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M. Theodorou, R. Clement, R. McClean, F. Proudlock, I. Gottlob; Quantitative Assessment of Factors Affecting Visual Acuity in Congenital Nystagmus. Invest. Ophthalmol. Vis. Sci. 2007;48(13):879.
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© ARVO (1962-2015); The Authors (2016-present)
To investigate the relative contributions of factors affecting visual acuity in congenital nystagmus.
20 adult subjects (age range 27 - 55 years) with idiopathic congenital nystagmus were assessed . Binocular corrected LogMAR Visual Acuity (VA) was measured. Horizontal eye movements were recorded using a head mounted video system. Recordings were taken binocularly in 3 horizontal positions of gaze (straight ahead, 15 degrees right and left) and monocularly (straight ahead position only). Calibration was carried out using the 15 degree saccades. The minimum mean retinal eccentricity and velocity during a 50ms period of the nystagmus waveform was calculated. The visual acuity was predicted using a function which describes the drop in visual acuity with increasing retinal eccentricity, and a function which describes the drop in visual acuity with increasing velocity.
The predicted VAs based on the nystagmus waveform parameters were significantly better than the measured VAs in 17/20 subjects with both the position or velocity functions (Wilcoxon Signed-Rank Test, p<0.01). The range of the measured VAs were 0.02-0.62 (mean 0.33), and the predicted VAs 0.00-0.37 (mean 0.1) and 0.00-0.39 (mean 0.1) based on the position and velocity functions respectively. The Spearman Rank Correlation Coefficients between the predicted and measured VAs were not significant (0.12 and 0.14 for position and velocity based predictions respectively).
On the basis of clinical examination and electrophysiological tests, subjects with idiopathic congenital nystagmus are assumed to have normal visual systems. However, prediction of the effects of nystagmus on normal visual systems underestimated the VAs in a group of subjects with idiopathic congenital nystagmus by 2 lines on a standard LogMAR chart. We conclude that the effect of motion of the eye on the retinal image is not primarily responsible for the loss of acuity.
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