The document discusses aniseikonia, which is a difference in the perceived size of images between the two eyes. It can be caused by anisometropia (unequal refractive power between the eyes) or changes in the macula. Aniseikonia can lead to issues like amblyopia, headaches, or double vision. It describes methods for testing and treating optical aniseikonia, including calculating lens magnification, reducing vertex distance, contact lenses, lens-spectacle combinations, undercorrecting lenses, and designing specialized iseikonic spectacle lenses to equalize magnification between the eyes. Iseikonic lenses precisely alter lens properties but have disadvantages like uneven weight, poor cosmesis, and induced distortion

1. ANISEICONIC LENSES
Jithin Johney, Assistant professor of Optometry

2.  Aniseikonia occurs when there is a difference in an image's perceived size or
shape and is often caused by anisometropia.
 When aniseikonia is caused by anisometropia, it is called optical aniseikonia
 Aniseikonia can also be caused by changes in the shape and location of the macula
and is then called retinal aniseikonia
 Aniseikonia at a young age can result in amblyopia.
 Aniseikonia at a later age can cause asthenopia, headache, diplopia, dizziness,
imbalance, nausea, spectacle intolerance, ocular suppression, and distorted space
perception

3.  Aniseikonia can be subdivided into symmetrical and asymmetrical aniseikonia.
 Symmetrical aniseikonia further subdivides into spherical (overall difference in
magnification) and cylindrical (a difference in magnification in one orientation).
 Asymmetrical aniseikonia is also called distortion and occurs when perceived
image size differences are unequal in different parts of the visual field.

4.  In many cases, optical aniseikonia is predicted by calculating the difference in
spectacle magnification of the right and left spectacle lens.
 In clinical practice, the rule of thumb that every 0.25 diopter of anisometropia
causes about 0.25% to 0.5% of aniseikonia.

5.  In the case of prismatic distortion, the difference in image size increases in one
direction.
 In the case of pincushion distortion, the corners of one image are stretched more
outward than the other.
 In the case of barrel distortion, the corners of the image appear squeezed inward.
In the case of oblique distortion, one image is rotated relative to the other.
 Aniseikonia is most accurately measured with an eikonometer.

7. Testing for Retinal Aniseikonia
1. Amsler Grid.
 This test helps to detect barrel and pincushion distortion and evidence of dragging
or other types of distortion of the macula.
 This test measures distortion in the central 20 degrees of the macula.
2. Optical coherence tomography of the macula
3. Metamorphopsia include an experimental 3D digital device

8. Testing for Optical Aniseikonia
 In practice, the most common method to estimate aniseikonia is by calculating the
difference in spectacle magnification between the two eyes.
 The formula for spectacle magnification (SM) for a thin lens is SM = 1/(1-dFsp).
 d is the distance in meters from the spectacle lens to the entrance pupil.

9. Treatment of Optical Aniseikonia.
1. Reduction of vertex distance.
 The shorter the vertex distance, the less the spectacle magnification, and while it
can not eliminate it, this simple step often reduces the amount of aniseikonia to
acceptable levels.
2. Contact lens
 Contact lenses are positioned very close to the entrance pupil of the eye, and even
highly divergent lens powers will generate minor amounts of aniseikonia.

10. 3. Contact lens-spectacle lens combinations:
 Only one eye is fit with a contact lens, reducing the cost and risk of contact lens-
related infection and inflammation.
 A special pair of isometropic glasses are then prescribed to be worn in
combination with the contact lens.
4. Undercorrection of one spectacle lens.
 When neither surgical intervention, contact lenses, nor iseikonic spectacles are an
option, anisometropia may be reduced by under or over-correcting one of the
spectacle lenses.
 While this may cause one eye to see blurry, this may allow for better depth
perception.

11. 5. Iseikonic spectacle lenses
 Work by altering the vertex distance, base curve, center
thickness, and index of refraction of the spectacle lens
material to provide magnification of one lens compared to
the other.
 The formula for total magnification produced by iseikonic
lenses (Mt) equals the magnification due to lens shape
factor (Ms), which depends on the index of refraction (n),
base curve (D1) and thickness (t), and the lens power factor
(Mp), which depends on the dioptrical power of the lens
(Dv) and the vertex distance in meter (h).
 Mt = Ms x Mp
 Ms = 1/(1-(tD1/n))
 Mp = 1/(1-hDv)
MAGNIFICATION (Mt)
LENS SHAPE FACTOR (Ms)
REFRACTIVE INDEX (n)
BASE CURVE (D1)
THICKNESS (t)
LENS POWER FACTOR (Mp)
DIOPTRIC POWER (Dv)
VERTEX DISTANCE (h)

12. The impact in magnification by alteration of lens
variables is as follows:
1) Increasing vertex distance (h): increases magnification for plus and minification
for minus lenses.
2) Steepening the (front) base curve (D1): increases magnification and decreases
minification for lenses of all powers.
3) Increase lens thickness (t): increases magnification and decreases minification
for lenses of all powers.
4) Increase lens power (Dv): increases magnification for plus and minification for
minus lenses.
5) Increase index of refraction (n): decreases magnification and increases
minification.

13. Advantages of Iseikonic Lenses
 It can treat both optical and retinal aniseikonia.
 It can be the only option when surgical intervention or contact lenses are not
possible.

14. Disadvantages of Iseikonic Lenses
 It is hard to find a local provider of lenses. The advent of optical vendors of
iseikonic lenses on the internet that will assist with the design of the lenses
provides hope that this solution will be used more often.
 One lens is usually heavier than the other, creating an unequal distribution of
weight.
 Cosmetically unattractive and can make frame selection hard.
 The iseikonic lens induces prism and distortion.
 One eye will look larger than the other affecting acceptable cosmesis for the
patient.

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