I. INTRODUCTION

Definition - difference in image sizes between the two eyes

first recognized by Donders during the Civil War (1864) as a consequence of anisometropia and noted that these differences may disturb binocular vision single vision or even make it impossible.

In 1889, Green was the first to interpret the correction of astigmatism producing distortion in binocular space peception resulted from the changed disparities between the retinal images.

Walter Lancaster, prominent Boston ophthalmologist suggested the term aniseikonia (not equal images)

Dartmouth Eye Institute founded by Adelbert Ames, Charles Proctor, Kenneth Ogle, Robert Bannon, Gordon Glidden. Closed in 1947.

II. CLASSIFICATION

I. Origin

a. optical

1. type

i. axial

ii. refractive

b. basic, intrinsic or physiological

II. Symmetry

a. overall

b. meridional

 

III. PREVALENCE

a. 20-30% of the spectacle wearing population exhibits a measurable amount of aniseikonia

b. 3-5% may be clinically significant (>0.75% image size difference)

 

IV. CLINICAL IMPORTANCE

a. it is thought that >0.75% is clinically significant and can produce symptoms

b. 1-3% aniseikonia is thought to produce definit symptoms and binocular fusion difficulties

c. >5% aniseikonia is not compatible with binocular vision

 

V. SYMPTOMS

characteristic symptoms reported by 500 patients refered for aniseikonia examination at Dartmouth Eye Institute:

a. headache (67%)

b. astenopia (discomfort, pulling, burning, etc.) (67%)

c. photophobia (27%)

d. nearpoint asthenopia, slow reading, poor concentration (23%)

e. nausea, vertigo or dizziness, car sickness (15%)

f. diplopia (11%)

g. nervousness (7%)

h. fatigue (7%)

i. spatial distortion (6%)

a. tilted wall and floors

b. slanted ceilings

c. difficulties with size and distance discriminations

i. most patients are able to suppress their incorrect spatial localization in familiar surroundings, but they expeience discomfort, which may be due to he conflict between the perspective and the incorrect stereoscopic clues in aniseikonia.

ii. unless the patient has some basis for knowing what is normal, they are inclined to believe that what they see is normal.

iii. beware trained observers

VI. INDICATIONS OF REFERRAL FOR ANISEIKONIA EVALUATION OR TREATMENT

a. patient with a long history of asthenopia

1. associated with visual tasks that has not been successfully eliminated by conventional methods

b . patient with marked ocular complaints

1. with no manifest ocular anomaly accounting for these symptoms

c. patient with apparent binocular vision anomalies that have not responded to properly applied and complied with vision therapy programs

d. patient with strabismus with difficulties establishing motor and sensory fusion

e. patient with strabismus with horror fusionis

f. patient with anisometropia of >2.00 diopters, spherical or astigmatic

g. patient with onset of spatial disorientation coinciding with spectacle Rx change

h. patient with improperly adjusted spectacleß Rx

 

Note the date>>>   Things are much different now with intraocular lens implants! Think of trying to get a monocular apkakic patient to fuse wearing a +12.00 lens unilaterally! Nowadays, think about aniseikonia issues surrounding pseudophakia and refractive surgery.

VII. WHEN TO PRESCRIBE

a. when a definite aniseikonia can be measured with a sensitivity smaller than the size difference measured

b. when there are significant symptoms related to the use of the eyes

c. when other corrections have failed to give relief

d. when monocular occlusion relieves symptoms and accommodative/vergence dysfunctions are R/O

e. when symptoms are relieved by wearing a size lens for a trial peiod.

VIII. WHEN NOT TO PRESCRIBE

a. when symptoms are not related to use of the eyes

b. when measurement(s) of aniseikonia are inconsistent and not repeatable

c. beware calculated or predicted aniseikonia

IX. SCREENING TESTS (these are of some value but may have a sensitivity of only 2-5%)

a. alternate occlusion

b. vertical dissociation

c. double light - single Maddox rod

X. ANISEIKONIA TESTS

a. eikonometry

1. standard or direct comparison eikonometer

2. office model space eikonometer

XI. PROGNOSIS

a. age

b. type of refractive error

c. level of binocularity

d. measured vs. calculated aniseikonia

e. type of aniseikonia

 http://www.angelfire.com/ca2/eyedoc/

XII. CLINICAL MANAGEMENT

 

a. Knapp's law

b. Contact lenses and the fallacy of Knapp's law (the photoreceptor hypothesis)

c. Small frame size

d. Minus cylinder form

e. Reduce cylinder power (maintain equivalent sphere)

f. Rotate cylinder axis toward x90 and x180 (reduce cylinder power)

g. Spectacle lens modification

1. front surface lens power

2. center thickness

3. vertex distance

4. lens material index of refraction (crown 1.523, high index 1.66, polycarbonate 1.586, other plastic 1.49)

5. to increase magnification

i. increase front surface power (F₁)

ii. increase center thickness (t)

iii. increase vertex distance (h) for plus power

iv. decrease vertex distance (h) for minus power

v. decrease index of refraction (n)

 

6. formulas:

SM = (M_S)(M_P)

 

SM = [1/(1-(t/n)F₁)] [1/(1-hF_v)]

Case 1:

21 year old college student.