One obvious way to measure sight is to ask the patient to identify letters that are graded in size. This is the basis of the standard Snellen test for visual acuity (Figure 3.1). This test only measures the function of a small area of retina at the posterior pole of the eye called the macula. If we stare fixedly at an object, for example a picture on the wall, and attempt to keep our eyes as still as possible, it soon becomes apparent that we can only appreciate detail in a small part of the centre of the field of vision. Everything around us is ill-defined and yet we can detect the slightest twitch of a finger from the corner of our eyes. The macula region is specialised to detect fine detail, whereas the whole peripheral retina is concerned with the detection of shape and movement. In order to see, we use the peripheral retina to help us scan the field of view. The peripheral retina can be considered as equivalent to the television cameraman who moves the camera around to the relevant views and allows the camera (or macula) to make sense of the scene. If the macula area is damaged by, for example, age-related macular degeneration, the patient might be unable to see even the largest print on the test type and yet have no difficulty in walking about the room. Navigational vision is largely dependent on the peripheral field of vision. On the other side of the coin, the patient with marked constriction of the peripheral field of vision but preservation of the central field might behave as though blind. The same patient could read the test chart down to the bottom once he has found it. This situation sometimes arises in patients with advanced chronic simple glaucoma.
It should be becoming clear that measuring the visual acuity, although very useful, is not an adequate measure of vision on its own. For a proper clinical examination, we need to assess the visual fields and colour vision. A number of other facets of visual function can also be measured, such as dark adaptation or the perception of flicker.
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