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M. M. Marzban, E. D. Weber, S. A. Newman; Heijl-Krakau Fixation Monitoring and the Duration of Automated Static Perimetry. Invest. Ophthalmol. Vis. Sci. 2008;49(13):6018.
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Heijl-Krakau’s strategy was originally designed to monitor fixation. It has also been used as a reliability index. With the advent of automated perimetry tracking systems and video monitoring we undertook to determine how much extra time was added by utilizing the fixation monitor.
Initial pilot study was done on 15 normal volunteers to assess the reduction in duration by turning off the fixation monitor The visual field of one eye of each patient was mapped twice using the Humphrey Visual Field Analyzer, program 24-2 SITA FAST, with the blind spot fixation monitor function on and off. The patients were randomized to one of two groups such that half the subjects took their first test with the fixation monitor on and the other half took the test first with the monitor off. The data was then analyzed using a Student’s unpaired t-test.
Visual field testing time with the blind spot fixation monitor off ranged from 126 to 178 seconds (mean=149.6, sd=16.2), compared with a range from 142 to 206 seconds with the monitor on (mean=178.4, sd=20.8). The reduction in time was statically significant (p=.004). There were no significant differences noted in mean defect (p=0.9) or pattern standard deviation (p=0.21) done with the monitor on or off.
Based on this data, using the blind spot fixation monitor clearly increases the length of time for a field study. While the average increase in testing time for normal subjects is less than a minute and does not affect test outcome, in subjects with abnormal tests this increase in time could be longer and potentially have significant affects on test reliability. With a conscientious perimetrist observing the patient, the blind spot monitor appears to be unnecessary.
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