Figure 3-24 High false-positive rate. The top visual field contains characteristic “white scotomata,” which represent areas of impossibly high retinal sensitivity. Upon return visit 3 weeks later, the patient was carefully instructed to respond only when she saw the light, resulting in the bottom visual field, which shows good reliability and demonstrates the patient’s dense superior visual field loss (Humphrey 30-2 program).
Interpretation of a Series of Visual Fields
Interpretation of serial visual fields should meet 2 goals:
1.separating real change from ordinary variation
2.using the information from the visual field testing to determine the likelihood that a change is related to glaucomatous progression
Several methods can be used to analyze a series of visual fields for glaucomatous change. Point-by- point analysis performed with statistical programs available from the major instrument manufacturers (eg, the Humphrey Glaucoma Progression Analysis or Octopus Delta programs) is a valuable aid in progression analysis. Application of these programs is described clearly in the owner’s manual accompanying the program packages. The point-by-point approach is better at identifying regions of change within a visual field.
Another method used in visual field series analysis is comparison of visual field indices, which can reveal global trends that may be missed with point-by-point analysis. Raw perimetric data can also be transferred to independent software programs for change analysis. Even when computed statistical methods are employed, however, separating true pathologic progression from normal test- to-test variability remains a difficult challenge. Moreover, the clinician interpreting a series of visual fields must keep in mind that test variability is increased as part of the pathophysiology of glaucoma.
Whatever method the clinician uses, the fundamental requirement for adequate interpretation over time is a good baseline visual field. Often the patient experiences a learning effect, and the second visual field may show substantial improvement over the first (Fig 3-25). At least 2 visual fields should be obtained as early as possible in the course of a patient’s disease. If they are quite different, a third test should be performed. Subsequent visual fields should be compared with these baseline fields. If a follow-up visual field appears to differ from baseline, the test should be repeated for confirmation.
Figure 3-25 Learning effect. These 3 visual fields were obtained within the first 3½ months of diagnosis in a patient with very early, clinically stable glaucoma. They illustrate the learning effect between the first and the second visual fields. The third visual field is similar to the second, and the second and third visual fields provided a baseline for follow-up of the patient (Humphrey 30-2 program).
Progression
No hard-and-fast rules define what determines visual field progression. But, generally, the methods used in the assessment of progression employ regional or global analysis and an event-based or trend-based approach.
Progressive cases such as that shown in Figure 3-26 are easy to recognize. A general decrease in sensitivity may be secondary to glaucoma or may be related to media opacity, and clinical correlation is required, which is often difficult. Two causes of general decline in sensitivity that may confuse interpretation are variable miosis (often related to use of eyedrops) and cataract (Fig 3-27).
Figure 3-26 Progression of glaucomatous damage. The 3 visual fields shown illustrate the development and advancement of a visual field defect. Between the first and second visual fields, the patient developed a significant inferior nasal step (see the section Patterns of Glaucomatous Nerve Loss). The third visual field illustrates the extension of this defect to the blind spot, as well as the development of superior visual field loss (Humphrey 30-2 program).
Figure 3-27 Pupil size (marked on each visual field). The first visual field shown was obtained before the patient began pilocarpine therapy. The second visual field was obtained with a miotic pupil (Humphrey 30-2 program).
Suspected new defects or progression of existing defects should be reproduced on subsequent visual fields to determine their validity. Definitions of progression have varied in clinical trials; these definitions will continue to be refined in the years to come with further improvements in computer software.
The Glaucoma Change Probability (Fig 3-28) uses an event-based analysis, which is based on the analysis performed in the Early Manifest Glaucoma Trial, to provide a sensitive assessment of possible progression. It compares the current visual field with a baseline composed of 2 separate visual field tests. The operator must choose the 2 baseline visual fields. Thus, with this method, if progression occurs, a new baseline must be established for future analysis.
