This document discusses aniseikonia, a condition where the two retinal images differ in size. It notes that anisometropia above 2 diopters can cause symptoms or issues with binocular vision. While anisometropia was thought to be refractive in origin, studies found subjects had axial anisometropia due to differences in vitreous chamber depth. Contact lenses produced less aniseikonia than spectacles, and this is explained as contact lenses providing optical magnification or minification that better matches the retinal receptor sampling in myopic or hyperopic eyes compared to spectacles. The document concludes that naturally occurring anisometropia is typically axial in origin, and contact lenses are better than

1. Aniseikonia A condition in which there exists a relative difference in the size of the 2 ocular images

2. Image Size Retinal image: the physical size of an image formed on the retina Ocular Image: the impression which is brought to consciousness

3. Retinal Projection

4. Ocular Image Size Retinal image Projection of retina on to the cortex Higher cortical representation

5. Ocular Image Size Anatomical Ocular components Optical correction Anisometropia Pathology

6. Axial Anisometropia

7. Refractive Anisometropia

8. Obstacle to Fusion 1% can cause symptoms 3-5% will place stress on BV ~5% will prevent fusion >5% may contribute to strabismus and amblyopia

9. Retinal Image

10. Retinal Image

11. Spatial Distortion: Size Lens

12. Correction of Aniseikonia Size Lenses Knapp’s Law Equate retinal image sizes?

13. Evidence of Correction Spectacle vs Contact Lenses Many studies Contact lenses always produced less aniseikonia than spectacles Anisometropia must be refractive in origin??

14. Aetiology of Anisometropia Sorsby et al 1960s Anisometropia >2D Corneal power constant Anterior chamber depth constant Lens curvature and thickness constant Axial length variable Anisometropia is AXIAL in origin

15. Paradox Anisometropia is axial Contact lenses give best optical correction Studies must have used refractive anisometropes? Perhaps theory is incorrect?

16. Anisometropia Study 18 subjects >2D of anisometropia Refractive error Aniseikonia with specs and contacts Keratometry Ultrasound

17. Biometry: Corneal Power

18. Biometry: Axial Length

19. Biometry:VCD

20. Ratio of AL:K

21. Aniseikonia: Spectacles

22. Aniseikonia: Contact Lenses

23. Improvement in Aniseikonia

24. Experimental Evidence Subjects are all axial anisometropes Due to vitreous chamber depth Spectacle lenses give EQUAL retinal image sizes Contact Lenses give UNEQUAL retinal images sizes Contact lenses produce LESS aniseikonia

25. Retinal Receptors

26. Retinal Sampling

27. Emmetropia

28. Myopia

29. Myopia

30. Explanation: Anisomyopia Contact lenses give optical magnification in more myopic eye Myopic eye is bigger than fellow eye To cover same % of eye a bigger image is required to stimulate same number of receptors Visual pathway produces neural minification

31. Explanation: Anisohyperopia Contact lenses give optical minification in more hyperopic eye Hyperopic eye is smaller than fellow eye To cover same % of eye a smaller image is required to stimulate same number of receptors Visual pathway produces neural magnification

32. Image size in myopia Do bigger eyes need bigger images? Will VA be better with specs of CLs? Is best VA constant for all refractive errors? Experimental study was needed

33. Models of Ocular Growth

34. Predicted VA Spectacles and CLs

35. Visual Acuity

36. VA and Rx

37. Summary Anisometropia main cause of aniseikonia Size difference can cause problems with BV Naturally occurring anisometropia is axial in origin Contact lenses produce less aniseikonia Unilateral aphakics have refractive anisometropia Contact lenses produce less aniseikonia Use contact lenses in practice