This document discusses aniseikonia, which is a difference in the size or shape of retinal images between the two eyes. It can be caused by refractive errors, corneal issues, lenticular problems, or axial length differences. Aniseikonia affects 2-3% of the population and can induce symptoms like headaches, eyestrain, and difficulties with depth perception. The document provides guidelines for evaluating and managing aniseikonia, including formulas for calculating spectacle and contact lens magnification to help reduce the perceived size difference between eyes. It also includes examples of calculating lens parameters to minimize aniseikonia for different refractive error scenarios.

1. Aniseikonia
Gauri Shankar Shrestha,
M.Optom, FIACLE
Lecturer
BPKLCOS, IOM, TU

2. It’s true, aniseikonia
Is less lethal than pneumonia.
Doctors, nonetheless, if choosing,
Would find pneumonia less confusing
The fine art of prescribing glasses

3. Definition
A relative difference in the size and/or
shape of the ocular images between the two
eyes to the visual cortex
Image should be equal in brightness, size
and form
Vocabulary:
 Aniseikonia:
Difference in size/shape
 Anisometropia: Difference in Rx

4. Aniseikonia
Affects 2-3% of population
Anisekonia produced by glass=2%/D

5. Estimation of Amount of
Aniseikonia
Cateogy
Linksz &
Bannon
Ogle
refractive
1.50% /D
1.5% /D
corneal
2% /D
lenticular
0.25% /D
Axial
Partly axial
1.4% /D
> 1% /D

6. Aniseikonia
Two types on basis of etiology
 Physiological
 Pathological
 Anatomical
 Optical
Induced
 Inherent (neural aniseikonia)


7. Anatomical
Can occur in emmetropes

40% of all emmetropes have at least 0.8%
Two retinal images are physically equal in size,
yet perceived to be different sizes
Caused by innate, anatomical differences between
the two eyes

8. Induced Aniseikonia
Difference in retinal image size between the
2 eyes that may be caused by:
 Axial
Anisometropia
 Refractive Anisometropia
 Induced Aniseikonia
 Caused
by external optical factors (telescopes)

9. Neural/Essential Aniseikonia
Due to anatomically distorted map of
corresponding elements
Displacement of the retinal element towards the nodal
point
 Separation of the neuro-epithelial elements
 Stretching of the retina or edema

Meridional Rx
 Causes
meridional stretching of retinal
elements

10. Types on basis of image
shape
Symmetrical type

Overall

Meridianal

Compound
Asymmetrical type

in all meridian

in one meridian

11. What Does This Do?
Disrupts fusion
Causes Diplopia
Possible Suppression
Possible Amblyopia
Possible Strabismus

12. Perception of Aniseikonia
Distorts our 3D perception
 reduces
stereopsis
 Floor may appear slanted
Cause: incorrect localization of objects in space
as judged by stereopsis
May induce binocular suppression
Spatial distortion
 Physiologic
mechanism

13. Indications
Symptoms unrelieved by the proper
refractive and / or prismatic correction
Need for an anisometropic prescription
Patient more comfortable with monocular
vision
Spatial perception difficulties
Difference in keratometry finding

14. Prism Distortion
Progressive Increase in magnification
toward the apex
Base-Out OU: concave perception
Base-In OU: convex perception
Vertical OU: declination

15. How Much Can We Tolerate?
Clinical symptoms can occur with 1-2%
magnification differences
Aniseikonia beyond 5-7% begins to
influence stereopic thresholds
Aniseikonia within the 20% range is
incompatible with binocular vision
 Aphakic
lenses
patients corrected with spectacle

16. Clinical Problem?
Yes and No!!!
Depends On:
 Patients
tolerance (adaptability)
 Nature of Aniseikonia
 Type of Rx

17. Patient’s Tolerance
Patient learns to ignore stereoscopic
distortion and relies on monocular cues
However, distortion is still present
 Revealed
available
when monocular cues are not
When will the patient most likely suffer?
 Driving
at night-eye strain, slow responses
 Using binoculars-Artificial fusing condition

18. Frequency
o
o
o
o
o
o
o
o
o
o
Headache- 67%
Asthenopia (fatigue, burning sensation, tearing, ache ,
pain, pulling etc)- 67%
Photophobia- 27%
Reading difficulty –23%
Nausea – 15%
Motility (diplopia)- 11%
Nervousness – 11%
Vertigo & dizziness- 7%
General fatigue- 7%
Spatial projection & perception- 6%

19. Prescription Guideline
The use of spherical equivalent is often useful.
Theoretically, this places the circle of least of
confusion on the retina.
Prescriptions should not be given primarily on the
basis of measurement.
Prescribe in accordance with the nature, duration,
frequency and severity of the symptoms

20. When it should be corrected
When anisekonia measured with a sensitivity
smaller than size differences measured
When patients symptoms relieved by wearing
temporary correction for reasonable length of time
'Provocative test ' putting lens in wrong eye
increase discomfort and then place it to proper eye

21. When not to prescribe
anisekonic corrections
o
When symptoms not related to use of the
eyes, refractive or heterophoric correction
o
Measurement of size difference when
inconsistent
o
Patient if comfortable with partial correction
regardless of anisekonia

22. Spectacle Magnification
Defn: Change in retinal image size brought
about by correcting lens
Formulas:
.
SM=
Retinal image size in corrected state
Retinal image size in uncorrected state
%SM= (SM-1)100%
SM(AM)=Ms x Mp = 1/(1-tF1) X 1/(1-dFv)
SM increases as F1 & t increases
For +ve lens:

SM increases as Fv & d increases

23. Clinical Application of SM Formula
Useful in predicting retinal image size
change in pts Rx
 change in vertex distance
 change in thickness of lens
 change in the bend or form of lens

.
Note: SM>1 for +ve lens
<1 for -ve lens
=1 if lens is kept in entrance pupil of
eye and Ms = 1

24. Contact lens magnification
Contact lens magnification= Image corrected
with contact lens/image corrected with
spectacle lens

25. Comparing contact lens and
spectacle image size
CLM= 1-dFsp
d= 14mm
+10.00D, CLM=0.86
-10.00D, CLM= 1.14
These derivations suggest the biggest
single advantage of contact lenses over
spectacles

26. Relative Spec Magn (RSM)
Defn: size of retinal image in corrected
ametropia as compared with that of
emmetropic eye

27. Relative spectacle
magnification
Refractive ametropia
RSM= 1+d2Fsp
With spectacle d2=d=vertex distance=14mm
RSM=unity
With contact lens
d2=1.55
RSM= unity
d= distance from spectacle point to first
principle plane of the eye

28. Relative spectacle
magnification
Axial ametropia
RSM= 1-gFsp
With spectacle g= 0
RSM=unity
With contact lens
g= feye-(d+1.55)
RSM= unity
g= distance from first focal point of eye to
the spectacle point

29. RSM in Axial Ametropia
Knapp’s Law: for an axially ametropic eye if
correcting lens is placed so that its sec
principal point coincides with ant. Focal point of
eye, size of retinal image is same as if it were
std emmetropic eye

30. General rule
Axial Ametropes should be corrected
with spectacle lenses
Refractive Ametropes should be
corrected with contact lenses

31. How To Reduce Aniseikonia
Limitation
Refractive error can’t be ascertained that
it is purely axial despite longer axial
length
Astigmatic difference are often refractive
Correcting spectacle may be of different
shapes
Wearing spectacle at anterior focal plane
is coincidence rather than by design

32. How to ascertain aniseikonia
reduction in spectacle lens
Draw power diagram to compare
corresponding meridian
 Right
eye to left eye
Draw 1% of mag difference/D of
anisometropia
Start with eye that doesn’t need
magnification
 Reduce
shape magnification in this eye to
a minimum

33. Give magnification to other eye (with most
minus or less plus)

Subtract from predicted amount 0.50-0.75% from
predicted amount
Increase shape mag of more (-) eye until it is
greater than the other eye amount given
above
Calculate it using shape nomograph
 Many combinations of CT and F1 is possible
 To control power magnification

On (-) lens use thicker and flatter lens
 On (+) lens use thinner and steeper lens


34. Example 1

36. OS: Calculation of magnification

38. Example 2

45. Example 3

51. Sample 4

52. Frame selection

53. References
Optics of contact lens, IACLE module II
Milder B, Rubin ML. Anisometropia,
Fine art of prescribing glasses.
Fannin TE, Grosvenor TP. Clinical
optics
Polasky M. Instructional media center,
College of optometry, The Ohio State
University

54. Thank you