SIZE LENS

[DR.P]

I know that many of you out there are much brighter than I am concerning optics. Here is the situation that I am sure you have encountered.
Since I can prescribe, I have a slight edge on the typical optician, however, that does not make me the least bit smarter than you all are.
A patient has LASIK surgery hoping to be free from glasses. Final outcome:
O.D. Plano (maybe -0.25cyl, but its anyone's guess)
O.S. -2.50 (unoperated eye)
Patient is 53ish. sees fine, but would like driving glasses, and oh, by the way, perhaps a bifocal (if she could)
seems simple enough...not over 4D of imbalance. No need for that messy slab off prism...but she hates the glasses. And oh, by the way, they are of course rimless air titanium that cost a fortune to drill.
So, being the intelligent O.D. that I "think" I am...I re-refract her. Begin to mess with the power in the left eye, making single vision lenses just to rule out the bifocal as the culprit.
So far I made a -2.00 and a -1.50 for the left eye...still no luck. She hates them all
I recommend she get the other eye operated on to even it out...she likes the mono, but hates the distance RX.
So...she goes back to the surgeon...he assures her that all she needs is a -1.75...right?
I doubt it! What I need is of course my trusty "space eikenometer" to measure the difference in magnification between the two eyes...or, given I don't have one,perhaps an optician with some experience in changing the thickness of the lenses to even out the image size...Daris where are you when we need you?

Awaiting your reply.
Hope you folks have some excellent ideas.
Signed...
stumped optometrist Houston Texas

[Texas Ranger]

Dr. P, how's things in houston? Actually, i thought the "rule" on slab-off was 4 percent, not diopters, of course i'm not always right! but at 2.5 D, that 5% mag difference. you could flip flop base curves, say use a 4D BC on the OD and a 6D on the OS; at 53, she about a 2.00 add; perhaps you could under plus the od add and over plus the os add, say like a 1.50 and a 2.00, letting her accomodate a little. keep the lenses small, and preferably a very close vertex. do a slab off on the os., don't do a reverse slab on the od, it adds too much mag. with the extra thickness. we're just adjusting for vertex and elininating unwanted prism. hope this helps! ain't lasiks great! This does work with a varilux, also. Al.

[chip anderson]

How bout a bifocal contact O.S. spherical Rx should be no problems in soft. Forget the quarte in the O.D.

[Darris Chambless]

Hello Dr.P,

Let me see if I understand what you are trying to get at. The patient wants to offset the monocular vision with glasses for driving so that both eyes focus for distance after having had LASIK?

If this is what you are saying I hate to be the barer of bad news but it won't work. Here's why. You can put a conventional myopic script on her (which you've tried) and she will have a perceptual problem similar to anisometropia (but it isn't anisometropia) A size lens, as you know, is used for anisekonia which is believed to originate in the brain and not the eye so using a "size lens" or anisekonic lens will not solve the problem either because the problem lies in the perceptual difference in refractive power that has been created by the surgery.

Basically your suggestion to have the other eye done is the only option your patient has for good binocular vision. You were 100% correct in suggesting she have it done. You can mess around with trial and error with lower powers until you get one that your patient can tolerate, but that is the best you will be able to do under the circumstances. I would go into why this happens after LASIK but you already know why I'm sure.

There are those that may argue this based on optical theory and mathematics, but believe me when I say you will only be able to make the patient minimally comfortable (at best) but never happy. It's one of the many downsides to LASIK and there will be some "You ain't seen nothin' yet" in the future for LASIK patients.

I'm not trying to be the bringer of doom, but you're going to be stuck right where you are no matter what. Remember; it isn't your fault and the only cure is LASIK on the other eye ;-)

Sorry I can't bring you any positive answers or solutions to your problem.

Darris Chambless

[DR.P]

I knew I could count on you to cheer me up.
I was not involved in recommending Lasik for this patient...I am just the one to pick up the pieces. And of course, you can't convince the patient that they have just made a very permanent mistake. And of course, they refuse to do the only reasonable thing and mess up the other eye to match. "why, then I wouldn't be able to see up close."
Well, duuuuhhhhh, did you give that any thought before you let them "fix" the first eye.
I realize the size lens idea is thrashing around in the dark, but that is what you do when someone else has messed someone up and they come to you to be fixed.
It is especially tough when you have sold them the most expensive frame in the store and now you realize you have to eat it and its not even your fault.
Help me remember Knapp's law. I actually published an article on this in the late 70s.
If the corneal curvatures are the same, and there is a difference in rx, it means its axial..Better corrected with specs. Now since we know her axial lengths are probably the same, the obvious answer is a contact lens....problem is, a contact lens is the same as having the eye "fixed?" Not much help for the person who wants to see up close when she takes off her glasses. I think its time to refund her money and send her packing.
It will only cost me more time and money than I can reasonably justify.
Thanks

[Darris Chambless]

Sorry Dude :-)

Remember also that axial lengths being equal will not always make visual perception equal so problems will exist. All due to the butchering of the eye for "quality of life" sake :-)

Darris "Why can't they leave well enough alone?" Chambless

[Darryl Meister]

Hi Dr. P,

Knapp's rule applies to anisometropia that is axial in origin, and basically states that a spectacle lens placed at the anterior focus of the eye (around 15 mm) will produce the same image size, no matter what the degree of error. Since your patient has induced anisometropia from refractive surgery, this really doesn't apply (i.e., she is a refractive anisometrope at this point).

Honestly, I would guess that your best bet -- as I think Chip pointed out -- is a contact lens, assuming that your patient is receptive to that. This will produce very little aniseikonia, no spectacle-induced anisophoria (prism imbalance), and no cosmetic issues.

Size (iseikonic) lenses are difficult to produce for minus powers, since the "size component" of the lens (i.e., base curve, thickness, and index) has little effect on these flat, thin lenses -- without using extreme base curves and thicknesses. Most of the minification comes the "power component" of the Rx, which will provide about 3.6% minification for a -2.50 DS. Playing with the base curve, vertex distance, and thickness will help to some extent, but you might end up producing a lens that looks horrible and provides very poor peripheral vision. If you do, though, you should probably get good results if you can knock the minification down even 1% to 2%.

Don't lose too much sleep over your trusty Space Eikonometer... AO discontinued the service to fill the iseikonic lens orders for it years ago! ;) Anyway, good luck to you.

Best regards,
Darryl

[This message has been edited by Darryl Meister (edited 07-18-2000).]

[Jerry Thornhill]

DR.P
Are you sure you are not in Calif? We have a patient identical to yours, including the Rx.
She is not complaining of image size or thickness just the reading imbalance. She doesn't want the slab or a pair of distance and a pair of reading and she doesn't want to wear contacts. We gave her those choices and then sent her back to the doctor. This is the second one I've ran across and a slab solved the first one.

Jerry