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Design of Iseikonic Lenses, Part One

June 30, 2017

Reprinted with permission, Opticians Handbook, Jobson Medical Information LLC, all rights reserved February 2014

Eyes/Optics

 

The design of iseikonic lenses is a tool given to opticians to correct aniseikonia.

Aniseikonia is defined as a difference in the size and/or shape of the ocular images corresponding to each of the two eyes. To make a person see clearly, the images that are appearing on the retina and then transferred to the brain by the optic nerve have to be the same size and overlapping. If the signals or images do not concur, aniseikonia occurs.

Patients wearing glasses with large differences in the prescription for the right and left eye are the most likely candidates to experience aniseikonic symptoms. The large difference in the amount of refractive error is known as anisometropia. An antimetropic Rx is a prescription that has opposite signs for the right and left eye.

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For example:

OD     +2.00
OS     -2.00

(OD, occulus dexter, is Latin for right eye, OS occulus sinister, is Latin for left eye.)

The question now is whether the large difference in refractive error, or the spectacles used to correct it, induces aniseikonia. The answer is given by a principle of geometric optics: Any lens placed at the plane of the entrance pupil of any optical system produces no magnification.

If we could place our spectacle lenses at the plane of the iris, they would not produce any magnification. Eyeglasses need to be carried on the nose and need to leave a certain space for the eyelid to open and close. Therefore, the glasses correcting for the large difference in refractive error, and not the refraction itself, are responsible for the aniseikonic symptoms.

However, these symptoms only occur when the patient can achieve binocular vision.
The weaker eye has to achieve a visual acuity of 20/60 or better. Aniseikonia is caused by the difference in image sizes produced on the retina. To notice the difference, both eyes have to be able to recognize it. If one eye sees clearly, the other very blurry, the brain will only accept the clear image, only one image, no magnification difference.

second-pictKnowing when aniseikonia occurs, the next question would be: Why does it occur? Each spectacle lens has a certain amount of magnification, which varies with the power or shape of the lens. A +10.00 lens magnifies more than a +2.00 lens. This explains the power factor. The shape factor is determined by the thickness and the curvature of a spectacle lens; the thicker the lens and the steeper the curves, the more it magnifies. The power and the shape factor combined determine the magnification of a spectacle lens. If the image size produced by the right eye varies in magnification from the left eye’s image by 2 percent or more, ainiseikonia may occur.

Identifying Anisokonia
The first step is to look at the Rx. A large difference between the right and left eye might be an indication. An example for an anisometropic Rx would be

OD    +1.00
OS      +5.00

A difference of 4 diopters would certainly raise suspicion. To simplify the calculations for astigmatic Rxs, the spherical equivalent is used. To get the spherical equivalent, the amount of cylinder is cut in half and added to the spherical value.

For example
OD     +2.50     +0.50 x 090
OS      plano    +5.00 x 090

The spherical equivalents are

OD    +2.75
OS    +2.50

and it is not an anisometropic Rx.

The patient’s complaints will give further indication of aniseikonia being present. Headaches, eyestrain, reading difficulties, dizziness and photophobia are possible signs, or the patient might state, the glasses just “don’t feel right”. In two patients with the exact same prescription, these statements might not be the same, because tolerance for magnification differences is subjective. Therefore, just seeing an anisometropic prescription alone does not indicate aniseikonia. Many patients have adjusted to large magnification differences over the years. The glasses for those patients should be more or less a duplication of their current Rx. Same frame style, same frame size, same vertex distance, same base curve, same lens material and same lens thickness. Any changes might result in problems with the new eyewear.

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A very small percentage of patients will be symptomatic for refractive aniseikonia. And unless contraindicated, contact lenses are always the treatment of choice for aniseikonia when it is caused by anisometropia. Therefore, an optician will not have many opportunities to practice designing iseikonic lenses. However, the knowledge and understanding of how to treat aniseikonia with spectacle lenses will identify the expert optician.

 

Design of Iseikonic Lenses, Part Two

The magnification created by any ophthalmic lens is determined by:

  • Dioptric power
  • Vertex distance
  • Base curve
  • Center thickness
  • Lens material

Plus lenses will create magnification which in turn creates a larger than normal retinal image.

Minus lenses create minification (negative magnification) which will create a smaller than normal retinal image.

Magnification due to the vertex power and vertex distance of a lens is called the POWER FACTOR. Magnification produced by the shape of the lens – base curve, center thickness and index of refraction – is referred to as the SHAPE FACTOR. Together they determine the TOTAL MAGNIFICATION of an ophthalmic lens.
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Let us now analyze the individual components.

POWER FACTOR
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All ophthalmic lenses produce magnification. Magnification is expressed as positive or negative. A plus lens magnifies (+), a minus lens minifies. (has negative (-) magnification or minification.) The greater the power, the more it magnifies for a plus lens, or minifies for a minus lens.  Furthermore, the vertex distance influences the magnification of a lens; moved away from the eye, magnification increases – moved closer to the eye, magnification decreases.

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THE SHAPE FACTOR is determined by the base curve of the lens, its thickness and the material used. The following formula applies:

7-pict

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Rules of Thumb
Aniseikonia only requires correction if there is good correctable vision in both eyes. The general rule of thumb is that if one eye is not correctable to 20/60 or better then fusion of the images will not take place anyway, so a special lens design to correct for image magnification differences is not appropriate. A second concept to note is that the lens design is not intended to either remove magnification or to exactly match the magnification from each lens. It is only necessary to decrease the magnification differences to the point where binocular vision is restored.

Case Study
After cataract surgery in the right eye only, the patient presents the following distance Rx:
OD    + 1.00            VA 20/20
OS    + 5.00            VA 20/25

The optician suspects that the difference in image size is more than the patient can adapt to and considers designing iseikonic lenses. The following steps need to be taken:

1. Collect lens data
Rx    OD    + 1.00
OS    + 5.00

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For the right eye:

10-pict

For the left eye:

11-pict

The magnification difference is 8.2%   (Mag%OS – Mag%OD)

We know that clinical symptoms may show up with differences of three to five percent. Therefore, 8.2 percent magnification difference surely calls for the design of iseikonic lenses.

There are four variables we can change to reduce the magnification difference:

  • Vertex distance
  • Material – index of refraction
  • Base curve
  • Center thickness

We want to increase the magnification of the right eye, and reduce the magnification of the left eye. The starting parameters are listed in red.

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The resultant magnification difference is three percent. This will enable the patient to wear the new prescription comfortably, achieving good binocular vision.

While you may not design many iseikonic lenses in your optical career, it certainly makes you a more knowledgeable and well-rounded optician to understand why and how. To work through the magnification formula stimulates your brain and to some of us, it is fun.

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Maggie Sayers is a professional development coach and a Master Optician. Her optical career started over 30 years ago in a family business in Germany.

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