Vertical Anisometrope

[Hayde]

Have a patient whose most previous Rx was
+2.25 +1.00 180
+2.25 DS
Add: +2.75

New Refraction improves VA on the left eye to 20/30
+2.50 +1.00 180
+1.75 DS
Add: +2.75

Alas, we have a new issue of prismatic imbalance. To boot, my patient likes low segs in her FT28s, so I estimate her reading line to be 12-13mm below her distance gaze.

The lit I've read says to worry about prismatic differentials on the 90 above 1.50D/cm. Me and my calculator have her at ~2.2D.

A small slab off is possible, but I'm wondering if differing ADDs (assuming the doc's ok with it) would address this issue more cosmetically?

I've seen a rule of thumb of .25 bump in add power per 4 diopters difference in [adjusted] sphere powers. That would suggest a fairly minute differential of add powers for my patient and perhaps not all that feasible.

Not sure what "the real" equation is here--figured there's 2 dozen eyeballs attached to genius brains around these parts who can tell me if and why I'm on a dead-end tangent.

I might be lucky enough that one of them does!

[Robert Martellaro]

There's probably pathology, a unilateral decrease in hyperopia, maybe due to a nuclear sclerotic cataract. So there's that. The client had about 1.25 Vertical Imbalance with the habitual eyeglasses, increasing to 2.25 VI with the new Rx, at a reading depth of 13mm. Frequency of near tasks is unknown, but is pertinent.

I'm not familiar with using different adds for anisometropia, although different segment designs are one solution. I'm not aware of the rule of thumb WRT increasing the add power for anisometropia.

In general, most folks will be nonsymptomatic at this level of VI, especially when they've adapted to the majority of the imbalance in their habitual eyeglasses. Moreover, with the low segment placement, the distance OC is probably well below the pupil, minimizing VI somewhat on the near gaze, with some degree of VI in the primary (straight ahead) gaze.

The best course of action depends on answers to the above unknowns, that is, distance OC placement and near task frequency. But in general, at this level of VI, one might nudge the seg height up, place the distance OC carefully for best effect, and take a wait and see attitude, telling the client that there may be increased fatigue with frequent and/or extended near tasks, and if so, report the symptoms so that you can take the next step- slab prism, OC position compromise, dissimilar segments (rarely used for cosmetic reasons), or separate SVNO.

[Hayde]

Hmmm...lemme see if I can find where I originally read about bumping an add in one lens for anisometropia. Maybe I'm remembering it wrong.

But you're thinking a jump from 1.2 to 2.2 diopters is probably too little to worry about? Dare to give me your best guess as to the odds? (I won't hold you to it.) If the jump hadn't been as much, I wouldn't be so concerned. (Patient is an avid reader, but she has me convinced not to raise her segs.)

Distance OC--drop it say 1.8mm to keep the VI under .33D?

Thanks Robert. I'll try to dig out the article and see how bad my memory is.

[Robert Martellaro]

Hmmm...lemme see if I can find where I originally read about bumping an add in one lens for anisometropia. Maybe I'm remembering it wrong.

But you're thinking a jump from 1.2 to 2.2 diopters is probably too little to worry about? Dare to give me your best guess as to the odds? (I won't hold you to it.) If the jump hadn't been as much, I wouldn't be so concerned. (Patient is an avid reader, but she has me convinced not to raise her segs.)

Distance OC--drop it say 1.8mm to keep the VI under .33D?

Thanks Robert. I'll try to dig out the article and see how bad my memory is.

Your welcome.

I know that manipulating the base curve has an effect on the vertical imbalance, and that Prentice's rule will underestimate the prismatic effect with plus powers. Any additional knowledge you can bring forth is always appreciated.

Odds are they'll be nonsymptomatic, I'd guess three or four to one. It's certainly worth trying. Keep the client informed.

Check how far the old OC is below the pupil with the old eyeglasses. It's probably best to keep it the same unless it's center pupil. Just don't lower it so much that it introduces too much VI on the distance gaze (lowering the OC below the pupil increases the VI distance, but reduces the VI at near).

[wmcdonald]

Average reading depth is only 8.5 mm- 10 mm. I suspect 12-13 may be a bit much, despite the low segs. If it is 10, you are below 2D, and should not be concerned unless she is symptomatic. Is there a subjective complaint?

[Hayde]

She loves her old Rx--no complaints on habitual. I dotted her pupil at 5mm above her eyelid and dropped the seg 2 below that (after discussion and matching her old segs)--knowing the average was ten I estimated based on the drop. This seemed consistent with the reading task she most prominently reported for her main bifocals: reading a hymnal in her lap.

But if I'm overestimating the reading angle, that'd be great. Even if not, I'll take 3:1 odds that she'll be asymptomatic without any proactive redress.

Thank you gentlemen! Will dig out that article when I get to the office.

[Hayde]

http://www.college-optometrists.org/...ctive_d5_5.pdf

Page 8. Will scrutinize it later to see how I misread it, but since it's in conversation anyway--I'll share my learning curve to the world.

[HarryChiling]

The 0.25D bump in add for 4D difference is used by the prescriber to adjust the accommodation. The reason for the adjustment is that the distance Rx is tuned for an object at optical infinity, the reading prescription should be tuned for an object at 30 to 33cm on average. For a distance lens the power of the object entering the lens system is negligible or close to zero, for a reading Rx the object vergence is -3.00 to -3.25D which is significant and should be accounted for. Just as a note, using vertex in those calculations instead of using the measure to the crystalline lens has a negligible amount of error as well.

As nice as that formula may be it is impractical since most doctors are not using computations to determine the accommodation for patients, it is done empirically during the exam.

I have attached an excel spreadsheet that I quickly whipped up with the examples in the paper to show the calculation used by the author in the article since he does not give a step by step making the assumption that the reader is familiar with Swaine's procedure for ray tracing.

Also to answer your question, Roberts answer was flawless. Lowering the "OC" to reduce the vertical imbalance is a practical and preferential method. If this was a progressive lens we would be calling that lowering "prism thinning", like Robert mentioned take care not to lower too much, your ultimately going to balance the lower imbalance by moving it into the distance gaze. As a rule of thumb I would say place 1/3 of the imbalance into the distance gaze as a maximum.

[Robert Martellaro]

http://www.college-optometrists.org/...ctive_d5_5.pdf

Page 8. Will scrutinize it later to see how I misread it, but since it's in conversation anyway--I'll share my learning curve to the world.

I think that accommodation and vergence considerations are usually ignored because most folks who are candidates for slab-offs are older, and their accommodation is pretty much kaput. I'm certainly not seeing unequal Rx adds, however, most of my RXs come from MDs and eye institutes. It's certainly worth a look for a younger presbyope though.

[Robert Martellaro]

I have attached an excel spreadsheet that I quickly whipped up

Awesome. Using data from Hayde's client (object .36m OD +3.00 OS +1.75) reveals that the accommodative demands are close enough to be clinically insignificant, I think.

Thank you, Harry C.

Hayde,

It's usually best to measure the reading depth instead of using averages. There are many different methods, but my favorite is the mirror method.

http://www.optiboard.com/forums/show...g-progressives

[D_Zab]

To all in this thread, thank you. I greatly appreciate the educational content of this discussion.

[Hayde]

Optiboard at its best! Thanks for the insights, all. I will definitely be trying out the mirror measurement to gauge reading levels. (In fact, I'll try to get it on my patient at dispense to see how close or far I called it.)

I'm very obliged for all the good guidance. Thanks again. Will let y'all know how she likes her new specs.

[Hayde]

Update: 12 days from dispense, a follow-up call, and so far all seems well with the specs.

Robert, I love the mirror method! A light mirror is sure handier than a bigger one for patients with shaky hands, but with some care we confirmed her reading line at 12mm (+/-0.5) under her pupil. She did report double vision at that angle, but so far the patient has reported no adjustment issues.

Thanks for the assist, all.

[Robert Martellaro]

Update: 12 days from dispense, a follow-up call, and so far all seems well with the specs.

Robert, I love the mirror method! A light mirror is sure handier than a bigger one for patients with shaky hands, but with some care we confirmed her reading line at 12mm (+/-0.5) under her pupil. She did report double vision at that angle, but so far the patient has reported no adjustment issues.

Thanks for the assist, all.

Thanks for the update. I wish more threads had them.

To be clear, you did not use a slab-off prism, even when there was a break (temporary?) in fusion with the trial frame, but the client is comfortable at near with the eyeglasses. Sounds like a close call.

I may be mistaken, but I believe that the mirror method was introduced by a German optician. Simple and effective.

[Hayde]

Thanks for the update. I wish more threads had them.

To be clear, you did not use a slab-off prism, even when there was a break (temporary?) in fusion with the trial frame, but the client is comfortable at near with the eyeglasses. Sounds like a close call.

I may be mistaken, but I believe that the mirror method was introduced by a German optician. Simple and effective.

Yeah, I'm the same way--wish a lot of these mysteries we read on here had the last page attached.

Yep, no slab off. In this case the 'trial' was just me dispensing her order and gauging any degree of preliminary success. I assume she's tilting a little more to read to keep herself north of the 12mm line.

The jury's always out for a few months (you know how patients can be procrastinating to complain), but I think we successfully averted the expense and aesthetic issues of a slab-off.

If something crops up, the doc approved reverting to her old Rx if need be--losing accuity in OS but minimizing the vertical imbalance. A slab off was probably the safer bet, but I enjoyed learning a new option to help the odds in the patient's favor.

Thanks again!

[HarryChiling]

Thanks for the update. I wish more threads had them.

To be clear, you did not use a slab-off prism, even when there was a break (temporary?) in fusion with the trial frame, but the client is comfortable at near with the eyeglasses. Sounds like a close call.

I may be mistaken, but I believe that the mirror method was introduced by a German optician. Simple and effective.

Here's a suggestion in the Ophthalmic Optics thread start the thread with CASE STUDY:, this should signify that the poster is making a commitment to provide a recommended minimum amount of data and a follow up conclusion.

I would recommend old spec info, new spec info, problem as explained by patient, dispensers hypothesis, and tried plans with corresponding outcomes.

In return I would be willing to not only provide my recommendations but provide citation to ophthalmic texts for peer review.

That could mean high quality content with academic significance.

[Hayde]

Here's a suggestion in the Ophthalmic Optics thread start the thread with CASE STUDY:, this should signify that the poster is making a commitment to provide a recommended minimum amount of data and a follow up conclusion.

I would recommend old spec info, new spec info, problem as explained by patient, dispensers hypothesis, and tried plans with corresponding outcomes.

In return I would be willing to not only provide my recommendations but provide citation to ophthalmic texts for peer review.

That could mean high quality content with academic significance.

I love this suggestion.

How about a template?

***

Case Study:
[OP Presentation]

Historical Specs (please consider including monocular OCs and segs, pantoscopic tilt, wrap, vertex, lens design, material & treatments, and frame type):

Current Specs (please consider including monocular OCs and segs, pantoscopic tilt, wrap, vertex, lens design, material & treatments, and frame type):

Patient Report:

Hypothesis & Treatment Attempt (Initial, second, etc. Please include accumulated diagnostic data):

***

A template surely isn't required, but just in case it's handy to copy and paste...and I know I could use the reminders of inclusions as I'm typing.


Harry, you're six shades of awesome.

[HarryChiling]

Harry, you're six shades of awesome.

That's your template casting a shadow of awesome. ; )