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Medical Forum / General / Vision / October 2006RGP residual astigmatism
I've got my RGP's dialed in pretty well. The doc "maxed out" the diameter and the OZ and fixed most of the trouble I was having with night vision issues and seeing the edges. The last small complaint I have is that the vision is slightly variable in one eye (tack sharp perfect in the other). More often than not, I think I have about 0.25D of astigmatism remaining, which I can occasionally blink away, and that seems to come out more when my eyes are more dry. I intend to ask my doc the same questions, but to the docs on the list, what do you think might be causing this, and are there things that can be adjusted to try and fix it? I don't think I had this when the lenses were smaller (but the problems with them were far more annoying). My layman's guess would be that maybe the larger lenses may cause this if they cover more severe astigmatism toward the edge of the cornea, and flex around it (?). Might a stiffer lens material help? -- > I've got my RGP's dialed in pretty well. The doc "maxed out" the > diameter and the OZ and fixed most of the trouble I was having with [quoted text clipped - 11 lines] > cause this if they cover more severe astigmatism toward the edge of the > cornea, and flex around it (?). Might a stiffer lens material help? Geez, you ARE demanding, eh? Are you down to 20/12 yet? Ready for wavefront? Meanwhile...... Your theory of the lens flexing and using a stiffer material is a definite consideration. If your lens is not already aspheric optically, you might try that. If it is so, then you might want to *reduce* the asphericity in the periphery leaving the OZ the same. LB, O.D. > Geez, you ARE demanding, eh? Are you down to 20/12 yet? Ready for > wavefront? Meanwhile...... [quoted text clipped - 5 lines] > > LB, O.D. Just for sake of me learning something, in what way might this help address residual astigmatism? -- Sorry Charles your layman's theory is wrong. Residual cyl is from lenses flexing or lenticular astigmatism. Larger lenses will do nothing for the cyl. Depending on whether your cyl is central or peripheral, a spherical lens may correct the cyl or you may need a back surface toric if the toricity extends to the limbus. I would not bother to correct the additional -0.25, it is pretty well irrelevant and will not improve vision any in most cases. grant > Sorry Charles your layman's theory is wrong. > > Residual cyl is from lenses flexing or lenticular astigmatism. Larger > lenses will do nothing for the cyl. Depending on whether your cyl is > central or peripheral, a spherical lens may correct the cyl or you may > need a back surface toric if the toricity extends to the limbus. Are we saying the same thing then? Lens flexure? I was just trying to understand why I didn't have this with the small lenses. Of course, having changed docs in-between, I don't know what else also changed. > I would not bother to correct the additional -0.25, it is pretty well > irrelevant and will not improve vision any in most cases. Well, I guess that's subjective. I can read the 20/20 line with both eyes, but I always have my "perfect" right eye to compare against, which makes the blur in the left noticeable. With both eyes open, I can tell I'm leaning on my right eye to make out street signs far away. I'm not complaining, but if I can dial it in, why not? -- Charles schreef: > More often than not, I think I have about 0.25D > of astigmatism remaining, which I can occasionally blink away, and that > seems to come out more when my eyes are more dry. Residual not corrected astigmatism of 0,25 diopters when wearing RGP's is a marvelous result. Theoretically your may try to compensate it with a toric lens if you are wearing a spherical one now but the outcome is more worse, believe me. Why? The toric lens gives a perfect view in just one position, however when the lens rotates out of this exact position you create a so called crossed cylinders situation which create another optical error. Also the used optical zone gets an oval shape and this also can lead to more problems. I almost forgot to mention the needed prisma ballast, another possible complain issue. And as a result you, Charles, are the one that shall complain even more as you are already. Jan (normally Dutch spoken) > Charles schreef: > > More often than not, I think I have about 0.25D [quoted text clipped - 14 lines] > > Jan (normally Dutch spoken) Agreed, toric is not even on the table for only 0.25D. The funny thing is that the eye with the residual error is the one with 0.5D less astigmatism in the glasses Rx. -- Charles schreef: > Agreed, toric is not even on the table for only 0.25D. I can order such a lens, no problem. > The funny thing is that the eye with the residual error is the one with > 0.5D less astigmatism in the glasses Rx. Could be quite normal, depends on the cornea curvature related to the error in refraction (your astigmatism). If the astigmatic curvature of the cornea is optically speaking corresponding exactly with the prescribed cylinder, you can compensate the amount off astigmatism about 90%. If the two do not match you have to deal with the rest astigmatism caused inside the eye. In this way you can imaging a spherical prescription for glasses and a toric one for a RGP contactlens. (toric cornea versus spherical error in total refaction) Jan (normally Dutch spoken) > Charles schreef: >> More often than not, I think I have about 0.25D [quoted text clipped - 17 lines] > And as a result you, Charles, are the one that shall > complain even more as you are already. Not sure if this is even related but I had 0.25 astigmatism in my right eye when I started with my RGPs over 6 months ago. I had him do another refraction last week and it seems it's gone now. No change in prescription or noticeable difference in vision, but it's gone (orthok effect I guess?) -Quick > Not sure if this is even related but I had 0.25 astigmatism > in my right eye when I started with my RGPs over 6 months [quoted text clipped - 3 lines] > > -Quick Good data point. I was wondering if it might go away with time. -- Charles schreef: >> Not sure if this is even related but I had 0.25 astigmatism >> in my right eye when I started with my RGPs over 6 months [quoted text clipped - 5 lines] > > Good data point. I was wondering if it might go away with time. Small changes back and forth are quite normal over a longer period of time. And yes, RGP lenses changes the cornea a little bit due to the massage effects and the little metabolism changes when changing from wearing RGP lenses to not wearing those. This is one off the reasons why you can not prescribe exact correcting spectacles for wearing occasionally next to this type of lenses. Jan (normally Dutch spoken) > Small changes back and forth are quite normal over a longer period of > time. And yes, RGP lenses changes the cornea a little bit due to the [quoted text clipped - 6 lines] > > Jan (normally Dutch spoken) After only a month or so in my new lenses, my astigmatism is almost totally gone between wearings. In well lit places, I have perfect vision with no lenses (I was as high as -1.5D cyl in one eye). In the dark it's not as good, but still probably less than 0.5D I'd guess. The reshaping must be more pronounced in the center of the cornea. This would make sense since my dark night vision is still imprefect even with the lenses on (apparently I have a large pupil). It's worked so well that I asked again whether overnight ortho-k was an option. If it works this well as an accidental side effect, shouldn't it work better if you design for this effect on purpose? The answer still came back "no" though. -- |
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