Aniseikonia

❚ A condition in which there exists a relative difference in the size of the 2 ocular images

Image Size

❚ Retinal image: the physical size of an image formed on the retina

❚ Ocular Image: the impression which is brought to consciousness

Retinal Projection

Ocular Image Size

❚ Retinal image❚ Projection of retina on to the cortex❚ Higher cortical representation

Ocular Image Size

❚ Anatomical❚ Ocular components❚ Optical correction❚ Anisometropia❚ Pathology

Axial Anisometropia

Refractive Anisometropia

Obstacle to Fusion

❚ 1% can cause symptoms❚ 3-5% will place stress on BV❚ ~5% will prevent fusion❚ >5% may contribute to strabismus and

amblyopia

Retinal Image

Retinal Image

Spatial Distortion: Size Lens

Correction of Aniseikonia

❚ Size Lenses❚ Knapp’s Law❚ Equate retinal image sizes?

Evidence of Correction

❚ Spectacle vs Contact Lenses❚ Many studies❚ Contact lenses always produced less

aniseikonia than spectacles❚ Anisometropia must be refractive in

origin??

Aetiology of Anisometropia

❚ Sorsby et al 1960s❚ Anisometropia >2D❚ Corneal power constant❚ Anterior chamber depth constant❚ Lens curvature and thickness constant❚ Axial length variable❚ Anisometropia is AXIAL in origin

Paradox

❚ Anisometropia is axial❚ Contact lenses give best optical correction❚ Studies must have used refractive

anisometropes?❚ Perhaps theory is incorrect?

Anisometropia Study

❚ 18 subjects❚ >2D of anisometropia❚ Refractive error❚ Aniseikonia with specs and contacts❚ Keratometry❚ Ultrasound

Biometry: Corneal Power

Biometry: Axial Length

Biometry:VCD

Ratio of AL:K

Aniseikonia: Spectacles

Aniseikonia: Contact Lenses

Improvement in Aniseikonia

Experimental Evidence❚ Subjects are all axial anisometropes❚ Due to vitreous chamber depth❚ Spectacle lenses give EQUAL retinal

image sizes❚ Contact Lenses give UNEQUAL retinal

images sizes❚ Contact lenses produce LESS aniseikonia

Retinal Receptors

Retinal Sampling

Emmetropia

Myopia

Myopia

Explanation: Anisomyopia

❚ Contact lenses give optical magnification in more myopic eye

❚ Myopic eye is bigger than fellow eye❚ To cover same % of eye a bigger image is

required to stimulate same number of receptors

❚ Visual pathway produces neural minification

Explanation: Anisohyperopia

❚ Contact lenses give optical minification in more hyperopic eye

❚ Hyperopic eye is smaller than fellow eye❚ To cover same % of eye a smaller image is

required to stimulate same number of receptors

❚ Visual pathway produces neural magnification

Image size in myopia

❚ Do bigger eyes need bigger images?❚ Will VA be better with specs of CLs?❚ Is best VA constant for all refractive

errors?❚ Experimental study was needed

Models of Ocular Growth

Model 1EquatorialStretching

Model 2GlobalExpansion

Model 3PosteriorPole

Predicted VA Spectacles and CLs

50-5-10-15

-0.1

0.0

0.1

0.2

SL

CL

Refractive Error (D)

LogMAR Acuity Difference

Visual Acuity

50-5-10-15

-0.3

-0.2

-0.1

0.0

0.1

0.2

Spectacle Correction

Contact Lens Correction

Refractive Error (D)

LogMAR Visual Acuity

y = -0.153 - 0.011x; r = 0.50

y = -0.185 - 0.021x; r = 0.78

VA and Rx

50-5-10-15

0

100

200

300

Equatorial

Global

Posterior

C/mm SL

C/mm CL

Refractive Error (D)

Resolution (cycles/mm)

Summary

❚ Anisometropia main cause of aniseikonia❚ Size difference can cause problems with BV❚ Naturally occurring anisometropia is axial in

origin❚ Contact lenses produce less aniseikonia❚ Unilateral aphakics have refractive

anisometropia❚ Contact lenses produce less aniseikonia❚ Use contact lenses in practice