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Medical Forum / General / Vision / June 2006Toric lens and accomodation
A quick query, anyone know if while wearing a toric lens, does it alter the eyes accomodation abilities? TIA Gary >A quick query, anyone know if while wearing a toric lens, does it alter the >eyes accomodation abilities? I'm not an eye doctor, but ... I do wear toric lenses. My first question would be "Why?" It seems like you're experiencing something, and you're trying to understand what's causing it. If so, perhaps you should elaborate. What's the issue you're having? What's your prescription? What's your age? These might lead you to a better answer.... Neil > My first question would be "Why?" It seems like you're experiencing > something, and you're trying to understand what's causing it. If so, > perhaps you should elaborate. The reason I ask is that I'm presbyopic and my close focussing abilities vary between wearing contact lenses and glasses. It's better with the latter. I've assumed this is purely because of the physical distance from the eye when being corrected by glasses. Or does/can a toric contact lens constrict the eye's accomodation abilities? Regards Gary >> My first question would be "Why?" It seems like you're experiencing >> something, and you're trying to understand what's causing it. If so, [quoted text clipped - 3 lines] >vary between wearing contact lenses and glasses. It's better with the >latter. Contact lenses REDUCE the amount of accommodation that a FARSIGHTED person needs. They INCREASE the amount of accommodation that a NEARSIGHTED person needs. It *could* be just what you alluded to below (known as the "vertex distance"). IF the vertex distance wasn't factored in appropriately when either or both your glasses or contact lenses were made, then you would effectively have different prescriptions. If that's the case, you really *don't* have an apples-to-apples comparison between specs and contacts. You might want to get back with your optometrist to see if your contacts need the prescription tweaked. Depending on your Rx, it could be that you're due for reading glasses *over* the contacts. Hopefully, one of the docs can sweep up behind me if need be ;-) >I've assumed this is purely because of the physical distance from the eye >when being corrected by glasses. Or does/can a toric contact lens constrict >the eye's accomodation abilities? > >Regards >Gary >Contact lenses REDUCE the amount of accommodation that a FARSIGHTED >person needs. They INCREASE the amount of accommodation that a >NEARSIGHTED person needs. ...compared to eyeglass lenses. In addition, the prismatic effect (induced base-in prism) with minus power lenses assists with convergence. Regards, Robert Martellaro ~~~~~~~~~~~~~~~~~~ Optician/Owner Roberts Optical ~~~~~~~~~~~~~~~~~~ "If a million people believe a foolish thing, it is still a foolish thing." - Anatole France > Contact lenses REDUCE the amount of accommodation that a FARSIGHTED > person needs. They INCREASE the amount of accommodation that a > NEARSIGHTED person needs. I thought it was the opposite, Highly myopic individuals need to accomodate slightly more with their contact lenses and vice versa when presbyopic. Base in prism when reading with spectacles also help. Roland Izaac > > [quoted text clipped - 16 lines] > > I think you guys are comparing apples to oranges. Myopes have to accommodate more in full distance Rx glasses than they do in full distance Rx contacts, although certainly less when not wearing any correction. Hyperopes have to accommodate less in their full distance Rx glasses than they do in full distance Rx contacts, although certainly more when not wearing any correction. Now is that perfectly clear? w.stacy, o.d. >Myopes have to accommodate more in full distance Rx glasses than they do >in full distance Rx contacts, although certainly less when not wearing [quoted text clipped - 5 lines] > >Now is that perfectly clear? As mud ;-) I can't find the two original cites that I used to post, but ... here's a quote and the cite from which I pulled it: "The accommodative demand on hyperopic patients is less with contact lenses than with spectacle lenses, and so they are more readily accepted" FROM: Fitting youngsters with contact lenses From children to teenagers http://www.optometry.co.uk/files/16b584a2c9ab1d496644a3d8e3b2184c_bansai20040924.pdf OR: http://tinyurl.com/mxxsf Interesting, and only tangentially related is: http://aglasser.opt.uh.edu/publications/KV&G2003.pdf I also found this: "In this respect, it is interesting that wearers of glasses accommodate less and have lower convergence demands than emmetropes or wearers of contact lenses due to optical reasons" [presumably implying the prism effect of the spectacle lenses, BUT ... CL's (primarily RGP's) have this, too, no??] FROM: http://www.agingeye.net/myopia/3.2.1.2.php Which seems to cite this: http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=1175259&blobtype=pdf OR: http://tinyurl.com/zj67f Now THIS should clear it right up ;-) >I also found this: > >"In this respect, it is interesting that wearers of glasses >accommodate less and have lower convergence demands than emmetropes or >wearers of contact lenses due to optical reasons" [presumably implying >the prism effect of the spectacle lenses, Hyperopes wearing glasses have the highest accommodative requirements, emmetropes and contact wearers have lower accommodative requirements, and myopes wearing glasses have the lowest accommodative requirements. The prismatic effect is a separate issue from vergence considerations. For example, look at three eyes, a -5.00DS myopic eye, an emmetropic eye, and a +5.00DS hyperopic eye, all gazing at an object at a distance of one third of a meter, with the spectacle plane (except for the emmetrope) 15mm from the eye. Using U + D = V (for a thin lens system U is the vergence of the light entering the system (U=1/u where u is the measure from the lens to the object. D is the dioptric power of the lens and V=1/v is the vergence of the light in image space as it leaves the lens) the -5.00D eye (vergence power entering the spectacle lens = 3.15D, leaving the lens -8.15D, and at the cornea -7.25D), (lens power -4.65 at the cornea), requires +2.60D of accommodation (-4.65 - (-7.25) , the emmetropic eye +3.00D, and the +5.00D eye +3.50D of accommodation (+5.40 - (+1.90). Regards, Robert Martellaro ~~~~~~~~~~~~~~~~~~ Optician/Owner Roberts Optical ~~~~~~~~~~~~~~~~~~ "If a million people believe a foolish thing, it is still a foolish thing." - Anatole France >>I also found this: >> [quoted text clipped - 20 lines] >(-7.25) , the emmetropic eye +3.00D, and the +5.00D eye +3.50D of accommodation >(+5.40 - (+1.90). I *think* I'm following you, Robert, and/but I'm very much intrigued. When you speak of the vergence, or prismatic effect, even of a thin lens, how is it calculated whether the prismatic effect is BI or BO? Is it purely a function of whether the lens is minus or plus?? If so, then is *that* the basis for the delta in accommodative demand introduced by spectacles as different between myopes and hyperopes? I'm curious because--if I DO understand you correctly, and--for example--my spectacles (which do NOT have "added" prism correction) INDUCE prism that is the *opposite* of what I need (inducing BO, for example, when I'm exotropic), then I would need to force additional accommodation simply to compensate for the inherent prismatic effect of the thin lens, no? Do I have that right? If so, and if it is such a readily quantifiable issue, then couldn't/shouldn't compensatory prism be introduced in spectacle lenses--particularly higher power--to offset this effect and minimize "spurious accommodative load?" Thanks much, Robert. > When you speak of the vergence, or prismatic effect, even of a thin > lens, how is it calculated whether the prismatic effect is BI or BO? I think Robert was using the term vergence to describe spherical refractive power as opposed to prismatic effects. The term is confusing because convergence can mean light focused by a plus lens, and it can also mean turning the eyes inward (binocularly). At any rate, you can tell if prismatic effect is base in or out by looking at the profile of the lens. If it is a plus lens and you are converging binocularly toward a near object, you are looking through base out prism (the centers of your lenses are thick, the nasal edge is thin, so your induced prism is base out). Just the opposite for minus. Plano lenses have zero prism induced, no matter where you look. > Is it purely a function of whether the lens is minus or plus?? If so, > then is *that* the basis for the delta in accommodative demand > introduced by spectacles as different between myopes and hyperopes? Nope. That effect is due to the differences in effective powers of plus and minus lenses that are placed a finite distance from the eye, e.g. spectacle lenses. > I'm curious because--if I DO understand you correctly, and--for > example--my spectacles (which do NOT have "added" prism correction) > INDUCE prism that is the *opposite* of what I need (inducing BO, for > example, when I'm exotropic), then I would need to force additional > accommodation simply to compensate for the inherent prismatic effect > of the thin lens, no? If your lenses are plus, and you converge toward something near, you indeed get a certain amount of base out prism. But if you added compensatory prism for this, it would give you more base in prism in the primary position, something you might or might not want. > couldn't/shouldn't compensatory prism be introduced in spectacle > lenses--particularly higher power--to offset this effect and minimize > "spurious accommodative load?" If you are exo, you might indeed want some base in prism. The effect of converging to a near object means that whatever base in prism you have, it will be effectively less and less as you bring the object nearer and nearer. Again, I don't think it has much to do with accommodation. w.stacy, o.d. >> When you speak of the vergence, or prismatic effect, even of a thin >> lens, how is it calculated whether the prismatic effect is BI or BO? [quoted text clipped - 39 lines] >it will be effectively less and less as you bring the object nearer and >nearer. Again, I don't think it has much to do with accommodation. Thanks, Doc. Interesting ... and ... good stuff. I would have to think, though, that virtually any unintended/unwanted intervention into the near vision triad should have some impact on accommodation, no? Whether it's undercorrecting a hyperope for reading to ease CI or adding plus to a near-point esophore... Aren't these strategic interferences with the near vision triad to "strike the balance?" I my case (always eso-, but exo- 8yrs post-op), that inherent BO prism in a high-plus lens might just *be* a bad thing. Might be something for others with strong Rx's and/or alignment issues to consider. From what you're saying, it would *seem* that ... to be *really* properly corrected for a phoria/tropia ... the practitioner would have to add the combination of the findings from the prism bar exam AND the prism inherent in a stronger spectacle lens. I'm guessing that this latter could easily amount to several diopters, no? Thanks again. >I would have to think, though, that virtually any unintended/unwanted >intervention into the near vision triad should have some impact on >accommodation, no? Whether it's undercorrecting a hyperope for >reading to ease CI or adding plus to a near-point esophore... > > Well sure, but that's just straightforward math: if you add plus at near, accommodation is relaxed by exactly the amount of the addition, and vice versa, assuming there is accommodation (phakic eyes under 55 years old or so). >Aren't these strategic interferences with the near vision triad to >"strike the balance?" [quoted text clipped - 3 lines] > > Agreed, and it is correctable as Robert pointed out. >Might be something for others with strong Rx's and/or alignment issues >to consider. From what you're saying, it would *seem* that ... to be [quoted text clipped - 6 lines] > > Assuming you mean prism diopters, it's dependent on Prentice rule as Robert pointed out, easily calculable from power and decentration. Unfortunately, you can't easily do different p.d.s for distance and near in one lens. w.stacy, o.d. I just need to be sure that this piece wasn't overlooked in my case, and ... to raise it, generally, for other outliers like myself ;-) The glasses that I wear for distance ... when I'm NOT in cl's ... IIRC don't have any added prism. At my Rx, though, they may be imparting a fair bit of BO ... unintentionally ... and detrimentally. The last thing *I* need is to be exo-. If this issue is a contributing factor, I'll simply want to raise it with my ophth. For my particular case, this *could--* at least in part--explain why I seem to fatigue much more quickly with my glasses than with cl's. You know ... despite our buddy, Otis constantly posting things like "It is not hard to be "smarter" than a man who only requires 2 months of FUNCTIONAL training to put a kid into an excessive minus." ... you guys really have to juggle an amazing number of parameters ... all delicately balanced (and the associated geometry, math, and physics) ... to achieve optimal outcomes. I'm starting to wonder if ... maybe ... Otis isn't wrong after all ;-) Thanks again. >I just need to be sure that this piece wasn't overlooked in my case, >and ... to raise it, generally, for other outliers like myself ;-) [quoted text clipped - 4 lines] > > You can avoid that by having a pair made up with the optical centers set to match your monocular p.d.s at near. However, this will induce a fair amount of base in prism when you are looking at far objects through them, which if you are exo may not bother you. But another problem with unwanted prism is chromatic aberration. For this reason you might want to split the difference and set the o.c.s between your distance and near mono p.d.s. w.stacy, o.d. >>>I also found this: >>> [quoted text clipped - 24 lines] >When you speak of the vergence, or prismatic effect, even of a thin >lens, how is it calculated whether the prismatic effect is BI or BO? Prentice's rule. P (Prism) = (distance from optical center in cm)(power of the lens in diopters). If the gaze is nasal of the optical center or prism reference point, the prism induced is base-out for plus power and base-in for minus power. Think of a plus lens as two prisms with their bases joined at the middle and minus as having their apexs joined at the center. >Is it purely a function of whether the lens is minus or plus?? If so, >then is *that* the basis for the delta in accommodative demand >introduced by spectacles as different between myopes and hyperopes? There are two issues here, vergence power, and fusional vergence, the latter is related to the prismatic effect from induced base-in or base-out prism. >I'm curious because--if I DO understand you correctly, and--for >example--my spectacles (which do NOT have "added" prism correction) [quoted text clipped - 4 lines] > >Do I have that right? It's not accommodation per se, the base-out prism puts strain on the negative fusional vergence system of the eyes. The differences in accommodative requirements between plus and minus lenses are related to vergence power discussed above. >If so, and if it is such a readily quantifiable issue, then >couldn't/shouldn't compensatory prism be introduced in spectacle >lenses--particularly higher power--to offset this effect and minimize >"spurious accommodative load?" The eyes fusional reserves usually handle this quite well, if there are no vergence disorders present. Prism can be placed in the near zones buy using press-on prisms, prism segs, decentering the segs for prism, or separate glasses for near only. >Thanks much, Robert. Your welcome. Robert Martellaro ~~~~~~~~~~~~~~~~~~ Optician/Owner Roberts Optical ~~~~~~~~~~~~~~~~~~ "If a million people believe a foolish thing, it is still a foolish thing." - Anatole France >>>>I also found this: >>>> [quoted text clipped - 64 lines] >press-on prisms, prism segs, decentering the segs for prism, or separate glasses >for near only. Thanks again, Robert, but ... maybe only in my case (accommodative and binocular dysfunction) ... wouldn't one--out of an abundance of caution ... want to factor in the inherent BO prism in a strong plus lens? I seem to be a good 10-12d exo- at near now (by measurement over the contact lenses). I would imagine that prescribing 12d of BI prism in reading glasses (no cl's) might *undercorrect* the exo- exactly because of what you describe above then, no? If so (as I referenced in my post to Bill Stacy), then would(n't) it be prudent to factor in this inherent prismatic effect when prescribing (at least in a case like mine)? Realistically, I'm thinking that ... since we're likely talking about people with a high degree of hyper/myopia ... then even IF there are (as you say) no vergence disorders present ... this sounds like a rather non-productive amount of "risk" with the only reward being slightly thinner lenses (that can--as *you* surely know) be mitigated with appropriate choice of materials. I feel like this is another "vertex distance" issue in higher powers and vergence issues: something that may not get factored into the equation consistently and accurately, but something that could dramatically impact outcomes. Follow? Thanks again. You're all nuts! Any eye corrected to focus infinity unaccommodated will need to accommodate 2 diopters to focus text at 0.5 meter, etc. "Toric" does not change anything, except for being another one of your misty concepts. -- Dicky > You're all nuts! > [quoted text clipped - 3 lines] > "Toric" does not change anything, except for being another one > of your misty concepts. The topic shifted. You simply didn't keep up. The topic shifted into the relative accommodative demand induced by: Glasses in hyperopes; Glasses in myopes; Contact lenses in hyperopes; Contact lenses in myopes. > The topic shifted. You simply didn't keep up. > [quoted text clipped - 4 lines] > Contact lenses in hyperopes; > Contact lenses in myopes. Yeah, well, I really got messed up by that relative accommodative demand inducement shift. You are really getting the hang of this stuff, Neil Brooks! Have you ever considered going to Professional School? Maybe you could rise to do something important in the interest of mankind, like writing speeches for the President, for instance. -- Dicky Thanks Robert, I have stated this fact some time ago and asked that Otis relates this to his theory. Otis was suprised at my statement and said that there should be no difference between the accomodation of a high myope wearing galsses and that of an emetrope. He also feels that distant objects are brought closer when wearing the dreaded minus lens. and therefore increases the accomodative stress. Can you clarify this for us. Are objects brought closer then the myope's far point when looking at distant objects? Thanks again Roland J. Izaac > >I also found this: > > [quoted text clipped - 30 lines] > "If a million people believe a foolish thing, it is still a foolish thing." > - Anatole France I disagree with you William. Myopes do not accomodate more in full distance Rx glasses when compared to full distance Rx contacts. The fact is they accomodate less, the higher the Rx, the greater the vertex distance, the lower the accomodative demand. Therefore Myopes when wearing glasses accomodate the least, even less then emetropes. Hyperopes are just the opposite. So OTIS, please understand what I am saying here, The bases for your theories is wrong. And by the way, how are objects brought closer when the minus lens brings distant objects to focus at the patients far point, not any closer. Do you understand that at this far point the patient does not need to accomodate. I am expecting a rebutal from you Otis, Oh wow, I'm kidding myself, Otis never answers. Previous Post:"I thought it was the opposite, Highly myopic individuals need to accomodate slightly more with their contact lenses and vice versa when presbyopic" CORRECTION TO PREVIOUS POST. I meant to say hyperopic, not presbyopic. I appologise for the confusion. Philip D Izaac wrote: I thought it was the opposite, Highly myopic individuals need to accomodate slightly more with their contact lenses and vice versa when presbyopic. Base in prism when reading with spectacles also help. Roland Izaac I think you guys are comparing apples to oranges. Myopes have to accommodate more in full distance Rx glasses than they do in full distance Rx contacts, although certainly less when not wearing any correction. Hyperopes have to accommodate less in their full distance Rx glasses than they do in full distance Rx contacts, although certainly more when not wearing any correction. Now is that perfectly clear? w.stacy, o.d. Yes, you will accomodate. If you are myopic and remove your glasses to read, you are not accomodating fully. After Wearing a corrected contact lens prescription, you should begin to accomodate normally again. The same applies to lasik. If you are hyperopic (farsighted), your are accomodating too much already. A corrected toric lens will have you accomodate less. -Dr. Tom > A quick query, anyone know if while wearing a toric lens, does it alter the > eyes accomodation abilities? > > TIA > Gary > A quick query, anyone know if while wearing a toric lens, does it alter the > eyes accomodation abilities? > > TIA > Gary The short answer is "no". If you're experiencing difficulty with close focussing while wearing a toric lens it could be due to the lens being a slightly different strength to your glasses or various other factors. In my opinion the difference in convergence required between specs and contact lenses isn't really significant but I know that others disagree. Dom |
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