Vision loss reduces the ability to cope with daily living activities, and affects the safety and quality of life. 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 can be done” about many of the causes of vision loss, “much can be done” about its consequences. This chapter will deal with ways to alleviate the consequences of vision loss through comprehensive rehabilitation (also see Chapter 24).
Vision is a complex, multi-stage process. Dysfunction of the different stages causes different problems and requires different solutions.
The first stage is the optical process that puts an image of the outside world on the retina. This stage can be disrupted by refractive errors or media opacities, such as cataract. A good tool to evaluate this stage is visual (letter chart) acuity which measures the MAgnification Requirement (MAR) relative to the 20/20 reference standard. (MAR is usually known as minimum angle of resolution.) Magnification devices (see Chapter 24) are the natural choice to counteract this type of vision loss.
The second stage is the receptor stage that translates the optical image into neural impulses. If this stage is defective, vision is disrupted in a different way. A blind spot in the central retina (central scotoma) may necessitate shifting fixation to a less central retinal area (the preferred retinal locus, PRL) where the receptor mosaic is less dense. This causes reduced visual acuity, which can be counteracted by magnification (see Chapter 24). However, visual acuity tells only part of the story, since the condition at the PRL tells us nothing about the condition of the surrounding retina. Since normal vision involves constant eye movements, the object of attention may move in and out of the scotoma. This scotoma interference is not quantified by visual acuity, although it may be apparent during testing, and cannot be remedied by magnification devices. The patient needs training and practice to improve 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 make the appropriate referral.
The third stage is that of neural processing. This process starts in the inner retina and proceeds via the visual cortex to higher cortical centers, where it eventually gives rise to visually guided behavior. This stage is undoubtedly the most complex and awareness of vision problems related to the processing of visual information is increasing, such as the perceptual consequences of traumatic brain injury (TBI) and cerebro-vascular accidents (CVAs), and in children the importance of the ...