Oblique Cylinders

[epandi]

Dear optiboarders,

The last few weeks in my practice I had some customers with oblique cylinders (C-0.75 axis 135 and 45) which I prescribed the Hoyalux iD. Four of those customers really had problems adapting to the PAL. The following problem occured in all those 4 customers: when looking down they had the feeling the ground was more near then without the lenses; the world was spinning around when walking around. They had no problems just looking straight forward, but the problems occured when turning the head or walking. When we removed the cylinders from the lenses the problem was resolved.

Did anyone encounter this problem? Or does anybody know some research articles about oblique cylinders and progressive lenses?

I hope you guys can help me out.

Greetings,

Eelco

[drk]

You have experienced an underappreciated fact.

Think of bilateral spherocylinder lenses, plano-2.00x180.

I can't draw an optical cross, but realize the horizontal meridian is plano and the vertical is -2.00.

That means that vertical minification occurs; a circle would look like a squatty horizontal oval to the wearer, initially.

Since both eyes see a symmetric image, the brain can easily resize, and the understandable visual discomfort subsides.



Now, envision your patient:
OD: about one diopter axis 45
OS: about one diopter axis 135

The optical crosses will show the right eye has minification on a slant, (from the top of his ear to the tip of his nose, if you will), and the left eye will get a slant in the opposite direction, like an "X"-shaped minification pattern.

If you think of what the wearer experiences, it will be pretty neat: the top of his vision will get "uncrossed diplopia" (which occurs to farther background objects when you are looking at something closer) and the bottom of his vision will get "crossed diplopia" (which occurs to nearer background objects when you are looking at something farther away).

The net effect is that the top of his vision looks slanted away and the bottom of the vision will look slanted closer (I may be backwards on that, but spare me the details). A wall will look like it's tilted away at top and closer at the floor, like a ramp.

You can see why that would be visually disconcerting.


The general condition is "anisekonia" (unequal image sizes), and the cause most often is "anisometropia" (unequal vision). That is commonly known.

But, here's the "secret kicker": oblique astigmatism axes EVEN WITH EQUAL CYLINDER AMOUNTS causes anisometropia! Let that sink in!

That means any time your patient is, say, 20 degrees off horizontal or vertical (that's 160-020 for WTR and 70-110 for ATR), and the cylinder isn't trifiling (I'd say 1 diopter is a good red alert), you are going to get anisometropia and potentially anisekonia.

The solution in your case? Probably to drop the cyl. That guy was way sensitive. But in general, you cut the cylinder as much as feasible, prescribe the spherical equivalent if you're so inclined, and very possibly adjust the axes of astigmatism towards whichever principal meridian and away from "obliquity".

Carry on!

[epandi]

Hi Drk,

Thank you for your excellent response. It was very nice reading it and really did explain the cause of the problem to me!

[drk]

Your english is quite excellent, but if necessary I could re-write my response to avoid slang.

Please continue to post here!

[Wes]

Reminds me of a piece in system for ophthalmic dispensing in the aniseikonia chapter. Possibly better explained than in the book. Kudos drk!
Wes

[FVCCHRIS]

Had a patient a few years ago, last name- Sikonia. Can you guess what her 1st name was(drum roll)......you guessed it, Anni ! When I saw the chart i thought the Doc was playing a joke but she was real. Alas, she was a simple hyperope! :)

[JanMueller]

Goeden avond to the Netherlands
:cheers:

If your customer is used to those cyl. with SV lenses he will make it also with progressives. If he is not do not even try!
I would then even try to stay with a spherical prescription.

Tot ziens! :D

[rolandclaur]

You have experienced an underappreciated fact.

Think of bilateral spherocylinder lenses, plano-2.00x180.

I can't draw an optical cross, but realize the horizontal meridian is plano and the vertical is -2.00.

That means that vertical minification occurs; a circle would look like a squatty horizontal oval to the wearer, initially.

Since both eyes see a symmetric image, the brain can easily resize, and the understandable visual discomfort subsides.



Now, envision your patient:
OD: about one diopter axis 45
OS: about one diopter axis 135

The optical crosses will show the right eye has minification on a slant, (from the top of his ear to the tip of his nose, if you will), and the left eye will get a slant in the opposite direction, like an "X"-shaped minification pattern.

If you think of what the wearer experiences, it will be pretty neat: the top of his vision will get "uncrossed diplopia" (which occurs to farther background objects when you are looking at something closer) and the bottom of his vision will get "crossed diplopia" (which occurs to nearer background objects when you are looking at something farther away).

The net effect is that the top of his vision looks slanted away and the bottom of the vision will look slanted closer (I may be backwards on that, but spare me the details). A wall will look like it's tilted away at top and closer at the floor, like a ramp.

You can see why that would be visually disconcerting.


The general condition is "anisekonia" (unequal image sizes), and the cause most often is "anisometropia" (unequal vision). That is commonly known.

But, here's the "secret kicker": oblique astigmatism axes EVEN WITH EQUAL CYLINDER AMOUNTS causes anisometropia! Let that sink in!

That means any time your patient is, say, 20 degrees off horizontal or vertical (that's 160-020 for WTR and 70-110 for ATR), and the cylinder isn't trifiling (I'd say 1 diopter is a good red alert), you are going to get anisometropia and potentially anisekonia.

The solution in your case? Probably to drop the cyl. That guy was way sensitive. But in general, you cut the cylinder as much as feasible, prescribe the spherical equivalent if you're so inclined, and very possibly adjust the axes of astigmatism towards whichever principal meridian and away from "obliquity".

Carry on!

Very informative post!!

Can anyone point me in the direction of any textbook or scholarly materials that can elaborate more on the topic of the relationship between power and image size (magnification, minification, etc). Thanks

[Robert Martellaro]

Very informative post!!

Can anyone point me in the direction of any textbook or scholarly materials that can elaborate more on the topic of the relationship between power and image size (magnification, minification, etc). Thanks

http://books.google.com/books?id=Qzv...tropia&f=false

[Java99]

Thank you so much for this post! You've just solved a mystery I've been been puzzling over for two days. I was blaming a switch from a Physio 360 to the Enhanced for the problem, but the pt is a moderate/high hyperope with oblique cylinders over 1D OU. You just saved me and the pt another week waiting on yet another set of lenses to try.

[snotbagel]

I think that the magnification caused by the cylinders is NOT the problem. More than likely, it is vertical imbalance caused by the version of the eye from center with oblique astigmatism. Try this yourself..take a -1.00 cyl, align it at 45 degrees,center it in the vertometer, and slide it in the 180 . You will see the target move vertically, even though the lens is moving horizontally.

There's no fix for this, but the better fit the lenses and frames are (and smaller) the less it will trouble the patient.

[braheem24]

Target will move equally vertically and horizontally at 45 and 135. A 0.75cyl ou, the vertical imbalance cannot be more the .75 vertical imbalance per 10mm of reading depth no matter the axis.

[snotbagel]

but the prism is changing..and does cause problems for first time cyl wearers. Yoked prism..right? causes the feeling of walking up or down a hill? (many first time progressive wearers in moderate to high powers feel this as a result of prism thinning, but it gets lost in all the other changes).