4.2.1 Advises on use of and dispenses simple LVAs & 4.2.2 Understands application of complex LVAs Flashcards
What is the aim of LVA?
Aim of LVA is magnification (Image enlargement)
What assistive technology is availble to help low vision pxs?
- BlindSquare – App describes environment and announces points of
interests/streets/specified points as you travel - iDentifi – Voiceovers objects in the camera screen
- Be My Eyes – App matches visually impaired user with sighted volunteer for help
- Kindle
- Tap tap see - uses camera and voiceover to identify things
What non-optical LVAs are available?
- Tints – Dark for albinism or RP, Yellow filters for CS in AMD
- Caps/Visors to reduce excess glare and reflections
- Typoscopes - Reduce back scatter, can increase reading fluency, same as negative contract on CCTV
- Extra Illumination for AMD, Glaucoma, DR, RP
- Reduced illumination for Albinism, Anridia, Cataracts
Describe hand magnifiers as LVAs?
- Used with distance Rx because emergent light is parallel
+ves: - portable & lightweight - easy to use
- low cost
- can come w/ light
-ves: - must be held at v specific point - difficult for pxs with arthritis or Parkinson’s etc
Describe stand magnifiers as LVAs?
- Use with readig glasses as light is divergent coming out of it
+ves: - hands free - great for hobbies e.g. knitting - often comes with light attached to enhance viewing
-ves: - bulky - fixed focus - px must hold object at particular point under magnifier to achieve clear image (but often length of stand is where object needs to be)
Describe dome/flat field/bar magnifiers as LVAs?
- Use with reading glasses
+ves: - light gathering so illuminates page - great for instantly enlarging text on book
- rests on page - does not need held by px
-ves: - need to use on flat surface - not abailable in high mag powers
- can be heavy & fiddly - not ideal for pxs with arthritis
Describe Field of View with LVAs?
To expand the field of view achieved by a stand or hand magnifier, once the image is in
focus the patient can choose to move closer to the magnifier. This will bring in more of
the image from the periphery
FoV = y/ (Fm x z) – y (lens diameter (m)), Fm (lens power), z (distance eye to mag(m))
Describe spectacle magnification as LVAs?
§ Advantages
* A high plus rx for reading may replace a magnifier (+4.00), a
convenient option
§ Disadvantages
* Lenses are very heavy
* focal length is very short
Difficult to maintain binocularity over 12D. if more mag needed use near vision telescope.
Describe telescopes as LVAs?
o Used in Px with Intermediate or distance vision demand, 2 types: Galilean telescope (-ve eyepiece, +ve obj),
Keplerian (+ve eyepiece and objective).
o Enlargement = -Fe/Fo. To compensate of Rx: Wear SVD, add Rx to eyepiece, alter telescope length (myope
shorten telescope, hyperope lengthen telescope).
o Adjustment for near use add Reading Add to eye piece (using end cap). Enlargement using an end cap = E
(telescope) x (end cap power/4).
o Exit Pupil - (Larger the better) – Exit pupil diameter = Fo / Enlargement
Describe Galilean telescopes as LVAs?
– Cone shaped, fixed focus, small compact and lightweight, produce an upright image. Optically not
as good as Keplerian. Have virtual exit pupil within the system closer the exit pupil to the entrance of pupil
gives better FoV. Light intensity and image brightness. Un modified poor for near
Galilean telescopes are made up of 2 lenses.
The objective lens is a convex (plus) lens (which is held close to the object you are
viewing) and the ocular lens is a minus lens and closer to the eye.
Advantages
§ Widens the visual field, great for patients with peripheral vision loss
§ Lighter, shorter and cheaper
Disadvantages
§ Quality of visual image and detail is sometimes poor
Describe Keplerien telescopes as LVAs?
– Inverted image require prism to erect image – new compact design is lightweight with variable
focus (N, I, D) by adjusting tube length. Small FoV but better than Galilean equivalent.
o Exit pupil is posterior to eye piece therefore able to align with pupil entrance of the eye. Needs to be held very
close and very still in front of the eye.
use two convex (plus) lenses. The objective lens is of a smaller diopter power than the
ocular lens. This produces a real image but it is inverted & requires a prism to reverse the
image.
Advantages
§ Larger visual field than Galilean
§ Better optical image quality than Galilean
Disadvantages
§ Longer, heavier & more expensive than Galilean
What is disability glare?
impairs Px visual performance, filters can improve VA, visual comfort and mobility
o Increase in contrast threshold due to scattering veiling luminance across the retinal image
o Short wavelength cut off filter (UV absorbing)
o Polaroid filter
o Shade or Broad brimmed hat
o Contrast enhancement
What is discomfort glare?
Visual discomfort but has no effect on VA
o Broadly neutral tint
o Photochromatic grey or brown (Transitions)
o Drop in grey filter
o Short wave cut off filter have no benefit
o Shading less effective but still useful
Describe photophobia in low vision?
– Severe pain and discomfort in the presence of light – not to be confused with discomfort glare
generally photophobia is symptom of damage to corneal nerve endings in Low vision Px most like due to glare
Describe light and dark adaptation in low vision?
reduced visual response when going from light to dark environment and vice versa
o Dark Adaptation (Light to Dark) – Grey filter before going outside, photochromatic don’t react fast enough,
Filters lower adaptive state when re-entering dark environment.
o Light Adaptation (Dark to Light) – Minimise Rod bleaching as only functioning photoreceptor (Cone
Dystrophy) – Require specialist dark red lenses.
Describe large print, PCs/large monitors, lighting, auditory options and tactile options in low vision?
- Large Print – Large print text book, Heavy, poor contrast, limited availability. Bills, statement and correspondence
- PCs and Large Monitors – font size, type and colour can be me made to meet Px needs. Enlargement software
systems can increase size of displayed image but screen size may restrict portion of image visible. Large monitor
used in conjunction would be best combination. - Key top stickers, copy holder and clipboards, writing guides and black marker pens
- Lighting – doesn’t increase object contrast, changes adaptive state of the retina thus lowering threshold (increased
sensitivity) decreases central scotoma, permits smaller pupil size. Distance of task to illumination is halved.
Illuminance of the source increase by factor of 4 (inverse square). Angle poised lamps - Auditory – Talking books, newspaper, daily equipment: clock, watch, microwave, scales , audio description TV
- Tactile – Braille and Moon
Describe complex LVAs? EVES and CCTV?
- electronic vision enhancement system/ closed circuity TV
+ves: - good if tremor or other e.g. Parkinson’s disease - good if extensive VF defects - px can use eccentric fixation
-ves: - expensive, bulky, servicing/repair - practice required (reading speed) - 15-20 sessions - hard to coordinate moving book while looking straightahead. Advise px to try before buying - Visibility in Glasgow.
- control positions
- depth of field
Portable EVES/mouse magnifiers/headmounted also available
EVES best for younger pxs with worse vision
Calculation for nominal mag? Trade mag?
F/4 = Magnification Power of the lens
F: Dioptres
e.g. 10/4 = 2.5x – a 10 dioptre lens has a magnification power of 2.5x
Trade mag: F/4 + 1
Calculation for focal length?
1/F = Focal Length of a lens (m)
F: Dioptres
e.g. 1/10 = 0.1 m – a 10 dioptre lens has a focal length of 10cm (the object has to be held 10
cm away from the lens to be perfectly in focus)
Calculation for required mag based on N reading ability?
Current ‘N’ reading ability/Desired ‘N’ reading ability =
magnification needed to achieve this
e.g. Patient currently reads N10 with reading rx, but wants to read N5:
10/5 = 2
Therefore we need to supply the patient with a 2x magnifier to achieve this
Calculation for magnification with acuity reserve?
Acuity Reserve is calculated at 2:1 for the patient to achieve fluent reading.
Example:
A patient currently sees N24. They would like to read N12 fluently.
On the basis of a 2:1 acuity reserve, this means the patient must be able to spot read N6.
(N12/2 = N6)
Therefore, you must tailor the ‘current reading ability’ magnification equation listed just
above to
N24/N6 = 4
(rather than N24/N12 as this will not allow for fluent reading)
Therefore the patient will need a 4x magnifier to fluently read N12, and spot read N6.
Fluent reading can also be further enhanced by proper lighting and good contrast.
