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Aniseikonia <ul><li>A condition  in which there exists a relative difference in the size of the 2 ocular images </li></ul>
Image Size <ul><li>Retinal image: the physical size of an image formed on the retina </li></ul><ul><li>Ocular Image: the i...
Retinal Projection
Ocular Image Size <ul><li>Retinal image </li></ul><ul><li>Projection of retina on to the cortex </li></ul><ul><li>Higher c...
Ocular Image Size <ul><li>Anatomical </li></ul><ul><li>Ocular components </li></ul><ul><li>Optical correction </li></ul><u...
Axial Anisometropia
Refractive Anisometropia
Obstacle to Fusion <ul><li>1% can cause symptoms </li></ul><ul><li>3-5% will place stress on BV </li></ul><ul><li>~5% will...
Retinal Image
Retinal Image
Spatial Distortion: Size Lens
Correction of Aniseikonia <ul><li>Size Lenses </li></ul><ul><li>Knapp’s Law </li></ul><ul><li>Equate retinal image sizes? ...
Evidence of Correction <ul><li>Spectacle vs Contact Lenses </li></ul><ul><li>Many studies </li></ul><ul><li>Contact lenses...
Aetiology of Anisometropia <ul><li>Sorsby et al 1960s </li></ul><ul><li>Anisometropia >2D </li></ul><ul><li>Corneal power ...
Paradox <ul><li>Anisometropia is axial </li></ul><ul><li>Contact lenses give best optical correction </li></ul><ul><li>Stu...
Anisometropia Study <ul><li>18 subjects </li></ul><ul><li>>2D of anisometropia </li></ul><ul><li>Refractive error </li></u...
Biometry: Corneal Power
Biometry: Axial Length
Biometry:VCD
Ratio of AL:K
Aniseikonia: Spectacles
Aniseikonia: Contact Lenses
Improvement in Aniseikonia
Experimental Evidence <ul><li>Subjects are all axial anisometropes </li></ul><ul><li>Due to vitreous chamber depth </li></...
Retinal Receptors
Retinal Sampling
Emmetropia
Myopia
Myopia
Explanation: Anisomyopia <ul><li>Contact lenses give optical magnification in more myopic eye </li></ul><ul><li>Myopic eye...
Explanation: Anisohyperopia <ul><li>Contact lenses give optical minification in more hyperopic eye </li></ul><ul><li>Hyper...
Image size in myopia <ul><li>Do bigger eyes need bigger images? </li></ul><ul><li>Will VA be better with specs of CLs? </l...
Models of Ocular Growth
Predicted VA Spectacles and CLs
Visual Acuity
VA and Rx
Summary <ul><li>Anisometropia main cause of aniseikonia </li></ul><ul><li>Size difference can cause problems with BV </li>...
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Aniseikonia

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Aniseikonia

  1. 1. Aniseikonia <ul><li>A condition in which there exists a relative difference in the size of the 2 ocular images </li></ul>
  2. 2. Image Size <ul><li>Retinal image: the physical size of an image formed on the retina </li></ul><ul><li>Ocular Image: the impression which is brought to consciousness </li></ul>
  3. 3. Retinal Projection
  4. 4. Ocular Image Size <ul><li>Retinal image </li></ul><ul><li>Projection of retina on to the cortex </li></ul><ul><li>Higher cortical representation </li></ul>
  5. 5. Ocular Image Size <ul><li>Anatomical </li></ul><ul><li>Ocular components </li></ul><ul><li>Optical correction </li></ul><ul><li>Anisometropia </li></ul><ul><li>Pathology </li></ul>
  6. 6. Axial Anisometropia
  7. 7. Refractive Anisometropia
  8. 8. Obstacle to Fusion <ul><li>1% can cause symptoms </li></ul><ul><li>3-5% will place stress on BV </li></ul><ul><li>~5% will prevent fusion </li></ul><ul><li>>5% may contribute to strabismus and amblyopia </li></ul>
  9. 9. Retinal Image
  10. 10. Retinal Image
  11. 11. Spatial Distortion: Size Lens
  12. 12. Correction of Aniseikonia <ul><li>Size Lenses </li></ul><ul><li>Knapp’s Law </li></ul><ul><li>Equate retinal image sizes? </li></ul>
  13. 13. Evidence of Correction <ul><li>Spectacle vs Contact Lenses </li></ul><ul><li>Many studies </li></ul><ul><li>Contact lenses always produced less aniseikonia than spectacles </li></ul><ul><li>Anisometropia must be refractive in origin?? </li></ul>
  14. 14. Aetiology of Anisometropia <ul><li>Sorsby et al 1960s </li></ul><ul><li>Anisometropia >2D </li></ul><ul><li>Corneal power constant </li></ul><ul><li>Anterior chamber depth constant </li></ul><ul><li>Lens curvature and thickness constant </li></ul><ul><li>Axial length variable </li></ul><ul><li>Anisometropia is AXIAL in origin </li></ul>
  15. 15. Paradox <ul><li>Anisometropia is axial </li></ul><ul><li>Contact lenses give best optical correction </li></ul><ul><li>Studies must have used refractive anisometropes? </li></ul><ul><li>Perhaps theory is incorrect? </li></ul>
  16. 16. Anisometropia Study <ul><li>18 subjects </li></ul><ul><li>>2D of anisometropia </li></ul><ul><li>Refractive error </li></ul><ul><li>Aniseikonia with specs and contacts </li></ul><ul><li>Keratometry </li></ul><ul><li>Ultrasound </li></ul>
  17. 17. Biometry: Corneal Power
  18. 18. Biometry: Axial Length
  19. 19. Biometry:VCD
  20. 20. Ratio of AL:K
  21. 21. Aniseikonia: Spectacles
  22. 22. Aniseikonia: Contact Lenses
  23. 23. Improvement in Aniseikonia
  24. 24. Experimental Evidence <ul><li>Subjects are all axial anisometropes </li></ul><ul><li>Due to vitreous chamber depth </li></ul><ul><li>Spectacle lenses give EQUAL retinal image sizes </li></ul><ul><li>Contact Lenses give UNEQUAL retinal images sizes </li></ul><ul><li>Contact lenses produce LESS aniseikonia </li></ul>
  25. 25. Retinal Receptors
  26. 26. Retinal Sampling
  27. 27. Emmetropia
  28. 28. Myopia
  29. 29. Myopia
  30. 30. Explanation: Anisomyopia <ul><li>Contact lenses give optical magnification in more myopic eye </li></ul><ul><li>Myopic eye is bigger than fellow eye </li></ul><ul><li>To cover same % of eye a bigger image is required to stimulate same number of receptors </li></ul><ul><li>Visual pathway produces neural minification </li></ul>
  31. 31. Explanation: Anisohyperopia <ul><li>Contact lenses give optical minification in more hyperopic eye </li></ul><ul><li>Hyperopic eye is smaller than fellow eye </li></ul><ul><li>To cover same % of eye a smaller image is required to stimulate same number of receptors </li></ul><ul><li>Visual pathway produces neural magnification </li></ul>
  32. 32. Image size in myopia <ul><li>Do bigger eyes need bigger images? </li></ul><ul><li>Will VA be better with specs of CLs? </li></ul><ul><li>Is best VA constant for all refractive errors? </li></ul><ul><li>Experimental study was needed </li></ul>
  33. 33. Models of Ocular Growth
  34. 34. Predicted VA Spectacles and CLs
  35. 35. Visual Acuity
  36. 36. VA and Rx
  37. 37. Summary <ul><li>Anisometropia main cause of aniseikonia </li></ul><ul><li>Size difference can cause problems with BV </li></ul><ul><li>Naturally occurring anisometropia is axial in origin </li></ul><ul><li>Contact lenses produce less aniseikonia </li></ul><ul><li>Unilateral aphakics have refractive anisometropia </li></ul><ul><li>Contact lenses produce less aniseikonia </li></ul><ul><li>Use contact lenses in practice </li></ul>

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