Visual impairment (or vision impairment) is vision loss (of a person) to such a degree as to qualify as an additional support need through a significant limitation of visual capability resulting from either disease, trauma, or congenital or degenerative conditions that cannot be corrected by conventional means, such as refractive correction or medication. This functional loss of vision is typically defined to manifest with:
- best corrected visual acuity of less than 20/60, or significant central field defect,
- significant peripheral field defect including homonymous or heteronymous bilateral visual, field defect or generalized contraction or constriction of field, or
- reduced peak contrast sensitivity with either of the above conditions.
Eye disorders which can lead to visual impairments can include retinal degeneration, albinism, cataracts, glaucoma, muscular problems that result in visual disturbances, corneal disorders, diabetic retinopathy, congenital disorders, and infection." Visual impairment can also be caused by brain and nerve disorders, in which case it is usually termed cortical visual impairment (CVI).
The American Medical Association's Guides to the Evaluation of Permanent Impairment attempts to provide "a standardised, objective approach to evaluating medical impairments." The Visual System chapter "provides criteria for evaluating permanent impairment of the visual system as it affects an individual's ability to perform activities of daily living. "The Guide has estimated that the loss of one eye equals 25% impairment of the visual system and 24% impairment of the whole person; total loss of vision in both eyes is considered to be 100% visual impairment and 85% impairment of the whole person.
Visual impairments have considerable economic impact on even developed countries. 'A major proportion of global visual impairment is preventable'. An update based on census data of 2010 in the United States projects that 13 million Americans aged 40 and older will have a visual impairment or be blind by the year 2050.
Types of Visual Impairment
Visual impairment is usually classified as either ’sight impaired’ or ‘severely sight impaired’. These classifications are based on the results of the tests described above.
Sight impairment, previously called ‘partial sight’, is usually defined as:
- having poor visual acuity (3/60 to 6/60) but having a full field of vision, or
- having a combination of slightly reduced visual acuity (up to 6/24) and a reduced field of vision or having blurriness or cloudiness in your central vision, or
- having relatively good visual acuity (up to 6/18) but a significantly reduced field of vision
Severely Sight Impaired
The legal definition of severe sight impairment (which was previously called ‘blindness’) is when ‘a person is so blind that they cannot do any work for which eyesight is essential’.
- This usually falls into one of three categories:
- having very poor visual acuity (less than 3/60), but having a full field of vision
- having poor visual acuity (between 3/60 and 6/60) and a severe reduction in your field of vision
- having slightly reduced visual acuity (6/60 or better) and a significantly reduced field of vision
There are also levels of visual impairment based on visual field loss (loss of peripheral vision).
Causes of Visual Impairment
Most causes of visual impairment in the UK are conditions that develop as you get older. About 8 in every 10 people with visual impairment are over 65.
However, losing your vision is not an inevitable part of ageing. It is often the result of a condition that can either be treated or sometimes even prevented. This is why it's so important to have regular check-ups with your optometrist.
Some of the most common causes of visual impairment include:
- Age-related macular degeneration (AMD) – where the central part of the back of the eye (the macula, which plays an important role in central vision) stops working properly
- Cataracts – where cloudy patches can form within the lenses of the eyes
- Glaucoma – where fluid builds up inside the eye, damaging the optic nerve (which relays information from the eye to the brain)
- Diabetic retinopathy – where blood vessels that supply the eye become damaged from a build-up of glucose
In some of these cases, such as cataracts, treatment can at least partially restore your vision.
Vision loss caused by AMD, glaucoma and diabetic retinopathy cannot usually be reversed. However, there are several treatments that can prevent further damage to vision, or at least slow down the progression of these conditions.
The vast majority of patients with low vision can be helped to function at a higher level with the use of low vision devices. Low vision specialists recommend appropriate low vision devices and counsel patients on how better to deal with their reduced vision in general. Many government and private organisations exist to aid the visually impaired.
Improving far sight: works best with static objects
- Contemplating scenery
- Seeing the bus number
Improving near sight: the person must work closer to the object
Improving sensitivity to contrast: the person must use special optical filters
- Book stands
- Special lights
- Grid paper
- Magnified games
- Watches, audio thermometers, special phones, etc.
Efficacy of Optical Aids
In a study performed by this specialist on 1,000 patients, all subjects with a visual acuity above 0.02 decimal (20/1000 feet) significantly improved their vision. From this group, 48% were very satisfied with their visual aids, 44% were satisfied, 5% little satisfied and 3% unsatisfied. Adaptation process to visual aids In the patient's first visit, the most adequate options for their particular case are studied, taking into consideration their psychological, cultural, social and work factors, and the degree of improvement experienced with the selected aids, advising the patient on which aids improve quality of life. After the specific adaptation, there is a follow up to ensure the patient is correctly using and taking the best advantage of the visual aids. In some cases (approx. 4%), the initial visual aids must be changed. Once the patient is released, a report on their first visit and follow up is given to their eye doctor or the professional who made the referral. We believe that low vision, as a complementary technique to ophthalmology, has a great future, due to the progress of science, the increase of life expectancy, and the increasing need people have to access information.
For the totally blind, there are books in braille, audio-books, and text-to-speech computer programs, machines and e-book readers. Low vision people can make use of these tools as well as large-print reading materials and e-book readers that provide large font sizes.
Computers are important tools of integration for the visually impaired person. They allow, using standard or specific programs, screen magnification and conversion of text into sound or touch (Braille line), and are useful for all levels of visual handicap. OCR scanners can, in conjunction with text-to-speech software, read the contents of books and documents aloud via computer. Vendors also build closed-circuit televisions that electronically magnify paper, and even change its contrast and color, for visually impaired users.
In adults with low vision there is no conclusive evidence supporting one form of reading aid over another. In several studies stand-based closed-circuit television and hand-held closed-circuit television allowed faster reading than optical aids. While electronic aids may allow faster reading for individuals with low vision, portability, ease of use, and affordability must be considered for people.
Children with low vision sometimes have reading delays, but do benefit from phonics-based beginning reading instruction methods. Engaging phonics instruction is multisensory, highly motivating, and hands-on. Typically students are first taught the most frequent sounds of the alphabet letters, especially the so-called short vowel sounds, then taught to blend sounds together with three-letter consonant-vowel-consonant words such as cat, red, sit, hot, sun. Hands-on (or kinesthetically appealing) VERY enlarged print materials such as those found in "The Big Collection of Phonics Flipbooks" by Lynn Gordon (Scholastic, 2010) are helpful for teaching word families and blending skills to beginning readers with low vision. Beginning reading instructional materials should focus primarily on the lower-case letters, not the capital letters (even though they are larger) because reading text requires familiarity (mostly) with lower-case letters. Phonics-based beginning reading should also be supplemented with phonemic awareness lessons, writing opportunities, and lots of read-alouds (literature read to children daily) to stimulate motivation, vocabulary development, concept development, and comprehension skill development. Many children with low vision can be successfully included in regular education environments. Parents may need to be vigilant to ensure that the school provides the teacher and students with appropriate low vision resources, for example technology in the classroom, classroom aide time, modified educational materials, and consultation assistance with low vision experts.
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