Vertical Imbalance and Plano Lenses

[EyeFitWell]

Perhaps this is a silly question, but I thought I'd ask anyway.
Take the following Rx:
OD +2.00 -2.00 x180
OS +2.00 sph
Add +2.50

So, since the right lens is essentially plano on the 90 meridian, is there still a vertical imbalance? Obviously, there is still an image size difference, but would this situation involve vertical prism that would disturb fusion?

Thanks!

[drk]

Yes, sir, there is.

Compare OD and OS vertical meridia:
plano and +2.00

consider downgaze 10mm: 2 pd BU OS vertical imbalance. Ouch.

The sneaky thing about anisometropia is that you have to consider each meridian.

Look at that Rx in plus cylinder notation, BTW. The anisometropia's more obvious:
plano +2.00 x 90
+2.00 D.S.

For a nice practical experience, take a 2 pd prism from the trial lens set and hold it in front of your left eye, base up and hold a plano lens over the right eye (or nothing, of course :)). You'll feel it!

[EyeFitWell]

Yes, I agree DRK, but my question is whether or not one lens being essentially plano changes the senario.
Plano doesn't really have prism away from the "OC" and that's why I was thinking it might not apply here. I do see the difference in powers.
If you were talking about SV lenses (yes, I know anisometropia doesn't come into play here b/c the wearer looks through the OC) but theoretically,
SV lenses, one plano, one +2. How much prism is there looking 10mm above the OC? I don't know, because on one hand, there's 2D prism in the left lens. On the other hand, you can look anywhere in a plano lens w/o prism, so that's why I thought this case might not be a fussion issue.

[Fezz]

Plano doesn't really have prism away from the "OC" and that's why I was thinking it might not apply here. I do see the difference in power.


But, plano CAN have prism.

[EyeFitWell]

Hmm...
I've seen a plano prism, with no power except prism power, and I guess that's what you're referring to.
But, if you take a stock SV plano lens, nowhere in that lens is there prism.

[Dave Nelson]

Use Prentice's rule to calculate the prism, then compare the imbalance. As drk says, put a prism over one eye, and you can see and feel the imbalance, even if there is no prism in the other eye. to further demonstrate, you could put a plano lens over the other eye, and you will still see and feel the imbalance, and Prentices rule will still indicate a prism imbalance.

[EyeFitWell]

I'm sorry, maybe you're answering my question and I don't even realize, but that just doesn't make sense....(I'm following you that there is prism in one eye, even if it's not in both eyes. I also understand the prentice rule, etc.)
When looking through the bifocal, the left lens has BU prism, moving the image down. But, the right lens has no prism and no power.

What if you got the RX:
OD plano
OS +2.00 2D BD prism
SV

what would keep the right eye from turning up with the left?

[Dave Nelson]

Wer'e talking about prism imbalance, not prism. When one image is displaced, and one is either not displaced, or is displaced a different amount, prism imbalance results, and results in working the ocular muscles harder to maintain fusion. This can result in eyestrain, and, where the imbalance is greater, a loss of fusion, resulting in double vision. People can generally tolerate far more horizontal prism imbalance than vertical. I hope this answers your question.

[chip anderson]

Unless the patient has a phoria or other muscle problem. Almost zero patients will have any sensitivity to a 2 diopter imballance. At 3 or 4 this becomes a problem.

Chip

If it is a problem it can be solved by using a straight top bifocal on the lens with the most minus power (base up prism here) and a round top on the lens with the most minus (base down prism here). Of course you can't do this if you are one of those people that feels cosmetics is more important that vision.

[Fezz]

Chip-

I could maybe agree with 2 horizontal, but vertical? You folks from the south must be waaaay more liad back then us Northerners!

[Darryl Meister]

I think you guys might be talking about two different situations...

When verifying the lens, you are not likely to find any vertical imbalance, since you would generally start by neutralizing the lens with the strongest vertical meridian at the optical center, first. When sliding the eyewear over to measure the second lens, which has no power and produces virtually no prism in the vertical meridian, you would therefore not find any significant vertical imbalance.

However, when wearing the lenses, since the left eye will not necessarily be looking through the optical center of the left lens, you may in fact experience a significant amount of vertical imbalance at any given moment as the eyes traverse the lenses.

[EyeFitWell]

ok, got it. thanks!

[HarryChiling]

I would like to add that when measureing the power why use the (90o meridian)? Don't most here decenter the seg about 1.5 mm per eye? If so then the meridian changes since the eye is not only gazing down but slightly in. So if the 10mm as metioned earlier were to apply the the position we would measure for prism at would be 10 down and 1.5 in or

tan (ang) = 1.5/10
tan (ang) = 0.15
ang = tan-1(0.15)
ang = 8.5o

so the meridian you should check for would be 98.5o the power in this meridian being 0.04, not that it matters much but just a side thought. In the given scenario the amount of prism imbalance should not make a difference, but that is up to the wearer to really decide, I bet you if you mention the price of a slab off they may decide that it is an acceptable amount of prism to deal with. :)

alberto correia just posted a program in the download sectiopn that would be of use to you in this scenario. Also if you go to www.opticiansfriend.com (http://www.opticiansfriend.com) Al Cordova has a calculator that would help you calculate the amount of imbalance.

[Darryl Meister]

I would like to add that when measureing the power why use the (90o meridian)?
Keep in mind that since the principal powers are at 180 and 90 in this particular prescription, the vertical prism component will only be due to the power through the 90 deg meridian, regardless of the horizontal decentration. You can verify this by entering this prescription into this induced prism calculator (http://www.opticampus.com/tools/induced.php).

[drk]

I think Harry's nitpicking over convergence. Nobody likes a geek, Harry

[Darryl Meister]

I suspect that every single one of us reading this thread is probably a geek, DRK... ;)

[HarryChiling]

I think Harry's nitpicking over convergence. Nobody likes a geek, Harry

:(

[Fezz]

Hello, hello...I'm right here. Did somebody call for the geek?

:D:cheers::D

[EyeFitWell]

LOL, Harry I completely agree with you, and I thought about that when I was taught how to calculate the vert. imbalance in school. However, this particular question was more about the theory and less about the math. I had a pt. that Doc said "might" need a slab, and I wondered in my head about the one lens being plano making a difference.

Thanks for the insight!

[GOS_Queen]

I think Harry's nitpicking over convergence. Nobody likes a geek, Harry

I like a geek, Harry. (heck, I already knew I was a nobody)

[LENNY]

Can I trow this in?

What if the patient is monocular?

[EyeFitWell]

then fusion isnt' a concern, right?

[Fezz]

Can I trow this in?

What if the patient is monocular?

Tell them to buy a blasted patch, throw a parrot on their shoulder, scream AARRGGHH, and get the blue blazes out of your dispensary.



Monocular patients......why do I always think of Colonel Klink(Hogans Heroes) and his Monocle?


:D