Zeiss HUMPHREY 720i, User Manual

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APPENDIX A A-21
REV. B 02/03 PN 51680-1 HUMPHREY FIELD ANALYZER
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HUMPHREY FIELD ANALYZER
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To better explain how SITA works, we will use analogies. Think of perimetry testing
as taking a patient’s case history. Think of SITA as an experienced doctor. Think of
current perimetry strategies as student doctors. With these analogies in mind, we
will describe how SITA reduces test time in the following four ways:
1. SITA Asks Smart Questions
The importance of asking smart questions is familiar to any experienced doctor. When students
take a patient’s case history, they often ask questions that are off the mark and do not yield
critical information. They sometimes miss clues that the patient is offering—information which
could lead directly to the proper diagnosis if pursued. After many years of taking histories,
however, experience teaches practitioners precision and economy in framing their questions.
Good perimetry is similar to taking a good history. It is a matter of getting information from the
patient in a quick and efficient manner. In perimetry, the most critical factors are the following:
A.
Start with stimuli at each point that are already very near the threshold, thus avoiding
the long, inefficient process of gradually brightening or dimming the stimulus while
searching for the threshold.
B.
Make optimal use of the information contained in the patient’s responses to those
stimuli. This is important both in terms of calculating the threshold at the point being
tested, and in terms of determining how bright the initial stimulus should be at the
next point to be tested.
SITA considers many factors in determining what stimuli to present at each point during the
test. These factors include age, normative data, detailed characteristics of abnormal and
normal tests, and patient responses so far in the test. They are combined and weighted into the
SITA visual field model, which continually updates calculations of the threshold at each point.
2. SITA Tailors the Testing Pace to the Individual
When taking a history, all doctors know that some patients are quick to respond to questions
and others are not. If they rush elderly patients, they will not get much good information. If
they go too slowly with young, bright patients they may lose both their interest and their
cooperation.
In a threshold test, fewer than half of the stimuli will be seen. Thus, the perimeter must decide
how long to wait after stimulus presentation before moving to the next point. The test must
allow a reasonable amount of time between presentations, but waiting too long will unneces-
sarily prolong testing and make the test uninteresting. Thus, it is very important to know how
quickly a particular patient reacts to stimuli and to make careful use of that information. If a
test proceeds too slowly, the patient may get frustrated and fatigued, and the results may,
therefore, be inaccurate.
Appendix G: How SITA Works / Acknowledgments
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