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New perimetry algorithm three times faster than other systems
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SAN LEANDRO, Calif. — A new perimetry algorithm from Humphrey Instruments uses artificial intelligence to complete visual field tests for glaucoma patients in about one-third the time it takes conventional machines. Results are available in under 3 minutes and, moreover, practitioners who have used the new system say it is highly accurate.
Swedish Interactive Threshold Algorithm (SITA) is an individualized, self-correcting, self-directing visual field test operating system designed to run on all Humphrey HFA-2 perimetry machines.
--- J. James Thimons, OD
"This is a new and innovative technology to enhance visual field assessment," said J. James Thimons, OD, chair of the Department of Clinical Sciences at the State University of New York College of Optometry. "It addresses several of the problems inherently present in any psycho-physical examination: compliance, fatigue, variability of response and cost of technology."
"Everyone knows the importance of automated perimetry," said Anders Heijl, MD, PhD, head of the team that developed SITA in Malmo, Sweden. "But those of us who have been working with automated perimetry since its start in the early 70s have been painfully aware of how patient unfriendly it is regarded as being."
Instead of using a fixed analysis criteria, as in current perimetry algorithms, the SITA system is flexible. By searching for consistent responses from patients and altering its diagnostic mission to conform to individuals, the SITA system reduces evaluation time and im proves the accuracy of its findings. Its developers point out that SITA differs from conventional perimetry in four major ways.
First, the algorithm asks "smart" questions. A variety of questions regarding normal age-corrected threshold values, patterns of loss typically found in a particular disease, patient responses at the test point so far in the test, patient responses at nearby test points, and the rate at which patients respond to questions all are factored into the system's decision-making process. SITA then uses the responses to produce a threshold estimate for each point of the visual field.
At the start of the test, estimates are based on normative data compiled from Humphrey's database and the Baltimore Eye Survey. However, as the test proceeds, the patient's own responses are factored into the algorithm's conclusions. The threshold estimate at any given point on the test is continuously updated according to the patient's actual answers at adjoining points.
"If a policeman were interviewing someone and the person gave fuzzy responses or was unclear, then the officer would be prompted to ask more questions," said Michael Patella, OD, director of glaucoma products business group at Humphrey. "That's what SITA does."
Exam pace tailored
Second, SITA tailors the pace of the exam to individual patients. Doctors report that patients do not feel rushed, but that the machine does not waste time, either. By analyzing reaction time of individual patients and comparing that data to a normative base, the system accurately predicts the rate at which a patient is comfortable being tested.
--- Alan Robin, MD
"A normal perimeter allows for slow, medium and fast settings," said Alan L. Robin, MD, an associate professor of ophthalmology at Johns Hopkins University and the University of Maryland. "This machine looks at what an age-matched normal patient would do, then goes to the test patient and judges his or her response by comparison and adjusts the rate."
Knows when to quit
Third, the new algorithm knows when to stop testing at each location. By looking for consistency in patient responses and cross-checking the results, the machine knows when it can proceed and can determine when test points require further examination.
A Bayesian posterior probability function, which compares what is known about the general patient population and the chances of obtaining a specific response in a random set of variables, is integrated into the algorithm to estimate error in the threshold at each test point. When the error estimate falls below a predetermined value, testing at that point is discontinued and the point is "closed."
"Other systems have fixed criteria for determining when to stop the measurement process at a given point," Thimons said. "But in SITA, this allows increased speed of testing while maintaining the accuracy found in other algorithms."
Examines the big picture
Last, to complete its individualized analysis, the SITA bases diagnostic conclusions on the complete pattern of patient responses at each point on the test. By examining the "big picture," SITA - analyzed visual fields are "spatially interactive," meaning each point depends on patient responses at each adjacent point. "This permits a more accurate and improved clinical analysis," Thimons said.
Robin said the SITA machine will benefit clinicians whose patients are uncomfortable with conventional perimetry tests. Because even Fastpac algorithms can take 10 minutes to complete, the attention span of many patients wanes long before the procedure is over.
Robin said that visual field tests with conventional algorithms can take up to 1 hour to compete, including the time it takes to seat a patient, complete the test and analyze the results. With SITA, tests can be completed in about 15 minutes.
"We will see that patients who were thought not to be suitable for automated perimetry really are if you try not to tax them as hard," said Heijl, professor and chairman of the Department of Ophthalmology at Malmo University Hospital.
After running SITA clinically for the past 2 years, Heijl said, he found that patients did not mind being retested as much as with earlier perimeters. "It means a lot of clinical difference to administer two tests per year instead of one," he said.
The SITA operating system runs in two modes. SITA Fast uses the same accuracy and reproducibility standards as the Fastpac protocol, currently available on Humphrey visual field analyzers. SITA Standard's accuracy and reproducibility are as good or better, than the full threshold algorithm mode of current Humphrey machines.
"If a doctor is performing a test 24-2, and that test usually takes 8 minutes on the full threshold strategy, it will probably take about 5 minutes with the Fastpac strategy and just slightly over 2 minutes with SITA," said Foster.
The SITA algorithm debuted at the International Vision Expo West meeting in Anaheim, Calif., last month. SITA upgrade software will be distributed free of charge to all current owners of Humphrey HFA-2 perimeters. New HFA-2 machines will include the new algorithm without additional charge for a limited time.
For Your Information:
- Alan L. Robin, MD, is an associate professor of ophthalmology at Johns Hopkins University and the University of Maryland. He can be reached at (410) 377-2422; fax: (410) 377-7960.
- J. James Thimons, OD, can be contacted at SUNY College of Optometry, 100 East 24th St., New York, NY 10010; (212) 780-4980; fax: (212) 780-4980.
- For additional information on SITA, contact Humphrey Instruments Inc., 2992 Alvarado St., San Leandro, CA 94577; (510) 297-4181; fax: (510) 483-8025.