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Algorithm for short-wavelength perimetry reduces test time without sensitivity loss
The SITA algorithm makes the early-detection perimetry mode easier on patients, clinicians say.
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Short-wavelength automated perimetry is effective in detecting glaucoma earlier than standard white-on-white perimetry, but because of the length of the test it has mostly been used only in academic settings to date. With the application of a faster algorithm, test time has now been considerably reduced with no sacrifice in sensitivity, researchers have found. The addition of the faster algorithm makes the short-wavelength test practical for widespread use in the evaluation of glaucoma suspects, according to clinicians familiar with the test.
The new testing strategy is a combination of two existing tests, explained Boel Bengtsson, PhD, of Sweden. She and colleague Anders Heijl, MD, modified the Swedish Interactive Thresholding Algorithm (SITA) for use with the Short Wavelength Automated Perimetry (SWAP) test, also known as blue-on-yellow perimetry, which is more sensitive to early glaucomatous damage.
The SITA SWAP test will be included on the Humphrey Field Analyzer from Carl Zeiss Meditec. It was introduced by the company this year and will be available in early 2005, according to a company spokeswoman.
Drs. Bengtsson and Heijl originally developed the SITA testing strategy in the 1990s.
“By using powerful computers and advanced statistical mathematical techniques, we developed new, faster interactive test algorithms for threshold estimation. These algorithms were based on models of the normal and the abnormal white-on-white visual field,” Dr. Bengtsson said.
Now the researchers have taken the SITA strategy that they developed for white-on-white perimetry and applied it to the blue-on-yellow testing mode.
“By constructing models of the blue-on-yellow visual field and adjusting test algorithms in computer simulations and real time [clinical] measurements, we developed SITA SWAP,” Dr. Bengtsson said.
In SWAP testing, blue light stimuli are shown on a yellow background to assess damage to a specific subset of retinal ganglion cells, said Pamela Sample, PhD. Dr. Sample and Christopher A. Johnson, PhD, developed the original full threshold SWAP system, a precursor to the SITA SWAP system.
“SWAP works by using a short wavelength (violet) target on a bright yellow background to isolate only one subtype of retinal ganglion cell, the small, bistratified cell,” Dr. Sample said. “We believe this allows us to pick up loss of visual function 2 to 5 years earlier than standard white-on-white perimetry and to follow changes over time more effectively in patients with glaucoma.”
Christopher A. Girkin, MD, a Birmingham, Ala., researcher, began using the new SITA SWAP system earlier this year, both in his practice and in an ongoing multicenter glaucoma study.
“What SWAP does is bleach out red and green cones with a yellow background light and then shines a blue light on the retina. It should just be selectively picked up primarily by this subset of cells, so you have fewer cells in the eye responding to the stimulus. This makes the test more sensitive,” Dr. Girkin said.
The SITA SWAP combination
Image: Bengtsson B |
Dr. Girkin said the original SWAP test could detect glaucoma defects as much as 5 years earlier than standard perimetry. The combined test is just as sensitive as the original SWAP full threshold test, but faster, he said.
The second-generation test reduced testing time from the 15-to-20-minute range to about 3 to 5 minutes, making it more conducive for clinical use, he said.
“With the first generation of SWAP, we had a visual field test that was more sensitive but was limited because the test was too long and the intra-test variability too high – a lot of noise in the measure when looked at serially. It was hard to separate what was noise and what was progression,” Dr. Girkin said.
Dr. Girkin said he believes the new test has great potential for identifying glaucoma in its early stages.
“Since SITA can reduce testing time and, at the same time, reduce testing variability, hopefully what you have created in that marriage is a test that is more sensitive to glaucomatous damage and also less variable and quicker,” he said.
Surpassing first generation
Dr. Bengtsson conducted a study to compare the SITA SWAP combination to full threshold SWAP. The study showed that the two tests were comparable and that reproducibility of threshold values was almost the same.
The test produces probability maps that graphically display test point locations that fall outside of normal limits, Dr. Bengtsson said.
“In probability maps, the measured visual field of a patient is compared to a normal database of fields obtained in healthy normal subjects,” she said.
As in standard automated perimetry, the test points at which the measured threshold sensitivity varies from the normal range are flagged, and a map showing the deviations is produced.
Best for early detection
SITA SWAP is most appropriate for detecting and following glaucoma in its early and middle stages, Dr. Sample said. In late stages of the disease, the bistratified ganglion cells may no longer function, she said, and white-on-white perimetry is then best for following progression.
The test loses sensitivity in patients with significant cataract, Dr. Sample said. In addition to glaucoma, the test is useful in assessing visual fields of patients with diabetes mellitus, migraine, optic neuritis and HIV, she said.
For Your Information:
- Boel Bengtsson, PhD, can be reached at the department of ophthalmology of Malmo University Hospital, Lund University, SE 205-02 Malmo, Sweden; 46-40-333230; fax: 46-40-336212.
- Christopher A. Girkin, MD, can be reached at UAB Ophthalmology Service Fdn, 700 South 18th, Suite 601, Birmingham, AL 35233; 205-325-8110; fax: 205-325-8654.
- Pamela Sample, PhD, is a professor in residence in the department of ophthalmology and the director of the Visual Function Laboratory at the University of California, San Diego. She can be reached at Shiley Eye Center, Room 180, University of California, San Diego, La Jolla, CA 92093-0946; 858-534-6629; fax: 858-534-1625.
- Carl Zeiss Meditec Inc., maker of the SITA SWAP test and the Humphrey Field Analyzer, can be reached at 5160 Hacienda Drive, Dublin, CA 94568-7562; 925-557-4593; Web site: www.meditec.zeiss.com.
- Jeanne Michelle Gonzalez is an OSN Staff Writer who covers all aspects of ophthalmology, specializing in practice management, regulatory and legislative issues. She focuses geographically on Latin America.