Vision is the special sense by which the qualities of an object (as colour, luminosity, shape, and size) constituting its appearance are perceived through a process in which light rays entering the eye are transformed by the retina(light sensitive inner layer of eyeball) into electrical signals that are transmitted to the brain via the optic nerve .
Visual acuity is a measure of resolution of the visual processing system. It is measured commonly with an eye chart e.g. Snellen chart. A common Snellen chart is printed with eleven lines of block letters. In visual acuity testing on Snellen chart, person normally sits at a distance of 6 meters from the chart on which letters of standard decreasing size are there in rows. Top line has a single letter which should be readable at a distance of 60 meters. Visual acuity is recorded as a ratio with numerator of 6 (distance at which patient sits.) and the denominator of the smallest possible size being read by the patient. For example, a vision record of 6/60 means that patient sitting at 6 meters could read letter in top line only , which should normally be read at 60 meters. Instead of distances being in meters, it may also be measured in feet. 6 meters is about 20 feet. Therefore, a vision of 6/6 in meters, is equivalent to a vision of 20/20 in feet. Similarly, a vision of 6/60 in meters, is equal to 20/200 in feet.
In spite of best medical, surgical and/or optical correction, there is :-
- Difficulty recognising objects at distance.
- Difficulty in reading or close work.
- Difficulty in colour differentiation.
Causes of Low Vision
- Age-related Macular Degeneration – Macula , the area on the retina responsible for sharp central vision, is affected causing diminished vision.
- Refractive Errors like High Myopia
- Strabismic Amblyopia
- Cataract – A cataract is clouding of lens in eye, thereby decreasing vision. Though treated surgically, few associated with ocular diseases may benefit with Low vision aids.
- Diabetic Retinopathy – In Diabetes, due to hypoxia ( less oxygen supply to retina ), there are vascular changes leading to haemorrhages (bleeding) and exudates ( leaks from vessel ). These changes in Macula may produce diminution of vision.
- Glaucoma – In Glaucoma, there is rise in intraocular pressure leading to damage to the optic nerve and diminution of vision.
- Retinal Detachment – Retina may separate from its underlying layer and leads to diminution of vision.
- Retinitis Pigmentosa – An inherited disease, usually affects peripheral vision to begin with, leads to night blindness.
- Retinopathy of Prematurity –Retinal vessels are not fully developed in premature infants. Premature infants are kept on oxygen in incubator. There is relative hypoxia in peripheral part of retina, which leads to new vessel formation and other complications, leading to diminution of vision.
- Optic Nerve Diseases like Optic Atrophy
- Heredomacular degeneration (Stargardt’s disease)
- Brain damage – There may be injury or disease of brain which may affect vision.
Most people requiring visual aids have macular problems. Tests are done to assess the degree of vision and the visual field.
It includes :-
- Refraction – As full a refraction as possible, should be attempted, and a subjective test should be performed with special test types. It is done to check the visual status of an individual. A trial frame and lenses should always be used, not a large refracting unit, which may interfere with eccentric viewing. Similarly, for near vision, special charts indicate reading vision and the magnification or addition required to attain a particular level.
- Visual fields – It is done to assess the peripheral vision.
Many of the patients are old and frequently relatively immobile so that the retention of some ability to read is of vital importance. Low vision aids improve the reading capability and speed of reading. Visual aids for distance are less practical than those for near.
Accurate refraction and appropriate illumination should be ensured for optimal retinal image.
Magnification is the basis of most techniques used in low vision clinic. There are three methods of enlarging the retinal image :-
- By bringing the object nearer to the eye.
- By magnification of the retinal image.
- By increasing the size of the object to be viewed.
Bringing the object nearer to the eye, leads to larger visual angle being subtended at the eye.
Magnification, though increases the size of retinal image, leads to diminished visual field and reduced depth of focus. Due to this, lighting should be ample and directed on the fixation area. Therefore, lowest magnification compatible with useful vision should be employed .
Low vision aids may be –
- Optical devices, such as magnifiers and telescopes.
- Adaptive non-optical devices, such as large-print reading material and talking watches.
- Non-magnifying visual aids.
- Electronic devices – computer software.
OPTICAL DEVICES :-
Hand-held Magnifiers :- Hand – held magnifiers are the simplest type of magnifiers. The hand-held magnifier design involves a handle that is attached to a plus lens through which the user views an object. Since user must support the weight of magnifier, hand-held magnifiers are helpful for short term spotting task, like reading a prescription bottle or a price tag. There are illuminated or non-illuminated magnifiers with a variety of lens designs e.g. aspheric , aplanatic , biconvex , and diffractive lens.
Stand Magnifiers :- Stand Magnifier is a plus lens mounted on legs that fix the distance from the lens to object. Stand Magnifiers are good for extended, near tasks such as reading a book, newspaper or magazine and making small notes. There are illuminated or non-illuminated magnifiers. There are around-the- neck magnifiers , for jobs which require both hands to be free, like sewing and knitting.
Some magnifiers may use variations like :
- Combination of lenses are also extensively used in magnifiers e.g. Hastings triplet magnifier.
- Stepped lenses introduced by Fresnel can also be used as magnifiers. One type is essentially a plastic sheet with concentric ridges forming a series of prisms of increasing power from the axis to the periphery and has the great advantage of eliminating the thickness and marked aberration that would be unavoidable in a single large lens with a continuous surface.
Magnifying Reading Glasses :- Magnifying reading glasses have plus lenses for short term use as a replacement, but cannot substitute the prescription glasses. These may be having constant power or are progressive to be used for varying reading distances . These may be of illuminated type, which can be used in the dark.
Spectacle Magnifiers :- Spectacle magnifiers are ideal for extended near tasks like reading or working on hobbies because they allow users to have ‘ hands free’. These may be:
- Clip– on system :- To be worn over the glasses.
- Prismatic eye wear :- Prisms are incorporated in spectacle to aid monocular or binocular vision. These may be bifocal, with normal reading power in upper part, and prismatic high power in the lower part for reading fine print.
- Noves spectacle magnifier :- It uses diffractive lenses which are much thinner than the lenses of similar power.
- Spectacle microscope magnifiers :- Have lenses of greater power than a traditional lens which forms a microscopic system when mounted on a spectacle lens. By holding the object closer, it allows an increase sized image with large field of view and clarity.
Telescopes :- Telescopes as vision aids, are ideal for tasks at any distance ( near, intermediate or far ) and for any duration of time ( short term or long term ). Telescopes may be monocular or binocular, focusable or fixed-focus, hand-held or spectacle mounted.
Technical Magnifier :- Technical magnifiers are ideal for extended near tasks such as inspection, quality control or viewing objects. Technical magnifiers may be hand-held magnifiers or stand magnifiers.
High power magnifiers are sometimes mounted in a cylindrical or conical holder with no handle. This is called a Loupe. These may be monocular loupe e.g. jewellers’ and watchmakers’ loupes or binocular.
Limitations of Low Vision Services :-
- Provision of low vision services is generally not favoured by eye-care providers because of the low economic gain.
- The need for low vision services is often not fully recognised, owing to inadequate data.
- There is little evidence for the cost-effectiveness of low vision care interventions.
- Persons with low vision are often unaware that they can be helped.
- In some, there may be lack of encouragement on the part of the tester; in others, there may be poor motivation on the part of the patient.
- Some patient may feel that forcing himself to see may accelerate a deterioration of his condition.
- Communication and referral between eye-care, special education, rehabilitation and low vision services are often inadequate.
- Planning for low vision services is inadequate.