Anisometropia refers to the condition where there is a variation in refractive power between the left and right eyes. A significant amount of anisometropia can lead to problems. In young children, an unnoticed difference in refractive error between the two eyes can result in the blurred eye failing to develop good visual acuity - a condition known as amblyopia. Even when the refractive error is fully corrected, an amblyopic eye will not be able to obtain 20/20 vision. Therefore, timely correction of anisometropia detection is crucial. Correcting anisometropia with spectacle lenses may not always be the end of the problems. The lenses themselves can create difficulties due to their magnifying or minifying effects on everything viewed through them. Each lens power magnifies objects by different amounts. When one lens has a different power than the other, the image of an object seen through one lens is not the same size as the image of that same object seen through the other lens. The brain tries to fuse these two images into one single object, but a highly powered lens in one eye may cause a greater difference in image size. This difference in image size can result in diplopia, which is called Aniseikonia. This discussion is mainly about those problems that arise as a result of correcting anisometropia with spectacles and what can be done to resolve them with spectacle lenses.

1. SANTANU RAY M.Optom (BVDU). Fellow (LVPEI)
Assistant Professor – Optometry
The Neotia University

2. • Iseikonia - Greek; is(o) = equal + eikon = image
• It is a state where the left and right eyes produce
equally sized images on the retina.
• Aniseikonia is a relative difference in the size and/or the
shape of the images seen by the right and the left eyes.

3. • Physiologic aniseikonia
– Natural, present even with the eyes identical to each other.
• Anomalous aniseikonia - can be caused by
– Anatomic structure of the eye.
– by the optics of either the eye or the correcting spectacle
lens.

4. • Anatomic Aniseikonia- Caused by an unequal distribution of
the retinal elements (rods and cones) of one eye compared with
the other.
• Optical Aniseikonia
– inherent optical aniseikonia: Result of the optics of the eye.
– induced aniseikonia: Results from correcting lenses

5. • Image in one eye is symmetrically larger than the other
eye.
– Meridional aniseikonia: meridional size difference in a
meridian of one eye compared with that of the other eye.

6. • There is a progressive increase or decrease across the
visual field.
• Due to the variable base-towards-the-center effect of
plus lenses and base-towards-the-edge prismatic effect
of minus lenses.

7. • Diplopia - Due to a magnification difference of more
than or equal to 3%.
• Prismatic effect
– Anisophoria
– Vertical imbalance
• Asthenopic symptoms
• Suppression
• Amblyopia
• Lac of stereoscopic depth perception

8. • Contact lenses
• Refractive surgeries
• Spectacles lenses
– Iseikonic lenses

9. • Magnification change induced by a single spectacle lens
is called spectacle magnification.

10. • Shape factor- thickness,
r.i. and front curvature
• Power factor - wearing
distance and BVP

11. • SM = Shape factor X Power factor

12. • Axial and Refractive ametropia
– Axial is due to alteration in axial length
– Refractive is due to change in curvature
• Knapp's law
– When a correcting lens is so placed before the eye that its
second principal plane coincides with the anterior focal point
of an axially ametropic eye, the size of the retinal image will
be the same as though the eye were emmetropic.

13. • Rabin J et al conducted a study “on the relation between
aniseikonia and axial anisometropia” and found that
aniseikonia is still present when axial ametropia is
corrected with ordinary spectacle lenses that are placed
at the theoretically correct position.

14. • Modifications of the spectacle lenses that change
relative spectacle magnification

15. • Decreasing Vertex Distance
– by selecting a frame of short vertex distance & small eye size
– by adjusting the nosepad
• Flattening the Basecurve
• Selecting a thin lens
– by selecting high index lens material
• Selecting an Aspheric design

16. • Benifits of Aspheric design
1. Optically correct
aberration
2. Make the lens flatter
3. Make the lens thinner
4. Ensure tight fit in the
frame
5. Make a lens with
progressive optics

17. • First Pass method - A concerned about Aniseikonia
without clear evidence.
• Directionally correct magnification changes - Presence
of Aniseikonia but can not be measured.
• Estimating Percent Magnification Differences
• Measuring Percent Magnification Differences

18. 1. Selecting a frame with a short vertex distance & further
reducing the vertex distance with nosepads.
2. Selecting a frame with a small eye size.
3. Using an aspheric lens design.
4. Using a high-index lens material.

22. • Estimation of magnification difference from the
prescription.
• 1% change in magnification per diopter is rule of thumb.
• Can not suggest how to make exact magnification
changes
• Problem in specifically changing lens parameters

23. • Space Eikonometer
– Used to quantitatively measure image size differences.

24. • Both testing and lens design
into a computer-based
software program
• Subject wears red-green
glasses
• the screen image is adjusted
until both halves of the image
are of equal size

27. https://youtu.be/mxNpB2Mo5xc
http://www.opticaldiagnostics.com/products/ai/demo.html

28. • 93% of the studies showed that subjects preferred the
spectacles modified to correct for aniseikonia over
traditional spectacles.
• Achiron et al compared corrections for 34
anisometropes and found that modifying lens design to
equalize relative spectacle magnifcation both reduced
aniseikonia and improved subjective comfort and
performance.

29. 1. Books W., Borish M. 2007 System for Ophthalmic Dispensing, third
edition. St. Louis, Missouri: Butterworth-Heinemann
2. Fennin E., Grosvenor T. 1987 Clinical Optics. Stoneham, MA: Butterworth
Publishers
3. Rabin J, Bradley A, Freeman RD: On the relation between aniseikonia and
axial anisometropia, Am J Optom & Physiol Optics 60:553-558, 1983.
4. http://www.opticaldiagnostics.com/products/ai/demo.html