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Chad E. Davis, Carrie K. Doyle, Gideon J. Zamba, Chris A. Johnson, Michael Wall; The Effect of Stimulus Size on Repeatability in Glaucoma Using Goldmann Sizes III, V, VI and STP. Invest. Ophthalmol. Vis. Sci. 2011;52(14):5508.
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Standard automated perimetry (SAP) is limited by an exponential rise in variability with decreasing sensitivity. We therefore studied the effect of stimulus size on repeatability in glaucoma patients using Goldmann stimulus sizes III, V and VI and size threshold perimetry (STP) - a test that estimates threshold by finding the smallest sized differential light sensitivity stimulus a subject can detect at the 24-2 test locations.
We tested 20 glaucoma patients with Humphrey program 24-2 using stimulus sizes III (SITA), V, VI, and STP each 5 times over a 5 week period. Mean deviation for size III was -13.2 ± 6.7. Point-wise limits of test-retest variability were then established from the empirical 5th and 95th percentiles of the distribution of retest values, stratified by the value of at the first test (Fig). The log differences between test and retest values were linearly regressed onto the averages of the two tests to determine the relationship between variability and sensitivity.
The average sensitivities were SAP size III: 14.7 ± 6.6 dB; SAP size V: 21.5 ± 8.0 dB, SAP size VI: 25.2 ± 6.9 dB, and STP: 17.3 ± 6.5 dB. There was increasing variability associated with lower visual field sensitivity but the rise in variability was less with the larger stimulus sizes, with STP having the least (Fig). After eliminating the values subject to a floor effect, we found the following correlations: with size III, sensitivity explained 21% of the test-retest variability (r2), while corresponding figures for size V, size VI and STP were 9%, 10% and 1%, respectively.
Large sized perimetric stimuli have a diminished increase in variability with decreasing sensitivity. These stimuli show promise for use in moderate to severe glaucoma.
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