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Joshua Pratt, Harold Bedell, Joy Ohara, Stanley Woo; Retinal Fixation Locus in Patients with Bilateral Central Scotomas: Letter Targets are Better Suited for Visual Testing than Targets that Fill in. Invest. Ophthalmol. Vis. Sci. 2013;54(15):2184.
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© ARVO (1962-2015); The Authors (2016-present)
Training eccentric viewing has been widely implemented as a regular part of low vision services for patients with central scotomas. In this experiment we investigated whether target type affects the retinal fixation location and stability, and specifically whether targets expected to perceptually fill in are imaged at the vestigial fovea. If targets expected to fill in are imaged at or near the vestigial fovea, then fixation on these targets would allow the projected location of the vestigial fovea and the pathological scotoma to be located accurately in visual space during eccentric viewing training, even without the use of a fundus imaging device.
The retinal location and stability of fixation were measured using the Nidek MP-1 microperimter in twelve patients with bilateral central scotomas for six types of fixation target, three expected to fill in and three that included letters. The approximate position of the vestigial fovea was delineated either by using residual retinal landmarks or by locating the residual foveal pit in a dense macular scan, obtained with a Spectralis Optical Coherence Tomographer. Fixation location and stability were compared for the different target types and referenced to the position of the vestigial fovea.
All of the subjects except one fixated consistently on targets that included a letter using peripheral retinal locations outside of the central scotoma. Eleven of the twelve subjects used a retinal location closer to the vestigial fovea to fixate targets expected to fill in, compared to letters. Although four of the subjects imaged the fill-in targets at or within a half degree of the vestigial fovea, six other subjects imaged the fill-in targets at a retinal locus removed from the vestigial fovea. Target type produced no overall significant difference in fixation stability, specified in terms of bivariate contour ellipse area. However, in some individual subjects fixation tended to be more stable on letter targets than on fill-in targets.
In patients with central field loss, letter targets generate more consistent fixation behavior than fill-in targets and should be used for eccentric viewing training and perimetry.
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