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Thread: PAL wearer/anisomyope with vertical imbalance issues...help!

  1. #1
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    Confused PAL wearer/anisomyope with vertical imbalance issues...help!

    To preface: I've been fitting glasses on-and-off for five years; I started my most recent optical job 2.5 years ago. I now work at an optical located inside a medical clinic with a large ophthalmology department. The learning curve has been steep for me, and while I feel competent enough to address most patients' issues, I still feel adrift without a paddle when it comes to issues like recommending multifocal lenses for anisometropes. So, any advice you all can offer is greatly appreciated.

    I saw a patient today who complains of near vision diplopia and general difficulty focusing at objects closer than 6-8 feet. The doctor consulted with me (yikes!) about how to approach prescribing prism. Here's how the patient's Rx came out today:
    OD -0.75 +0.50 x005 +2.00 add, 1.5 PD up
    OS -2.50 +0.50 x163 +2.00 add, 1.5 PD down
    Because of the degree of anisomyopia, I thought he may be on the cusp of needing a slab off. I mentioned this to the doctor, and he recommended it on the prescription.
    Here's the thing: my gut tells me that we should either grind prism or slab off, but not both. I am reluctant to order slab off for this patient, because it severely limits the progressive lens options available, and other complaints he had about his vision could be better remedied by newer/digital PALs (which my labs tell me they can't slab off).

    This patient has never previously been prescribed prism, but in neutralizing the Rx of his current pair of glasses, I found (approximately):
    - no prism at the OD PRP
    - 2 PD up at the OD NRP
    - 1.5 PD down at OS PRP
    - 1.5 PD down at OS NRP
    He has been wearing Comfort 2 DRX in poly since last October. He has not had issues with his distance vision.

    His prescription has not changed very much since last year, and I'm worried that the amount of prism prescribed by my doctor will still create imbalance for the patient since it's not really taking into account the amount of prism induced by a PAL add.

    What are your recommendations? Should I specify a lower OC and go with a digital (camber) lens? Go with the slab-off in a Physio and place it partway down into the intermediate zone?

    Thanks!

  2. #2
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    Welcome, ASW!

    I calculated a vertical imbalance of your patient of 1.79 Diopters/cm. That you're sensitive to it shows you're on the ball, but while I've read in clinical literature that symptoms may start surfacing with as little as 1.5, I've honestly only seen it surface below 2.0 once. Far more often I've seen patients well into the 2.0-3.0 range with no problems. I honestly haven't had much cause to worry unless it's north of a 2.25.

    I would suggest neutralizing the patient's old specs (assuming they've been multicofals as well) and seeing what/how much change the new Rx represents. If it's comparable, and the patient hasn't needed a slab off before, then you can safely write it off.

    If there is an increase of vertical imbalance, then I advise the patient of the issue and risk of going without a slab-off--and the cost of replacing a lens should it turn out to have been needed after s/he declined. They'll appreciate that you warned them of the uncertainty and left the choice to them, and that they were advised of the risks.

    In the case of your patient, I think the prism is more than enough to worry about. High likelihood the doc will need to tweak the Rx anyway. I don't see any appreciable risk warranting a slab-off.

    If the Comfort worked well for the patient, I wouldn't worry about re-inventing the wheel into the digital upgrades. However, a short channel lens may work in the patient's favor...and diminish any risk of vertical imbalance issues as well. Be sure the patient's complaints are really appropriate for a 'digital solution' before reflexively upgrading. asking the patient to adapt to changing models/brands carries its own risks.

    Good luck!
    Last edited by Hayde; 11-10-2016 at 02:02 PM.

  3. #3
    One eye sees, the other feels. OptiBoard Gold Supporter
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    Dear fellow beer town ophthalmic optician-

    Welcome.

    According to your findings, the habitual glasses have 1.5 PD BD OS, likely prescribed. The new Rx will have, depending on the reading depth, roughly 2 PD BD OS of optically induced vertical imbalance. The CC is diplopia at near only.

    The new Rx doubles the prescribed OS BD prism from 1.5 PD to 3 PD. The VI remained the same. Use trial lenses over the glasses and evaluate. I would not recommend slabbing a PAL, especially with prescribed prism.

    Please confirm my conclusions...I'm short on time and may not be able to review this until tomorrow. Also, include the old Rx. Unless there's a change in the dioptric power OD, there's a discrepancy in your findings (BU at near, no prism at the PRP, and little if any power at ninety). (Edit- the near optical center might not be at the NRP, even with a plano distance. It will probably be above the NRP, and would account for the BU prism you measured).



    Hope this helps,

    Robert Martellaro
    Last edited by Robert Martellaro; 11-10-2016 at 07:28 PM.
    Roberts Optical Ltd.
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    ~~~~~~~~~~~~~~~~~~
    Science is a way of trying not to fool yourself. - Richard P. Feynman

    Experience is the hardest teacher. She gives the test before the lesson.



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    Thanks for your feedback, Hayde.

    The prescription he wears currently is:
    OD -1.00 +0.75 x003, +2.00
    OS -2.50 +0.50 x166, +2.00
    So, negligible change.
    His eye doctor re-refracted this past May and found a similar result. Somebody at that office clearly did not understand what the origin of his issues is; therefore, I think they ignored his need for some kind of prism for near vision.

    I'm still a little bit unsure of what to do, but I'm thinking I'll have him try ground-in prism and hope for the best.

  5. #5
    One eye sees, the other feels. OptiBoard Gold Supporter
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    Quote Originally Posted by andshewas View Post

    The prescription he wears currently is:
    OD -1.00 +0.75 x003, +2.00
    OS -2.50 +0.50 x166, +2.00
    <snip>
    I'm still a little bit unsure of what to do, but I'm thinking I'll have him try ground-in prism and hope for the best.
    Thats's one way to do it. However, if you have trial lenses, you can rule in or out whether the increase in prescribed prism eliminates the vertical diplopia, or if their fusional reserves are presently insufficient to overcome the VI. My guess is the latter, that they'll need SVRO or a slab.

    Good luck,

    Robert Martellaro
    Roberts Optical Ltd.
    Wauwatosa Wi.
    www.roberts-optical.com
    ~~~~~~~~~~~~~~~~~~
    Science is a way of trying not to fool yourself. - Richard P. Feynman

    Experience is the hardest teacher. She gives the test before the lesson.



  6. #6
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    Just to clarify, andshewas, since I may have made a bad assumption about your post:

    When you say PD down, are you meaning "Base Down" measured in prism diopters, or what we typically abbreviate as "BD" (& "BU" for Base Up)?

  7. #7
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    Quote Originally Posted by Robert Martellaro View Post
    Thats's one way to do it. However, if you have trial lenses, you can rule in or out whether the increase in prescribed prism eliminates the vertical diplopia, or if their fusional reserves are presently insufficient to overcome the VI. My guess is the latter, that they'll need SVRO or a slab.

    Good luck,

    Robert Martellaro
    Thanks for your feedback, Robert!
    (An aside: back when I started lurking around here a couple of years ago, I was relieved to see that there is still at least one real professional here in Brew City.)

    Having read both of your responses--I missed your first while writing my first response in between patients--I'm more convinced that slab-off is the right way to go. In the patient's exam, he was given contact lenses for distance correction only and still experienced diplopia. My feeble brain tells me that could be corrected by vertical prism. When we introduce a multifocal spectacle lens, however, the issue will still exist with vertical imbalance at near, necessitating a slab-off.

    I'm going to try recommending a lined bifocal for him, and hope he acquiesces. He's a fairly active guy (ski patroller) yet seems pretty demanding of his intermediate and near vision, so if he pushes back, I'll probably recommend a Physio.


    One more aside: my mom is an anisohyperope with a 3.5 diopter difference, and 2 PD vertical total seems to do the trick for her...which is making this situation all the more confusing for me! Darn those eyeballs! (haha)
    I clearly have a lot to learn about prism and vertical imbalance. I brought my ABO study materials to work with me today.

  8. #8
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    Quote Originally Posted by Hayde View Post
    Just to clarify, andshewas, since I may have made a bad assumption about your post:

    When you say PD down, are you meaning "Base Down" measured in prism diopters, or what we typically abbreviate as "BD" (& "BU" for Base Up)?
    Yes, I meant base down.

  9. #9
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    Thanks!

    Robert's suggestion could save both you and your patient time & expense, if you have the resources. If so, go for it! If not, but your lab affords you a cost-free remake for Rx rewrites, you're still ok. (Just let the patient know ahead of time there could be some trial and error in their case.)

    I agree the likely culprit is a convergence insufficiency rather than vertical imbalance from anisometropic lenses, particularly with the BD correction in the most-minus OS. If the diplopia surfaces in the distance range as well as reading, you can take that to the bank.

    The patient may balk at dispense. If so, try to hang on to them for a while to acclimate & see if they grow more optimistic before they leave. If diplopia isn't resolved after a few days, it doesn't necessarily mean that leaving off the slab-off was the culprit. If it persists in the distance view, then the doc will need to do some more tweaking.

    There's still a theoretical possibility of VI necessitating a slab-off, but for this Rx that would really be one heck of a fluke.

    Interesting case, ASW! Let us know how it turns out?

    [Edit: May I also suggest dropping the seg .5mm OD & raising it .5mm OS from your measurements, to offset the ground prism?]
    Last edited by Hayde; 11-11-2016 at 12:09 PM.

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    I've done more thinking/reading and found a (pretty old) article suggesting different corridor lengths for each lens:
    http://www.sciencedirect.com/science...39625798000393
    The theory is interesting, especially considering the PALs out there that offer many corridor lengths. At my skill level, it's not something I'm inclined to try for this patient, but I might spend some time this weekend doing the math to see which corridor lengths might neutralize induced prism/imbalance. For this fella, I'm inclined to think this theory won't create a successful outcome, because it seems it might rely more on the patient's fusional ability as compared to slab-off.

    What do y'all think of the theory behind this article?

  11. #11
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    Does your patient's issue stem from disparate ocular altitudes?

    Either way, I would encourage you to spend your weekend on something more fun! 'Cos that math, factoring in the Anisometropia, would be more complicated than I'd care to tackle....

    Tell you what, let's both kick back & drink beer and make Robert do the math....

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    andshewas
    I've done more thinking/reading and found a (pretty old) article suggesting different corridor lengths for each lens:
    http://www.sciencedirect.com/science...39625798000393
    The theory is interesting, especially considering the PALs out there that offer many corridor lengths. At my skill level, it's not something I'm inclined to try for this patient, but I might spend some time this weekend doing the math to see which corridor lengths might neutralize induced prism/imbalance. For this fella, I'm inclined to think this theory won't create a successful outcome, because it seems it might rely more on the patient's fusional ability as compared to slab-off.

    What do y'all think of the theory behind this article?



    http://www.optiboard.com/forums/show...s-this-working

    There is a table in the link above where Harry has already the math. I had a couple instances the year before where this worked for me, ... not suggesting this is the way to go in this particular scenario, but sharing I've tried this.
    Trip

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