This presentation is prepared in the process of me teaching Visual Optics for graduate level optometry students. It narrates the type, clinical manifestations, clinical assessment and treatment of Aniseikonia.

1. ANISEIKONIA
By-
Sangita Sarma
Assistant Professor, optometry
Jain University

2. Definition
• The word aniseikonia derives from the Greek words "an," "is," and
"eikon," which mean "not," "equal," and "image," respectively.
Aniseikonia occurs when there is a difference in an image's
perceived size or shape and is often caused by anisometropia, which is a
difference in refractive error.

3. How much aniseikonia can cause
symptom?
• Aniseikonia at a young age can result in amblyopia. Aniseikonia at a later
age can cause asthenopia, headache, diplopia, dizziness, nervousness,
imbalance, nausea, spectacle intolerance, ocular suppression, and
distorted space perception.
• It is thought that over 0.75% of aniseikonia can start to cause symptoms,
that at 1 to 3% definite symptoms are present, and that more than 5%
(upto 2-3 D of anisometropia) of aniseikonia is incompatible with
binocular vision.

4. Symmetric vs Asymmetric
aniseikonia
• 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.
• Changes in the cornea and the crystalline lens can cause anisometropia
and as well as aniso-astigmatism, while changes in the length of the
eyeball (axial length) or a difference in the refractive index of the
vitreous cavity contents can only cause anisometropia and do not result
in aniso-astigmatism.

5. Knapp’s law
• According to Knapp's law, axial anisometropia does not cause
aniseikonia when the refractive difference between the eyes is solely due
to a difference in the axial length (distance from the cornea to the retina)
and when the correcting lens is placed at the anterior focal point of the
eye which is about 16 mm in front of the cornea.
• However, clinical applications of Knapp's law are limited as
anisometropia is usually not purely axial. The retina is usually stretched
in the eye with the increased axial length (a cause of retinal aniseikonia),
and placing a correcting lens at 17 mm in front of the eye is impractical.

6. Types- 1. Optical Aniseikonia
• Optical Aniseikonia. This is caused by a difference in refractive error. A
disparity in the eye's spherical refractive error is called anisometropia,
while aniso-astigmatism is reserved for dissimilarity in the amount or the
orientation of cylindrical error. Anisometropia causes a difference in size,
while aniso-astigmatism causes a difference in the shape of an image.
• Spectacle lenses may cause a change in the shape of objects as well.
High minus lenses will cause a barrel type of distortion (the corners of
the image appear squeezed inward), and high plus lenses will cause a
pincushion type of distortion (the corners of the image appear stretched
outward). Unequal barrel or pincushion distortion between the two eyes
can also result in optical aniseikonia.

7. Causes of Optical Aniseikonia
• Corneal---- Keratoconus, Corneal edema, post refractive
corneal surgeries
• Lens---- cataract(Inc. RI, causing index myopia),
unilateral aphakia post cataract Sx (earlier),
accommodative spasm,
• Vitreous---Silicone oil is used for retinal tamponade and
may cause hyperopic or myopic shifts based on posture
and phakic, pseudophakic, or aphakic status of the eye.
Intravitreal gas such as sulfur hexafluoride or
perfluoropropane is used for retinal tamponade and may
generate a large myopic shift of up to 50 diopters.[

10. • Axial length---Anisometropia and the resultant
aniseikonia, when due to an increase in axial length, can
be congenital and, when not treated, can lead to
amblyopia.
• Retinal---Macula swelling causes a pincushion type of
distortion as well as micropsia. For every 1000 microns
of elevation, about 3 diopters of increased hyperopia are
expected to occur. Shrinking of the macula will cause a
barrel type of distortion and macropsia.

11. • Altered Brain Function---This can be the cause of
symptoms of micropsia and macropsia seen in
conditions such as Alice in Wonderland syndrome,
Charles Bonnet syndrome, migraine etc.

12. Spectacle Magnification
• In practice, the most common method to estimate aniseikonia is by
calculating the difference in spectacle magnification between the
two eyes.
• SM for a thin lens is SM = 1/(1-dFsp). d is the distance in meters
from the spectacle lens to the entrance pupil.
• Vertex distance is generally close to 12 mm (0.012 m).
• The distance from the apex of the cornea to the entrance pupil is
assumed to be an additional 3 mm (0.003 m). Therefore, d is
usually close to 15 mm (0.015 m)
• Rule of thumb that 1 diopter of anisometropia generates about a
1 to 2% difference in image size

13. Some clinical tests-
• 1. Binocular balancing (Maginifaction controlled customized size lenses
can be used)
• 2. The Turville test evaluates aniseikonia in the vertical meridian. Using
a device developed by Morgan, in which the two eyes are dissociated,
each eye sees two horizontal lines separated by a vertical line. An
unequal distance between the horizontal lines on each side indicates
aniseikonia.
• 3. Maddox rod and two light sources-This test also called the Brecher
test, employs a Maddox rod placed over one eye to dissociate the two
eyes. Dissociation can be done with a Maddox rod placed over each eye
as well. This is called the Miles test. The two lights are separated
horizontally or vertically to measure horizontal or vertical aniseikonia,
respectively. Size lenses can then be used to eliminate a difference in the
separation of the lights.

14. • 4. The space eikonometer is a very accurate test and is
considered the gold standard for measuring aniseikonia. It
requires single binocular vision with visual acuity of 20/60
or better and the ability to perform a relatively complex task
of aligning four images of lines to be at an equal distance
around a central target. The device was developed and sold
by American Optical in 1951 and is no longer in production.
This device can measure up to 5% of aniseikonia

15. • 5. the (Awaya) New Aniseikonia Test (Good-Lite Company,
Tokyo, Japan) uses red-green glasses to dissociate the two
eyes and a booklet with red and green half-circles of
different relative sizes. The observer finds the image where
the half circles appear to be the same size. If, in reality, the
half-circles are a different size, a diagnosis of aniseikonia is
made.

16. •TREATMENT OF ANISEIKONIA

17. 1. Management of Optical
Aniseikonia
• 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.
• Contact lens(es). 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. Therefore, using contact lenses on one or both eyes to fully correct
the patient's refractive error is a highly effective way of reducing optical
aniseikonia to tolerable levels.
• Contact lens-spectacle lens combinations. Often it is most desirable to the patient
to eliminate anisometropia with a contact lens that corrects only the difference
between the anisometropic spectacle lenses. Spectacle lenses often
provide superior correction for presbyopia and astigmatism, are generally more
effective in filtering out different light wavelengths (sunglasses, blue-light
blocking lenses), and spectacles provide protection as well.

18. cntd
• 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.
• Neuroadaptation. There is evidence that rapid neuroadaptation can occur
to adjust for the onset and after surgical reversal of
aniseikonia. Therefore, in some cases, when other solutions are
unavailable, prescribing an anisometropic spectacle, Rx may allow
neuroadaptation and some degree of binocularity.
• Refractive surgery (e.g., LASIK, PRK, RK) is a common way to
correct optical aniseikonia.
• Clear lens exchange, also called refractive lens exchange, is when the
crystalline lens is surgically removed and replaced with an intraocular
lens implant. This procedure is used less often as a solution for
aniseikonia, but with the advent of good multifocal intraocular lenses,
many surgeons and patients alike feel the benefit outweighs the relatively
small risk of complications such as infection and retinal detachment.

19. • 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.
• Advantages of Contact Lenses for Aniseikonia
• It is the most effective and convenient way of eliminating optical aniseikonia.
• It can be combined with a spectacle lens to create a magnifier to treat retinal
aniseikonia.
• Contact lenses can usually immediately be dispensed from trial lens inventory at
the initial visit, and the lens power can be changed easily and quickly in the case
of changes of refractive error or aniseikonia, making it a convenient solution for
unstable conditions and those that require immediate intervention.
• Disadvantages of Contact Lenses for Aniseikonia
• Many patients may not be able to wear contact lenses due to allergies, dry eye, or
problems handling the lenses.
• Contact lenses alone can not treat retinal aniseikonia.
• ide magnification of one lens compared to the other.

20. • 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.
• 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.
• One lens is generally thicker than the other. This is 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.

21. Differential diagnosis
• Anisometropia causes induced prism. When anisometropia is
accompanied by symptoms of asthenopia, diplopia, nausea, imbalance,
and headache, the assumption is generally made that aniseikonia is the
cause.
• However, anisometropic spectacles can cause similar symptoms due to
either optical aniseikonia (from a difference in spectacle magnification
between the eyes) or from induced prism when looking away from the
optical center of the lenses (Prentice rule).
• To add to the potential confusion, contact lenses will treat both induced
prism and optical aniseikonia.
• Separate single vision distance and reading glasses to alleviate the
symptoms (doesn’t require looking away from optical center).
• Altered brain function can cause micropsia and macropsia, and other distortions and can
occur in conditions such as Alice in Wonderland syndrome, Charles Bonnet syndrome,
migraine, and other types of hallucinations but they are temporary in nature.

22. Prognosis
• Optical aniseikonia generally has an excellent prognosis as most
refractive conditions can be treated with surgery or contact lenses.
Iseikonic spectacles are also available.
• Retinal aniseikonia has a good prognosis in most cases. In the case of
retinal edema, central serous chorioretinopathy is generally a self-limited
condition that reverses without treatment.
• If aniseikonia has resulted in amblyopia and the patient is youfg age
(,10years), then treatment for amblyopia according to the Pediatric Eye
Disease Investigator Group guidelines will significantly improve visual
outcomes.

23. References
• Borish Clinical Refraction
• Clinical Procedures in Primary Eye Care, Elliott

24. • Thank yu