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Module 12 Section 1 |
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Module 12: Visual Field Testing Section 1: Basic Concepts |
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In order to better understand visual field testing, a brief review of visual physiology is in order.
The human eye has two types of vision: central vision and peripheral vision. Central vision is located in a small area of the retina called the macula. The macula contains a large number of cones cells which specialize in seeing detail in color and in relatively bright light. |
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Above, in the center of the photo: The macula of a 70 year old white female. If it does not look like a normal macula to you, you are correct. There are some retinal pigment epithelial (RPE) changes that are perhaps a precursor to macular degeneration. |
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The remainder of the retina contains rod cells which do not see detail. These cells are more sensitive to low light and, as a group, are very sensitive to movement. It is the primary job of the peripheral vision to make us aware of the location of objects in our visual space and to direct our central vision toward a particular object if desired. It is sometimes desirable to measure the function of the peripheral vision because there are disease process that can affect it. Visual field testing is most often performed on glaucoma patients. Glaucoma affects the peripheral vision long before it affects the central vision. The progress of the disease can be followed by making periodic maps of the peripheral vision immediately surrounding the central vision. |
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| The primary measure of central vision is the visual acuity as determined by reading a Snellen chart. The peripheral retina is not sensitive enough to see much detail on a Snellen chart. Another method must be used. |
E H B P H T C V L N E D A O F |
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As stated earlier, the peripheral retina is sensitive to relatively low light and to movement. The science of perimetry has evolved a variety of procedures and instruments to measure these properties and to quantify the extent and the sensitivity of the visual field. |
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The zenith of manual perimetry was reached with the invention of the Goldmann perimeter, an engineering masterpiece that standardized much of the measuring process. Automated perimeters have taken the process a step farther by eliminating the operator as a major variable. Although the Goldmann perimeter is rarely used these days, a familiarity with its basic design and operation gives us a good understanding of basic visual field testing. |
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| Visual field testing is simply a method by which we measure the extent of our visual space and our sensitivity to it at any given moment. This is accomplished by increasing the intensity of a stimulus to a known area of the retina until the stimulus is seen. In order for the results to be meaningful, we must be able to reproduce the circumstances of the test. In other words, the procedure and the instrument must be standardized. | ||||
| The Goldmann perimeter standardized the background by having the patient look into a bowl that was illuminated exactly the same way for each test. One eye was patched so that only one eye was being tested at a time. The patient was instructed to maintain fixation on a target at all times. This ensured that the the exact same area of the retina could be repeatedly stimulated. The stimulus could be varied by precise increments of brightness and size, and the stimulus could be easily presented to any area of the retina by use of a projection system. | ||||
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| Above: The bowl of the Goldmann perimeter. Notice the fixation target in the center of the bowl. The small arm holds a corrective lens if necessary. The black housing at the top of the photo has a bulb that precisely illuminates the bowl. The black arm to the left holds a projector that can project a stimulus of varying intensity and size to any point in the bowl. | ||||
| There are two basic types of stimulus presentation: static perimetry and kinetic perimetry. Static perimetry involves the presentation of stimuli to one spot of the retina in increasing levels of intensity until the stimulus is seen (the threshold). This method is good for determining the sensitivity of the visual field in a particular area of interest on the retina. Static perimetry is time consuming and is not a good method for determining the extent (boundaries) of a visual field because of the large number of spots that would need to be tested. The Goldmann perimeter was rarely used for static perimetry because of the tedious nature of the procedure. Automated perimeters have standardized strategies which shorten the testing time and permit the use of this sensitive procedure to detect small changes in the visual field. | ||||
| Kinetic perimetry involves the presentation of a moving stimulus of a particular size and intensity. The stimulus is moved from a non-seeing area toward the fixation point until it is seen. | ||||
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| A boundary (isopter) is mapped for the particular stimulus size and intensity. The size and/or intensity of the stimulus is then changed and another boundary is mapped. Viewed together on a chart, these boundaries (isopters) give a picture of the overall extent of the visual field and give a measure of the sensitivity of the visual field, depending on how far the boundary extends for a particular size and intensity of stimulus (see the isopter map below). | ||||
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It is sometimes assumed that all areas inside of the isopter boundary would respond to the isopter simulus and brighter stimuli. This is not a good assumption, because areas of defective vision can be contained within the boundaries of an isopter (i.e., a scotoma). Scotomata can be found by testing statically within the boundary of the isopter using a kinetic perimeter. The technique is to turn the stimulus on and off at strategic points (grid) within the isopter boundary. Scotomata can also be found kinetically by moving the stimulus around inside the isopter boundary and having the patient indicate if the stimulus dims or disappears at any point. A scotoma found could then be mapped by presenting the stimulus in a non-seeing area and moving outward until seen. |
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The Goldmann perimeter is best suited for kinetic perimetry. The Goldmann perimeter is still the perimeter of choice when a boundary visual field, such as for disability determination, is desired. The downfall of the Goldmann perimeter is that the movement of the stimulus in the kinetic procedure is controlled by the operator and cannot be standardized. This limits the accuracy of the machine as far as getting an accurate determination of the sensitivity of the visual field. The sensitivity of the visual field is important when following glaucoma patients, thus the automated perimeter has become the standard. |
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| Figure 1 represents a visual field map of a normal right eye as measured kinetically with a Goldmann perimeter. Point A represents foveal fixation. The black spot at B is a map of the blind spot (optic nerve entry). Points C and D represent the horizontal limits of the visual field. Point C is about 60 degrees nasal from point A and point D is about 90 degrees temporal from point A. The vertical limits are about 60 degrees superiorly (E) and about 75 degrees inferiorly (F). | ||||
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Figure 1 |
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Each line represents an isopter and is the boundary map of a stimulus of a particular size and intensity. The stimulus was moved from outside the boundary, toward point A, until the patient indicated that the stimulus was seen. The point was marked on the graph, and at the end of the test the dots were connected to form a boundary line (isopter) for each stimulus. The smaller isopters represent stimuli that are smaller in size and/or less bright. The stimulus travels closer to the central vision before it is seen by the patient. This indicates increased sensitivity. This map tells us that the central part of the field is more sensitive than the periphery, with the most sensitive point corresponding to the fovea at A. There is zero sensitivity on the optic nerve head, indicated by the small black circle. Another way to look at the results is to plot sensitivity on the vertical axis of a graph and plot position on the horizontal axis, as in Figure 2. The fovea, at point A, has the greatest sensitivity, and sensitivity drops off as we move toward the nasal (C) and temporal (D) periphery. Sensitivity drops to zero for a short distance at the location of the optic nerve head (B). |
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Figure 2 |
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| Figures 1 and 2 can be combined to produce a "3-D" map of the visual field (Fig. 3). The is called the "island of vision analogy". The area taken up by the island is an indication of the boundaries or the extent of the visual field. The height of various points on the island is an indication of how sensitive the retina is to light at any particular point in the field. The peak of the mountain represents the fovea at point A. Sensitivity drops off into a black hole at the optic nerve (B). | ||||
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Figure 3 |
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The object of visual field testing is to produce an accurate "island of vision" map. Notice that the light sensitivity of the retina gradually drops off as you move farther into the periphery. The sensitivity of the retina also normally decreases with age. This would be characterized by an island of the same shape, but it would a lower height across the board. |
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