Ординатура / Офтальмология / Английские материалы / A Visual Field Evaluation with Automated Devices 2nd edition_Reddy_2006

22 A VISUAL FIELD EVALUATION WITH AUTOMATED DEVICES

FIGURE 2.19

FUNDAMENTALS OF AUTOMATED PERIMETRY 23

THRESHOLD TESTING STRATEGIES

The test “strategy” refers to the method of presenting stimuli to the patient to attain the desired information. There are two types of strategies: screening or suprathreshold testing strategies and threshold testing strategies. In a suprathreshold screening test, each stimulus is intense enough to be seen by nearly all normal patients. This strategy provide only general, qualitative information about the visual field and consumes less time. While threshold strategies consume more time, they provide detailed, quantitative information to find out the threshold at each selected point of retina. There are different threshold strategies available to the Humphrey field analyzer.

The testing threshold strategies are of 2 types:

(SITA means Swedish Interactive Threshold Algorithm)

Threshold determination at a visual field point P, open circle indicates patient response to stimulus. Filled circle indicates no response to stimulus. Down arrow indicates stimuli decreasing in 4 dB steps. Up arrow indicates stimuli increasing in 2 dB steps. Numbers indicate order of stimulus presentation.

FIGURE 2.20

Staircase method (Bracketing method) of detecting threshold.

This method is used in full threshold strategy.

24 A VISUAL FIELD EVALUATION WITH AUTOMATED DEVICES

DETERMINATION OF THRESHOLD

During determination of threshold at each point in the test grid, stimulus intensities will vary in such a way that some stimuli will be suprathreshold (brighter than necessary to elicit a response most of the time) and some will be infrathreshold (too dim to elicit a response most of the time). Threshold is thus bracketed by the stimuli as the examination proceeds. This method of bracketing is also known as staircase method for determining threshold.

Illustration of staircase method (bracketing) for determining threshold (Figure 2.20)

First stimulus: The first stimulus presented at the point P is seen and hence it is labeled as open circle 1.

Second stimulus: Because the first stimulus was seen a intensity of light was dimmed by 4 dB In this example, the second stimulus was also seen and labeled as open circle 2.

Third stimulus: As the second stimulus was also seen, the third stimulus was dimmed further by 4 dB. The third stimulus is too dim to elicit a response and hence indicated by closed circle 3. This is the first crossing from seeing zone to nonseeing zone.

Fourth stimulus: Now the machine makes the stimulus brighter but in 2 dB steps. The fourth stimulus was also too dim to elicit a response and hence indicated by closed circle 4.

Fifth stimulus: The fifth stimulus is further brightened by 2 dB and this time, the patient pushes the button and hence labeled as open circle 5. This is the second crossing of the threshold from nonseeing zone to seeing zone. Since the step size was 2 dB, threshold is the value that lies between the intensity values of the last 2 stimuli. Obviously if the first stimulus presented to the point was not seen, the machine will make the stimuli brighter in 4 dB steps until the threshold is crossed and then dimmer in 2 dB steps until the second crossing. This standard method is adopted in determining threshold in the full threshold strategy. Thus the theoretical best threshold resolution is 2 dB.

The newer threshold strategies reduce the test time of the threshold tests by more than 40% by measuring threshold with an alternative strategy to the standard staircase method. In the newer strategies change the stimulus in 3 dB steps and crosses the threshold only once. The threshold assigned to each tested point lies between the two stimulus intensities where the threshold is crossed. The theoretical best threshold resolution is 3 dB rather than the theoretical 2 dB resolution of the full threshold (algorithm). The threshold values determined by the newer fast methods were found to be similar to those found on the standard testing strategies (Full Threshold strategy and FAST PAC).

It is very important to note that the Full Threshold strategy is a confusing terminology because there are no half threshold strategies. Because the standard staircase method is adopted in detecting the threshold in Full Threshold strategy it should have been named as standard threshold strategy rather than as full threshold strategy.

FUNDAMENTALS OF AUTOMATED PERIMETRY 25

FIGURE 2.21

Typical test time ranges (minutes) comparative test times for four main threshold test algorithms.

Percentages are relative to typical test time for full threshold 30-2 testing.

From the above figure, you understand the test time for each selection of the test is as follows:

Please see pages 6369 for further details on the threshold testing strategies.

26 A VISUAL FIELD EVALUATION WITH AUTOMATED DEVICES

IMPORTANT THRESHOLD TESTS TO DETECT

GLAUCOMA FIELD DEFECTS

Now, we know the available point patterns and the testing strategies. Whenever we ask for a visual field test we have to specify the point pattern and the testing strategy.

The usual available central threshold test point patterns are as follows:

30-2 central field, 24-2 central field, 10-2 central field, macular program and nasal step.

Available threshold testing strategies:

Full threshold, FAST PAC, SITA Standard, SITA Fast.

Each of the above test point pattern can be tested with any of the above threshold strategy. For example, the 30-2 central field with Full Threshold strategy, 30-2 central fieldSITA standard, 30-2 central field - SITA Fast and central field 30-2 FAST PAC. Like that the 24-2 central fields and 10-2 central fields test can be done with all these strategies. Please note that both SITA Standard SITA Fast testing strategies are not available for macular program and nasal step field test point patterns.

We can tabulate the above information as follows:

28 A VISUAL FIELD EVALUATION WITH AUTOMATED DEVICES

Selection of the point patterns in patients of glaucoma according to the stage of glaucoma

30-2 central is the first test to be selected.

If no pathology is seen and peripheral 30° is free of scotoma, the following test can be selected during followup tests

24-2 central field

For situations like advanced glaucoma selection of the test would be

10 -2 central field

If the field is so advanced and most of the points sensitivity is less than 10-15 dB, and if macular split is present, the selection of the test would be

10-2 Central field with size V stimulus or macular program with size V stimulus

We can represent the selection of the test to detect glaucoma field defects in a tabular form as follows:

Classification3 of Visual

Field Printouts

HUMPHREY VISUAL FIELD TEST PRINTOUTS

The normative data is available to the tests that are conducted with the following parameters. So the STATPAC (The Humphrey field analyzer’s statistical package) will analyse tests that fall within the parameters listed below.

First, we will discuss in detail the single field analysis printout with STATPAC analysis. Later we will talk about printouts without STATPAC analysis and follow-up tests.