The document provides information on low vision, including definitions, causes, assessments, and aids. It defines low vision according to the WHO and Indian standards. Common causes that can benefit from low vision aids are discussed. Assessment of low vision patients involves testing visual acuity, visual fields, contrast sensitivity, and other factors. A variety of optical and non-optical low vision aids are described, including magnifiers, telescopes, illumination devices, software, and filters to reduce glare. The goals of low vision management and global prevalence of low vision are also summarized.
3. DEFINITION (INDIA)
According to the Person with Disabilities Act 1995, “A person with
low vision means a person with impairment of visual functioning
even after treatment of standard refractive correction but who uses
or is potentially capable of using vision for the planning or execution
of a task with appropriate assistive device.”
4. DEFINITION(WHO)
WHO (ICD-10) definition
“A person with low vision is one who suffers visual acuity between
6/18 to 3/60 in the better eye after the best possible correction or a
field of vision between 20 to 30 degrees.”
Used for reporting and comparison of data
The WHO working definition of Low Vision (Bangkok definition,
1992)
“A person with low vision is one who has impairment of visual
functioning even after treatment, and/ or standard refractive correction,
and has a visual acuity of less than 6/18 to light perception or a visual
field of less than 10 degrees from the point of fixation, but who uses, or
is potentially able to use, vision for the planning and/or execution of a
task .”
Defines population in need of low vision services
5. category Corrected WHO working Indian
VA- better eye definition definition
0 6/6 – 6/18 Normal Normal Normal
1 <6/18 – 6/60 Visual Low vision Low vision
impairment
2 <6/60 – 3/60 Severe visual Low vision Blind
impairment
3 <3/60 – 1/60 Blind Low vision Blind
4 <1/60 - PL Blind Low vision Blind
5 No PL Blind Total blindness Total blindness
7. FUNCTIONAL EFFECTS OF LOW VISION
Loss of central vision (eg. macular degeneration, toxoplasma scar etc.)
Difficulty reading
Problems writing/ completing paperwork
Inability to recognize distance objects and faces
Loss of peripheral vision (eg. Retinitis pigmentosa, glaucoma etc. )
Difficulty in mobility and navigation
Difficulty reading if there is constricted central visual field
Visual acuity may not be affected until very advanced disease
Cloudy media (eg. Corneal scar, vitreous hemorrhage etc.)
Blurred vision
Reduced contrast
Problems with glare
8. GOALS OF LOW VISION MANAGEMENT
Increase functionality
Make the most of the remaining vision
Provide link to community resources and support services
Education
9. STRATEGIES
Be oriented towards activities of daily living
Use appropriate technology
Be cost effective
Utilize appropriate educational and vocational adaption
Focus on target groups
10. GLOBAL PREVALENCE OF LOW VISION
True magnitude not known because :
No uniform definition of low vision
Incomplete surveys
Low vision definition does not include standards of near vision,
which is the main area dealt with low vision patients.
Current Data *
No. of visually impaired: 180 million
No. of blind: 45 million
Those with residual vision: 171 million
Of these 171 million:
Those with vision from PL to 3/60 : 36 million
No. with vision from 3/60to 6/18: 135 million
No. who can benefit from treatment: 103 million
True low vision patients: 68 million
*Ramachandra Pararaiasegaram. Low vision care: the need to maximise visual potential. Community Eye Health. 2004; 17: 1-2
11.
12. WHAT ARE LOW VISION AIDS AND HOW DO THEY
WORK ??
Devices which help the people to use their sight to better advantage
Can be optical devices like magnifiers or telescopes, or non optical
devices like stands, lamps and large prints.
Alter the environment perception through
BBB – bigger brighter and blacker
CCC – closer color and contrast
13. DISEASES WHERE LOW VISION AIDS ARE HELPFUL
Retinitis pigmentosa
Glaucoma
Macular degeneration
Corneal scar
Albinism and aniridia
Retinal detachment
Diabetic retinopathy
Chorioretinitis
Optic atrophy
14. TYPES OF MAGNIFICATION
Low vision aids make use of angular magnifications by increasing :
Relative size
Relative distance
15. Angular : it is the apparent size of the object compared with true
size of the object seen without the device.eg. Telescopic system
Angular magnification M = ω’/ ω
16. Relative size: by making the object appear bigger (no
accommodation required) eg. CCTV
17. Relative distance: by bringing the object closer (requires good
accommodation) eg. magnifiers
19. HISTORY
Ocular history:
To know cause of low vision
To know the progression of disease
Systemic diseases that may pose difficulty in using certain devices
eg. arthritis, tremors
Task analysis
20. VISUAL ACUITY
Distance visual acuity:
Lighthouse distance visual acuity test chart is preferred over the standard
snellen’s chart as it has :
Equal line difficulty
geometric progression of optotype size from line to line
5 letters on each line
More lines at lower level of visual acuity
Test distance of 2 meters can be used to cover visual acuity upto 20/400
21.
22. Near visual acuity:
Text samples are better than single letter acuity charts
Metric notations are used
1M symbol subtends an angle of 5 minutes of arc at 1 meter and is
roughly equal to the size of the newsprint
Visual acuity is recorded as distance of reading material (in meters) over
the letter size (in M units)
Snellens equivalent can be calculated from the metric notations
23.
24. OTHERS
Contrast sensitivity
Visual field analysis:
Peripheral field: using Humphery or octopus perimetry
Central field: using Amsler grid
Glare :
History
Measuring visual acuity both with and without illumination in the chart
Colour vision
Look for dominant eye:
by testing contrast sensitivity monocularly and binocularly
25. LOW VISION AIDS
OPTICAL
DISTANCE
Hand held telescopes
Mounted telescopes
NEAR
Spectacles
• Prismatic ½ eyes
• Bifocals
Magnifiers
• Hand held vs. stand
• Illuminated vs. non-illuminated
Electronic Devices
28. MAGNIFYING SPECTACLES
High plus reading glasses to
magnify the images
Given as an add to the best distance refraction
Reading distance is calculated by 100 divided by add
Magnification is 1/4th the power of the lens.
Used for near work
Amount of add needed depends on the accommodation and the reading
distance
29. Reading add can be predicted using the Kestenbaum rule i.e the amount
of add needed to read 1M print is the inverse of the visual acuity fraction
However usually greater add is required than predicted as the patient also
has reduced contrast sensitivity
If the patient is monocular, the poorer eye may be occluded if it
improves the functioning
When binocular corrections are needed :
Base in prisms are added to compensate for convergence angle.
Optical center may be decentred
Aspheric lenses may be used to reduce lenticular distortion
30. Advantages :
Hands are free
Field of view larger when compared to telescope
Greater reading speed
Can be given in both monocular and binocular forms
More portable
Cosmetically acceptable
Disadvantages:
Higher the power, closer the reading distance
Close reading distance causes fatigue and unacceptable posture
Patients with eccentric fixation are unable to fix through these glasses
31. MAGNIFIERS
Useful for near work
Designed to be held close to the reading material to enlarge the image
The eye lens distance should be minimum to achieve larger magnification
Two types:
Hand magnifier
Stand magnifiers.
32. HAND MAGNIFIERS
Available from + 4.0 to + 68.0 D.
Available in three designs:
Aspheric – reduces thickness and peripheral distortion
Aplantic – flat and wide distortion free field and good clarity
Biaspheric – eliminating aberrations from both surfaces
Most patients accept upto 6x magnification
33. Advantages
The eye to lens distance can be varied
Patient can maintain normal reading distance
Work well with patients with eccentric viewing
Some have light source which further enhances vision
Easily available, over the counter
Disadvantages:
It occupies both hands
Patients with tremors, arthritis etc have difficulty holding the magnifier
Maintaining focus is a problem especially for elderly
Field of vision is limited
34. STAND MAGNIFIERS
The magnifiers are stand mounted
The patient needs to place the stand magnifier on the reading material
and move across the page to read
Has a fixed focus
Advantages :
They are a choice for patients with tremors, arthritis and constricted
visual fields.
Disadvantage:
Field of vision is reduced
Too close reading posture is uncomfortable for the patient
Blocks good lighting unless self illuminated
35. CLOSED CIRCUIT TELEVISION SYSTEM
Closed circuit television system (CCTV) consists of a monitor, a
camera and a platform to place the reading text
It has control for brightness, contrast and change of polarity
Magnification varies from 3X to 60X
37. TELESCOPES
Work on the principle of angular magnification
Telescopes with magnification power from 2x to 10x are prescribed
They can be prescribed for near, intermediate and distant tasks
Field of view decreases with magnification
Types:
Hand held monocular
Clip on design
Bioptic design: mounted on a pair of eyeglasses
38. Principal
Telescopes consist of two lenses (in practice two optical systems) mounted such
that the focal point of the objective coincides with the focal point of the ocular.
Objective lens is a converging lens
Galilean telescope Keplerian telescope
The eye piece is a negative lens and Both eye piece and objective are
the objective is a positive lens positive lens
Resultant image is virtual and erect Resultant image is real and inverted.
Prisms are incorporated to erect the
image
Loss of light reduces brightness of Loss of light is more in this system
the image
Field quality is poor Field quality is relatively good
39. Magnification of a telescope is given by the formula M = fo/fe
Telescopes can be used to focus near objects by
changing the distance between objective and ocular lens
Increasing the power of the objective lens
43. Advantages:
Only possible device to enhance distant vision
Disadvantage:
Restriction of the field of view
Appearance and apprehension
Expensive and costly
Depth perception is distorted
45. ILLUMINATION
Positioning
Light source should be to the side of better eye
Moving light closer will yield higher illumination
Higher levels of illumination is needed in patients with
Lost cone functions (macular degeneration)
Glaucoma
Diabetic retinopathy
Retinitis pigmentosa, Chorioretinitis
Reduced illumination
Albinism
Aniridia
49. TYPOSCOPE / READING GUIDE
Masking device with a line cut out from an opaque, non reflecting
black plastic or thick paper.
Reduces glare and controls contrast.
50. NOTEX
It is a rectangular piece of cardboard with steps on top right corner
which helps in identifying the currency of the note
1st cut indicates Rs. 500, 2nd cut indicates Rs.100, 3rd cut indicates
Rs 50 and so on.
51. RELATIVE SIZE DEVICES
Larger object subtends a larger visual angle at the eye and is thus
easier to resolve
Large print material
Large type playing cards, computer keyboards
Enlarged clocks, telephones, calendars
52. COMPUTER SOFTWARE
Jaws screen reading software
Connect out loud internet and email software
Magic 8.0 screen magnification software and speech
53. GLARE REDUCING DEVICES
Glare is described as unwanted light
It is disabling in patients with cataracts, corneal opacities, albinism,
retinitis pigmentosa
Devices to prevent glare:
Sunglasses
Caps
Umbrella
Polaroid glasses
NoIR filters
Corning photochromic filters
(CPF glasses)
54. CPF GLASEES
o Attenuate 100% of UVB wavelengths.
o Block 99% of UVA wavelengths.
o The blue light portion of the visible spectrum is most likely to scatter
in the eye, causing discomfort and hazy illusion.
o Attenuate 98% of high-energy blue light, with exception of CPF
450, which is 96% of high-energy blue light.
o The number of the CPF glasses correspond to wavelength in
nanometers above which light is transmitted
55. CPF® 550 (red) Lens colour varies from retinitis pigmentosa
orange-red when lightened albinism
to brown when darkened.
CPF® 527 (orange) Orange-amber lens darkens retinitis pigmentosa
to brown in sunlight, giving diabetic retinopathy
individuals better visual
function and reduced glare
CPF® 450 (yellow) enhances contrast and helps optic atrophy
control glare indoors albinism
pseudophakia
CPF® 511 (yellow Medium-range filter macular degeneration
orange) provides moderate blue light glaucoma
filtering aphakia
pseudophakia
optic atrophy
developing cataracts
56. NOIR FILTERS
Absorbs the short wavelengths of the visible spectrum that can
scatter within the ocular media,
Also absorbs ultraviolet light (to 4000 nm) and infrared light
Manages overall visible light transmission (VLT) to allow the proper
amount of light energy to reach the eyes.
57. Includes a full range of lenses (spanning 90% to 1% VLT)
2% dark amber: 100% UV, infrared and blue light protection, helpful on very
bright days
13% standard grey: good for postoperative cataract, glaucoma, diabetics and
those who had corneal transplants
20% medium plum: good in low light situations and can be worn indoors
58% light grey: reduce indoor glare especially under fluorescent light
65% yellow: retinitis pigmentosa and macular dgeneration
58. COLOR AND CONTRAST ENHANCEMENT
Maximize contrast by using a light color against black or dark color
Choose colors in the room or working area which have high contrast
59. PINHOLE GLASSES
Multiple holes of approximately 1mm size are made in the glasses
The distance between the holes should be atleast 3-3.5 mm or
approximately the size of the pupil
Used in patients with corneal opacities or conditions with irregular
reflexes
Not used in patients with central field defects as it reduces
illumination and visual acuity
60. MOBILITY ASSISTING DEVICES
Patients with low vision suffer a major problem of mobility
Long canes
Strong portable lights
61. FIELD EXPANDING DEVICES
As the magnification increases, the field of view decreases
Three methods of increasing the field:
Compress the existing image to include more of available area
Provide prisms that relocates the image from a non seeing to a seeing
area
Use a mirror to reflect an image from a non seeing area
Reverse telescopes: they are usually not accepted due to minification
Fresnel lenses with power of 10-15D with base in the direction of
field loss
63. BIONIC EYE
Designed for patients who are blind due to diseases like retinitis
pigmentosa or AMD
Can also be tried for those with severe vision loss
Relies on patient having a healthy optic nerve and a developed visual
cortex
Cannot be used for people who were born blind
The prosthesis consists of :
A digital camera built into a pair of glasses
A video processing microchip built into a hand held unit
A radio transmitter on the glasses
A receiver implanted above the ear
A retinal implant with electrodes on a chip behind the retina
64. Camera captures an image
Send image to microchip
Convert image to electrical impulse of light and dark pixels
Send image to radiotansmitter
Transmits pulses wirelessly to the receiver
Sends impulses to the retinal implant by a hair thin implanted wire
The stimultaed electrodes generate electrical signals that travel to the visual cortex
65. Requires training by the subject to actually see an object
Subjects have to learn to interpret the array of white and dark dots as
object
It is still in clinical trial stage
72. Various forms are available
1. Powers usually available are +4.0, +5.0, +6.0, +10.0 , +12.0, +16.0,
20.0 and +24.0
2. Binocular corrections are needed –Base in prisms are added to
compensate for convergence angle.
Optical quality of the lens should be an aspheric design to eliminate
peripheral aberration and provide reasonable field.
The reading glass should be prescribed as an addition over the distance
correction.
76. OPTICS OF LOW VISION AIDS
Principle : Magnification = D/4
on the assumption that the patient can sustain just enough
accommodation to hold the matter at 25 cm.
Modified formula : M = D + A-h AD/2.5 where
A is the amplitude of accomodation
h is the eye lens distance in meters.
To increase magnification:
Eyes should be kept close to the lens (reduce h)
Object should be as close to the patient’s eye as his accomodation
allows
78. IMPACT OF OCULAR DISEASE ON THE PATIENT
Visual disorder
Anatomical changes in the visual organ caused by the disease of the eye
Visual impairment
Functional loss that results from the visual disorder
Visual disability
Refers to vision related changes in the skill and abilities of the patient
Visual handicap
Psychosocial and economic consequences of visual loss
79. Legal Blindness
Best corrected distance visual acuity not exceeding 6/60 in the
better eye
Visual field of 20 degrees or less at widest point in the better eye
Low Vision
Best corrected visual acuity between 6/60 to 6/18
Significant field loss
Impaired function
All these definitions however do not consider
Near vision
Scotoma, hemianopia
Visual performance like contrast
Left column metric notation . Vision is distance divided by this.Next column gives the snellens equivalenceRight column log units
Left column metric notationRight is snellens equivalent and the amount of add required to read 1M line
Superior monocular performance suggests interference from a poorer functioning dominant eye and supports use of monocular aids
Formula of magnifivcTIION
Magnifying glasses, standmagn.
Images 7.5 7.6
Availability
Not only is this absorption of energy the most effective way to reduce glare, it also has the effect of enhancing contrast by highlighting visual distinction.
Bloom portable light
Reverse telescope door bellWhen the individual directs his eyes into the prism area, low contrast image from the missing field will come into view
??? Understand…American academy
remove
For visual acuity the commonly used threshold of 20/40 (0.5, 6/12) is accepted. For visual fields a binocular field of at least 120° horizontal and 40° vertical is suggested. .Contrast sensitivity screening is listed as desirable. India 6/18.