# Optical Forums > Ophthalmic Optics >  Case Study: PAL seg height adjustments for vertical prism

## Hayde

Case Study
Px prescribed:
+0.50 ds 4.00BU
-2.50 +0.75 080 4.00 BD
+2.75 add

Diplopia is a recent symptom caused by surgery complications.  First time prism wearer; historical specs can't really shed any light--but he's currently in a polycarb Varilux Comfort.  He's opted for a trivex Varilux Physio in his new pair.

Seg heights for progressives measured in new frames at 24.0/25.0.

The ol' "System" bible tells us to adjust bifocal segs in cases of vertical prism .3mm per prism diopter.  In this case I'd be dropping the OD seg to 21.6; or if I broke my habit of adjusting alternate segs lower instead of raising any: OD 22.8/OS 26.2



Insights and experience in this sort of situation is greatly appreciated--whether it confirms B&B or not.  Is this seg offset for vertical prism as advisable in PALs?  Different answers in conventional versus digital designs?

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## optical24/7

Yes, I'd advise it more in a PAL than a FT or sv in fact. More importantly for a resent onset case of diplopia. If they had been wearing similar fitting heights for years with diplopia I usually won't adjust the fitting height due to probable habituation. You're correct, adjust .3mm (.28) for every D of prism in the apex direction.

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## Hayde

Thanks, neighbor. Will play it by the numbers and let y'all know how it turns out.  Going with 22.8/26.2.

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## Robert Martellaro

Hayde,

I like to use a trial prism, verifying that the deviation occurs as expected. 

This Rx, with a 4mm drop PAL, will induce VI of 1.2∆ BU OS. Subtract that from your calculations. 

Depending on the diagnosis, this might be time to switch to segmented multifocals.  

There's a possibility of diplopia at near due to VI at the near point. Test for it if you can, neutralizing the induced/unwanted prism with a bicentric grind (slab-off), or supply separate SVNO. 

Warn the client that there's a two week adaptation period. There might be diplopia during and after adaptation, the latter only when fatigued or after an eccentric gaze. 

Falling is a real concern, especially with the elderly. It might be best to wait until they get home before wearing the glasses. Tell them to be on guard for perspective changes, and please be extra careful when walking and driving.

Hope this helps,

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## Hayde

That is very helpful, Robert.  Thank you.

Yeah, a script like this is a real crapshoot.  The functionality of the same prism correction in the near is an open question.  Honestly no outcome here would surprise me much.

I think you bring up a good point about perhaps switching to segments.  I might have been better off suggesting it from the get-go and confering with the doc beforehand.

Great reminders on safety issues!  Thank you!

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## Robert Martellaro

> That is very helpful, Robert.  Thank you.
> 
> Yeah, a script like this is a real crapshoot.  The functionality of the same prism correction in the near is an open question.  Honestly no outcome here would surprise me much.


Your welcome. 

Hopefully the doc measured the cardinal positions of gaze. 

http://vimeo.com/74467883

A trial frame can help, but in this case it might be difficult to determine if any diplopia at near is due to the induced vertical imbalance, or a nonconcomitant deviation. Worse case is that you'll need to supply SVRO with a different amount of prism, using the PALs for distance and probably intermediate use.

Best regards,

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## Hayde

Update!

Patient is exceedingly satisfied with distance vision. He reports binocular fusion breaking at about 3 feet out, where diplopia still plagues his reading. He's scheduled for a doctor's visit and anticipates getting a separate Rx for reading zone.

For purposes of determining correct seg height placement, the patient didn't report any symptoms suggesting any problem. (I'm assuming that he would have noticed an issue in the outer intermediate at least if not distance range had there been a problem there.)

An interesting experiment of a strong vertical imbalence in a PAL, but I think another example suggesting a justified preference of treatment with multiple pairs of SV lenses or line BF.

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## Speed

> Update!
> 
> Patient is exceedingly satisfied with distance vision. He reports binocular fusion breaking at about 3 feet out, where diplopia still plagues his reading. He's scheduled for a doctor's visit and anticipates getting a separate Rx for reading zone.
> 
> For purposes of determining correct seg height placement, the patient didn't report any symptoms suggesting any problem. (I'm assuming that he would have noticed an issue in the outer intermediate at least if not distance range had there been a problem there.)
> 
> An interesting experiment of a strong vertical imbalence in a PAL, but I think another example suggesting a justified preference of treatment with multiple pairs of SV lenses or line BF.


You forgot the slab?

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## Wes

> You forgot the slab?


Slab?  What's slab?










JK

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## Hayde

> You forgot the slab?


Correct me if I'm wrong, but a slab off would be for the purpose of mitigating unwanted prismatic balance--in this case the prism is prescribed. (It makes perfect sense to me that the correction should be different in the reading and perhaps a slab might be appropriate, but I'm not inclined to step in front of the doc prescribing the Rx.)  Did I misunderstand you?

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## Wes

> Correct me if I'm wrong, but a slab off would be for the purpose of mitigating unwanted prismatic balance--in this case the prism is prescribed. (It makes perfect sense to me that the correction should be different in the reading and perhaps a slab might be appropriate, but I'm not inclined to step in front of the doc prescribing the Rx.)  Did I misunderstand you?


No. The prescribed prism is used to correct his surgery-induced diplopia, but the RX itself (+.50, -2.50) has 3 D of difference.  It is that difference at near gaze (several mm's away from the PRP) that is likely causing his fusion to break at 3 feet out, resulting in diplopia plaguing his reading.  Good luck with this one.

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## Hayde

> No. The prescribed prism is used to correct his surgery-induced diplopia, but the RX itself (+.50, -2.50) has 3 D of difference. It is that difference at near gaze (several mm's away from the PRP) that is likely causing his fusion to break at 3 feet out, resulting in diplopia plaguing his reading. Good luck with this one.


Thank you!  Yes, the vertical is almost 3D at 1cm.  I think the induction is a good one.

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## Speed

> Slab?  What's slab?
> 
> 
> JK






 Sorry.  Missed the JK
A slab is needed here.  Rx prism has nothing to do with the need for a slab. Diplopia at the reading level can be corrected with a slab off.  Consult your lab.  They will take care of this for you.

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## Robert Martellaro

> A slab is needed here.


Probably maybe. See post #6. 

There's a chance that the diplopia at near will be the same, or worse, when a supplemental 3∆ or more BU is added to the near in the left eye, if the prescribed prism needed at near is different than the prescribed prism on the primary gaze. 

For example, the prescribed prism for Hayde's client is 8∆ BD (split) in the left eye. If the prism need at near is 12∆ BD left, then the spectacle induced VI will add to the distance BD prism providing the needed amount of prism at near. Adding a 3∆ BU slab-off in the left eye will create an imbalance, and subsequently diplopia or asthenopia, when doing nothing would have been the correct course of action. However, if the prismatic needs at near are less than the primary gaze, then the only way to decrease the BD prism in the left, with a slab-off, is to use a BU slab-off for the fellow eye.

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