Normality or Abnormality of the Visual Field
The next aspect to be assessed is the normality or abnormality of the visual field. A normal visual field demonstrates the greatest sensitivity centrally, with sensitivity falling steadily toward the periphery. Figure 6-3 shows a single field analysis of a visual field obtained with the HFA. The results are presented as a series of numerical plots and probability maps, including a threshold sensitivity map with the numerical threshold sensitivities for each location and a corresponding grayscale map; a total-deviation plot, showing deviations from age-corrected normal sensitivities; a total-deviation probability map, showing deviations that fall outside the statistical range of normal sensitivity; a pattern-deviation map, showing the localized loss after correcting for overall decreases in sensitivity; and a pattern-deviation probability map.
The Humphrey perimeter also provides a series of summary indices, including the following:
Mean deviation (MD). MD is a weighted average of the total deviation values. Zero equates to no deviation from normal, and more negative values indicate more advanced loss.
Pattern standard deviation (PSD). PSD is a summary index of localized visual field loss.
Glaucoma Hemifield Test (GHT). This index categorizes eyes as within normal limits, borderline, or outside normal limits based on a comparison of visual field sensitivities at corresponding areas of the superior and inferior hemifields. Because glaucoma frequently causes asymmetric damage to the superior and inferior hemifields, the GHT is a powerful tool for identification of glaucomatous visual field defects (Fig 4-6).
Figure 6-3 Printout of a visual field test obtained with a Humphrey field analyzer using the Swedish Interactive Threshold Algorithm (SITA) Standard 24-2 test.
Several criteria have been proposed for identification of visual field abnormalities. The system employed by the Ocular Hypertension Treatment Study (OHTS) is simple and widely accepted. In the OHTS, an abnormal visual field was defined by the presence of a PSD with P < 5% or the presence of a GHT with a result outside normal limits. The abnormality had to be present in 3 consecutive visual field tests. In the analysis of a visual field printout, it is also important to evaluate the probability maps, especially the pattern-deviation probability plot. The presence of a cluster of at least 3 abnormal points (P < 5%) on the pattern-deviation plot, with at least 1 of those points with P < 1%, is also commonly used to define a visual field as abnormal.
Figure 6-4 The Glaucoma Hemifield Test (GHT) compares pattern-deviation probability values in 5 predetermined zones in the superior hemifield with corresponding zones in the inferior hemifield.
(Courtesy of Michael Boland, MD.)
It is important to emphasize that the clinician should examine the visual fields to verify whether the defect is repeatable and present in approximately the same location, and that the abnormalities are not due to the presence of artifacts. Although the points that are abnormal will not be exactly the same in all confirmatory tests, the area of visual field abnormality should be similar among the tests.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.