3.3 Field of Vision Tests |
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3.3 Field of Vision Tests
The procedure of testing a patient’s field of vision is known as perimetry. The procedure can be performed by an ophthalmologist but is most commonly practiced by a hospital optometrist or a high street optician. A range of methods and instruments are available for perimetry and some of the more common types are described below. In each case the subject is asked to view a central fixation target and different targets presented at various locations in the visual field. The subject responds by pressing a hand-held button if the stimulus is seen.
3.3.1 The Normal Visual Field
If one fixates on a central target then we are able to detect objects in our peripheral vision out to certain limits. A normal subject should be able to detect objects or test stimuli placed within 60◦ in the superior field, 75◦ in the inferior field, 100◦ in the temporal field and 60 degrees in the nasal field. The normal visual field is almost elliptical in shape and this is illustrated in Figure 3.4.
Note that the extent of the normal visual field is dependent on the intensity of the test target. Another important feature of the normal visual field is the location of the physiological blind spot. This is the optic disc area of the retina which is devoid of photoreceptors and is where the nerve fibres bundles pass through to the optic nerve to transmit signals to the visual cortex. The blind spot is located at approximately 15.5◦ in the temporal field and is a vertical oval approximately 8.5◦ by 5.5◦.
Figure 3.4. Normal visual field plot
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Figure 3.5. Tangent screen
3.3.2 The Tangent Screen
This is a simple and effective method of performing perimetry. A black felt screen (Figure 3.5) of 1 or 2 m2 is usually used. The centre of the screen has a central fixation mark. Targets are attached to long matt black handles which the operator uses to present the target in the desired location. Targets are usually white in colour and can vary in size from 1–5 mm. The targets are usually black on the reverse side allowing the operator to flip the stimulus quickly to check the reliability of the subject’s response.
An isopter defines the boundary between the area where the stimulus is detected and the area outwith the field of vision where the stimulus is not detected. In practice the operator will use large targets to map the blind spot area. The target is moved from non-seeing areas to seeing areas with the patient simply responding that the object has ‘gone’ or come ‘back’. Smaller targets will then be used to assess the visual field. The isopters obtained are expressed as a fraction with the numerator representing the target size and the denominator the screen size. 3/2000 W indicates that a 3 mm white target was used on a 2000 mm screen.
3.3.3 Kinetic Perimetry
Modern perimetry is usually performed with computer controlled automated instruments. The Goldmann perimeter is shown in Figure 3.6. This is a kinetic perimeter which utilises moving stimuli. The principle of operation is similar to that described for the Tangent Screen.
The operator will move stimuli from seeing areas to non-seeing areas with the patient giving a verbal response indicating if the stimulus has ‘gone’ or come ‘back’. The operator will manually document the changes on a chart. The normal visual field from this perimeter was shown in Figure 3.4 and an abnormal example
3.3 Field of Vision Tests |
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Figure 3.6. Goldmann perimeter
Figure 3.7. Abnormal visual field plot from the Goldmann perimeter
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from a patient with the hereditary degenerative condition of retinitis pigmentosa is shown in Figure 3.7.
3.3.4 Static Perimetry
The Humphrey perimeter is a common machine used in hospital eye departments and is shown in Figure 3.8. Like most modern perimeters, the device consists of a projection bowl and test stimuli are presented on the surface of the bowl using fibre optics, light emitting diodes or light projections. The bowl design ensures even background illumination of a known intensity which will be consistent between repeated measurements over a period of time. The Humphrey instrument uses light projection to present the stimuli. Protocols vary in complexity but a range of stimulus intensities will be used in each test location in order to determine the threshold for detection. Various algorithms are employed to alter the intensity of subsequent test stimuli based on the subject’s previous responses.
Test stimuli will also be placed within the blind spot area to monitor the subject’s fixation. Infrared cameras are also used to give the operator an indication of the patient’s eye and this is used to monitor fixation throughout the procedure. Throughout the procedure stimuli are presented at pre-determined points in the visual field. If the stimulus is detected then the patient presses a button to indicate the stimulus has been detected.
Figure 3.8. Humphrey perimeter