Vision is the most important source of information about our environment. Loss of vision reduces the ability to perform activities of daily living, and affects safety and quality of life. Most of this book deals with reducing the causes of vision loss. Vision rehabilitation deals with reducing its consequences.
In developed countries, and increasingly in developing countries, the majority of irreversible vision loss occurs in the elderly and will represent an ever increasing part of ophthalmic practice (see Chapter 20). Unfortunately, many patients and caregivers still consider vision loss as an inevitable result of aging and often do not seek the help that is available. It is the task of the ophthalmologist to tell them that even if “nothing more can be done” about their reduction of vision, “much can be done” to deal with the consequences of vision loss for the person.
STAGES OF VISUAL PROCESSING
When dealing with the consequences of vision loss, it is important to recognize that vision is a complex, multistage process. Dysfunction at the different stages of visual processing causes different problems that require different solutions. The first is the optical stage, which puts an image of the outside world on the retina. The second is the receptor stage, which translates the optical image into neural impulses. The third stage is neural processing, which starts in the inner retina and proceeds via the visual cortex to higher cortical centers, where it eventually gives rise to visually guided behavior.
The optical stage can be disrupted by refractive errors and media opacities. Letter chart acuity is a good tool to evaluate this stage, and magnification devices (see Chapter 24) are the natural choice to counteract this type of vision loss.
The receptor stage can be disrupted by retinal disease. If foveal function is reduced, visual acuity is reduced. If foveal function is absent, causing a central scotoma, a pseudo-fovea or preferred retinal locus (PRL) must take over fixation. This eccentric area will have a reduced receptor density, which causes further reduction of visual acuity.
For retinal disorders, letter chart acuity is important but tells only part of the story, since it describes only the function at the point of fixation and tells us nothing about the condition of the surrounding retina (even a 20/200 letter covers less than a 1° area). Normal vision involves constant eye movements, which may move the object of attention in and out of the best-functioning area. This scotoma interference, which may be apparent as hesitation during testing, is not quantified by visual acuity and cannot be remedied with magnification devices. Patients need training and practice to improve their fixation stability. This may be provided by occupational therapists or vision rehabilitation specialists, but it is up to ophthalmologists to recognize the need for this training and to make ...