Target luminance

100

Fig. 9-3  Temporal summation related to background luminance. Using the 10 dB background curve, approximately a 3 dB increase

({) can be anticipated by increasing stimulus exposure time from 0.1 to 0.2 seconds.

(Modified from Aulhorn E, Harms H. In: Jameson D, Hurvich LM: editors: Handbook of sensory physiology, vol 7, New York,

Springer-Verlag, 1972. With kind permission of Springer Science and Business Media.)

kinetic perimetry; the stimulus size and testing strategy should be duplicated­ for sequential static examinations. Other variables such as pupil size, technician, visual acuity, and testing equipment should be standardized as much as possible to reduce artifactual field fluctuations and to facilitate recognition of true pathologic change.

Tangent screen

The tangent screen may be used at 1 or 2 meters.45 It should be large enough to allow testing of the full 30º of the central field at whichever distance is chosen and should have a uniform illumination of 7 foot candles. Examiners can use the tangent screen for either kinetic or static testing. For static testing, the test object is placed on the side of the test wand so that it can be obscured by being rotated out of view. The wand should be covered by black felt material similar in composition to the tangent screen so that the wand itself is largely unseen during the test.46 While the patient maintains steady fixation, the wand is moved to the area of interest and the target is quickly rotated in and out of view.The patient responds to the on/off presentation.

The smallest test object the patient can see consistently just temporal to the blind spot is used for testing the central field. For a 1-meter test distance, this is usually a white test object that is 1 mm in diameter.

Fixation can be tested repeatedly by concealing and exposing the test object in the blind spot area. If the patient sees it, fixation has shifted.The test strategy is the same as that used for the central 30º of the visual field during Goldmann perimetry. The tangent screen is generally not sensitive enough to diagnose early field loss in glaucoma patients. It finds its greatest usefulness in patients with established field defect who are being following in centers that do not have access to Goldmann or automated perimeters. Tangent

screen fields are also useful for patients who cannot use a bowl perimeter for physical or other reasons.

Bowl perimetry

In the mid-twentieth century, Goldmann developed the first bowl perimeter that provided standardized background and stimulus intensity. He improved fixation monitoring by allowing the technician to see the patient’s eye through a telescope. He also provided a system for simplified test recording and facilitated reproducible test object positioning and movement.The perimeter that bears his name is the standard for manual bowl perimetry (Fig. 9-5). Many of its features have been incorporated into computerized visual field machines.

Using the light meter provided, the machine should be calibrated each day before the initial examination. The maximum stimulus, V4e, should be 1000 apostilbs; the background, or bowl, illumination should match theV1e stimulus, equivalent to 31.5 apostilbs.

Preparing the patient

Patient preparation for the examination is similar whether the bowl perimeter is to be used for manual or computerized testing. Refractive correction with the appropriate addition should be inserted into the lens holder for examination of the central portion of the visual field.

The patient should be comfortably seated so that the chin and forehead are firmly against the supports and the eye is centered in the observer’s telescope or display screen. After the patient is positioned, the lens holder should be placed as close as possible to the patient’s eye without touching the lashes. The eye should be centered in the lens holder. Corrective lenses should be ‘full-field’ type with thin rims so that they do not interfere with peripheral vision.

For manual machines, the patient should be instructed to push the button on the patient response indicator if one is available. If not, the patient should tap the table with a coin to indicate when he or she sees the test object. Verbal responses are discouraged because they move the head and adversely affect fixation and concentration. For automated machines, the patient uses the patient response button.

It is important to encourage the patient throughout the test, even if the computer is doing the testing. Many patients are more comfortable interacting with another person rather than with a machine; reliability is increased when a technician is present during computerized perimetry.9

Technique of manual bowl (Goldmann) perimetry (see Fig. 8.8)

Aulhorn and Harms,44 Armaly,47,48 Drance and co-workers,49 and others50 contributed concepts that provide the basis for techniques in

glaucomatous visual field testing and analysis. For the central field, a test object that is just detectable at the temporal horizontal meridian at 25º eccentricity, 15º above and 15º below this point, is appropriate. This threshold target is used to define the limits of the central isopter and blind spot by kinetic perimetry. Particular attention is given to the nasal and temporal meridians in looking for a step. Careful investigation of the 5º, 10º, and 15º isopters is necessary to reveal the isolated scotomata that are characteristic of early glaucoma. These three central isopters are examined by static and, if necessary, by kinetic perimetry (from non-seeing to seeing). Any paracentral field defects found with the threshold target should be checked at least twice, because artifacts are possible in any subjective test. Hesitant responses (especially in the Bjerrum area) should be noted.The I2e is the established standard test stimulus for the central visual field and provides a comparison for other patients and eyes. Selected higher intensity objects will determine the density of a defect. An I4e test

(A)

(B)

Fig. 9-5  Goldmann perimeter for visual field examination as viewed from the patient’s side (A) and the examiner’s side (B).

object is used to test the far periphery, and a V4e stimulus will outline the maximum area of the visual field.

Technique of computerized bowl perimetry

The Octopus was the first computerized visual field machine that provided enough flexibility to allow accurate detection and quantification of visual field defects.This machine used only static testing strategies. The Humphrey and some other computerized perimeters offer a kinetic option,51,52 but the overwhelming majority of clinical tests are performed using static techniques only.

Each of these machines provides a variety of testing programs for different situations.The physician should designate the program to be used for each patient. In practice, it is appropriate to have a standard program that is used by default. If a particular patient has special needs or circumstances, the physician can order a test that addresses that patient’s needs. If a wide variety of tests are used routinely, it is difficult to gain sufficient experience with any one test to interpret the results optimally. We have found it much better to be comfortable with a few tests we know well than to try to master the entire menu offered by the manufacturer.

Visual field defects of stage II fequency distribution

in 400 eyes of 361 patients ( 1953–76 )

Fig. 9-6  Frequency distribution of the location of early glaucomatous visual field defects found in 400 eyes of 361 patients.

(From Aulhorn E. In: Krieglstein GK, Leydecker W, editors: Glaucoma update, Berlin, Springer-Verlag, 1979. With kind permission of Springer Science and Business Media.)

In addition to perhaps dozens of standard options, computerized machines offer custom programs that allow the examiner to tailor the area tested as well as the testing algorithm. This sounds like a good feature, but using custom programs is a risky practice. Remember that the hallmark of successful serial perimetry is consistency. For a custom test to be useful in the future, it must be duplicated exactly on subsequent examinations. It is not practical in most busy clinics and practices to take the time to reprogram the machine before each test. It is also difficult to remember to do this for the occasional patient.

For glaucoma testing, most visual field defects are located in the central 30º of the visual field (Fig. 9-6). Programs that test the central 24º produce adequate results for most patients, and save substantial testing time. Testing out to 30º may allow better definition of peripheral nasal step defects in the occasional patient.We tend to use the 30º program for new patients and when patient acceptance is good and testing time is not critical.The number of points tested varies but is roughly 60 to 70 depending on the program. In each case, the test grid is chosen to gather sufficient information without tiring the patient excessively. For the most part there is a direct relationship between the amount of time spent testing and the amount of information obtained. But overly long tests produce inconsistent results. Patient fatigue leads to errors that may be clinically significant. There is a point of diminishing returns with automated perimetric tests.53–55 Several programs allow the machine or the examiner to shorten the test if certain parameters are met (indicating that further testing is unlikely to improve the quality of the result or change the ultimate interpretation). For example, the patient may be a young adult with no obvious pathology and a previously normal baseline examination. Rechecking a representative sample of the previous test points may be enough to reassure the examiner that no change has occurred.The most sophisticated

programs, like the Humphrey Swedish Interactive Thresholding Algorithm program, spend more time testing suspicious areas of the field and less time on normal or clearly defective areas.

Defects that approach fixation are especially worrisome and can be plotted within 1.0º or 1.5º spatial frequency using macular programs. Central fields can also be plotted using the Goldmann or tangent screen perimeter in glaucoma patients who have only a central island remaining. Full-field automated tests are very timeconsuming and discouraging for these patients, and they do not provide enough useful information to warrant the time and aggravation they cause.

Glaucomatous defects may occur solely in the peripheral visual

field. This is rare, however, and probably occurs in 0–8% of cases depending on the examination technique.56–63 Many of the

­computerized perimeters supply a separate program that carefully examines the nasal area to detect most isolated peripheral defects.

Other machines have screening programs that test the periphery. If the periphery is tested, the lens holder and lens should be removed from in front of the eye.

The constant dilemma facing physicians with computerized perimeters is matching a patient’s ability with the program(s) to be used.64 Younger, more vigorous patients can tolerate longer tests. Most people are bored or fatigued after 20 minutes of consecutive testing and need a rest. The machine never gets tired and is capable of testing the visual field in minute detail. Patient reliability decreases, however, with increasing fatigue. The physician must select the appropriate tests to obtain the most accurate information for each patient. Healthy patients can generally tolerate a full-threshold visual field that measures the central 30º visual field (Fig. 9-7). Patients with visual field loss may have a difficult time with such a test.This is partly because of the length of time it takes to map the defective field, and partly because sitting in front of