ANISEIKONIA

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.

 

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 persistent history of asthenopia

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

    2. with no manifest ocular anomaly accounting for these symptoms

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

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

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

 

 

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

d. when symptoms are relieved by wearing a size lens for a trial peiod (if you have them).

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. significant differenecs between calculated & measured aniseikonia

 

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

a. alternate occlusion

b. vertical dissociation

 X. MEASUREMENT

a. Eikonometer

 

XI. PROGNOSIS

a. age

 

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:

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

 

6. formulas:

SM = (MS)(MP)

 

SM = [1/(1-(t/n)F1)] [1/(1-hFv)]

 

Case 1:

21 year old college student.

Since getting new glasses, patient has been bothered by headaches, dizziness and nausea which disappear soon after removing glasses. He noticed while wearing the glasses things feel "unnatural" for several minutes after putting on glasses and taking them of. Does not want to wear glasses constantly because they make him nervous and give marked local eye discomfort and a frontal headache if worn more than one-half hour at a time.
O.D. -2.25 - 0.75 x 120 O.S. -2.25 -0.75 x 60 20/15
Binocular vision: WNL
Aniseikonia with Rx: R 1.05% x 135 L 1.05% x 45
Aniseikonia with -2.50 equiv. sphere R <0.2% L <0.2%
Rx: 20/20 O.D., O.S. O.D. -2.50 O.S. -2.50

After 1 month of wearing the spherical correction, the patient reported comfort.

Case 2:

Accountant, 33 years old

Eye-strain and headaches with GI disturbances, dizziness and nervousness for the past 10 years associated with the use of the eyes. Has worn glasses since age 10 years, constantly for the past 15 years; has had previous care with partial relief of symptoms; symptoms reduced on weekends or when close work is avoided.
O.D. +1.50 - 2.00 x 90 (habitual) O.S. + 3.50 - 1.50 x 90 20/20; 20/25
O.D. +2.00 - 2.00 x 90 (refraction) O.S. + 4.00 -1.50 x 90 20/20; 20/25
Aniseikonia with Rx: O.D. 1.60% OA with 1.00% x 22
Rx given: O.D. +2.00 - 2.00 x 90 with 2.00% overall
O.S. +4.00 - 1.50 x 90

Overall magnification given only. The patient reported after one month and one year that he was considerably aided by the new glasess. He has been able to do more than his normal amount of work with comfort.

Case 3:

Homemaker, age 21 wearing no glasses

Asthenopic symptoms and headaches associated with close work, movies, television, and driving. Three pairs of different glasses and convergence therapy for two months had brought no significant relief.
No Rx: O.D. 20/20 O.S. 20/20
Refraction: O.D.: plano 20/20 O.S.: +0.25 20/20
Aniseikonia: R 2.85% x 130 L 2.10% x 40
Rx given: O.D.: plano with 1.5% x 130 O.S.: +0.25 with 1.0% x 40

After wearing the correction for more than one month, the patient reported considerable relief from her symptoms for the first time in two years. She had tried to go without the glasses on a few occasions, only to experience a return of the headaches and local eye discomfort.