Aniseikonia

Aniseikonia

1. DR.ANKITA MAHAPATRA 2 ND YR PG RESIDENT DEPARTMENT OF OPHTHALMOLOGY VSSIMSAR,BURLA ANISEIKONIA

2. INTRODUCTION ANISEIKONIA(= UNEQUAL IMAGES) Anomaly of binocular vision , where ocular images are unequal in size or shape or both . Aniseikonia considered to be clinically significant when a patient's VISUAL SYSTEM has difficulty combining two images of different size and/or shape in a single perceived image.

3. TYPES :

4. STATIC

5. DYNAMIC

6. AETIOLOGY

7. OPTICAL : Causes: 1.Secondary to anisometropia caused naturally 2.Secondary to refractive surgery 3.Pseudophakic IOL implantation 4. Aphakia Aniseikonia is frequently associated with anisometropia. Isometropic aniseikonia may exist if one eye is simply larger than the other, without a refractive error difference and without a compensating redistribution of neural elements however such a condition is rare.

8. Aniseikonia can be described by means of ANISEIKONIC ELLIPSE. Overall aniseikonia describes the minimum aniseikonia. Meridional aniseikonia describes the direction dependent part (Usually induced by cylindrical glass correction)

9. RETINAL : Spacing of retinal elements The retinal elements that receive the optical image carry image size information The density and distribution of retinal receptors would therefore be expected to influence perceived image size If the spacing or density of these retinal elements differs between the two eyes, the perceived image sizes may also differ as a result of the differential spacing of these retinal elements. 2.Compression,stretching or damage to retina can cause light projected on retina by a perceived image to appear larger(Macropsia) or smaller (Micropsia) , as a variable number of photoreceptors are stimulated.

10. CAUSES: 1.Macular edema 2.Epiretinal membrane 3.Macular hole 4.Retinal detachment 5.Retinal tears 6.Retinoschisis

11. Retinal aniseikonia varies with field angles field-dependent aniseikonia . The field angle is defined as the angle between the gaze direction and the direction of a peripheral point Schematic presentation of the ocular sources determining aniseikonia and the visualization of a visual field angle α (for simplicity the optical and visual axis are assumed equal here).

12. If there are stretching or compression forces close to the fovea, this would have little effect on the receptor distribution far away in the periphery. Therefore, a change in the receptor distribution close to the fovea will be non-uniform. Receptors are compressed at the center while a little further peripheral they are stretched to finally being unaltered in a position even further out in the periphery. The aniseikonia in this example would be field-dependent with a larger effect for smaller field angles

13. CORTICAL : Distribution of cortical nerve fiber Difference in the retinal image size may be obtained if there is alteration in the visual system after the retina

14. CLINICAL TYPES 1.Symmetrical Spherical image may be magnified or minified equally in both meridia Cylindrical image is magnified or minified symmetrical in one meridian

15. 2.ASYMMETRICAL : i.Prismatic Image difference increases progressively in one direction ii.Pincushion Image distortion increases progressively in both directions, as seen with high plus correction in aphakia iii.Barrel Image distortion decreases progressively in both direction, as seen with high minus correction iv.Oblique Image size remains the same but there occurs an oblique distortion of shape A – Spherical B – Cylindrical C – Prismatic D – Pincushion E – Barrel distortion F – Oblique distortion

16. CLINICAL FEATURES : Clinical aniseikonia defined as the amount of aniseikonia that is necessary to correct to eliminate the symptoms. Occurs when image size difference between two eyes approaches 0.75%. Symptoms are more common among those with meridional magnification differences, more likely among patients with astigmatic anisometropia. Oblique meridional aniseikonia causes rotatory deviation between fused images of vertical lines in the two eyes : DECLINATION . It becomes clinically troublesome when it approaches 0.3%

17. 1. Asthenopia symptoms Occurs when the differences in image size of the two images is between 0.75 to 5.0% Headache , difficulty reading, photophobia, difficulty of fixation, vertigo, etc 2. Disturbances of binocular vision Diplopia occur only if the difference exceeds 5%. Escape attained by suppression of one eye at an early stage. 3. Disturbances in depth perception and spatial disorientations

18. MEASUREMENT: 1.ROUGH ESTIMATE: RULE OF THUMB : Based on Knapp's law, which deals with an image size difference as projected onto the retina in anisometropia . If the difference in image size associated with anisometropia is primarily refractive in origin, the aniseikonia produced will be about 1.5% per diopter of anisometropia, but since anisometropia may be partly axial, an estimate of 1.0% per D is useful clinically.

19. 2. SPACE EIKONOMETER Formerly produced by the American Optical Company Consist of size lens that could produce magnification in two rotatable primary meridians. The display consisted of four vertical rods positioned at the four corners at the center of the cube is another vertical rod, with a cross consisting of two diagonal lines intersecting at the middle Performs its function not by actual measurement of the ocular images but by measuring the rotations of the observed field as affected by binocular clues alone.

20. SPACE EIKONOMETER

21. 3.STANDARD EIKONOMETER Contains the following elements: 1.Device for securing the fixation of the eyes in the desired planes by fixing the head and presenting the targets along the primary plane and the normal depressed plane for near 2.Devices which alter the size of the target without affecting the refraction and measure discrepancies in percent 3. Projector which presents a target which has certain elements polarized so that each eye sees some part of the target alone and some parts simultaneously 4. Polaroid filters before each eye to select the target elements for each eye

22. The patient looks through red-green glasses at the computer screen, which gives a series of test images. For each test image the patient's task is to identify which of the two I-shaped bars is perceived as larger (a forced choice procedure). Because of the red-green glasses, one eye sees only the left I-bar and the other eye sees only the right I-bar. If all images of a series have been presented, the data can be plotted in a 'psychometric curve’. Using a maximum likelihood mathematical method, the aniseikonia value can be obtained from this data. . ANISEIKONIA INSPECTOR VER 3

23. One of the advantages of this forced choice method compared to a method of adjustment (such as used in the Basic Aniseikonia test and the 'New' Aniseikonia Test) is that the forced choice method also provides information about the consistency/accuracy of the measurement. This is also very useful when the patient self-administers the test and the results are analyzed elsewhere.

24. ANISEIKONIA VALUES Represents how much the right eye should be magnified or minified to cancel the aniseikonia. Ex: -5% aniseikonia means the image in right eye is perceived as approx. 5% larger than left eye. Thus, Aniseikonia can be corrected by Minifying image in right eye by 5% Magnifying image in left by 5% Combination of both

25. SIMPLE TESTING FOR ANISEIKONIA Size Lenses Maddox Rod Penlights Prism Bar

26. SIZE LENSES : For subjective testing. Size lenses are lenses without refractive power (the image stays sharp), but with an optical magnification. They may provide the patient with a sense of what it would be like to have a certain aniseikonia correction.

27. MADDOX ROD TEST : As the patient views two penlights (or, better yet, the face of the examiner who is holding the penlights on either side) One eye sees streaks of light, while the other sees the penlights

28. Special Testing for Aniseikonia 1. The Leaf Room Effects The leaf room is a cube with no size cues, perhaps covered with patterned wallpaper, a leaf patter n. This cubical “Leaf Room” doesn’t look square if you have aniseikonia! C an use the Leaf Room to simulate aniseikonia caused by high monocular astigmatism, even in normal , With planocylindrical magnifiers (size lenses) in front of one eye only, as follows

29. This test, developed by Dr. Awaya of Japan, is very easy to administer and interpret. The test uses a very clever direct comparative approach with the patient viewing pairs of adjacent calibrated half-moon targets - one red and one green. 2. Awaya Aniseikonia Test

30. Management OPTICAL: i . Contact lens : 1.Iseikonic lenses- a correcting lens or device that generally acts by altering magnification due to thickness of the lens and base curve w/o altering its power. 2.Toric lenses- front or back surfaces are made toric or both 3.Doublet lenses- combination of two lenses, one is telescopic and the other on microscopic lens. 4.Fused bifocal lenses. ii. Aneiseikonic spectacle

31. 2.RETINAL : aniseikonia cannot be fully corrected by optical means, because an optical correction is approximately field independent. Nevertheless, a partial correction often improves the vision comfort considerably. If an optical correction is not possible or does not provide enough correction, a partial occlusion may be tried. Some (field dependent) aniseikonia patients have gained some relief by occluding part of the visual field (for example, placing a (removable) dot in the center of one of the spectacle lenses or (partially) shielding the periphery in one eye, creating kind of a tunnel vision for one eye). Instead of occluding part of the visual field, another approach could be to occlude the whole visual field, but with a partial transparent foil.

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