Cupula distances, or bowl radii, differ in various machines. For example, the Octopus uses 100- and 85-cm bowls; the Humphrey Field Analyzer, like the standard Goldmann perimeter, uses a 33-cm cupula.^18,19 These values dictate the warranted corrective lens for central field examinations.

Background illumination also varies among perimeters and is important in its effect on the level of retinal adaptation as well as the degree of contrast that may be generated. The level of background illumination in the Octopus perimeter is 4 apostilbs; the Humphrey Field Analyzer’s is 31.5 apostilbs, similar to the Goldmann perimeter.^18,19 Dimmer backgrounds allow a machine to present brighter stimuli to the visual system with respect to background light, which is helpful for evaluating patients with markedly reduced sensitivities. The disadvantage is the risk of shifting retinal sensitivity from the photopic range, with a subsequent alteration in retinal sensitivities. The effect of media opacities to decrease retinal sensitivities may be more pronounced in instruments using lower background illuminations.²⁹ The possible advantages of the brighter background include the shorter time required for adaptation from ambient lighting conditions before test initiation and a testing situation in which the examination is less sensitive to aberrant light. Although white light is typically used, the color of the background illumination can be altered. For example, in short-wavelength automated perimetry (SWAP), discussed later, yellow background illumination is used.

Instruments use three basic stimulus sources. The first is a fiberoptic cable system, with the advantages of low cost, no moving parts, and quiet operation. Its disadvantages include minimal flexibility in that stimuli are at fixed locations, are of fixed size, and cannot individually alter their intensities during any one examination. The light-emitting diode (LED) system has the same advantages and disadvantages as the fiberoptic system but is somewhat more flexible in that intensities of stimuli can be altered individually during a program. An additional disadvantage of both systems is a potentially increased variability of results compared with projection perimeters.^30,31 The projection system, used by both the Octopus and Humphrey perimeters, allows the greatest flexibility and versatility in the selection of test point locations as well as stimulus size, intensity, and color. Its disadvantages include expense, noise production, and need for more frequent maintenance. Unlike the fiberoptic and LED systems, the projection method allows for possible adaptation to kinetic capabilities.

Most instruments incorporate a stimulus size equal to that of a Goldmann III target (4 mm², 0.431°). Projection perimeters allow for the use of larger targets, which may allow quantification of focal defects in the presence of generalized field depression and identification of small remaining portions of the visual field in cases of extensive loss.³² LED and fiberoptic cable systems do not allow for the changing of target sizes. Stimulus size may be varied in patients with reduced retinal sensitivities or in algorithms designed to assess the field of vision at a lower spatial resolution (e.g., SWAP).

The perimeter may be programmed for testing at a fixed intensity or at a few levels of intensity, or to vary intensities for determining threshold sensitivities at test locations. Ranges of stimulus intensities vary among machines. The Humphrey Field Analyzer generates stimuli that range between 0 and 10,000 apostilbs (51–0 dB, respectively); the Octopus provides a range of 0 to 1000 apostilbs (40–0 dB, respectively).^18,19 A fixed stimulus intensity in apostilbs has a different value in decibels on machines with different maximum stimuli, because decibels are relative measurements depending on maximum stimuli. A patient’s ability to perceive a stimulus depends on the intensity of the stimulus in addition to the level of background illumination.

Stimulus durations usually range between 0.1 (Octopus) and 0.2 (Humphrey) seconds,^18,19 but they can be altered by the examiner with certain instruments. These values have been chosen because it has been shown that duration times >0.5 s have no effect on the ability to detect stimuli by temporal summation, and exposure times >0.2 s correspond to a patient’s reaction time to the stimulus and might cause a shift in fixation.²⁷

Reliable fixation is essential for automated perimetry. Available techniques include viewing the patient’s eye, electronic eye motion detectors, and blind-spot monitoring. Combinations of these systems are often used by various perimeters. The technician can view the patient’s eye directly either with a telescope or a closed-circuit television monitor. Images of the eye may be taken from a camera coupled to a bowl-centered telescope and then easily viewed by the examiner. Eye movement sensors are the most sensitive fixation deviation detectors. Photoelectric detectors of corneal reflectivity and pupil image monitors are used. The disadvantage of fixation monitoring by electronic sensors is that it may be too sensitive, responding to minor physiologic alterations in fixation such as respirations or blinking. The blind-spot projection technique, the Heijl-Krakau method,³³ assumes that if fixation is unchanged during an examination, the blind spot should remain constant. During an examination, the machine presents suprathreshold stimuli into the presumed or previously mapped blind spot. If the stimuli are seen, fixation has changed; if not, fixation has been maintained. The disadvantages include increased testing time and unknown status of fixation between points tested. Another disadvantage is that the visual field being tested may be only a central 10° island (as in advanced glaucoma or retinitis pigmentosa), in which case a physiologic blind spot cannot be localized. If there exists a scotoma contiguous with the blind spot and deviations from fixation occur, if blind-spot checking stimuli are presented in the scotoma, fixation losses will not be detected. The advantage of the blind-spot projection technique is that it is less expensive and sensitive than electronic sensors and does not require technician input. This technique is usually supplemented by a telescope or video monitor, allowing the examiner to oversee fixation. When these techniques are used in combination, as they often are, monitoring of fixation is excellent.

In general, fixation is improved with random presentations of stimuli where patients cannot as easily anticipate the location of the next stimulus. For patients with central scotomas who cannot fixate on a central target, most instruments direct fixation by having patients look at the center of a diamond or cross pattern.

Although not all perimeters have this capability, one of the greatest advantages of computerized automated perimetry is its capacity for data storage. Data are stored on hard disk or a floppy disk; this prevents data loss when the hard copy is inaccessible. Storage of data also permits statistical analysis and manipulations of individual visual fields or serial examinations of the same patient, as well as transfer of information to a central computer or another clinician or researcher. With greater use of electronic medical records, networked transferring of perimetric results has become more important. Most automated perimeters allow networked data transfer of visual field results in both a portable document format (.pdf) and raw data format.

Most instruments make available a series of basic testing patterns, allowing the examiner to choose the area of field to be tested and the density and distribution of points within. The options for areas to be tested include the central 30°, the peripheral 30° to 60° area, full 60°, central 10°, and central 5°. Special test configurations are also available, including glaucoma patterns (central 30° plus peripheral nasal step area and temporal periphery), peripheral nasal step area, temporal crescent region, and neurologic patterns for examining the vertical meridian. Custom tests for obtaining additional detail in certain abnormal areas may be added to any portion of the field in different patterns such as grids, clusters, or individual points. With certain instruments, the examiner has the option of combining several different patterns to increase the density of testing locations or total area tested. Custom tests can be stored and reused in subsequent testing.

Two basic categories of testing strategies are offered by most instruments: suprathreshold testing and threshold determinations.³⁴ Either of these modalities of testing can be used with most of the previously discussed patterns or variations thereof.

Suprathreshold (supraliminal) screening enables rapid discovery of gross field defects. However, the degree of the resolved sensitivity decrease is not determined. A fixed stimulus that is presumed to be threshold at approximately 30° is used for the entire test area, without compensating for the normal contour of the hill of vision—that is, without adjusting for decreased sensitivity with increasing eccentricity (one-level or single-intensity strategy). These suprathreshold stimuli of fixed intensity are presented throughout the designated testing area. The tested points are recorded as either seen or not seen.

An alternative technique is the threshold-related screen. This strategy involves using a varying stimulus intensity at different test points, taking into account the effects of eccentricity on the expected threshold levels. The expected values are determined by either thresholding at two or four points in the field at the initiation of the test (prethresholding) or estimating the height of the hill of vision using age-matched normals. The intensity of the stimuli presented at any given point in the field is suprathreshold, usually 5 to 6 dB brighter than the expected threshold value. The points can be recorded as either seen or not seen (Figs. 109-2 and 109-3). Unseen points can then be tested with maximal stimuli to determine whether they are absolute or relative defects (three-zone strategy). These unseen points may also be thresholded to determine actual sensitivity at these defective locations. Certain strategies allow for testing of additional points around any abnormal points to delineate the size and density of the detected scotoma. The prethresholding strategy compensates for generalized depression, unlike using age-matched normals, better enabling the detection of localized abnormalities.

To designate the sensitivity level at which the screening test was run, or the height of the patient’s hill of vision, the perimeter reports the central reference level. This represents the extrapolated sensitivity at fixation determined by thresholding several reference points away from fixation.

Threshold determination gives the most accurate and detailed information concerning the quality of a visual field but is the most tedious and time-consuming testing strategy (Figs. 109-4 and 109-5). For example, a normal full-threshold test of the central 30° requires approximately 11 to 13 minutes; a significantly abnormal test may take 50% longer to complete, because thresholding an abnormal point requires more stimulus presentations. This testing modality involves performing the bracketing (“staircase,” “up-and-down”) process at every point tested. A stimulus brighter than the patient’s expected threshold is presented at a point. If the stimulus is seen, stimuli of successively reduced intensity are presented until the stimulus is not seen. Then stimuli of increasingly higher intensity are presented until one is seen. Most instruments use 4-dB increments until threshold is crossed for the first time. Subsequent changes in intensity are usually of 2-dB steps. A point somewhere between suprathreshold and infrathreshold is determined to be threshold for that point. Most perimeters bracket until threshold is crossed twice, from the suprathreshold and infrathreshold directions.

Several points in the field are in the process of being thresholded at the same time. Completely thresholding only one point at a time would encourage anticipation and possible loss of fixation by the patient as well as altering local retinal adaptation. At each point examined, a value for retinal threshold sensitivity is generated. Certain instruments begin the full threshold program by determining threshold values at several primary points and then use these points to extrapolate probable threshold levels at adjacent points. These estimated threshold levels are the starting point for full thresholding. Others begin thresholding at an age-corrected expected level.

Several perimeters offer rapid thresholding strategies. One such strategy involves not fully thresholding a point if a 4-dB stimulus above expected threshold is seen. An additional testing strategy involves thresholding of each point in the field, beginning at 2 dB greater than those determined during a previous examination. Another alternative is to screen the field for areas of significantly decreased sensitivity from those established during a previous examination, and then threshold only these areas. The latter strategies save time but risk overlooking small, gradual decreases in sensitivities.

Humphrey Instruments, Inc., developed a rapid thresholding strategy, FASTPAC, which uses a 3-dB step size and estimates the threshold after only one crossing but thresholds the four primary seed points in a similar manner to standard full-threshold testing. Half of the points start at 1 dB brighter and the other half start at 2 dB dimmer than the predicted threshold values. After a single crossing, the threshold is estimated. Thresholds of locations that differ from expected values by 4 dB or more are then re-estimated. Recent studies have indicated that FASTPAC, although considerably faster than conventional standard threshold strategies, underestimated mean deviation, pattern standard deviation, and corrected pattern standard deviation when compared with the standard algorithm.^35,36,37

In an attempt to create shorter threshold tests with preserved accuracy, a family of new threshold strategies, SITA (Swedish Interactive Thresholding Algorithm), was developed. The SITA strategies use continuous estimation of threshold values and measurement errors throughout the test, using advanced visual field models and mathematical analyses performed in real time. Models are initially based on prior visual fields. Staircase procedures are used to alter stimulus intensities at predetermined test point locations, with interruption of staircases when measurement errors have been reduced to a preset level. This is the main factor involved in test time reduction. Test time is further reduced by eliminating catch trials to determine the frequencies of false-positive answers and by using a more effective timing algorithm. Several studies have demonstrated that SITA strategies reduce automated perimetry testing time while achieving the same or better test quality than standard full-threshold and FASTPAC strategies.^38,39,40 The SITA Standard and SITA Fast strategies are available as software implemented on the Humphrey Field Analyzer II.

Screening programs display, in a framework of symbols corresponding to the pattern used, whether a stimulus was seen or not seen at each point tested (Figs. 109-2 and 109-3). Some perimeters automatically determine whether additional points around detected abnormal points were seen. A multizone strategy delineates, usually by symbols, whether a maximally bright stimulus was seen or not seen, indicating an absolute versus relative scotoma. Actual retinal sensitivities at the abnormal points may be determined and displayed in numeric form (dB) with certain strategies. (See Testing Strategies, Screening.)

Threshold strategies report actual retinal sensitivities at each point tested in numeric form (dB). As in the screening strategies, the sensitivities are displayed in a pattern corresponding to that used in the test. Quadrant totals can also be reported, which is helpful in comparing quadrants within the same examination, those of fellow eyes, or subsequent fields of the same eye (Fig. 109-4). Along with actual values, a display of differences from expected values or depth of defect may be provided. Expected threshold values may be those determined by thresholding several points and extrapolating the remainder of the tested loci or those of age-matched controls collected by the manufacturer. If the actual values are within 4 to 5 dB of expected values, they are considered within the limits of normal variability and may be recorded as “normal,” without a value for difference from expected (Figs. 109-4 and 109-5). This type of display may be reported in numeric decibel values or in symbols, where a specific symbol corresponds to a range of reduction in decibel level. This table is valuable because it is a way to determine abnormal points rapidly. Its drawback is that localized areas that are defective with respect to a patient’s actual field may not be recognized as abnormal if they are within the normal limits of the thresholds expected by the perimeter. (See Testing Strategies, Threshold Determinations.)

A gray-scale printout is generated by delegating different symbols to actual decibel values of threshold sensitivities, with a specific symbol representing a particular range of stimulus intensities. Symbols usually increase in density (darker) as the required threshold intensity increases. Therefore, darker areas indicate a lower differential light sensitivity and lighter areas indicate zones of higher sensitivity. The resultant printout appears to assign a retinal sensitivity to every area of the field tested, but in reality only a limited number of points were actually thresholded. To allow for continuity, values of presumed retinal sensitivity are interposed to unassessed areas of the field. Its appearance therefore resembles a picture simulating isopters. One should use this type of information display with its limitations in mind, because it gives only a gross assessment of the field. Although the clinician accustomed to manual kinetic perimetry results may find it more comfortable to rely on this type of graphic representation, important information may be missed when actual numeric values are not also considered (Fig. 109-5). Static profile printouts of interposed values along specified meridians may also be derived from the data (Fig. 109-6).