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 (1) assessing the quality or reliability of the visual field test, (2) assessing the normality or abnormality, and (3) identifying artifacts.
The first aspect of the visual field test to be evaluated is its quality or reliability. Reliability indices include the percentage of fixation losses, false-positives, and false-negatives. The false-positive response score measures the tendency of the patient to press the response button even when no stimulus has been presented. With the SITA strategy, patient responses that are made at impossible or unlikely times are used to estimate the false-positive response rate. These include responses made before or during the presentation or too soon after presentation, with consideration for the patient’s usual reaction times that have been gathered during the test. Of the reliability indices, a high percentage of false-positives is most detrimental to a visual field test. Visual fields with a false-positive rate greater than 15% are likely unreliable and nonrepresentative of the patient’s true visual function.
The fixation loss rate measures the patient’s gaze stability during the test. A patient who does not maintain correct fixation during the test will produce an unreliable assessment of his or her peripheral vision. Fixation losses are estimated by periodically presenting stimuli at the region of the physiologic blind spot, known as the Heijl-Krakau method. If a patient responds to a stimulus that should have been presented at the blind spot, this is an indication that some eye movement must have occurred. However, this method can sometimes fail in identifying fixation losses when the location of the blind spot has not been appropriately identified at the beginning of the test. Humphrey perimeters also possess a gaze tracker that uses infrared light to check pupil location throughout the test. A record of gaze stability is shown at the bottom of the printout. Lines extending upward indicate gaze error, while lines downward indicate that the tracker was not able to successfully track the gaze direction, for example, during blinks. A high fixation loss rate (>25%) is also indicative of an unreliable field, especially if accompanied by the lack of a well-demarcated blind spot.
Finally, the false-negative error rate was originally devised to assess inattention during the test. This indicates lack of response to stimuli that should have been seen. Although a high false-negative rate could indicate an inattentive patient, damaged areas of the visual field show increased variability, which can lead to a high false-negative rate. Therefore, false-negative response rates can be elevated in abnormal fields regardless of the attentiveness of the patient. Hence, visual field tests should not necessarily be disregarded because of high false-negative rates, and many testing algorithms no longer measure this parameter.
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.