ANISEIKONIA

Definition - difference in image sizes between the two eyes

first recognized by Donders during the Civil War as a consequence of spectacle correction of anisometropia

 

CLASSIFICATION

I. Origin
a. optical
type
1. axial

2. refractive

b. basic, intrinsic or physiological

II. Symmetry

a. overall

b. meridional

 

PREVALENCE

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

3-5% may be clinically significant (>0.75%)

 

CLINICAL IMPORTANCE

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

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

>5% aniseikonia is not compatible with binocular vision

 

SYMPTOMS

headache (67%)

photophobia (27%)

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

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

diplopia (11%)

nervousness (7%)

fatigue (7%)

spatial distortion (6%)

tilted wall and floors

slanted ceilings

difficulties with size and distance discriminations

 

INDICATIONS OF REFERRAL FOR ANISEIKONIA EVALUATION OR TREATMENT

1. patient with a long history of asthenopia

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

2. patient with marked ocular complaints

with no manifest ocular anomaly
accounting for these symptoms

3. patient with apparent binocular vision anomalies

that have not responded
to properly applied and complied with vision therapy programs

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

5. patient with strabismus with horror fusionis

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

7. patient with anisometropia in vertical meridian, first time bifocal wearer

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

9. patient with improperly adjusted spectacles 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.

 

WHEN NOT TO PRESCRIBE

1. When symptoms are not related to use of the eyes

2. When measurement(s) of aniseikonia are inconsistent and not repeatable

 

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

alternate occlusion

vertical dissociation

double light - single Maddox rod

 

ANISEIKONIA TESTS

eikonometry
standard or direct comparison eikonometer

RESULTS

office model space eikonometer
RESULTS

 

PROGNOSIS

age

type of refractive error

level of binocularity

measured vs. calculated aniseikonia

type of aniseikonia

 

 http://www.angelfire.com/ca2/eyedoc/

 

CLINICAL MANAGEMENT

 

1. Knapp's law

 

 

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

3. Small frame size

4. Minus cylinder form

5. Reduce cylinder power (maintain equivalent sphere)

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

7. Spectacle lens modification

a. front surface lens power

b. center thickness

c. vertex distance

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

e. 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)

 

f. formulas:

SM = (MS)(MP)

 

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