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Editors Selection IGR 14-4
Clinical Examination Methods: Stimulus size and visual field relaibility
Comment by Don Budenz on:
60796 The Effect of Stimulus Size on the Reliable Stimulus Range of Perimetry, Gardiner SK; Demirel S; Goren D et al., Translational vision science & technology, 2015; 4: 10
Following glaucoma patients for progression who have moderate severe disease presents a dilemma for the clinician. These patients are the ones most likely to go blind and surgical intervention is frequently considered to halt or slow worsening and preserve vision. Yet tools that are very helpful in diagnosing glaucoma and glaucoma progression at the early stages of glaucoma have limited usefulness at later stages due to the 'floor effect' of measuring retinal nerve fiber layer thickness using optical coherence tomography, detecting small changes in optic nerve cupping or rim thickness using stereoscopic disc photography, and measuring the visual field reliably. The study by Gardiner et al. compares the sensitivities of Size III and Size V stimuli in four moderate to severely depressed locations of the visual field in 35 subjects with glaucoma.
The clinical importance of this is that patients can be followed for a longer period of time with a Size V stimulus compared to a Size III stimulus, perhaps years longer.
They found that neither stimulus size had adequate reliability below the 15-19 dB range and that the Size V stimulus was no more useful in this range than a Size III stimulus. However, they did find that increased stimulus size results in better sensitivity measurements, suggesting that the dynamic range measurable with a Size V stimulus is wider than with a Size III stimulus. The clinical importance of this is that patients can be followed for a longer period of time with a Size V stimulus compared to a Size III stimulus, perhaps years longer.
The improved sensitivity and dynamic range of the Size V stimulus may be due to recruitment of adjacent, less affected areas of the receptive field being tested. The authors review several possibilities for improving visual field testing algorithms based on their results. These include changing all automated perimetry to Size V, increasing to a Size V stimulus automatically within a test when sensitivities with the Size III stimulus is below the reliable range, or following patients with a Size III stimulus until most of the thresholds are below a reliable range and then changing to all Size V stimuli in future tests. Until there is evidence that one of these strategies is superior to another, the third option seems like the best strategy for now.
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