iseikonic lens? need help.

[ifdog]

Hi,

Iam a independent Optometrist from China, and I am looking forward for your help.

We have a 6-year-old patient with congenitaleye disease, where one eye has high myopia, and the other still hassome hyperopia reserve. As it is known as anisometropia. The eye withmyopia has severe amblyopia, and the patient is undergoing visualtraining to improve their vision at medical institution. The currentprescription for the patient is as follows:

eye	sphere	cylinder	angle
OS	-6.25	-1.50	30
OD	+1.50	0	180

When using spectacles with such a large difference in sphere power, the difference in the size of objects viewed by the two eyes is significant, affecting the fusion of images and hindering visual recovery. However, due to the patient's age and cooperation, it is unable to use contact lenses for now.


We have tried domestically available technology, which adjusts magnification by slightly modifying the thickness of the lens, makingthe size of objects viewed by each eye closer. However, these lenses are not designed specifically for the patient, and the imaging difference is still significant. Additionally, there is an obvious prism effect in the vertical direction, causing significant height differences in the images.

Through the internet, I heard about slab-off (or any other ?) technology, which can optimize for above situation.

1, Is there any appropriate technology that can overcome the imaging difference problem in eyeglasses and provide them to patients with this condition?

2,Considering factors such as lens thickness or weight, to what extent can imaging differences be repaired?

3,If such a product exists, what is the approximate price, and where toorder them?

4,What measurements do we need to take to configure such lenses for thepatient?



any infomation may help ,thanks

[drk]

First, you have to have goals.

If the hyperopic right eye is left uncorrected, and the myopic left eye is undercorrected to, say, a residual -2.50 D (or even undercorrected to a -3.00 D residual refractive error) the amblyopic eye will be favored at nearpoint (reading, playing with toys, etc.).

This can lead to passive amblyopia therapy. You did not provide the best corrected visual acuity in the amblyopic left eye, but it should be in the range of 20/80.

So, undercorrection of the left eye serves two purposes. 1.) It minimizes the image size difference between the plano/+0.00D right lens, and the ~-3.50 left lens.

As to iseikonic lenses, it is generally unnecessary if you simply use the same base curve in the plano right eye as the -3.50 left eye (and I guess that'd be about +2.50 but I am open to correction on that), and simply order equal, minimal center thicknesses (no thinner than 1.5 mm in polycarbonate, for safety). This minimizes the image size difference without any additional expense. (Especially for a child who will destroy expensive lenses and will need more frequent prescription changes.)

As to slab-off design, that is only if there is a multifocal involved. When there is no need for a multifocal, the patient will instinctively position their head such that they don't get diplopia on downgaze--that is, they will depress their chin to look through the optical centers.

[drk]

Having said all of that, the best company I know of for iseikonic design is Shaw Lens in Canada: https://shawlens.com/

[ifdog]

First, you have to have goals.

If the hyperopic right eye is left uncorrected, and the myopic left eye is undercorrected to, say, a residual -2.50 D (or even undercorrected to a -3.00 D residual refractive error) the amblyopic eye will be favored at nearpoint (reading, playing with toys, etc.).

This can lead to passive amblyopia therapy. You did not provide the best corrected visual acuity in the amblyopic left eye, but it should be in the range of 20/80.

So, undercorrection of the left eye serves two purposes. 1.) It minimizes the image size difference between the plano/+0.00D right lens, and the ~-3.50 left lens.

As to iseikonic lenses, it is generally unnecessary if you simply use the same base curve in the plano right eye as the -3.50 left eye (and I guess that'd be about +2.50 but I am open to correction on that), and simply order equal, minimal center thicknesses (no thinner than 1.5 mm in polycarbonate, for safety). This minimizes the image size difference without any additional expense. (Especially for a child who will destroy expensive lenses and will need more frequent prescription changes.)

As to slab-off design, that is only if there is a multifocal involved. When there is no need for a multifocal, the patient will instinctively position their head such that they don't get diplopia on downgaze--that is, they will depress their chin to look through the optical centers.

Thanks for your reply，however there still be some doubts:

1, In my option, a difference of -3.50D is still big enough, to destroy the “fusion vision”, I heard it should be within -2.00D,will a -3.50D difference works?

2,sorry for not indicating the corrected vision of the patient. it is 20/500 two years ago and now 20/40, a bit higher than 20/80, will it affect the undercorrection recipe?

3,If we leave patient instinctively position his head to adjust the image height, will it lead to any other side effect? such as vertical strabismus or any other. If we take this way to make spectacles , how to determine the best center position for the lens?

thank you.

[optical24/7]

Iseikonic can help in image size differences, but do nothing for off primary viewing, due to prismatic differences between lens powers. Of course, a contact lens OS would be ideal, but to make eyeglasses slightly (and I mean slightly) more comfortable on image size difference, you could cut the cyl on the OS, the patient is getting a +1.6-2.00% difference in minification at 30 to 120 degree viewing, possible ghosting issue.

Increase the base curve ( not by much, you’ll get away from best curve theory), and center thickness of the OS ( maybe to 4.mm thick). And have the OS edged with the bevel as far to the + side of the lens as possible, this will decrease the vertex and induce more magnification to the minus lens…

These can “help” with the magnification/minimization in the above Rx, but glasses just aren’t going to be a good solution, I would highly recommend working toward a CL on the OS. Short of refractive surgery, ( definitely not so young of patient) CL’s are the future for this patient’s visual rehabilitation.


Zhù nǐ hǎo yùn, wǒ de péngyǒu!

[drk]

Thanks for that, optical 24/7.

As to looking off-axis, that is not considered a concern in young people with single vision lenses. There is no chance of permanent vertical strabismus. At worst, there will be prismatic imbalance on downgaze which will make the kid drop their head to avoid the diplopia or strain.

With 20/40 vision, studies show that passive therapy is adequate for refractive amblyopia. Or, at least worth a try. See the Shaw Lens link, or I can provide another.

The textbook amount of magnification difference that causes aniseikonia is somewhere in the range of 2-4% (probably on the higher end in this case due to plasticity at the young age).



Right eye = 0.4% with assumption of poly, plano, 1.5 CT, and +4.00 base.
Left eye = -4.1% with assumption of poly, -3.50, 1.5 CT, and +4.00 base.

You can mess around with the numbers. The important variable, though IS TO UNDERCORRECT THE LEFT EYE TO THE CORRECT WORKING DISTANCE SO THE KID PREFERS TO USE THAT EYE FOR NEAR WORK! That means you have about a 2.5-3.0 undercorrection window of opportunity.

Realize, though, in an undercorrection situation you are not requiring the patient to be binocular...you have already given up that goal. Without binocularity, you will not need equal image size, anyway, because you're not going to get fine stereoacuity development. The patient will have gross stereo, though. The kid's going to have underdeveloped fine stereo until you can get a CL on him, as soon as feasible. The critical age is around age 8, so you need to hurry to get a contact lens on.

http://opticampus.opti.vision/tools/magnification.php
May God bless Darryl, retroactively.

[ifdog]

Iseikonic can help in image size differences, but do nothing for off primary viewing, due to prismatic differences between lens powers. Of course, a contact lens OS would be ideal, but to make eyeglasses slightly (and I mean slightly) more comfortable on image size difference, you could cut the cyl on the OS, the patient is getting a +1.6-2.00% difference in minification at 30 to 120 degree viewing, possible ghosting issue.

Increase the base curve ( not by much, you’ll get away from best curve theory), and center thickness of the OS ( maybe to 4.mm thick). And have the OS edged with the bevel as far to the + side of the lens as possible, this will decrease the vertex and induce more magnification to the minus lens…

These can “help” with the magnification/minimization in the above Rx, but glasses just aren’t going to be a good solution, I would highly recommend working toward a CL on the OS. Short of refractive surgery, ( definitely not so young of patient) CL’s are the future for this patient’s visual rehabilitation.


Zhù nǐ hǎo yùn, wǒ de péngyǒu!

I see what you mean. You are right, CL is the final resort.

good luck to you too my friend

[ifdog]

yes the CL.
I will try your undercorrection way, also I think I should change my research direction to "how to train child to put on contract lens"
thank you very much!

[drk]

A child that age will need to have the parent put in the contact lens. It's not easy to have this done.