Zeiss HUMPHREY 720i User Manual - page 310

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APPENDIX A

REV. B 02/03 PN 51680-1

HUMPHREY FIELD ANALYZER II -

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series

The original Humphrey Full Threshold testing algorithm measured patient response time and

made small adjustments to test pacing. SITA takes this idea much further using patented timing

techniques. It is extraordinarily responsive to patient reaction times. One way to think about

SITA is that the patient runs the perimeter, rather than the reverse.

3. SITA Knows When to Quit

Student doctors are often given a list of questions to ask while taking histories. At first they will

adhere to that list, even when they already have more than enough information to make the

proper diagnosis. Later on, they start learning when to stop, and they also develop a sense of

when to probe further on issues which the patient did not make clear enough.

SITA does the same thing. SITA knows when enough is enough. The standard Full Threshold

algorithm used in the Humphrey Field Analyzer II crosses the threshold twice. It quits only

after the answer is near what is expected. When the answer is different from the expected

value, the measurement is repeated—again crossing the threshold twice.

Using such fixed criteria, sometimes too much information is gathered, and sometimes not

enough.SITA computes when to stop testing at each location, based on a patented “informa-

tion index.” This technique allows the instrument to spend extra time at test locations where

SITA is unsure about the result, and to spend less time at locations where the answers are

highly consistent.

When the information index reaches a predetermined value, testing at that point is discontin-

ued—the point is then closed. The information index depends not only on patient responses at

that location, but also on responses to stimuli presented at other locations nearby. Thus, it is

possible that a test point residing in a part of the visual field where all measured thresholds

were more or less in agreement might be closed earlier than a test point in a more variable

region. This might happen even if the responses at these two test points were otherwise

identical.

4. SITA Carefully Recalculates All Threshold Values at the End of Testing

Experienced doctors tend to be very skilled at putting the puzzle together. At the end of the

examination, they assess all of the information and make a diagnosis which is consistent with

all of the available data. SITA does the same thing, ignoring nothing. At the end of the test it

thinks the problem through, completely, one last time.

The original Humphrey algorithm—and other methods in current use—base the calculated

threshold on the last apparent crossing of threshold. All answers leading up to that final

crossing are ignored, and all answers at adjacent points are ignored as well. Such an approach

is highly vulnerable to patient response errors.

SITA looks at the complete pattern of patient responses at each tested point. During the test all

responses are considered, not just the last seen value. At the end of the exam, SITA again

considers the totality of the responses at each point and recalculates the whole field result to

produce a further refinement of its measurements.

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APPENDIX G

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