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Matt Dunn, Tom Margrain, Fergal Ennis, Christopher Harris, J. Woodhouse, Jonathan Erichsen; Visual acuity in infantile nystagmus in the absence of image motion. Invest. Ophthalmol. Vis. Sci. 2013;54(15):179. doi: https://doi.org/.
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Infantile nystagmus is associated with a reduction in visual acuity (VA), even in the absence of co-existing pathology. The purpose of this study was to assess the extent to which the motion blur induced by the eye movements contributes to this visual impairment. Since the intended outcome of most therapies for infantile nystagmus is a reduction in eye movement intensity, the result of this study will define the maximum possible benefits (to VA) that these interventions are able to provide.
Resolution thresholds were measured in 14 subjects with idiopathic infantile nystagmus and 24 controls, under flash and constant illumination conditions. Printed black & white gratings were viewed at a distance of 2 m and illuminated by either an incandescent lamp or multiple brief xenon flashes lasting 0.76 ms. The flashes were sufficiently brief to eliminate significant image motion. Subjects reported the tilt of the gratings (±5°) according to a 2AFC paradigm to determine resolution threshold. Viewing time was unrestricted; for the flash paradigm, the unit flashed at 2-6 Hz until the subjects made a response. Thresholds were determined for gratings oriented about both the horizontal and vertical axes in order to investigate the effect of flash presentations on orientation sensitivity.
As expected, acuity thresholds under constant illumination were consistently much poorer for nystagmats than controls (p < 0.001). However, in contrast to the control subjects, nystagmats’ acuity for near-vertical gratings was worse than near-horizontal (p < 0.01). Most importantly, although the flash caused VA to worsen in controls (p < 0.01), in nystagmats VA did not change for either the horizontal or vertical condition (p = 0.67 & 0.71).
Our results strongly suggest that the acuity deficit in people with idiopathic infantile nystagmus is not due to motion blur induced by eye movements. Therefore, the reduced VA in these individuals must be due to amblyopia and/or undetected pathology. This is clinically significant, as it suggests that therapies to slow the eye movements have only a limited potential to improve VA.
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