Visual field testing has continued to evolve because of advances in technology. In the past, the standard of excellence revolved around the manual Goldmann perimeter and the perimetrist. The visual field was plotted ( Fig. 19.1A ) after a lengthy examination during which both kinetic (moving) targets and static (stationary) targets were presented in random fashion. Technicians required extensive training and, perhaps more important, considerable patience to perform this task accurately. Kinetic testing has been almost exclusively replaced by static techniques ( Fig. 19.1B ) because the latter are more easily automated. Infrared and mosaic video cameras now monitor fixation. Patient consistency is constantly evaluated during the test by catch trials designed to elicit false-positive and false-negative responses. Improved testing algorithms have decreased the total test time.
In spite of these advances, perimetry remains a subjective test. The entire process depends on the patient’s ability to concentrate and respond to the test stimulus. The key person interfacing between the patient and the computer is the technician. It is his or her responsibility to ensure that the patient understands the test and that the testing process runs smoothly. It is also the responsibility of the technician to ensure that fixation remains aligned, that the patient is given a break, and that he or she is reinstructed if fatigue is observed. No automated perimeter available today has eliminated the need for a technician to administer the test and monitor its quality in real time. In this chapter, the basics of perimetry are reviewed, the technician’s duties are outlined and, when appropriate, suggestions are given to make the task easier and more relevant to the patient’s needs.
Understanding the principles of perimetry
There are two types of perimeters: kinetic and static. Both kinetic and static perimetry ultimately perform the same function: they test the field of vision, but they do so in different ways. Kinetic devices highlight moving targets to map out the visual field (see Fig. 19.1A ) and spend less time exploring the field with stationary or static techniques, whereas static devices depend on static techniques almost exclusively (see Fig. 19.1B ).
The analogy of the visual field to an island of vision sitting amid a sea of blindness is a useful one ( Fig. 19.2 ). The highest point near the center of the island corresponds to the area of greatest visual sensitivity (for the fovea of the retina). As a person moves toward the water’s edge, the sensitivity falls to zero. In other words, the farther away from the area of greatest visual sensitivity a person is, the brighter a light target must be to be seen. At the water’s edge, in the analogy, not even the brightest target can be seen.
The task of perimetry is to map the island of vision. The problem is that the island is enshrouded in a fog bank ( Fig. 19.3 ), and indirect methods of mapping it are required.
Kinetic testing can be likened to airplanes flying toward the visual island at different altitudes ( Fig. 19.4 ). If we know the altitude at which the planes fly and if we record the coordinates of each “crash site” of many planes flying at the same altitude, then we can draw an “isopter” line connecting the points. With several isopters, the shape of the island unfolds.
There are disadvantages with kinetic techniques. Flat sections of the island are hard to map unless we use many closely spaced isopters. In a similar fashion, hollowed-out valleys in the interior portion of the island are hard to identify, and we are often dependent on our luck in choosing the proper airplane altitude, or isopter, to detect its presence ( Fig. 19.5 ).
Static perimetry uses an entirely different method of visual field exploration. Continuing the analogy, instead of flying airplanes at a known altitude into the island, parachutes are dropped onto the island from above, and the altitude where each parachute lands is recorded. Once enough parachutes have landed, a topographic map of the island can be created because the coordinates of each parachute are known ( Fig. 19.6 ).
A typical test stimulus is a spot of light presented at a location with a specific size and intensity on a background. Goldmann perimetry uses an alphanumeric code to denote target illumination. Each letter step from a to e represents 0.1 log unit (1 dB) brighter in illumination, and each number step from 1 to 4 is a 0.5 log unit (5 dB) change in intensity. Roman numerals from I to V indicate stimulus size changes and not variation in brightness. Because most automated threshold perimetry uses only intensity and not size changes, the automated devices use a different unit of measure, that is, decibels (dB). The advantages of the Goldmann code are enhanced in decibel nomenclature with its direct logarithmic scale. For example, a 20-dB light source seems 10 times brighter than a 10-dB stimulus.
Unfortunately, not all manufacturers have adopted the decibel scale. Some have continued to use the apostilb (asb) notation, which is nonlogarithmic, whereas others have adopted the decibel notation. Compared with log units, the two scales are inversely related ( Fig. 19.7 ). Note that 0 dB on the automated devices corresponds to the maximal stimulus intensity, and 0 asb on the nonlogarithmic scale refers to absolutely no illumination. To add to the confusion, not all automated perimeters produce the same absolute levels of target brightness or use the same level of background illumination. Older Octopus perimeters used a dimmer background level of illumination (4 asb), whereas Humphrey perimeters and more recent Octopus perimeters adopted the traditional Goldmann level of background illumination (31.5 asb). The maximum stimulus of perimeters can also differ. The maximum stimulus for a Humphrey perimeter is 10,000 asb compared to 4000 asb for an Octopus perimeter. Other perimeters have several different options for background illumination and use light-emitting diodes (LEDs) for targets instead of a projected target source.
Because each machine is different, it is important for both the technician and the physician to understand what units of measure are used and the breadth of the scale. In one automated perimeter, the brightest possible stimulus, which is 0 dB, corresponds to 10,000 asb, whereas in another, 0 dB corresponds to 4000 asb.
Threshold is a relative term and represents the level where a stimulus can be seen 50% of the time. This means that the same stimulus will also be missed 50% of the time. Threshold is not an absolute number; rather it is a mathematical approximation described by the sigmoidal curve ( Fig. 19.8 ). As the brightness of the stimulus increases, the likelihood of detecting it becomes higher and higher. This percentage of seeing the stimulus does not approach 100% until it is brighter than threshold. In the example illustrated, a stimulus of 55 brightness is seen half of the time. When the stimulus is increased to 59 brightness, the likelihood of seeing it is 95%, yet it is still missed 5% of the time.