toric lenses - cylinder and power

[DougLassiter]

I was told by an optometrist that in toric lenses, "cylinder" and "power" are related. Such that when your cylinder (metric for astigmatism) gets bumped up, your power (metric for magnification) sometimes get bumped down. I do optics, but not contact lenses, and that doesn't make a lot of sense. I always though astigmatism and power were pretty much independent. Is that not true?

[Ron_AKA]

Yes, they are independent, but the conventions used in the industry are confusing. Ophthalmologists use positive cylinder and optometrists use negative cylinder. Conversion is not as simple as flipping the sign. The axis flips by 90 degrees, and the sphere changes as well.

And with contacts and in some cases with IOLs cylinder is not corrected, but instead partially compensated by changing the sphere power. Spherical equivalent is used which is the sum of sphere plus 50% of the cylinder. In contacts which require lower amounts of cylinder correction, a non toric can be used with minimal loss in visual acuity.

[John C Hagan III, MD, FACS, FAAO]

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[DougLassiter]

Thanks, that's consistent with what I was told that you can partially compensate one by adjusting the other. From a perspective of optical engineering, that doesn't make a lot of sense, but the definitions here don't seem to relate well to optical engineering.

[John C Hagan III, MD, FACS, FAAO]

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[Ron_AKA]

I look at it like a polar graph. As you go around 360 degrees of the graph the focusing power of the eye stays constant when there is no astigmatism. With astigmatism the power changes as you go around the clock. It increases to a peak, then down to a minimum back to a peak, and down to a minimum again. The peaks and valleys are 90 degrees apart with normal symmetrical astigmatism. The higher the peaks and valleys the higher the cylinder amount. So instead of a pure fixed power lens you have a varying power that is bringing light to a range of focus points rather than just one.

There are pros and cons to astigmatism. When you get older and start to lose the ability to focus nearer this stretched focus point can help with near vision. The con is that visual acuity suffers some. I call it a poor man's multifocal lens. If you have astigmatism you get it for "free".

CooperVision has a calculator on line that converts eyeglass prescriptions to contact lens prescriptions. See link. If you enter -2.0 D for sphere it will suggest non toric -2.0 D contacts. If you enter -2.0 D sphere and -0.5 D sphere it will suggest a -2.25 D non toric lens. This is the spherical equivalent of this prescription.

https://coopervision.com/practitioner/tools-and-calculators/optiexpert/optiexpert-web#/calculator

[DougLassiter]

That makes sense. I have in one eye, strong astigmatism, and was prescribed a high cylinder value (-3.75). These lenses refused to gravitationally orient, and I could see well only a fraction of the time. I understand this is a common issue with high cylinder lenses. I backed off on the cylinder, and now I have 100% vision, but the astigmatism is not completely corrected in that one eye. But as you say, it's sort of multi-focal, so the clear image is in there, but overlaid by a slightly less clear image. This works for me.  

[Ron_AKA]

When you get to the age that cataract surgery is necessary a toric IOL should be able to achieve a better correction than you can get now with contacts. With the natural lens still in place the overall astigmatism is the vector sum of the astigmatism in the lens plus the astigmatism in the cornea. After cataract surgery any astigmatism in the lens is gone, and it is only necessary to correct the remaining astigmatism in the cornea. Unless the lens and cornea have offsetting astigmatism, there will likely be less astigmatism to correct with the IOL. Standard toric IOLs can correct up to 4-5 D of cylinder.

So, getting cataracts may not be all bad news. There can be upsides...

[John C Hagan III, MD, FACS, FAAO]

Doug: You mean you tried contact toric lens and they would not orient. With glasses that is not a problem since the frame holds the cylinder lens in the proper orientation.  Have you looked into laser treatment of the cornea or LASIK or one of the newer hybrids. The technology is quite refined at this point.

[John C Hagan III, MD, FACS, FAAO]

You are right, they are independent variables, and changing one does not automatically change the other