Figure 3-20 Central 30-2 threshold test pattern, right eye. (Reproduced with permission from The Field Analyzer Primer. San Leandro, CA: Allergan Humphrey; 1989.)
Interpretation of a Single Visual Field
The clinician should exercise caution when interpreting perimetric results. Even with improved strategies, these remain subjective tests. Therefore, confirmation of a new defect or worsening of an existing defect is usually necessary to validate the clinical implication of the visual field in conjunction with all other pertinent data. Evaluation of the visual field involves the following: 1) assessing the quality or reliability of the visual field; 2) assessing the normality or abnormality; and 3) identifying artifacts.
Quality
The first aspect of the field to be evaluated is its quality or reliability. The percentage of fixation losses, the false-positives and false-negatives, and the fluctuations of doubly determined points are assessed. Damaged areas of the field demonstrate more variability than normal areas. Glaucomatous damage may cause an increase in false-negative responses unrelated to patient reliability. The clinician can evaluate patient reliability by looking at the least-damaged areas in a badly damaged visual field.
Normality or abnormality
The next aspect of the visual field to be assessed is its normality or abnormality. When tested under
photopic conditions, the normal visual field demonstrates the greatest sensitivity centrally, with sensitivity falling steadily toward the periphery. A cluster of 2 or more points depressed ≥5 dB compared with surrounding points is suspicious. A single point depressed >10 dB is unusual but is of less value on a single visual field than a cluster, because cluster points confirm one another. Corresponding points above and below the horizontal midline should not vary markedly; normally the superior field is depressed 1–2 dB compared with the inferior field.
To aid the clinician in interpreting the numerical data generated by threshold tests, field indices have been developed by perimeter manufacturers. The mean difference measures average sensitivity loss, and pattern standard deviation highlights localized loss.
These corrected indices help distinguish between generalized field depression and localized loss. An abnormal pattern deviation has greater diagnostic specificity than a generalized loss of sensitivity. An abnormally high pattern standard deviation indicates that some points of the visual field are depressed relative to other points in the visual field after correction for the patient’s moment-to- moment variability. Such a finding is suggestive of focal damage such as that occurring with glaucoma (and many other conditions). Although a normal pattern standard deviation in an eye with an abnormal visual field indicates a generalized depression of the hill of vision such as that occurring with media opacity, such generalized loss may also occur with diffuse glaucomatous damage.
The Humphrey STATPAC 2 statistical program provides a hemifield analysis. This test is designed only for glaucoma and involves comparison of corresponding clusters of points above and below the horizontal midline (Fig 3-21).
Figure 3-21 Superior visual field zones used in the glaucoma hemifield test. (Reproduced with permission from The STATPAC User’s
Guide. San Leandro, CA: Allergan Humphrey; 1989.)
Comparison of various perimetric techniques
As stated earlier, other technologies have recently become available that may be useful in evaluating the visual field. Two of the most commonly used newer techniques are short-wavelength automated perimetry and frequency-doubling technology, both of which are discussed below.
Other measures of the visual field include contrast sensitivity, flicker sensitivity, high-pass resolution perimetry, visually evoked cortical potential, and multifocal electroretinography. However, these tests are not commonly employed in the evaluation of patients with glaucoma. Several of these tests are discussed in greater detail in BCSC Section 12, Retina and Vitreous.
With the introduction of these newer perimetric techniques into the clinical arena, clinicians may be asked to derive important clinical information from several perimetric printouts. The association between glaucoma and short-wavelength (blue) color vision deficits has been known for some time. Sensitivity to blue stimuli is believed to be mediated by a small subpopulation of morphologically distinct ganglion cells, the small bistratified ganglion cells, that typically have large receptive fields, little receptive field overlap, and relatively large axon diameters. If early ganglion cell loss in openangle glaucoma preferentially affects either sparsely represented cell groups or those with larger axons, either scenario may produce reduced short-wavelength sensitivity. If special stimuli and background illumination conditions are used, it is possible to isolate and test the sensitivity of shortwavelength mechanisms throughout the visual field with short-wavelength automated perimetry (SWAP), also known as blue-yellow perimetry. Standard perimeters that can project a blue stimulus onto a yellow background are available, and SWAP is available on the HFA II (700 series) and the Octopus 1-2-3. A STATPAC procedure is employed to analyze the data, which are presented in the same layout as that used for standard automated perimetry.
This method is sensitive in the early identification of glaucomatous damage. Several studies suggest that the rate of development of perimetric defects in early glaucoma may be higher with blue- on-yellow testing than with conventional (achromatic) white-on-white visual fields. SWAP is suitable for identifying individuals likely to develop SAP visual field loss. Repeatable visual field loss on SWAP should be carefully monitored. Analyses of local loss such as the Glaucoma Hemifield Test (GHT) and pattern threshold deviations are the best statistical tools to identify glaucomatous SWAP deficits and to separate them from artifacts resulting from media opacities. The availability of SWAP using the SITA testing algorithm (SITA SWAP) has decreased the testing time for SWAP testing, making it much more clinically useful.
Frequency-doubling technology (FDT) perimetry is a visual field testing paradigm that uses a low spatial frequency sinusoidal grating undergoing rapid phase-reversal flicker. Commercially available instruments employ a 0.25 cycle per degree grating, phase-reversed at a rapid 25 Hz. When a low spatial frequency grating is presented in this manner, it appears to have twice as many alternating light and dark bars than are actually present—hence the term frequency doubling.
The FDT perimeter was developed to measure contrast detection thresholds for frequencydoubled test targets. The high temporal frequency and low spatial frequency attributes of the stimulus that causes the frequency-doubling stimulus likely preferentially activate the M cells. Whether it is because of the isolation of specific cell populations that are susceptible to early damage in glaucoma or because of the reduced redundancy, visual function tests that employ frequency-doubled stimuli may be useful for detection of early defects in some patients. Because of the small number of areas tested, the original FDT is limited in its usefulness for follow-up of glaucoma patients. The availability of the Humphrey Matrix 24-2 perimeter (Matrix, Carl Zeiss, Dublin, CA), which makes use of frequency-doubling testing with testing areas similar in number to those of SAP 24-2, potentially provides a new method to perform functional assessment in glaucoma.
Agreement between standard perimetry, SWAP, and FDT perimetry is limited. Results obtained with standard perimetry may be abnormal in early disease when those from SWAP and FDT testing are normal. Thus, each of these tests may identify or miss early glaucomatous damage in different patients.
Artifacts
Identification of artifacts is the next step in evaluation of the visual field. The following are common artifacts seen on automated perimetry:
Lens rim: If the patient’s corrective lens is decentered or set too far from the eye, the lens rim may project into the central 30° (Fig 3-22).
Incorrect corrective lens: If an incorrect corrective lens is used, the resulting field will be generally depressed. This appears to be less of a problem with FDT perimetry.
Cloverleaf visual field: If a patient stops paying attention and ceases to respond partway through a visual field test, a distinctive visual field pattern may develop. Figure 3-23 shows a cloverleaf visual field, the result of the testing order of the Humphrey 30-2 perimeter, which begins testing with the points circled in this figure and proceeds outward. This pattern may also be seen if a patient is malingering.
High false-positive rate: When a patient responds at a time when no test stimulus is being presented, a false-positive response is recorded. False-positive rates greater than 20% suggest an unreliable test that can mask or minimize an actual scotoma and can, in extreme cases, result in a visual field with impossibly high threshold values (Fig 3-24). Careful instruction of the patient may sometimes resolve this artifact.
High false-negative rate: When a patient fails to respond to a stimulus presented in a location where a dimmer stimulus was previously seen, a false-negative response is recorded. Falsenegative rates greater than 33% suggest test unreliability. A high false-negative rate should alert the clinician to the likelihood that the patient’s actual visual field might not be as depressed as suggested by the test result. However, it should also be noted that patients with significant visual field loss, including scotomata with steep edges, can demonstrate high false-negative rates that do not indicate unreliability. This effect appears to arise from presentation of stimuli at the edges of deep scotomata, where short-term threshold fluctuation can be quite variable.
Figure 3-22 Lens rims artifact. The 2 visual fields shown were obtained 9 days apart. The visual field on the left shows a typical lens rim artifact, whereas the corrective lens was positioned appropriately for the visual field on the right (Humphrey
30-2 program).
Figure 3-23 Cloverleaf visual field. The Humphrey visual field perimeter test is designed so that 4 circled points are checked initially and the testing in each quadrant proceeds outward from these points. If the patient ceases to respond after only a few points have been tested, the result is some variation of the cloverleaf visual field shown at right (Humphrey 30-2 program).
