DISP - Spectacle Magnification and Aniseikonia - Week 4 Flashcards

1
Q

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

A

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

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2
Q

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

A

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.

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3
Q

Do aspheric lenses increase or minimise eye magnification?

A

Minimise

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4
Q

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

A

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

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5
Q

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

A

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)

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6
Q

Name 6 things that increases spectacle magnification.

A

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)

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7
Q

In what way can we control spectacle magnification?

A

Adjusting the shape and power factor

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8
Q

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

A

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

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9
Q

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

A

±20.00D

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10
Q

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

A

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

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11
Q

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

A

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

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12
Q

Are prismatic distortions adapted to by patients?

A

Yesd

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13
Q

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

A

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.

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14
Q

Define aniseikonia and its cause.

A

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

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15
Q

Can aniseikonia occur in patients that are monocular?

A

No

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16
Q

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

A

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

17
Q

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

A

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

18
Q

Do vertical disparities contribute to stereoscopic depth perception?

A

No

19
Q

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

A

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

20
Q

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

A

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

21
Q

What does the oblique effect induce?

A

Cyclo-disparity

22
Q

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

A

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

23
Q

Briefly describe the leaf room.

A

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.

24
Q

How can aniseikonia be measured?

A

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

25
Q

What kind of lenses are used to treat aniseikonia?

A

Iseikonic lenses

26
Q

Describe iseikonic lenses.

A

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

27
Q

Can iseikonic lenses be used for the oblique effect?

A

Yes, using a bitoric design.

28
Q

Can emmetropes have aniseikonia? Explain.

A

Yes, its called essential neural aniseikonia

29
Q

List 4 symptoms of uncorrected aniseikonia.

A

Headaches
Asthenopia (eye strain)
Difficulty reading
Photophobia

30
Q

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

A

No

31
Q

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

A

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

32
Q

How is adaptation caused and what does it depend on?

A

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

33
Q

What are patients undergoing adaptation encouraged to engage in?

A

Fine motor tasks/hobbies

34
Q

What should be avoided in patients with aniseikonia?

A

Sudden changes in spherical and cyl power.

35
Q

What are the three forms of treatment for aniseikonia?

A

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

36
Q

What is required to design iseikonic lenses?

A

Must measure the amount of aniseikonia and calculate SF