Low vision or visually impaired is a term used to describe varying degrees of vision loss that cannot be corrected by medications, surgeries or conventional glasses. Vision loss may be due to:
- Decreased visual acuity
- Visual field defect
- Decreased contrast sensitivity
- Loss of colour perception
Some practitioners define low vision as a visual acuity of upto 6/24 or worse in the better eye using the best corrected spectacle 2correction or visual field of 20 degree or less. However, a more functional definition is that low vision comprises bilateral vision loss that adversely affects the performance of daily activities.
Thus a low vision patient has either poor Snellen's acuity or poor field of vision or both. With such subnormal vision, the subjects are unable to perform their task.
A low vision patient 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 (20/60) to light perception or a visual field of less than 10° from the point of fixation, but who uses or is potentially able to use vision for the planning and/or execution of a task.*
VISUAL DISTURBANCES PRODUCED BY LOW VISION
There are many eye diseases that can cause low vision, and can cause various kinds of visual disturbances. Some of the visual disturbances can be grouped as under:
- Loss of central vision: Loss of central vision affects the ability to see the objects or people in the direct line of vision. Individual may incur partial loss of central vision or loss ranging from a small sector to the total central loss depending upon the disease and its progression. In such cases colour vision may be affected while peripheral vision remains normal. The individual may also find it difficult to see details and suffer from distorted vision. Usually diseases like macular degeneration, albinism, stargardt diseases, toxoplasmosis, histoplasmosis, etc. cause loss of central vision (Fig. 1.2).
- Loss of peripheral vision: People with peripheral vision loss have various difficulties with independent travel, depending on the degree of vision loss.3A person who is just beginning to lose peripheral vision may find himself colliding with the obstacles on the sides, furnitures, etc. Some patients with advanced peripheral loss may not be able to detect steps, or other obstacles at all. One form of peripheral vision loss is called “tunnel vision”, in which the person sees the world as if looking through a tube with one eye. Retinitis pigmentosa, Hemianopia, Glaucoma, Juvenile Diabeties, etc. are the few diseases which can lead to loss of peripheral vision (Fig. 1.3).
- Overall blurred vision: Overall blurred vision affects the individual's ability to perceive the sharpness of details due to an alteration in the refractive media of the eyes. The individual may have blurred vision over the entire field, or over a partial field. The individual may even suffer from double vision, poor night vision, poor contrast and glare trouble. Congenital, traumatic and aging cataracts are some of the major reasons for overall blurred vision. Corneal opacities, high myopia and amblyopia can also lead to overall blurred vision (Fig. 1.4).
- Night blindness: Night blindness refers to the inability to see at night under starlight or moonlight or in dimly lighted areas like movie theatres, etc. Retinitis Pigmentosa, Diabetic Retinopathy, Glaucoma, etc. are a few diseases, which may lead to night blindness.
- Light and Glare: Patients with some specific diseases also live in a world where light and glare constantly interfere. Glare reduces the brightness difference and also impairs contrast sensitivity. For patients with retinitis pigmentosa, cataract, etc. walking outside into the sun may result in being overwhelmed by brightness to the point of temporary disability. However, light and glare symptoms will vary depending on the degree of damage (Fig. 1.5).
PSYCHOLOGY OF LOW VISION PATIENTS
A low vision patient undergoes definite psychological stages before he becomes a successful user of low vision aids. The first harsh reality that a low vision patient faces is that a single pair of glasses is not enough for his vision.6
The reality that the patient must hold a magnifying glass and hold materials very close to his eyes is often too much to bear. That is the reason that the first stage is characterized by his quick snap decision that “the low vision stuff is not for me”. They at this stage quickly reject any new informations or suggestions without taking time to think about some of the new options that are being presented to them. It is almost as if these new ideas are “too big” or “too scary” and often let them soak into their psyche. The reason could be that no one 7has ever told them about these devices, neither the doctor nor their family member. They are suddenly confronted with unusual devices and things that they have never heard of before. They experience a big “let down psychologically” and for some, this let down is so large that they must first push it away and get out of it. It is like handing somebody a big and heavy pumpkin and telling them to swallow that (Fig. 1.6). Some patients who cross over stage one will secretly go into their room, put on their reading devices for several minutes, try reading and then spend the rest of their day acting as if it never happened. And when they feel that even with these devices they cannot see as clearly, they experience their first surges of anger and often take it out on those who are closest to them (Fig. 1.7).
At the third stage, the low vision patient exposes them to many different low vision options and tools. Now they become a “hard working patient”. They start listening to what is being told them and also make an eye contact with the different devices being presented to them. They also ask questions about it. In fact they start slicing the pumpkin and sample a few bite size pieces (Fig. 1.8).
At this stage some patients often regret to their behaviours that were seen in earlier stage. They start feeling that they need to use the special devices to be independent. Now glimpses of acceptance and strong glimmers of hope become visible on their faces. These new glimmers of hope can be a very strong motivation to continue trying and refining the process of determining which visual aid work best for the patient in different situations.
After feeling strong and confident in the use of one or two visual aids, the patient will look beyond what they currently use and begin to open up to all the possibilities of the various types of devices and want to learn more on how they can be used. They begin to educate themselves by reading low vision newsletters or perhaps attending conventions where low vision lectures and exhibits are presented. Now they start accepting their disability and embrace the use of various low vision devices to become independent functionally. All person does not reach this stage, but once they meet this stage, they become your ambassador and often reach out to others to help them in any manner.
PRINCIPLES OF LOW VISION PRACTICE
The basic principle of low vision practice is magnification. Low vision patients donot recognize small and far off targets. When targets are magnified, their image covers more retinal areas, which may have more responsive visual receptors. Thus a low vision patient is assisted to use his remaining vision. It implies that some areas of the central retina should function. It is then only, the magnification works to a certain extent.
Low vision services are geared to help patients to maximize the use of residual vision, not helping to see better in general. It only means helping the patient function better and independently in everyday life. For instance make a list of all the tasks that frustrate the patient in his day-to-day life and try the use of various aids. There is no “quick fix” or “single pair of glass” that will do it for 9all. Instead, depending on the severity of the vision loss, a typical low vision patient may need 3 or 4 different aids to help his various tasks.
Another way to understand low vision service is that low vision service can not “help the bad eye”. It helps maximize the use of residual vision in the better eye to accomplish the task. Low vision is not medical service, so going for low vision does not mean that the patient will get treatment to fix their vision. In fact low vision specialist will not be doing things for you. They will accurately describe their role as someone who can work together with you and guide you on your work. It is a beginning of a life long process that continues for the rest of patient's life. As people's needs and vision change, low vision rehabilitation will also change to help adapt and make improvements so that they can maintain their “quality of life” as possible.
Unlike other eye examination, the low vision process can take many hours of directions and hard work—not only during examination but also afterwards. The patient has to be motivated to take the responsibility for whatever may be asked to get the maximum improvement. You just cannot “sit back” and expect the doctors to do it all.