Some measurements of the visual field can be made by sitting facing the patient and asking if the movement of one's fingers can be discerned. The patient is instructed to cover one eye with a hand and the observer also covers one of his eyes so that he can check the patient's field against his own. The test can be made more accurate by using a pin with a red head on it as a target. None of these confrontation methods can match the accuracy of formal perimetry. A number of specialised instruments of varying complexity are available. Using such equipment, the patient is presented with a number of different-sized targets in different parts of the visual field, and a map of the field of vision is charted. An accurate map of the visual field is often of great diagnostic importance. In the past, it was customary to map out the central part of the visual field using the Bjerrum screen, and the peripheral field using a perimeter. The Goldmann perimeter was then introduced, and this instrument allows both central and peripheral fields to be plotted out on one chart. The Humphrey field analyser is a further development in field testing. It provides an automated visual field recording system (Figure 3.3). It also
Figure 3.3. The Humphrey field analyser.!
records the reliability of the patient by showing false-positive and false-negative errors. In practice this is very useful, as poor reliability is often an explanation for poor performance.
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