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
