DISP - Spectacle Magnification and Aniseikonia - Week 4 Flashcards
Define spectacle magnification, including how myopes and hyperopes are affected.
When an ametropic patient puts on spectacles, clarity and size changes.
Myopes see clearer but smaller.
Hyperopes see clearer but larger.
Define eye magnification, including how myoptic and hyperopic eyes look.
It is the magnification of a wearer’s eyes from the viewpoint of another observer.
Corrected myopic eyes look smaller.
Corrected hyperopic eyes look larger.
Do aspheric lenses increase or minimise eye magnification?
Minimise
What happens to spectacle magnification with vertex distance? What happens when it is 0 (CL)?
Magnification effect increases with vertex distance.
If 0, then it is as close to unity as possible.
Describe shape factor and power factor in terms of spectacle magnification and note what happens if a lens has a shape factor of 0.
Shape factor - magnification due to the form and thickness of the lens
Power factor - magnification due to the back vertex power and vertex distance of the lens
If t=0 and SF=1 then all the effect comes from the power factor (thin lens)
Name 6 things that increases spectacle magnification.
Lens thickness increases
Refractive index decreases
Front surface power increases
Vertex distance increases for positive lenses
Vertex distance decreases for negative lenses
Back vertex power increases (more positive)
In what way can we control spectacle magnification?
Adjusting the shape and power factor
Consider the following parameters for spectacle magnification and note which are viably able to be modified. Comment why.
n
t
F1
F2
n is dependent on lens material
t affects aestheticc and should be kept minimum
F1 and F2 can be changed considerably
What is the steepest front and back surface power that is reasonable in a normal frame size (in D)?
±20.00D
How would BO prism affect an objective fronto-parallel plane (ie a wall) and the floor?
OFPP would appear to bow inward
The floor would appear to slope away
How would BI prism affect an objective fronto-parallel plane (ie a wall) and the floor?
OFPP would appear to bow outward
The floor would appear to slope upward
Are prismatic distortions adapted to by patients?
Yesd
Define anisophoria and its cause. What can it lead to and do patients adapt to this effect?
It is heterophoria that varies with gaze direction and is caused by anisometropia.
Can lead to distortions of space perception and binocular stress.
Most patients will adapt.
Define aniseikonia and its cause.
Usually caused by anisometropia, it is a difference in perceived image size/shapes between the two eyes.
Can aniseikonia occur in patients that are monocular?
No
Can you estimate perceived aniseikonia? What three additional factors are at play here?
No you cant, because perceived image size is more than just optics:
-receptor distribution
-neural processing
-adaptation
Define the following in terms of aniseikonia.
Geometric effect
Induced effect
Oblique effect
Geometric effect - magnification in the horizontal meridian only
Induced effect - magnification in the vertical meridian only
Oblique effect - magnification along oblique meridia
Do vertical disparities contribute to stereoscopic depth perception?
No
Consider an eye with the geometric effect looking at a wall. How is it perceived?
It appears to rotate away from the magnified eye, with the rotation about a vertical axis.
What happens with the induced effect? Do vertical disparities occur?
No, they are instead translated to horizontal disparities.
A vertical magnification in one eye will cause horizontal magnification in the other eye.
What does the oblique effect induce?
Cyclo-disparity
What will an overall magnification in one eye induce in the other?
Both geometric and induced effects, and they cancel each other out since theyre opposite
Briefly describe the leaf room.
A black box contains individually placed leaves that stand out from each wall but are not parallel.
It reduced monocular depth cues leaving only binocular cues.
Binocular individuals will see a cube.
Monocular individuals will see a shapeless concavity.
Aniseikonia will alter the shape of the room.
How can aniseikonia be measured?
An eikonometer, patient is asked if the test target is tilted, settings adjusted to eliminate it, and it reports the magnification required for this.
What kind of lenses are used to treat aniseikonia?
Iseikonic lenses
Describe iseikonic lenses.
Afocal lenses that contain a certain amount of spectacle magnification derived from shape factor alone.
Can iseikonic lenses be used for the oblique effect?
Yes, using a bitoric design.
Can emmetropes have aniseikonia? Explain.
Yes, its called essential neural aniseikonia
List 4 symptoms of uncorrected aniseikonia.
Headaches
Asthenopia (eye strain)
Difficulty reading
Photophobia
Do patients syptomatic of aniseikonia often report problems with spatial distortion?
No
At what percentage of aniseikonia does stereoscopic thresholds become affected and at what percentage is it incompatible with binocular vision?
5% begins to affect stereosopic thresholds
20% incompatible with binocular vision
How is adaptation caused and what does it depend on?
A change in the mapping between retinal disparity and perceived slant and depends on motor feedback.
What are patients undergoing adaptation encouraged to engage in?
Fine motor tasks/hobbies
What should be avoided in patients with aniseikonia?
Sudden changes in spherical and cyl power.
What are the three forms of treatment for aniseikonia?
Modify the prescription - sacrifice best VA for better binocularity
Alter base curves
Design iseikonic lenses - last resort
What is required to design iseikonic lenses?
Must measure the amount of aniseikonia and calculate SF
