LV - Field Expansion - Week 10

Question 1

Below what degree diamter of visual field do most people begin to have mobility issues?

Answer 1

<10 degrees

Question 2

List 5 aids that can be used to enhance visual fields.

Answer 2

Scanning techniques
Reversed telescopes
Concave lenses
Prisms
Mirrors

Question 3

What kind of visual field losses in particular do field enhancements apply to (3)?

Answer 3

Peripheral field constriction
Hemianopias
Sector losses

Question 4

Describe what is meant by scanning as a field enhancement (2).

Answer 4

Effective use of eye movements and scanning strategies
Systematic approach to scanning the environment in order to encompass the area of concern quickly

Question 5

What do you see if you look through a reversed telescope Dexcibe in terms of object size and field of view.

Answer 5

Objects look smaller and greater field of view

Question 6

Describe what the field of view through a reversed telescope is equal to. Describe this with a 2x telescope.

Answer 6

Apparent increase in field of view is approximately equal to the magnification of the telescope
A normal 2x telescope will give 2x magnification but reversed, it will have 2x the field of view, but 2x minification

Question 7

What power telescopes can be effectively used for reversal?

Answer 7

Only lower power telescopes

Question 8

What are reversed telescopes mostly used for?

Answer 8

Mobility

Question 9

Describe how to calculate the apparent VF increase and apparent VA a patient has when looking through a reversed telescope. Assume a 5 degree VF diameter, 6/12 VA, and a 3x reversed telescope.

Answer 9

Apparent field of view = VF degree diameter x magnification
= 3 x 5 -> 15 degree diameter
VA decrease = Snellen denominator x magnification
= 3 x 12 -> VA of 6/36

Question 10

What kind of image do concave lenses create and what effect does it have on the field of view.

Answer 10

Creates a minified image and a larger field of view

Question 11

What does the combination of a negative lens and accommodation form?

Answer 11

A reverse galilean telescope

Question 12

What is required when you use a negative lens to spot distances?

Answer 12

A second lens to act as an add (or accommodation if the patient can)

Question 13

Describe the apparent visual field of view increase for a concave lens.

Answer 13

Apparent VF = M = -F/A
It is equal to the magnification M of the system
F is power of the concave lens
A is the power of the add or accommodation

Question 14

Describe how a lens should be held if you need to use the entire lens (a concave lens for spotting).

Answer 14

To use the entire lens, it should be helf where it is at least as wide as V

Question 15

What base direction is used when trialling prisms for field enhancement? What does it allow for?

Answer 15

Base in the direction of the field loss
-image moves toward the apex
Allows for person to see object with smaller eye/head movements than otherwise necessary

Question 16

What are the two types of prism lenses generally used?

Answer 16

Full field grind or resnel prism type

Question 17

How many dioptres are generally needed for prisms?

Answer 17

15 to 25^D

Question 18

When are fresnel prism lens designs more useful (2)?

Answer 18

When needing to reduce weight and thickness

Question 19

How are prisms applied to ordinary lenses? How are they traditionally applied?

Answer 19

Simply applied to the back surface of the lens
-can be easily removed
Traditionally applied to a part of the lens
-edge just outside the line of sight in primary gaze

Question 20

What do more recent studies suggest of the benefits of increasing (or decreasing) prism dioptres for field enhancement. What is the suggested fitting for prisms (2)?

Answer 20

More prism - about 40^D base in the direction of field loss might give better results
Suggested fitting with 12mm gap above and below the pupil

Question 21

What is the advantage of fresnel prisms?

Answer 21

Most accepted method of field enhancement

Question 22

List 4 disadvantages of fresnel lenses.

Answer 22

Induced scotoma and jack in the box effect when patients gaze enters and leaves the prism
Chromatic aberration
Image degradation
-decreased contrast and VA
Difficult to clean

Question 23

Describe how mirrors can be used for field enhancement. What concept is it similar to?

Answer 23

Small plano mirror fastened to the frame to reflect image of an object in a non-seeing area close to the functioning area
Similar to fresnel prisms

Question 24

List 2 disadvantages of mirrors for field enhancement.

Answer 24

They block some of the remaining useful visual field
Image is reversed
-adapt quickly as in the use of mirrors for driving

Question 25

When are mirrors for field enhancement typically used?

Answer 25

When image displacement required is large

Question 26

What are 5 considerations when giving field enhancement to a patient?

Answer 26

Determine patients response while seated
Determine a wearing schedule
Determine patients response for mobility
Careful followup
May not need it in the future once adapted and learned to be spatially aware

Question 27

With what kind of hemianopia can it be difficult to read and why? Note 2 strategies for this.

Answer 27

Right hemianopia or substantial loss to the right can make it difficult as you are reading into the blind area
Can have vertical print or reading guides (typoscopes or ruler)

Question 28

Why may people who have had stroke or head trauma have difficulty reading as opposed to a field vield loss?

Answer 28

May be due to alexia, not simply field loss

Question 29

With what kind of visual field loss is eccentric viewing typically used?

Answer 29

Central field loss
-such as maculopathy
Some do it automatically, others need help

Question 30

What has been used to predict the best way for a patient to use eccentric viewing?

Answer 30

Scanning laser ophthalmoscope

Question 31

Describe how to detect the size and position of a scotoma before moving on to eccentric viewing. What is the most important question?

Answer 31

Tangent screen at 1 metre with 10mm white target
Use a large E as a target in the centre
Is the right field open is the most important question

Question 32

What is the preferred retinal locus? Describe it.

Answer 32

Eccentric viewing point used by the patient
It is the extrafoveal point the visual system has chosen for tasks normally performed by the fovea

Question 33

What should patients be told about eccentric viewing (2)?

Answer 33

They need reassurance that it is ok to use and that it will help

Question 34

How can the best eccentric viewing for a patient be determined (2)?

Answer 34

Have them put the scotoma in the centre of a clockface and ask them to report the clearest number they see
Can also have them view a target in the middle of the clockface eccentrically and ask them what clock-hour they are looking at (its direction)