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
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
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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.
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 WHO working Indian VA- better eye definition definition0 6/6 – 6/18 Normal Normal Normal1 <6/18 – 6/60 Visual Low vision Low vision impairment2 <6/60 – 3/60 Severe visual Low vision Blind impairment3 <3/60 – 1/60 Blind Low vision Blind4 <1/60 - PL Blind Low vision Blind5 No PL Blind Total blindness Total blindness
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 THEYWORK ?? 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
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 AIDSOPTICALDISTANCE Hand held telescopes Mounted telescopesNEAR Spectacles • Prismatic ½ eyes • Bifocals Magnifiers • Hand held vs. stand • Illuminated vs. non-illuminated Electronic Devices
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 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
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
GALILEAN TELESCOPE Objective Eye piece a β fo fe
KEPLERIAN TELESCOPE Objective Eyepiece fo fe α β
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
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
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 GLASEESo 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 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 glareCPF® 450 (yellow) enhances contrast and helps optic atrophy control glare indoors albinism pseudophakiaCPF® 511 (yellow Medium-range filter macular degenerationorange) provides moderate blue light glaucoma filtering aphakia pseudophakia optic atrophy developing 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 wireThe 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