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Princibles of low vision aids 2020
1. LOW VISION AIDS,
HOW TO PRACTICE
Abdelmonem M. Hamed, M.D.
Professor of ophthalmology
Benha College of Medicine
Fellow of Baylor College of Medicine,
Texas Medical Center, Houston, USA
2020
2. Definition of subnormal vision
Legal blindness:
An acuity rating in which
BCVA achieves no more
than 3/60 for distance in
the better eye.
Or, a defect in visual
field in which the widest
diameter of vision
subtends an angle no
greater than 20o
3. Definition of subnormal vision
Low vision Pt.:
Is a person who has the
following:
acuity of < 6/18 to 3/60 in the
better eye, from any cause.
visual field response bellow
normal
TheVA can not be corrected
with spectacles
4. Aim of low vision aids
to enhance vision-
related quality of life
for people with
functional low vision
5. Causes of low vision
Corneal opacity, dystrophy
Sub-luxation
Senile macular degeneration.
Optic atrophy
Etc.
16. 1. Relative Size Magnification
2. Relative distance Magnification
3. Angular Magnification
4. Real Image Magnification
Types of
magnifications:
17. Magnification is
(NOT) about making
objects (clearer) it is
simply about making
them (bigger).
Magnification
(M) = new retinal image
size / old retinal image
size
1. Relative Magnification
18. The magnification is calculated in comparison to a standard
size, e.g. if large print books are N24 and standard books are
N10, then the relative size magnification provided = 2.4X
1. Relative size Magnification
(object enlarged & dist. is fixed)
19. The simplest way to increase magnification is
to decrease the viewing distance.
2. Relative distance Magnification
(object fixed & dist. is decreased)
20. 3. Angular Magnification
Examples:Telescopes and
hand magnifiers
This optical system produces
a virtual image smaller than
the original object but much
closer to the eye.
The image has a larger
angular subtense than the
original object; therefore, the
objects appear larger when
seen through this optical
system .
M =
angle subtended at the eye
by image
image angle subtended at
the eye by object
21. Factors that increase
angular magnification
If the object lies directly at F1 of the lens
(maximum magnification)
If 2nd convex lens is placed between object
(which is brought closure to the eye) and the
eye.Total angular magnification= M1 + M2
22. 4. Real Image Magnification
size of real image
size of object
It is used with a CCTV device where a
magnified real image of the object is created
on aTV screen.
M = =
h4
h1
23. Calculation of required
magnification
Near point magnification
How to calculate?
print size in meter / distance of chart from
eye in centimeter = diopters needed
Ex.:
If Pt. read M2 at 40 cm
How much diopters to read m1?
Then M2= 2m= 200 cm
= 200cm/40cm
= 5 D of add needed to read M1
24. Calculation of required
magnification
Distance vision magnification:
Follow the following formula:
Ex.:
If real Pt.VA = 20/200
And desiredVA = 20/50
SO, 200/50 = 4 X magnification (rough
method) (as a starting point)
The Pt. real distance VA / desired VA = magnificationX
25. Practical points
Using the contact lenses as
low vision aids:
To correct irregular
astigmatism (hard CL)
Telescopic like action: positive
glass lenses with minus CL
Reversed telescope: minus
glass lenses with positive CL
Pine hole CL like in: coloboma
of iris, aniridia, etc.
26. Practical points
Magnifiers
Hand held magnifiers
Stand magnifiers
With or without
illumination
Pin hole spectacles: to
improveVA in Pt. with
irregular astigmatism
with good macular
function
Typo-scope
By enhancing the contrast
29. Practical points
Telescopes
Design:
KeplerianTs = plus ocular and objective lenses
(inverted image). It uses prism to reinvert the image.
30. Practical points
Telescopes
Wight:
Galilean is lighter than KeplerianTs
Field of vision:
Increasing the power in either design will
decrease the field
Ts 5X Ts 2X
31. Practical points
Telescopes
Power:
the spectacle mounted Galilean
Ts are relatively useful up to 4 X
beyond 4 X
the field is very small and
the light gathering ability
decreases
The Kaplerian is usually preferred
by Pts. at and above 4X
32. Practical points
Telescopes
Focus ability:
near,
intermediate, and
distance
Mounting position:
Ocular surface
Across the bridge (monocular
vision)
Types:
Hand held
Clip-on
mounted
34. Practical points
CCTV
Closed circuitTV
Advantages:
Greater brightness
Improved contrast
Reduced aberration and distortion
Longer viewing and reading distance
Greater magnification ( up to 60 times)
It is not the 1st aid, it is used when:
Field restricted <5 degrees
A higher magnification needed
35. Treatment of central scotoma
Causes:
CNVM
Chorio-retinitis
Diabetic retinopathy
Macular hole
Etc.
36. Treatment of central scotoma
The suspectedVA as we go away from
fovea:
VA at fovea = 6/6
VA at 2.5 degrees from fovea = 6/12
VA at 5 degrees from fovea = 6/12 – 6/24
VA at 10 degrees from fovea = 6/24
Treatment:
Ts, Magnifiers, CCTV, prism (move the
image towards the functioning retina), non
optical aids like large sized prints, filters to
enhance contrast.
fovea
37. How to prescribe a prismatic glass
Refract
Start with the better eye
Start with 10 ∆ diopter lens from the trial set
Put the base at 90o in the trial frame and testVA
Move the base of the prism at 45o interval around the
clock and test theVA in each position (at 2 meter or less)
Note axis of best acuity
Introduce identical power prism for the fellow eye at
identical axis
Test theVA while both eyes open and ask Pt. if he feels
comfort with the glass
Ex. OD -1.5 -1.25 ax 90 pr 10 ∆ base up at 120o
OS -2.0 -1.00 ax 95 pr 10∆ base up at 120o
38. Treatment of narrow field of vision
Causes:
RP
Glaucoma
Proliferative diabetic
retinopathy
Malignant myopia
Etc.
39. Treatment of narrow field of vision
Treatment
Use the optical aid that increase field of vision:
InvertedTs
Concave lenses e.g. -8 to -12 D
They increase the field but decrease theVA
41. Management of nystagmus
How to fit the fresnel
prism?
Leave 2 mm from the
visual axis for scanning
by the Pt.
Ask the Pt. to come back
after 2 weeks, then
adjust the apex of the
prism…..move it far
than 2mm, until
reaching the Wright
position
2mm
3mm