LV - Field Expansion - Week 10 Flashcards
Below what degree diamter of visual field do most people begin to have mobility issues?
<10 degrees
List 5 aids that can be used to enhance visual fields.
Scanning techniques
Reversed telescopes
Concave lenses
Prisms
Mirrors
What kind of visual field losses in particular do field enhancements apply to (3)?
Peripheral field constriction
Hemianopias
Sector losses
Describe what is meant by scanning as a field enhancement (2).
Effective use of eye movements and scanning strategies
Systematic approach to scanning the environment in order to encompass the area of concern quickly
What do you see if you look through a reversed telescope Dexcibe in terms of object size and field of view.
Objects look smaller and greater field of view
Describe what the field of view through a reversed telescope is equal to. Describe this with a 2x telescope.
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
What power telescopes can be effectively used for reversal?
Only lower power telescopes
What are reversed telescopes mostly used for?
Mobility
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.
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
What kind of image do concave lenses create and what effect does it have on the field of view.
Creates a minified image and a larger field of view
What does the combination of a negative lens and accommodation form?
A reverse galilean telescope
What is required when you use a negative lens to spot distances?
A second lens to act as an add (or accommodation if the patient can)
Describe the apparent visual field of view increase for a concave lens.
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
Describe how a lens should be held if you need to use the entire lens (a concave lens for spotting).
To use the entire lens, it should be helf where it is at least as wide as V
What base direction is used when trialling prisms for field enhancement? What does it allow for?
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
What are the two types of prism lenses generally used?
Full field grind or resnel prism type
How many dioptres are generally needed for prisms?
15 to 25^D
When are fresnel prism lens designs more useful (2)?
When needing to reduce weight and thickness
How are prisms applied to ordinary lenses? How are they traditionally applied?
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
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)?
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
What is the advantage of fresnel prisms?
Most accepted method of field enhancement
List 4 disadvantages of fresnel lenses.
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
Describe how mirrors can be used for field enhancement. What concept is it similar to?
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
List 2 disadvantages of mirrors for field enhancement.
They block some of the remaining useful visual field
Image is reversed
-adapt quickly as in the use of mirrors for driving
When are mirrors for field enhancement typically used?
When image displacement required is large
What are 5 considerations when giving field enhancement to a patient?
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
With what kind of hemianopia can it be difficult to read and why? Note 2 strategies for this.
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)
Why may people who have had stroke or head trauma have difficulty reading as opposed to a field vield loss?
May be due to alexia, not simply field loss
With what kind of visual field loss is eccentric viewing typically used?
Central field loss
-such as maculopathy
Some do it automatically, others need help
What has been used to predict the best way for a patient to use eccentric viewing?
Scanning laser ophthalmoscope
Describe how to detect the size and position of a scotoma before moving on to eccentric viewing. What is the most important question?
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
What is the preferred retinal locus? Describe it.
Eccentric viewing point used by the patient
It is the extrafoveal point the visual system has chosen for tasks normally performed by the fovea
What should patients be told about eccentric viewing (2)?
They need reassurance that it is ok to use and that it will help
How can the best eccentric viewing for a patient be determined (2)?
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)
