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low vision aids


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low vision aids

  1. 1. LOW VISION AIDS Astha Jain Shashi Sharma
  3. 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. 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. 5. 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
  6. 6. VISUAL DISABILITY CHARTCategory no. Good eye Worse eye Percent blindness1 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%
  7. 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. 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. 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. 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. 11. 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
  12. 12. DISEASES WHERE LOW VISION AIDS ARE HELPFUL Retinitis pigmentosa Glaucoma Macular degeneration Corneal scar Albinism and aniridia Retinal detachment Diabetic retinopathy Chorioretinitis Optic atrophy
  13. 13. TYPES OF MAGNIFICATION Low vision aids make use of angular magnifications by increasing :  Relative size  Relative distance
  14. 14.  Angular : it is the apparent size of the object compared with true size of the object seen without the Telescopic system Angular magnification M = ω’/ ω
  15. 15.  Relative size: by making the object appear bigger (no accommodation required) eg. CCTV
  16. 16.  Relative distance: by bringing the object closer (requires good accommodation) eg. magnifiers
  18. 18. 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
  19. 19. 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
  20. 20.  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
  21. 21. 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
  22. 22. LOW VISION AIDSOPTICALDISTANCE  Hand held telescopes  Mounted telescopesNEAR  Spectacles • Prismatic ½ eyes • Bifocals  Magnifiers • Hand held vs. stand • Illuminated vs. non-illuminated  Electronic Devices
  23. 23. NON-OPTICAL Glare reduction devices Contrast enhancement devices Computer software Accessory devices  Talking watches, clocks, etc  Writing guides  Tactile markers
  25. 25. 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
  26. 26.  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
  27. 27.  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
  28. 28. 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.
  29. 29. 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
  30. 30.  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
  31. 31. 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
  32. 32. 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
  34. 34. 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
  35. 35.  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
  36. 36.  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
  37. 37. GALILEAN TELESCOPE Objective Eye piece a β fo fe
  38. 38. KEPLERIAN TELESCOPE Objective Eyepiece fo fe α β
  40. 40.  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
  42. 42. 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
  43. 43. READING STAND Easy comfortable posture to the patient
  44. 44. WRITING GUIDE Black cards with rectangular cut outs horizontally along the card The patient can feel the empty cut out spaces and write
  46. 46. 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.
  47. 47. 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.
  48. 48. 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
  49. 49. COMPUTER SOFTWARE Jaws screen reading software Connect out loud internet and email software Magic 8.0 screen magnification software and speech
  50. 50. 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)
  51. 51. 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
  52. 52. 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
  53. 53. 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.
  54. 54.  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
  55. 55. 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
  56. 56. 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
  57. 57. MOBILITY ASSISTING DEVICES Patients with low vision suffer a major problem of mobility  Long canes  Strong portable lights
  58. 58. 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
  59. 59. FUTURE
  60. 60. 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
  61. 61. 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
  62. 62.  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
  63. 63. Help when there is no cure
  64. 64. Thank you
  65. 65. Thank you Thank you Thank you
  66. 66.  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.
  69. 69. 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
  70. 70. Left: simulated with cataracts. Middle: CPF 511 lenses. Right:normal eyes.
  71. 71. 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
  72. 72.  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
  74. 74. RETINOPATHY OF PREMATURITY Retinopathy of prematurity requires bright light and near additions required for near work
  75. 75. ANIRIDIA Tinted glasses and cap
  76. 76. ALBINISM Typoscope Dark glasses
  77. 77. CORNEAL DAMAGE Multiple pin hole glasses Hand magnifier
  78. 78. DIABETIC RETINOPATHY Diabetic Retinopathy with near glasses, hand magnifiers and a reading lamp
  79. 79. GALILEAN TELESCOPE Objective Eye piecea β F
  80. 80. KEPLERIAN TELESCOPE Objective Eye piece α β F