An Investigation into the Effect of Increasing Target Size for Visual Sensitivity Measurement in Normals and in Early Glaucoma
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The clinical standard method of measurement for clinical visual sensitivity is currently Standard Automated Perimetry with a size III target (SAP III). However, there are many factors that can make the results unreliable. While there is literature that confirms the benefit of increasing target size for measurement of visual sensitivity in the later stages of glaucoma, there is little literature that looks into the effect of increasing target size for the measurement of visual sensitivity in early glaucoma. Furthermore, the effect of increasing target size for the measurement of visual sensitivity in normals is largely undefined. We performed 2 studies to determine the effect of increasing target size for the perimetric measurement of both normals and in participants with very early glaucoma. In the first study (chapter 2), 40 normal participants (one study eye) performed 3 full threshold visual fields at 2 separate visits, no more than 90 days apart. The target sizes used were size III (0.43° diameter), size V (1.72° diameter) and size VI (3.44° diameter). We investigated the interaction of the different target sizes by regressing the average field threshold for each participant against age in both decibels and candelas. We found an expected difference in sensitivities between the different target sizes for the decibel analysis, but an unexpected difference in thresholds between the target sizes for the candela analysis. Possible reasons for this unexpected difference in total light energy are discussed. In the second study (chapter 3) we investigated the effect of increasing target size in 17 participants with very early glaucoma (perimetric mean deviation of equal to, or better than, -4.0dB). Each participant underwent 3 full threshold visual field tests, using 3 different target sizes, at 2 separate visits (no more than 90 days apart). We computed empirical probability plots for each participant and target size: Size III (0.43° diameter), size V (1.72° diameter) and size VI (3.44° diameter), where normal percentile limits were based on the first study - chapter 2. We then compared the number of normal and abnormal test locations at each defect depth (5%, 2%, 1% and 0.5%) between SITA-Std and the 3 different target sizes (full threshold) using a repeated measures ANOVA. We found there to be no statistical difference in the number of abnormal locations detected between SITA-Std and the 3 target sizes. However, when analysing the empirical probability plots there was an apparent clinical difference between the locations of abnormality detected between SITA-Std and the larger size VI target, with the size VI giving less consistent defect locations.