low vision- assistive devices
DESCRIPTION
ldTRANSCRIPT
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.”
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
category Corrected VA- better eye
WHO definition
working Indian definition
0 6/6 – 6/18 Normal Normal Normal
1 <6/18 – 6/60 Visual impairment
Low vision Low vision
2 <6/60 – 3/60 Severe visual impairment
Low vision Blind
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
VISUAL DISABILITY CHART
Category no. Good eye Worse eye Percent blindness
1 6/9-6/18 6/24-6/36 20%
2 6/18-6/36 6/60-nil 40%
3 6/60-4/60 3/60-nil 75%
4 3/60-1/60 CF 1 ft- nil 100%
5 CF 1 ft – nil CF 1 ft - nil 100%
6 6/6 nil 30%
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
GOALS OF LOW VISION MANAGEMENT
Increase functionality Make the most of the remaining vision
Provide link to community resources and support services
Education
STRATEGIES
Be oriented towards activities of daily living
Use appropriate technology
Be cost effective
Utilize appropriate educational and vocational adaption
Focus on target groups
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
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
DISEASES WHERE LOW VISION AIDS ARE HELPFUL
Retinitis pigmentosa Glaucoma Macular degeneration Corneal scar Albinism and aniridia Retinal detachment Diabetic retinopathy Chorioretinitis Optic atrophy
TYPES OF MAGNIFICATION
Low vision aids make use of angular magnifications by increasing :
Relative size
Relative distance
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
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
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
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
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
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
NON-OPTICAL
Glare reduction devices Contrast enhancement devices Computer software Accessory devices
Talking watches, clocks, etc Writing guides Tactile markers
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
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
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
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.
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
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
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
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
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
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 the objective is a positive lens
Both eye piece and objective are 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 the image
Loss of light is more in this system
Field quality is poor Field quality is relatively good
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
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
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
WRITING GUIDE Black cards with rectangular cut outs horizontally along the card The patient can feel the empty cut out spaces and write
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.
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.
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
COMPUTER SOFTWARE
Jaws screen reading software Connect out loud internet and email software Magic 8.0 screen magnification software and speech
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)
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
CPF® 550 (red) Lens colour varies from orange-red when lightened to brown when darkened.
retinitis pigmentosaalbinism
CPF® 527 (orange) Orange-amber lens darkens to brown in sunlight, giving individuals better visual function and reduced glare
retinitis pigmentosadiabetic retinopathy
CPF® 450 (yellow) enhances contrast and helps control glare indoors
optic atrophyalbinismpseudophakia
CPF® 511 (yellow orange)
Medium-range filter provides moderate blue light filtering
macular degenerationglaucomaaphakiapseudophakiaoptic atrophydeveloping cataracts
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.
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
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
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
MOBILITY ASSISTING DEVICES
Patients with low vision suffer a major problem of mobility Long canes Strong portable lights
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
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
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
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
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.
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
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
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
RETINOPATHY OF PREMATURITY
Retinopathy of prematurity requires bright light and near additions required for near work