Anisometria And Aniseikonia Flashcards
Define anisometropia.
Difference in refractive state of the two eyes
May occur in both meridians (spherical) or only one (cylindrical)
When is anisometropia considered to be clinically significant?
When the difference between the two eyes exceeds 1D
What are the symptoms of anisometropia?
Asthenopia (eye strain)
Headache
Photophobia
Reading difficulties
Nausea
Diplopia
Dizziness
Fatigue
Abnormal spatial localization
Preference for monocular vision (closing one eye)
What is the prevalence of clinically significant anisometropia (>1D)? How does this compare to ocular disease prevalence?
Spherical anisometropia= 10% of the general population
Cylindrical anisometropia= 5% of the general population
All ocular diseases combined= 5% of the general population
What is the hereditary etiology of anisometropia?
Congenital glaucoma, cataract, 3 nerve palsy
What is the acquired etiology of anisometropia?
Following trauma, monocular lens extraction, refractive surgery
What other differences in the eyes may be associated with anisometropia?
Vertical prism
Stimulus to ocular accommodation
Perceived image size
How do you solves the problems caused by difference amounts of vertical prism created by the different powered correcting lenses?
Switch to CL
Drop the optical centers for reading when looking down
Partial correction
Slab off prism
Franklin split bifocal
Stick on prism
How does switching to a CL help in minimizing the problems caused by correcting anisometropia?
With well fitted CL, lens moves with the eye so the patient is always looking through optical center and prism by decentration is zero
How does dropping the optical center for reading when looking down minimize the problems that occur when correcting anisometropia?
Prism by decentration when looking down now equals zero
How can we achieve slab off prism?
Removing base down prism portion of the lens bottom
Leaves base up prism in this area
When patient looks down they experience base up prism
What is reverse slab off?
For plastic lens
Base down prism is molded onto the front of the plastic lens
How does partial correction help with anisometropia?
Reduces prism by decentration and therefore differences between the eyes in prism by decentration P=cF
What is the Franklin Split Bifocal?
Created by taking two lenses, cutting each in half and using the top half of the first lens and the bottom half of the second lens to form a bifocal
Creates a bevel when glued together for stability
Describe stick on prisms (fresnel prisms).
Temporary
Stuck onto top or bottom or entire portion of lens
Made up of tiny prism arranged base to apex on plastic
Poor optical quality
Powers range 1 to 40PD
Loss of contrast sensitivity
Why don’t we perform refractive surgery on anisometropia patients?
Almost all myopia is axial
Refractive error can be corrected but the eye will still be longer, the retinal image size will be larger
How can we use stimulus to ocular accommodation to calculate the difference in ocular accommodation between two anisometropic eyes?
Calculate ocular accommodation for each eye
Take difference, if it exceeds DOF the patient will most likely not see clearly
What is perceived image size?
Different powered lenses will produce different sized images in two eyes
Brain must try to fuse these different sized retinal images into a single fused percept
What is spectacle magnification?
Shape factor X power factor
What is shape factor?
Varies with shape and thickness of the lens
Shape factor= 1/[1- (t/n’)F1]
What is power factor?
Power factor varies with back vertex power and the power of the eye
Power factor= 1/ [1- dF’v]
What is Knapp’s Law?
If the anisometropia is due to difference in axial length then the two eyes will have different image sizes
If the correcting lens is positioned at first focal length (1/power of eye) of the eye then the image size will be independent of axial length (2 eyes with different axial lengths will have same image size)
Longer eye, larger retinal image
What is the first focal point of the EYE?
Position of the object when the image is at infinity (1/power of eye)
Why is knapps law not practical?
Don’t know actual power of eye for first focal point
Position of first focal point varies with accommodation
Spectacles are not stable enough to maintain first focal point
Does not address differences in prism and AS
What is Aniseikonia?
Difference in the PERCEIVED size and/or shape of the images seen by two eyes
Effect of visual pathway
What is clinically significant Aniseikonia?
Size difference of at least 0.75% between the two ocular images
Estimated to occur in 3 to 5% of the general population
What are the symptoms of Aniseikonia?
Asthenopia
Headache
Photophobia
Reading difficulty
Nausea
Diplopia
Vertigo and dizziness
Physical fatigue
Abnormal spatial localization
Preference for monocular vision
How will the image appear to a patient with Aniseikonia?
The object will appear tilted away from the eye with the larger retinal image and closer to the eye with the smaller retinal image
Often occurs when patient gets new Rx may report that flor appear slanted
What are the methods for measurement of Aniseikonia?
Eikonometer
Maddox rod
Awaya test
Aniseikonia inspector
Describe an eikonometer.
Used to measure Aniseikonia
Shows vertical and horizontal lines (one set for left and one for right on top of each other)
Patient will see that the numbers do not line up in one direction or both
Describe how a Maddox rod can be used to measure Aniseikonia?
Correct phoria or tropia first
Introduce red Maddox rod before one eye, other sees light
Compare separation of the red lights with the separation of the lines
Describe the awaya test.
Patient wear red and green monocular filters
Pair of semicircles that appear of equal size indicates the % size difference
What is an Aniseikonia inspector?
A computer based version of the awaya test
How can we treat Aniseikonia?
Size lenses (lenses have no power, afocal, but provide optical magnification)
Vary spectacle magnification parameters
