Low Vision- Assistive Devices

Astha Jain

Shashi Sharma

LOW VISION AIDS

INTRODUCTION

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

Relative size: by making the object appear bigger (no accommodation required) eg. CCTV

Relative distance: by bringing the object closer (requires good accommodation) eg. magnifiers

VISUAL ASSESSMENT

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

LOW VISION OPTICAL DEVICES FOR NEAR

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

LOW VISION OPTICAL DEVICES FOR DISTANCE

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

aβ

ObjectiveEye piece

fo

fe

GALILEAN TELESCOPE

KEPLERIAN TELESCOPE

fo fe

α

Objective Eyepiece

β

TELESCOPE FOR NEAR

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

NON OPTICAL DEVICES

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

READING STAND

Easy comfortable posture to the patient

WRITING GUIDE Black cards with rectangular cut outs horizontally along the card The patient can feel the empty cut out spaces and write

SIGNATURE GUIDE

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

FUTURE

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

Help when there is no cure

Thank you

Thank you

Thank you

Thank you

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.

GALLELIAN TELESCOPE

KREPLERIAN TELESCOPE

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

Left: simulated with cataracts. Middle: CPF 511 lenses. Right: normal eyes.

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

EYE DISORDERS AND LOW VISION

RETINOPATHY OF PREMATURITY

Retinopathy of prematurity requires bright light and near additions required for near work

ANIRIDIA

Tinted glasses and cap

ALBINISM

Typoscope Dark glasses

CORNEAL DAMAGE

Multiple pin hole glasses Hand magnifier

DIABETIC RETINOPATHY

Diabetic Retinopathy with near glasses, hand magnifiers and a reading lamp

aβ

ObjectiveEye piece

F

GALILEAN TELESCOPE

βα

Objective Eye piece

F

KEPLERIAN TELESCOPE

KEPLERIAN TELESCCOPE

TELESCOPE FOR NEAR