DISP - Spectacle Magnification and Aniseikonia - Week 4

Question 1

Define spectacle magnification, including how myopes and hyperopes are affected.

Answer 1

When an ametropic patient puts on spectacles, clarity and size changes.
Myopes see clearer but smaller.
Hyperopes see clearer but larger.

Question 2

Define eye magnification, including how myoptic and hyperopic eyes look.

Answer 2

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.

Question 3

Do aspheric lenses increase or minimise eye magnification?

Answer 3

Minimise

Question 4

What happens to spectacle magnification with vertex distance? What happens when it is 0 (CL)?

Answer 4

Magnification effect increases with vertex distance.
If 0, then it is as close to unity as possible.

Question 5

Describe shape factor and power factor in terms of spectacle magnification and note what happens if a lens has a shape factor of 0.

Answer 5

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)

Question 6

Name 6 things that increases spectacle magnification.

Answer 6

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)

Question 7

In what way can we control spectacle magnification?

Answer 7

Adjusting the shape and power factor

Question 8

Consider the following parameters for spectacle magnification and note which are viably able to be modified. Comment why.
n
t
F1
F2

Answer 8

n is dependent on lens material
t affects aestheticc and should be kept minimum
F1 and F2 can be changed considerably

Question 9

What is the steepest front and back surface power that is reasonable in a normal frame size (in D)?

Answer 9

±20.00D

Question 10

How would BO prism affect an objective fronto-parallel plane (ie a wall) and the floor?

Answer 10

OFPP would appear to bow inward
The floor would appear to slope away

Question 11

How would BI prism affect an objective fronto-parallel plane (ie a wall) and the floor?

Answer 11

OFPP would appear to bow outward
The floor would appear to slope upward

Question 12

Are prismatic distortions adapted to by patients?

Answer 12

Yesd

Question 13

Define anisophoria and its cause. What can it lead to and do patients adapt to this effect?

Answer 13

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.

Question 14

Define aniseikonia and its cause.

Answer 14

Usually caused by anisometropia, it is a difference in perceived image size/shapes between the two eyes.

Question 15

Can aniseikonia occur in patients that are monocular?

Answer 15

No

Question 16

Can you estimate perceived aniseikonia? What three additional factors are at play here?

Answer 16

No you cant, because perceived image size is more than just optics:
-receptor distribution
-neural processing
-adaptation

Question 17

Define the following in terms of aniseikonia.
Geometric effect
Induced effect
Oblique effect

Answer 17

Geometric effect - magnification in the horizontal meridian only
Induced effect - magnification in the vertical meridian only
Oblique effect - magnification along oblique meridia

Question 18

Do vertical disparities contribute to stereoscopic depth perception?

Answer 18

No

Question 19

Consider an eye with the geometric effect looking at a wall. How is it perceived?

Answer 19

It appears to rotate away from the magnified eye, with the rotation about a vertical axis.

Question 20

What happens with the induced effect? Do vertical disparities occur?

Answer 20

No, they are instead translated to horizontal disparities.
A vertical magnification in one eye will cause horizontal magnification in the other eye.

Question 21

What does the oblique effect induce?

Answer 21

Cyclo-disparity

Question 22

What will an overall magnification in one eye induce in the other?

Answer 22

Both geometric and induced effects, and they cancel each other out since theyre opposite

Question 23

Briefly describe the leaf room.

Answer 23

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.

Question 24

How can aniseikonia be measured?

Answer 24

An eikonometer, patient is asked if the test target is tilted, settings adjusted to eliminate it, and it reports the magnification required for this.

Question 25

What kind of lenses are used to treat aniseikonia?

Answer 25

Iseikonic lenses

Question 26

Describe iseikonic lenses.

Answer 26

Afocal lenses that contain a certain amount of spectacle magnification derived from shape factor alone.

Question 27

Can iseikonic lenses be used for the oblique effect?

Answer 27

Yes, using a bitoric design.

Question 28

Can emmetropes have aniseikonia? Explain.

Answer 28

Yes, its called essential neural aniseikonia

Question 29

List 4 symptoms of uncorrected aniseikonia.

Answer 29

Headaches
Asthenopia (eye strain)
Difficulty reading
Photophobia

Question 30

Do patients syptomatic of aniseikonia often report problems with spatial distortion?

Answer 30

No

Question 31

At what percentage of aniseikonia does stereoscopic thresholds become affected and at what percentage is it incompatible with binocular vision?

Answer 31

5% begins to affect stereosopic thresholds
20% incompatible with binocular vision

Question 32

How is adaptation caused and what does it depend on?

Answer 32

A change in the mapping between retinal disparity and perceived slant and depends on motor feedback.

Question 33

What are patients undergoing adaptation encouraged to engage in?

Answer 33

Fine motor tasks/hobbies

Question 34

What should be avoided in patients with aniseikonia?

Answer 34

Sudden changes in spherical and cyl power.

Question 35

What are the three forms of treatment for aniseikonia?

Answer 35

Modify the prescription - sacrifice best VA for better binocularity
Alter base curves
Design iseikonic lenses - last resort

Question 36

What is required to design iseikonic lenses?

Answer 36

Must measure the amount of aniseikonia and calculate SF