1. The document discusses principles of low vision aids, defining low vision and outlining causes and types of low vision. 2. It describes different types of optical and non-optical low vision aids including magnifiers, telescopes, filters, and assistive technology. 3. Guidelines are provided for calculating required magnification, prescribing prismatic glasses, and treating specific low vision conditions like central scotomas and nystagmus.

1. PRINCIBLES OF LOW
VISION AIDS
Abdelmonem M. Hamed, M.D.
Professor of ophthalmology
Benha College of Medicine
Fellow of Baylor College of Medicine,
Texas Medical Center, Houston, USA
‫حامد‬ ‫المنعم‬ ‫عبد‬ ‫اد‬
‫بنها‬ ‫جامعة‬ ‫العين‬ ‫وجراحة‬ ‫طب‬ ‫استاذ‬
‫بأمريكا‬ ‫بيلور‬ ‫طب‬ ‫كلية‬ ‫زميل‬
2019

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 all causes.
 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

5. Causes of low vision
 Corneal opacity, dystrophy
 Sub-luxation
 Senile macular degeneration.
 Optic atrophy
 Etc.

6. Types of low vision aids:
 Optical visual aids:
 Contact lenses
 Magnifying glasses
 Pin-hole spectacles
 Typo-scope
 High plus reading
lenses
 Telescopic lenses
 Prismatic lenses
 Projection devices

7. Types of low vision aids:
 Non-optical visual aids:
 Large printing books
 Large sized pens
 Talking books
 Line guide
 Illumination
 Reading stands
 filters

8. 1. Relative Size Magnification
2. Relative distance Magnification
3. Angular Magnification
4. Real Image Magnification
Types of
magnifications:

9.  Magnification is NOT about making
objects clearer it is simply about making
them bigger.
 Magnification (M) = new retinal image size
/ old retinal image size
Magnification

10. 1. Relative size Magnification
(object enlarged & dist. is fixed)

11. 2. Relative distance Magnification
(object fixed & dist. is decreased)

12. Ex. Relative Distance Magnification

13. angle subtended at the eye by instrument
image angle subtended at the eye by object
M =
 It refers to the magnification provided by optical
instruments (e.g. telescopes and magnifiers).
 It does not involve changing object size or viewing distance.
3.

14. 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 even though the virtual
image is smaller than the
object.

15. 4. Real Image Magnification
size of real image (h4)
size of object (h1)
 It is used with a CCTV device where a
magnified image of the object is created on a
TV screen.
M =

16. 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
 M=1m
 Then M2= 2m= 200 cm
 = 200cm/40cm
 = 5 D of add needed to read M1

17. Near point magnification
Ex. 1:
If Pt. read M2 at 40 cm
= 200cm/40cm
= 5 D of add needed to read M1
Calculation of required
magnification

18. Calculation of required
magnification
 Near point magnification
 Ex2:
 IF Pt read 4M at 40cm
 = 400 cm / 40 cm
 = 10 D of add needed to
read M1
 To convert diopter to X
magnification, divide the
diopter/4
 Ex.: 10D = 10/4 = 2.5 X

19. Calculation of required
magnification
 Distance vision magnification:
 Follow the following formula: the Pt. real distance
VA / desiredVA = Xmagnification
 Ex.:
 If real Pt.VA = 20/200
 And desiredVA = 20/50
 SO, 200/50 = 4 X magnification (rough method) (as a
starting point)

20. 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.

21. 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
Can u see now

22. Practical points
 High plus reading glasses
 Power from + 4 to + 40 D
 Focal distance?

23. Practical points
 Telescopes
 Design:
 GalileanTs = high minus ocular + high plus objective
(erect image)
 KeplerianTs = plus ocular and objective lenses (inverted
image). It uses prism to reinvert the image.

24. 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

25. 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 Kalerian is usually preferred
by Pts. at and above 4X

26. Practical points
 Telescopes
 Focus ability:
 near,
 intermediate, and
 distance
 Mounting position:
 Ocular surface
 Across the bridge (monocular
vision)
 Types:
 Hand held
 Clip-on
 mounted

27. Practical points
 Telescopes
 IntraocularTs
 Still not that effective
 Low vision
enhancement system

28. 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

29. Treatment of central scotoma
 Causes:
 CNVM
 Chorio-retinitis
 Diabetic retinopathy
 Macular hole
 Etc.

30. 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

31. 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

32. Treatment of narrow field of
vision
 Causes:
 RP
 Glaucoma
 Proliferative diabetic
retinopathy
 Malignant myopia
 Etc.

33. Treatment of narrow field of
vision
 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

34. Management of nystagmus
Base out prism spectacles to
 stimulate convergence
 dampens the nystagmus
2mm 2mm

35. 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

36. Thank You