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Faster threshold strategies improve patient care
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While second-generation processed data and developments in interpretation strategies have been of great value in our clinical management of glaucoma, they have not significantly addressed the needs of the test subject. A great albatross in threshold field testing has always been patient fatigue with resultant loss of reliability.
Manufacturers of field testing devices have been motivated to develop a method of gathering threshold perimetric measurements that obtain more information per unit time as compared with traditional second-generation, full-threshold algorithms. Investigators believed that by using field knowledge gathered in the 1980s and modern high-powered computers, one should be able to create a third generation of more efficient algorithms. In 1997, Haag-Streit and Humphrey introduced testing strategies that drastically reduce threshold testing time by approximately 80%.
Swedish Interactive Threshold Algorithm (SITA) by Humphrey uses artificial intelligence to shorten test time. The SITA algorithm takes advantage of current knowledge of visual field physiology and employs improved methods for estimation of false answers. Tendency Oriented Perimetry (TOP) strategy by Haag-Streit is based on the anatomical and topographical interdependence of visual field defects and establishes a “tendency” between the thresholds of neighboring zones. TOP attempts to assess the visual field status by asking only one question per test location and using every other answer for establishing thresholds and the level of questioning for the points in the neighboring area.
Accurate with fewer questions?
The obvious question that comes to mind when evaluating the clinical efficacy of these new strategies is: How could they be as accurate with fewer questions being asked? There have been and continue to be new studies that should specifically address this question. Previous multicenter investigations have demonstrated an extremely high correlation between TOP, SITA and conventional threshold strategies (De la Rosa G, Bron A, Marales J, Sponsel W. TOP Perimetry, a theoretical evaluation. Vision Res. 1996;36:88(Sup. Jermov); Bengtsson B, Olsson J, Heijl A, Rootzen H. A new generation of algorithms for computerized threshold perimetry. Acta Ophthalmology Scand. 1997;75:368-375). Current data also demonstrate that, with tendency oriented perimetry, we can expect an increase in mean sensitivity by 1 dB and a corresponding decrease in mean defect by about 1 dB when compared to longer tests. This effect appears to be the result of the absence of the “fatigue effect” with the new shorter test. Second, the borders of deep scotoma are somewhat less sharp.
My impression is that there may be some decrement in specificity with the new faster test modalities. This decrement may only be demonstrable given true comparisons with perfect test subjects. Because none of my patients are perfect, it makes sense that some small decrease in accuracy as the result of speed may be more than compensated for by more reliable and valuable patient response. In other words, a 20-minute test strategy may be too difficult for even the perfect patient. After all, how good is any test if our patients are unable to perform at an acceptable level?
Better patient care
The addition of the TOP strategy in our office has translated into greater office efficiency and more effective patient care. When 30-minute fields were the standard, we had to schedule thresholds weeks in advance. Now, our doctors can order a test on the same day without serious impact on scheduling. If a patient shows incongruous results, he or she can be retested in only 2 to 3 minutes per eye. Some of our patients used to require two appointments because some physical impairment prevented them from enduring laborious tests on both eyes on the same visit. The new faster TOP strategy appears to address many of these issues without significantly sacrificing accuracy.
Because of the significance of field testing in glaucoma management, financial issues cannot be our first priority. Nonetheless, the reduced 1998 Medicare reimbursement levels for threshold testing make 30-minute visual fields an inefficient use of office time. The cost of the software upgrade is quickly recaptured because the new strategy provides at least a threefold increase in efficiency. The more invisible financial reward is in the strong signal that we send to our patients who candidly convey their gratitude that we are willing to invest in their comfort and the very latest in patient care modalities.
For Your Information:
- Elliot M. Kirstein, OD, is in private practice, specializing in contact lenses and treatment of glaucoma and anterior segment disease. He can be reached at 11304 Montgomery Road, Cincinnati, OH 45249; (513) 530-0440; fax: (513) 530-0473. Dr. Kirstein has no direct financial interest in the products mentioned in this article, nor is he a paid consultant for any companies mentioned.
- Swedish Interactive Threshold Algorithm is available from Humphrey Systems, 5160 Hacienda Drive, Dublin, CA 94568; (925) 557-4100 or (877) 486-7473; fax: (925) 557-4101.
- Tendency Oriented Perimetry is available from Haag-Streit Group, 5500 Courseview Drive, Mason, OH 45040; (513) 336-7255; fax: (513) 336-7260.