Princibles of low vision aids 2020

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

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

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

Aim of low vision aids
 to enhance vision-
related quality of life
for people with
functional low vision

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

Types of low vision aids:
 Optical visual aids:
 Contact lenses

 Magnifying glasses

 Pin-hole spectacles

 High plus reading
lenses

 Telescopic lenses

 Prismatic lenses

 Projection devices

 Non-optical visual aids:
 Large printing books
 Large sized pens
 Talking books
 Illumination
 Reading stands
 Filters
 Line guide

 filters

 Typo-scopes

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

 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

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)

The simplest way to increase magnification is
to decrease the viewing distance.
2. Relative distance Magnification
(object fixed & dist. is decreased)

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

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

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

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

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

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.

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

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

Practical points
 Telescopes
 Design:
 GalileanTs = high minus ocular + high plus objective
(erect image)

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

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

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

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

Practical points
 Telescopes
 IntraocularTs
 Low vision
enhancement
system

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

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

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

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

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

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

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

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

Thankyouforyour
attention
DO YOUR BEST TO
COMPETE BLINDNESS

Princibles of low vision aids 2020

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Princibles of low vision aids 2020