Visual Fields: Field Interpretation

››Visual field measurement is of unquestionable importance in glaucoma management. The status of the visual field and its progression are major drivers of clinical decisions.

››The visual field analyses and printouts contain invaluable information. All portions of the printout should be used to make clinical decisions.

››Special attention should be given to examination quality.

››For individual patients, the various statistical analyses are guidelines to help the clinician. They should not be relied on automatically but used in conjunction with other indicators of the disease and its progression.

17.1  How Is Information on a Single Field Printout of the Humphrey Visual Field Analyzer Interpreted?

The individual (or single field) printout (Fig. 17.1) contains a wealth of information to help clinicians decide whether the visual field is abnormal and specifically the likelihood of it being a glaucomatous visual field. The printout can be divided into four convenient parts to aid interpretation (Fig. 17.1).

B. C. Chauhan

Department of Ophthalmology and Visual Sciences,

Eye Care Centre, Centennial Building, 1278 Tower Road, Halifax, Nova Scotia, Canada B3H 3L9

e-mail: bal@dal.ca

This section of the printout (see Fig. 17.1) contains information about the test type, demographic and performance data. The name, eye, date of birth, and test date should be verified before interpreting the printout. The visual acuity and refractive correction used should also be checked.

Interpretation depends on the test type; hence it is imperative that this information is carefully checked. In this case (Fig. 17.1), the test type is the central 24-2 threshold test with a Goldmann size III stimulus and the SITA (Swedish Interactive Threshold Algorithm) standard examination strategy. Other test patterns (e.g., 30-2 or 10-2), examination strategies (e.g., full threshold or SITA-Fast), or stimulus sizes (e.g., size V) can also be used.

The blind spot monitor can be turned on or off, as can the gaze tracking monitor (trace shown at the ­bottom of printout). These indices can help in determining the patient’s fixation reliability.

There are three reliability indices printed in Part 1, namely fixation losses (the frequency of positive responses when a bright stimulus is presented in the blind spot), false-positive errors (the estimated number of times, expressed as a percentage, the response button is pressed when no stimulus is presented or the frequency the patient responds too quickly after stimulus presentation), and false-negative errors (the estimated number of times, expressed as a percentage, the response button is not pressed when a stimulus of much brighter intensity than threshold is presented at a location tested earlier in the session). These indices are only a guide to indicate the reliability of the patient. Fixation errors can be high in spite of good fixation accuracy; for example, if the blind spot is small or if it

Fig. 17.1  The single visual field printout from the Statpac program of the Humphrey field analyzer. The printout is divided into four parts: (1) the demographic, test type, and performance

data; (2) the sensitivity and grey scale plot; (3) the total and pattern deviation analyses; and (4) the summary indices

is not optimally plotted. The technician’s notes (if available) or the gaze tracking monitor trace should also be consulted. A high false-positive rate is an indication that the patient may be anxious and “trigger happy.” A test with a high false-positive rate (>20%) may be unreliable, particularly if the test duration is especially long, or if the foveal or macular sensitivity values (shown in Part 2) are especially high.

17.1.2  Part 2 of the Visual Field Printout

Part 2 of the printout contains the foveal sensitivity (if measured), the individual sensitivities of test locations measured in decibels (dB), and the interpolated grey scale plot of the sensitivity values. Values with <0 dB indicate that the patient did not respond to a stimulus of maximal intensity. Although the grey scale provides a quick overview of the location of defects, it should not be relied on exclusively because of the extensive interpolation of the grey scales between locations not actually tested.

17.1.3  Part 3 of the Visual Field Printout

Part 3 of the printout contains two important measurements of visual field loss, namely total deviation and pattern deviation. The total deviation values shown for each location (top left of Part 3) indicate, in dB, in decibel, the difference between the measured sensitivity and the expected sensitivity in an age-matched healthy individual. Hence, a total deviation value of −9 dB indicates that the measured sensitivity was 9 dB lower than normal, while a value of 2 dB indicates that the measured sensitivity was 2 dB higher than normal.

The pattern deviation values shown for each location (top right of Part 3) indicate the deviations from normal after adjusting for the general height of the visual field. The general height is affected by cataract and also by diffuse or overall changes to the visual field. Hence, these values help the clinician evaluate the ­localized component­ of the visual field damage after adjusting for loss that may occur because of cataract or diffuse loss. A total deviation of −6 dB and a pattern deviation of −4 dB at the same test location (for example, as seen in the

superior nasal step area on Fig. 17.1) indicate that 2 dB of the loss is due to a reduction in the general height at that location.

The total deviation probability plot (bottom left) indicates the probability of the total deviation value occurring in a normal population. Therefore, a probability designation of <5% indicates that the total deviation at that location occurs in less than 1 in 20 normal subjects while a designation of <0.5% indicates that the sensitivity at that location is so significantly reduced that it occurs in less than 1 in 200 normal subjects. Similarly, the pattern deviation probability plot (bottom right) indicates the probability of the pattern deviation values occurring in a normal population.

17.1.4  Part 4 of the Visual Field Printout

Part 4 contains the summary indices of the Glaucoma Hemifield test (GHT), mean deviation (MD), and pattern standard deviation (PSD). Since visual field asymmetry above and below the horizontal meridian is a hallmark of glaucomatous visual field damage, the GHT compares the pattern deviation in five mirror-image supe­ rior and inferior retinal nerve fiber layer sectors. If at least one of the mirror-image sector pairs has asymmetry between them that is found in less than 1% of the normal population, the GHT is marked as “outside normal limits.” If the asymmetry occurs in <3% of the normal population, the GHT is marked as “borderline.” If the asymmetry occurs in >3% of the normal population but the sensitivity of points is either too high or too low, occurring in <0.5% of the normal population, the GHT is marked as “abnormally high sensitivity” or “abnormally low sensitivity,” respectively. In all other cases, the GHT is marked “within normal limits.”

The MD is a weighted mean of the total deviation values in the visual field, while PSD is the standard deviation of the total deviation values. Conceptually, MD indicates the overall amount of visual field damage. The probability of this value occurring in a normal population is less than 0.5%, in the case of Fig. 17.1, where MD is −12.00 dB. PSD is the variability of the total deviation values, with typically high PSD seen with localized damage. In this case, the PSD is 12.05 dB and occurs at this value in less than 0.5% of the normal population.