribute that to surgically induced astigmatism.

Admittedly, my experience is anecdotal, but having cataract surgery with same IOL and same surgeon for both eyes, but due to having surgeries one year apart, first was with blade and second was laser, I noticed significantly less eye irritation and faster recovery in the days after surgery. Irritation and recovery were no big deal with the blade surgery, but definitely better with the laser surgery.

I sent you a private message .

I am getting surgery on one eye. That eye is approx -7 D, with mature cataract. My other eye is approx 20/20 and plano. So, my two eyes right now are kind of like yours will be after you get the cataract removed and correct for distance vision. It doesn't bother me, but ymmv. Good luck!

That is great to know that you were able to handle -7 and 20/20.

I am curious if you were managing with glasses or contacts or either was ok for you?

I never wear glasses or contacts.

I have had monovision from LASIK for many years. Between my two eyes, I could see pretty much everything, without any correction. Over the past four years or so, my near eye developed the cataract which has slowly caused a large increase in myopia. So now I have extreme monovision, and I can still see pretty much everything without correction (though to read fine print, I do need to put it about six inches in front of my nose). I have no significant problem with my current vision, because I have had time to get accustomed to having two very different eyes. I imagine it will feel strange to you when you get your cataract surgery, but perhaps you will get used to it. I think you are probably right to be apprehensive. Hope it works out well for you.

Chris, I had a similar situation as you in that I had -8D or more in both eyes before cataract surgery, and had the surgeries done over a year apart. I had the first (dominant) eye adjusted (RxSight LAL IOL) to plano (last refraction shows -0.125D spherical equivalent factoring in very minor astigmatism), so I had an ~8D or more (myopic) difference in my eyes for over a year until I had the second surgery. I tolerated it very well (I could clearly tell that the presurgical eye had a blurry image, but I still saw a single sharp image at distance and could still read well at 6" or so with my other eye. Clear intermediate vision required a contact lens. However, I had been wearing contacts for years prior, and except during the weeks before my second pre-op where I could not wear a contact lens to make sure eye biometry wasn't affected, I wore a contact lens in my non-operated eye for about 1/2 waking hours most days.

Individual tolerance to monovision (temporary between surgeries or permanent intentional say mini-monovision) varies significantly by person. If you can have a contact lens already fitted for your 2nd eye (the one not getting the cataract surgery for some time) which includes seeing well at distance and near or distance only and plan on needing reading glasses, until the [presumed] cataract in your second eye warrants it, you should be able to avoid monovison tolerance issues.

In fact, you could use the opportunity to test your tolerance for mini-monovision for consideration when you need cataract surgery in your second eye. If instead of targeting plano (distance) for your non-operated eye, you try something like -1.5D, that may be enough to see near and intermediate, while being close enough to your post-surgical plano (distance) eye. That's about what I have now after cataract surgery in both eyes (1.5D difference: mini-monovison). I have excellent distance in all light levels, excellent intermediate and functional near in bright light, and use reading glasses as needed for near in lesser light or for extended reading. Most people tolerate up to 1.5D difference in eyes without noticing the difference when using both eyes. My corrective lens (glasses or contacts) dependence is substantially reduced from pre cataract surgery, though not eliminated.

If you don't tolerate contact lenses well, that could be challenging for you because glasses with that much difference (or only one lens) would be very lopsided and may not be well tolerated.

what IOL do you have in both eyes?

RxSight LAL (Light Adjustable Lens) IOL in both eyes. RxSight has a lot of information on their website about it. Implanted as a simple monofocal lens, but when used for medically indicated purpose (0.75D-2D of corneal astigmatism) ends up a toric lens with refractive properties that can be adjusted post-operatively for tuning (more accurate than surgical targeting) and to trial different vision options (for instance, mini-monovision at different amounts of difference between eyes). Especially useful in the following situations:

1. Reduces risk of rotation of toric lenses, since the cylinder adjustment is added after surgical recovery when the lens should be more firmly implanted.
2. More accurate targeting vs pre-surgical biometry and estimation formulas.
3. Allows trialing different visual refraction options (near, intermediate, distance, mini-monovison, etc) once natural accomodation is already gone (since IOLs are not currently accomodative as a natural lens is, even though the amount of natural accomodation reduces with age).
4. Especially helpful for trialing for someone with advanced cataracts which prevent accurate pre-surgical refractive trials of different options.
5. Doesn't have the light artifacting issues (halos, starbursts, etc) of multifocals or strong EDoFs (a small amount of EDoF is supposedly added for intermediate vision targetting, but it is not asserted in the FDA claims).

If i am selecting a monofocals and like to set to distance, and i am high myopic -11.5 in that eye while -10.5 in the other, what is a reasonable target the surgeon will target? one surgeon suggested -0.2, the second surgeon targets -1.0. Why a big difference? how accurate will target be after surgery?

Given your situation, I would recommend selecting mini-monovison (near plano, say -0.25D in one eye, and as much myopia as you tolerate well and need in order to see reasonably well near, typically up to around -1.5D, in the other).

The reason is that it can be quite disconcerting to simply trade pre-surgical near vision for post-surgical distance vision and become heavily dependent on reading glasses as a result. Mini-monovision, if you tolerate it well, can give you good distance and intermediate vision and some near vision without correction (though that also requires treating any significant astigmatism either surgically or with a toric lens) depending on how much difference you can tolerate and need. It may not eliminate the need for reading glasses post-surgically, but it can significantly reduce it. Ron has a great thread on this forum on the topic, called "The Pros and Cons of Mini Monovision". I recommend reading through that, including all the responses, to decide if it is a good option for you or not.

I suspect the surgeon who recommended -1.0 target is concerned you'd be unhappy with that much loss of near vision if you went for optimal distance only at the expense of near vision.

As to accuracy of surgical tarrgeting, Ron has written a lot on that, in the thread I referenced above and elsewhere in the forum. For simple monofocal lenses, it depends a lot on the skills and experience of the surgeon and the quality of the biometry used. Ron can explain that a lot better than I can.

However, if you are a good candidate for the RxSight Light Adjustable Lens (LAL), the refractiction can be adjusted after surgery to address up to 2D of corneal astigmatism, correct any surgical targeting error, and help you fine tune your vision once you no longer have the cataract interfering with your vision, or whatever accomodation is still left in your natural lens (age related loss of accomodation is presbyopia, but IOLs are generally non-accomodative, so you suddenly lose whatever accomdation you had left).

The direct link to the mini-monovision thread is here: https://patient.info/forums/discuss/the-pros-and-cons-of-mini-monovision-798485

thank you for the explanation and link for Ron's post.

is the target of -0.2 and -1.5 almost true for everyone? i seem to see these numbers for everyone that wants minimono vision despite their before surgery RX.

-0.25D and -1.5D is likely the most common surgical target for mini-monovision, because it provides a local maximum of pros at a somewhat minimum of cons. But it is not the only configuration. There are several considerations and Rob's thread covers most of them.

First off, there is surgical targeting error, whose risk is substantially reduced with a post-operatively adjustable lens, like RxSight LAL. One of the reasons non-LAL distance lens is usually targeted at -0.25D is to account for potential targetting error. Any post surgical +D is unfortunate (except with LAL since it can be fixed) as it hurts distance, intermediate, and near, so by targetting a bit towards near, that risk is reduced.

Next, there is mini-monovision bias towards distance (the above common configuration), near, or intermediate. This is because full monovision would be something like 0D and -2.5D, which gives sharpest reading at distance and near, but may create a blurry intermediate distance and is often not well tolerated due to too much difference between the eyes. Full mono-vision is rarely used any more. Bias would be done by selecting which vision range (near or far) was more important, determining optimal refraction for that and selecting the refractive target for the other eye to not be too far from the bias eye (typically not more than 1.5D apart, but depends on individual tolerance, which you can often approximately test with contacts - only approximate because of natural lens accommodation which disappears with non-accommodative IOLs and any cataract interference). So, for instance standard distance biased mini-monovision might be -0.25D and -1.5D, whereas near biased mini-monovision might be -1D and -2.5D. Basically, similar to the way eye surgeon would ask if you want glasses independence at near or distance, but this way you select distance and intermediate or near and intermediate. You may be lucky and not need glasses for all three, but you definitely cannot count on that. The way to think about mini-monovision is that it reduces glasses dependence compared to straight monofocal single focal length for both eyes, but likely won't eliminate glasses dependence completely. Biasing for near or far determines which distances you're most likely to still need corrective lenses.

And don't forget about correcting any significant astigmatism if you want glasses independence at any distance. LAL takes care of up to 2D of corneal astigmatism as part of adjustments (becomes toric via adjustment), but otherwise requires a toric lens or surgical astigmatism correction unless your corneal astigmatism is small (lenticular astimatism can increase or decrease pre-surgical net astigmatism, but only corneal matters post-surgically).

sounds good! Thank you for such detailed explanation. i gained a lot and will help me in my discussion with my cornea specialist at preop.