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L. Mcilreavy, D. G. Hunter, P. J. Bex; Quantification of Spatial Distortions in Juvenile Amblyopia. Invest. Ophthalmol. Vis. Sci. 2010;51(13):1840.
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Qualitative spatial distortions are frequently reported in amblyopia. Animal models of amblyopia indicate that correlated binocular vision is essential for the successful recovery from experimentally-induced amblyopia. Spatial distortions de-correlate binocular vision and may therefore play a critical role in the prognosis of amblyopia. We aim to quantify the prevalence and magnitude of spatial distortion in vision of juveniles with amblyopia and to determine their role in the severity and outcomes of treatment as determined conventionally by visual acuity.
Spatial distortions were estimated in subjects (mean 6.2 yrs; range 5-7 yrs) with strabismic or anisometropic amblyopia at the first clinical appointment, prior to commencing occlusion therapy, and in control subjects (mean 10 yrs; range 8-12 yrs). Subjects wore stereo glasses to provide 120Hz dichoptic viewing. Subjects fixated a central fixation point (0.125° diameter) with compliance enforced with an eye tracker. Single white circular targets (0.125° diameter) were presented in random order to the amblyopic eye, on a notional 4x4 matrix subtending an 8°, centred on fixation. A black cross-hair (subtending 5°, with a 3° open centre to avoid inter-ocular suppression) was presented to the fellow eye and was under the control of a mouse. The subject’s task on four repetitions was to align the centre of the cross-hair with the target, a hyperacuity task, and to click the mouse to register a response.
The distance and angle between the physical and perceived location of each target provided an estimate of the local inter-ocular spatial distortion. The mean perceived horizontal shift was calculated and removed as a global estimate of strabismus. The mean of the strabismus-corrected local distortions provided an estimate of the overall level of visual distortion and was much greater in amblyopic (mean 1.44°; standard deviation 1.2 deg) than normal (mean 0.27°; standard deviation 0.26°) eyes. Optically-corrected anisometropia produced systematic magnification errors in dichoptic alignment.
Significant spatial distortions are present in juveniles with amblyopia, but the sample size was too small to identify any relationship with visual acuity. The spatial distortions included a mixture of predictable (strabismus and optical magnification-induced) and idiosyncratic perceptual misalignments. The presence of significant distortions caused by optical correction of anisometropia introduces binocular de-correlation with unknown consequences for the prognosis of amblyopia.
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