A number of factors can affect a visual field test1–16 other than the disease being studied. It is important to minimize the influence of these variables as completely as possible in order to assess accurately and document precisely the abnormalities present so that deviations from normal and future changes can be recognized easily. Fluctuation, the combination of normal physiologic variability and measurement error, complicates the recognition of pathologic change. If all other possible variables are eliminated, then change in the disease must be responsible for any alteration in the visual field. Unfortunately it is impossible to eliminate all other variables. Awareness of factors influencing the visual field, however, can help minimize these variables and improve interpretation (Box 9-1).

the test moves toward the periphery is also greater with age. The ­combined effect of these variables is a field with an increasingly steep slope as one moves away from fixation. The effect of age on the central field is gradual and can usually be ignored in the evaluation of individual patients. Mean sensitivity of the visual field decreases approximately 0.58–1.0 dB per decade.9 Increased age may be associated with increased variability in repeated test results over time.The standard deviation of the mean sensitivity of points tested ranges from about 1.0 to 2.0 dB in the normal central field of a young patient. Patients older than 60 years of age may have

standard deviations up to twice that amount centrally and up to 10 dB per point at 30º eccentricity.2,4,5,13,14,16,18,19

Patient variables

Age

With age, the visual field has a linear decrease in sensitivity and the slope steepens.13,17,18 The increase in fluctuation that occurs as

Box 9-1  Some artifacts that affect visual field results

Examination artifacts

Technician: results vary with different technicians Equipment: results vary with different equipment Test: results vary with different types of tests

Software: results vary with different testing or interpretation algorithms

Eye artifacts

Refraction: should have distance prescription with proper addition for near vision

Pupil size: should be 3 mm or more; must be consistent Fixation: results vary with quality of fixation control Media opacity: visual acuity should be recorded

Patient artifacts

Misunderstanding the test Fatigue

Inattentiveness Physiologic/pathologic/psychologic/mental status Systemic illness, hangover, anxiety, and so on

Analysis artifacts

Is the visual field normal? Requires standards for normal

Has the visual field changed? Requires knowledge of fluctuation Misinterpretation

Fixation

Most patients maintain acceptable fixation on the central target of the perimeter.20–22 Patients with poor fixation can be encouraged

to stabilize their fixation, but this does not always prevent them from looking around.23 Technicians and computerized systems usually monitor or grade patient fixation in some way. Machines that monitor fixation continuously, generally by some form of eye movement or pupillary reflex assessment, often have algorithms that disregard responses generated during fixation losses. These machines return to the same test location later during the examination and present the stimulus again. This programming feature helps ensure that the responses recorded by the machine occur during periods of steady fixation. Other machines use a monitoring system with blind spot fixation in which stimuli are projected on the physiologic blind spot at intervals throughout the test. If ­fixation is steady these stimuli will continue to land on the blind spot and will not be detected. If fixation has shifted the stimuli will land on photoreceptors and be detected.The machine records and/ or alerts the operator that a fixation loss has occurred. If the patient generates fixation losses more than 20–30% of the time, the test can be considered only an approximation of the true visual field.24,25

Reliability

In addition to monitoring fixation, patient reliability should be graded as good or poor by the technician. Computerized machines can provide some index of reliability based on false-positive or false-negative responses and fixation losses.26 Fatigue, drugs, age, and illness can all affect patient reliability and must be considered when assessing the accuracy of a given test. Even in well-rested patients, the test itself can be fatiguing, so that reliability tends to decrease with prolonged or extensive examination sessions.