Iseikonic Lenses.pptx

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

2. Iseikonia & Aniseikonia 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. Types of Aniseikonia 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 v ersus Optical Aniseikonia A natomic A niseikonia - C aused by an unequal distribution of the retinal elements (rods and cones) of one eye compared with the other. Optical A niseikonia inherent optical aniseikonia : Result of the optics of the eye. induced aniseikonia : Results from correcting lenses

5. Symmetrical Aniseikonia I mage in o ne 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. Asymmetrical aniseikonia T here is a progressive increase or decrease across the visual field. D ue to the variable base-towards-the-center effect of plus lenses and base-towards-the-edge prismatic effect of minus lenses.

7. Prblems with Aniseikonia 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. Correction options Contact lenses Refractive surgeries Spectacles lenses Iseikonic lenses

9. How a Spectacle Lens Changes the Image Size M agnification change induced by a single spectacle lens is called spectacle magnificatio n.

10. Factors affecting Spectacle Magnification Shape factor - thickness, r.i. and front curvature Power factor - wearing distance and BVP

11. Calculation of Spectacle Magnification SM = Shape factor X Power factor

12. According to theory the best correction 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. The incongruity between theory and practice 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. C orrecting A niseikonia with S pectacle lenses M odifications of the spectacle lenses that change relative spectacle magnification

15. Reducing magnification of Spectacle lenses 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. Aspheric lens Benifits of Aspheric design Optically correct aberration Make the lens flatter Make the lens thinner Ensure tight fit in the frame Make a lens with progressive optics

17. Ways to correct Aniseikonia 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. First Pass Method Selecting a frame with a short vertex distance & further reduc ing the vertex distance with nosepads. Selecting a frame with a small eye size. Us ing an aspheric lens design. Us ing a high-index lens material.

19. Directionally Correct method

20. Directionally Correct method

21. Directionally Correct method

22. Estimating Percent Magnification Differences 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. Measuring Percent Magnification Differences S pace E ikonomete r U sed to quantitatively measure image size differences .

24. The Aniseikonia Inspector B oth 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

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

26. Is the Eikonic lens rarely helpful? 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.

27. Bibliography Books W., Borish M. 2007 S ystem for O phthalmic D ispensing , third edition. St. Louis, Missouri: Butterworth-Heinemann Fennin E., Grosvenor T. 1987 Clinical Optics. Stoneham, MA: Butterworth Publishers Rabin J, Bradley A, Freeman RD: On the relation between aniseikonia and axial anisometropia, Am J Optom & Physiol Optics 60:553-558, 1983. http://www.opticaldiagnostics.com/products/ai/demo.html