Description

Visual impairment is seldom life threatening. Nevertheless, it is among the most disabling of medical conditions; more disabling than all chronic conditions except diabetes and cancer in older adults. Visual impairment is strongly associated with disability and dependency in daily activities, reduced physical activity, higher unemployment, social isolation, and depression. The quality of life of people with low vision has been compared to those with congestive heart failure or clinical depression. It is important to emphasize that the vast majority of visually impaired people retain some useful vision. Fewer than 10% are totally blind and over 75% can read newspaper headlines, according to a UK survey. One of the main goals of visual rehabilitation is to reduce disability and handicap by maximising the use of residual vision. Through the 1950s, the sight saving philosophy dominated visual rehabilitation. According to this view, the use of partial sight would hasten its demise. Although the perpetuation of this myth lead to some beneficial outcomes, such as the development of large print books and special educational programs for visually impaired children, it interfered with the establishment and acceptance of low vision rehabilitation. Even today, many eye care providers view visual rehabilitation as a service worthy of little attention. This is all the more surprising given that the majority of those who are visually impaired – other than those with cataract – cannot have their vision restored by current medical or surgical treatment. It is, therefore, especially significant that the emphasis of this feature issue is on new methods and challenges of visual rehabilitation.

There are many types of low vision devices and vision rehabilitation services that compete for scarce resources. Yet there are almost no studies of their effectiveness on which to base health care policy decisions. Surveys of patient satisfaction following vision rehabilitation have produced conflicting results ranging from 90% reporting that the service was sufficient to meet their needs to 50% reporting dissatisfaction. Despite the substantial number of people with visual impairment there have been only a few controlled trials of vision rehabilitation. The majority of these trials found rehabilitation to be effective but most were limited to older low vision patients, especially those with AMD. There is almost no evidence on the effectiveness of vision rehabilitation for children or adults of working age. Without solid evidence as to the benefits of vision rehabilitation, it is difficult to argue for the expansion of rehabilitation services. Similarly, there are wide discrepancies in the reported effectiveness of prescribed low-vision aids. Some studies report that 80% to 90% of patients find the devices useful for everyday activities, while others indicate that the majority of patients stop using the devices within 18 months. There are many published studies on the benefits of particular devices, but most of these are based on patient preference. If there is a consistent theme throughout this feature issue it is the need for better evaluation – of the patient, of new low vision devices, and of treatment strategies and interventions.

During the past twenty years there has been a dramatic increase in the number and variety of instruments for vision evaluation. These include everything from new charts for measuring visual function, such as acuity and contrast sensitivity, to questionnaires for assessment of vision-related quality of life. Clinicians may well ask why they should change their tried and trusted evaluation procedures to incorporate these new developments.

Most organised studies that assess vision use logMAR acuity charts. Numerous papers have been written about the advantages of logMAR tests over the venerable Snellen chart, including better standardisation, easier quantification of acuity scores, and improved test reliability, yet few clinicians use them in their daily practise. Contrast sensitivity testing was introduced into the clinic more than 30 years ago. Widely heralded as a replacement for visual acuity in refraction, screening, and diagnosis of ocular pathology, contrast sensitivity never lived up to its early hype. Nevertheless it has proven to be a useful adjunct to acuity in a wide variety of settings. Modem contrast sensitivity tests are easy to administer and interpret, and there is extensive evidence that they provide important information about visual function that cannot be learned from acuity tests alone. But once again, outside of a research setting, few low-vision practitioners use contrast sensitivity tests. The reason usually given for ignoring contrast sensitivity is that there is little that can be done to improve it. However, contrast sensitivity and visual acuity are inter-related and a visually impaired person with poor contrast sensitivity may benefit from extra magnification beyond that predicted by the level of acuity. Furthermore, recent developments in electronic low-vision aids (discussed in this issue) bring the prospect of affordable contrast enhancement ever nearer.

In addition to these “objective” psychophysical measures of visual function, heath care providers and policy makers have recognised the importance of “subjective” assessments of the impact of visual impairment on daily life. Vision-related quality of life (QoL) questionnaires are ubiquitous, and new instruments are being introduced all the time. Nowadays it is taken for granted that the psychometric properties of a new questionnaire must be well established before it can be incorporated into clinical research. Most have been evaluated for reliability and validity, and to a lesser extent, sensitivity. But less attention has been paid to the measurement properties of these instruments. Typical QoL questionnaires elicit ordinal responses such as “very difficult” or “less frequent”. It is not a simple matter to convert these responses into meaningful numerical scores, nor is it sufficient to merely add up the arbitrary numbers assigned to responses to individual items. Fortunately, there are well-established procedures for evaluating the measurement properties of ordinal instruments and several of the more recent QoL questionnaires have applied these techniques.

Performance-based tests are an important bridge between psychophysical vision assessment and subjective questionnaires. Simple tests of reading speed, face recognition ability, mobility performance, and other daily activities provide information about the level of performance that can be expected under standardised conditions. We have found these tests to be well correlated with real world performance and discrepancies between performance-based results and subjective questionnaires may be especially informative about patients who are risk for developing future disability.

With this expanded armamentarium of vision assessment tools, low vision practitioners and clinical researchers now have the opportunity and responsibility to work together to establish the effectiveness of low vision rehabilitation. Without this information we will be unable to meet the needs of the increasing low vision population and to provide improved rehabilitation services for visually – impaired persons of all ages.