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LOW VISION - ASSESSMENT AND AIDS
NP BALIWALLA
Professor, SNDT University, Mumbai.
The goals of low-vision rehabilitation are to help the patient understand the realistic possibilities of the rehabilitation process, and to enable the patient to perform tasks necessary for survival and for enjoying life. This review article describes the steps of assessment and types of low vision aids available. The estimated number of the blind in the world is 45 million whereas that of the partially sighted is 135 million. If the number of the blind in India has been estimated at 9 million, the partially sighted in India, by the world ratio, could be taken as 45 million. A country which is endeavouring its utmost to reduce the number of its curable blind can hardly divert its resources, both human and financial, to help its partially sighted population.
A person is said to have low vision if his vision in the better eye after treatment and refractive correction is < 6/18 or his field of vision is reduced to 10 degrees. The WHO categories of low vision are as under :
No impairment > or = 6/18
Visual impairment < 6/18 to 6/60
Severe visual impairment < 6/60 to 3/60Blind < 3/60 to NPL
The pace at which the problem of low vision is being remedied must take long to bear result. Low vision services for people with impaired vision are needed extensively to rehabilitate them to a level of self sufficiency in performing their daily tasks. Such rehabilitation demands ideally the expertise of inter disciplinary group of professionals, who work with low vision persons toward the goal of enhancing their level of visual acuity.
The most desirable system of vision rehabilitation services as described by Randall T Jose involves a team work of low vision specialists including ophthalmologists, optometrists, rehabilitation teachers / counsellors, orientation and mobility instructors, social workers, psychologists and occupational and physical therapists. There are 3 phases of such a model.
Phase 1 covers functional evaluation gathering information about the person’s level of social activities, psychological adjustments to the impairment, travel skills, functional acuities, fields, colour vision, glare, photophobia, medical care, financial needs, and the person’s own needs and goals. The data evaluates the functional problems.
Phase 2 is the clinical evaluation which assesses visual acuity, visual fields, refractive error and the magnification needed. The data collected in the functional assessment is compared to the data of clinical assessment and then a plan for training drawn up which is need based. It should not concentrate on training reading skills if the person’s main problem is orientation and mobility.
Phase 3 of the vision rehabilitation service involves instruction and training for the person to use the aid in the most beneficial way and to develop proficiency in using the aid.
Until a countrywide net work of low vision services can be developed, it becomes necessary for the individual ophthalmologists and the optometrists to help persons with low vision in their own clinic set up in the best possible manner. It may be of help to the practitioner to be associated with a social worker trained for counselling and for mobility training to render assistance when needed. Examination of a patient with low vision demands more time and patience as compared to the normal patient.
Initial Examination
Patients are often accompanied by a relative or a friend. Their presence during the examination is of help because they may remind the patient of specific problem that could be overlooked during the initial discussion. They also help at home in encouraging a hesitant patient by reminding him of the close working distance or the importance of adequate lighting. The initial discussion with the patient determines his needs, expectation and motivation. While examining the children, a friendly approach is needed taking care to set the child at ease. Examining children with a poor span of attention, it may be necessary to keep the initial discussion brief. A child at school needs to be asked about the classroom environment, the distance he sits away from the blackboard and the difficulties experienced at distance and near. It is to be ascertained whether the teacher is aware of the child’s problem and extends consideration to it. The type size of the text book and the reading difficulty experienced are noted. Elderly patients must not be rushed. Sometimes more than one visit may be needed to get a comprehensive picture of the patient’s daily living, work or educational requirements.
Ocular History
We need to establish not only the prevailing condition but also the length of time it has existed and how rapidly has the vision deteriorated. Patients with recent loss often respond in a different way to those, who have had an impairment for sometime. The practitioner should obtain full ocular history with regards to the patient’s health and general condition. He has to assess the existing state of impairment and its likely progression.
Observing the patient
General observation of the patient during the initial discussion can reveal nervousness or anxiety in some or in others a positive attitude. Motivation is the most significant factor.
A patient with reduced visual fields may reveal poor mobility in the clinic indicating a need for training whereas a patient with central visual field loss may navigate with less difficulty. Nystagmus, if present is noted, recording the particular head posture that the patient may have developed to arrive at the null point at which the nystagmaid movements are the minimal. Uniocular visual acuity of a patient with nystagmus can be considerably lower than binocular acuity.
Task Analysis to Assess Problems at Distance and Near
The practitioner should identify distance and near tasks to determine the visual acuity required for magnification. Vision of 6/18 may suffice to watch television at 2 or 3 meter distance but 6/9 may be needed to identify number of an approaching bus. For near work it is necessary to know the print size required to be read by the person. It is of help to know if a magnifier or any other device is being used.
At the start an accurate refraction is most essential. Snellen’s acuity measured at 6 meters does not often provide the desired information. In case the acuity is low, it is helpful if the chart is brought closer to 3 meters, 2 meters or 1 meter. At 2 meters the acuity is recorded as 2/36, 2/18, etc. Patient also feels more encouraged being able to read more letters, as compared to the 6 meter distance from the chart.
Near vision acuity is recorded with near vision charts using Times New Roman types in N notation from N 5 increasing progressively to N 60. There is a direct mathematical relationship between letter size notation. N 12 is half the size of N 24. N 18 is three times the size of N 6.
A person with N 12 will need magnification of 2X to be able to read N 6 and the person with N 18 will need 3X to get N 6. The chart gives theoretical estimate of magnification which does not always work. It becomes less predictable when the vision is poor. Patients with macular loss need higher magnification. Conventionally one unit of magnification is equivalent of 4 dioptres of power. Using a single lens magnifier in a spectacle frame reduces the working distance. Young patients adapt comfortably to higher magnification and learn to hold the reading matter close to the eye but elderly patients, who have been habituated to holding their papers at a fair distance find it difficult to work with the reading matter close to the eye.
LOW VISION AIDS FOR DISTANCE
Telescopes
The image of a distant object can be magnified by going closer to it as a person with low vision does when watching television. When it is not possible to move closer to a distant object the only other way to magnify it is by using a telescope. Telescopes for low vision generally are in magnifications 2X, 3X, 4X, 6X and 8X. Telescopes in Galilean system are with a negative eye piece and a positive objective separated in a tube that is of the same length as the difference in focal lengths of the two lenses. Galilean system may be used for lower magnifications upto 3X. Higher magnifications need astronomical telescopes (Kaplerian) where the eye piece and the objective are both positive. The image is inverted and hence erecting prisms are used. Astronomical telescopes are therefore more expensive.
Person with acuity of 6/36 will improve to 6/12 with magnification of 3X and to 6/9 with 4X. A telescope has to be prescribed guardedly. Lower magnification is easier to use. Stronger telescopes significantly reduce the field of view and increase the difficulty. Training in the use of telescope is necessary for its correct use. Locating the object and changing the focussing for different distances need training. A person with congenital loss may adopt readily whereas an elderly patient with ARMD, whose vision has dropped suddenly may find it difficult to adopt even after training. Telescopes may be hand held or spectacle mounted as may be appropriate for the user.
Magnification at near
Major need of the visually impaired is for near work. Reading is the main handicap and many devices are available for help.
Hand held magnifiers
Magnifiers may be illuminated or non illuminated. There are 3 conditions for consideration - magnification, field of view and distortion. Higher the magnification, lesser is the field of view. Higher the power, greater the distortion. Magnification is the prime condition. The lowest magnification is preferred in order to provide the widest field and minimum distortion. When higher magnification becomes necessary, aspheric lenses are used to minimise distortion that results towards the periphery of the lens. The magnifier is used preferably close to the eye and the reading distance is then adjusted to give maximum sharpness and magnification. Illuminated magnifiers have the advantage of ensuring optimum illumination for the reading field.
Stand Magnifiers
When a hand held magnifier is difficult to manage in case of unsteady hands of an elderly person, a stand magnifier is of help. Magnifier on a tripod is low priced. It is kept on the page to provide a reasonable magnification. Aspheric illuminated magnifiers of high magnification on a torch - head provide high magnificantion and are often found most useful. Such aspheric magnifiers are imported and are expensive.
Spectacle Mounted
Magnifiers can be mounted to a spectacle frame for monocular or binocular use. If vision in both the eyes has dropped to nearly the same level, the person prefers to use a binocular magnifier.
Magnification at a distance close to the eye requires excessive convergence which needs to be relieved by use of prisms. As a rule 1 degree of prism Base IN is incorporated with each dioptre of power to relieve 1 meter angle of convergence. Thus power of 5 dioptres needs 5 degree prism Base IN each eye or +8.00 needs 8 degrees of prism. Binocular magnifiers are available in +5.00, +8.00 and +12.00 dioptre powers. Lenses are mounted to half eye frames. Where higher magnifications become necessary only one eye is used. Magnifiers are usually fitted with lenses of the same power on both sides to allow the use of either eye. In case there is a marked difference between the vision of two eyes, only the better eye is used. The other eye is usually suppressed but if it is found to be disturbing, it is occluded. Patients often come to the clinic with the idea of getting stronger spectacles that would enable them to read. Whereas young patients may get adapted to a closer working distance, the older patients with a recent impairment find it difficult to accept reading at a close proximity. The practitioner may patiently persuade them to take the reading matter close enough to fall within the focusing distance of the lens but they often fail to get adapted. Such patients may find a telemicroscope of help. Telemicroscopes are telescopes for close work and allow reading at a distance longer than that of a single lens magnifier of the same strength.
Illuminated Magnifiers
Battery handle magnifiers with halogen illumination can be of great help to certain patients, specially those with ARMD. With appropriate illumination, contrast sensitivity is increased and the print becomes easier to read. Bullimore and Bailey (1995) showed that with the appropriate light, the scotoma produced by the macular lesion is reduced in size. On the other hand, high illumination may not be tolerated by a patient with media opacities due to intraocular scattering resulting in retinal image degradation.
High Add Bifocals
A low powered magnifier that is often found of practical help in reading, stitching or needle work is the Easy Vision magnifier of power +5.00 suspended by a cord around the neck with its feet resting against the chest. Its low power allows a large field, providing comfortable use for many tasks to help its user.
It is of help to note that where only a low magnification is needed for reading, bifocals with reading addition of +4.00 or more often help the user. Special low vision bifocals are available to give reading additions from +4.00 D to +16.00 dioptres. For binocular use the high add reading segments (upto +6.00 D) have to be rotated inwards to give the prism effect as required, above +6.00 add the use is monocular.
CCTV
Where higher magnification is needed for a person, a closed circuit television system is of help. A television camera is used to create a magnified image on a monitor screen. Reading matter is placed under the camera on a platform underneath and the text is viewed on the screen with a magnification which can be adjusted to the required level and can be as high as 60X. Some systems use a mouse which is placed on the text to pick up the image and display it on the screen. CCTVs provide higher contrast than other magnifying systems and hence are found useful to persons with reduced contrast sensitivity. A useful feature available is the reversal of polarity which transforms the text to white letters on black background. Such reversal reduces the intensity of the image and thus reduces the light scatter which helps patients with lens opacities. Imported CCTV systems are expensive but indigenous systems recently developed are reasonably priced.
Non Optical Aids
Optimum lighting conditions have to be assessed and demonstrated to the patient in the clinic drawing attention that similar lighting is needed at home. A single overhead tube light is often thought by the patient to be sufficient but he is to be made aware that it is not so. Additional adjustable reading lamp with high intensity fluorescent tube is often of great help.
Conversely, persons who are photophobic and light sensitive may prefer dim illumination. Absorptive filters can be of help to some. It is useful to have an arrangement to dim the clinic illumination when necessary.
Reading stands improve comfort and performance by positioning the reading matter at a convenient angle and at the required distance.
A typoscope is a matt black sheet with a rectangular slit cut in it. When kept on the page, it exposes only the required lines. It covers the surrounding page, so that the glare reflected from it is reduced. It also helps tracking the print and is used also for writing and signing a cheque.
Glare
Patients with extreme photophobia as with albinism, achromatism, corneal scarring, retinitis pigmentosa, retinal dystroplies and certain media opacities are adversely affected by bright surroundings causing discomfort and disability glare. They can be helped by filters. Corning CPS photochromic range comprises yellow orange CP S 511, a stronger orange CP S 527 and the red CP S 550. NOIR also makes a range of shades as clip - ons or visors. Such filters are costly and not readily available. Similar shades are available sometimes in sunglasses with side shields and those are of some help at low cost.
It is necessary for a low vision clinic to have different types of magnifiers and basic aids in varying magnifications for trial, so that the most suitable one can be selected. The patient has to be instructed in its proper use before it is handed over. Follow up visit is essential to ensure that it is used correctly to derive the desired benefit. It may be necessary to repeat instructions for use or to change the type of aid or to vary the magnification initially given. Loaner units are of help to create confidence and encourage the patient to derive the maximum use of his residual vision.
SUMMARY
A total service for rehabilitation of the partially sighted person is ideally available at only a few, well established institutions in the country. A comprehensive service demands a team work of professionals involving ophthalmologists, optometrists, rehabilitation workers, counsellors, orientation and mobility instructors, social workers, psychologists and occupational therapists. Until a network of such services can be established, ophthalmologists and optometrists can help persons with low vision within the set up of their own clinic. Such work demands professional time and attention for which assistants can be trained and directed.
The routine involves an initial examination, recording of ocular history and an analysis of the problems experienced by the patient. It is followed by a precise refraction. An assortment of magnifiers and some simple low vision aids is required for trial and assessment of magnification at distance and near. It is of help to provide units on loan to the patient to build up confidence. The patient needs to be instructed in the proper use of the aid. A follow-up is necessary to check how well the patient manages the device and derives the benefit intended. Rehabilitation of the visually handicapped is a satisfying service that should be organised and rendered wherever possible - at an ophthalmic department of a hospital or a private practice.
REFERENCES
1.Randall T Jose. Understanding low vision - American foundation for the blind, 1994.
2.Frank Eper Jesi. Low vision assessment part 4 - Optician. 2001; 22 : 5796.
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