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Y. Chopovska, C. Pieh, A. Fuchs, S. P. Heinrich, M. Bach; Vep-Based Acuity Assessment in Patients With Amblyopia and Cataract. Invest. Ophthalmol. Vis. Sci. 2008;49(13):2596.
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Recently we presented the Freiburg Acuity VEP test (FrAVEP). The method was validated in subjects with normal and blurred vision, and in patients with ocular diseases. It predicts subjective VA to 95% within ±1 octave. In amblyopes it is known that periodic stimuli can overestimate VA. We evaluated to what extent this applies to FrAVEP in subjects with amblyopia. Subjects with cataract served as a comparison group.
17 subjects with amblyopia (anisometropic and strabismic) and 17 subjects with unilateral or bilateral cataract participated in the study. Their decimal VA ranged from 0.03-1.69. Checkerboard stimuli with 6 check sizes covering 0.05-0.4° were presented in brief-onset-mode (40 ms on, 93 ms off) at 7.5 Hz. All VEPs were recorded with a Laplacian montage (2*Oz - (RO+LO)). Fourier analysis yielded the magnitude at the stimulus frequency, and the average of the neighbouring frequencies as noise estimate. Objective VA was estimated using the equation VA= SF0/17.6 cpd. Subjective VA was obtained with the automated Freiburg Visual Acuity Test (FrACT). Both types of VA were assessed twice using an "ABBA" scheme, eye with higher acuity was tested first.
The fully automated analysis algorithm succeeded in 88%. Test-retest reproducibility for FrACT was ±0.24 logMAR and for FrAVEP ±0.22 logMAR for a 95% confidence band. There was a highly significant (p<0.01) correlation for subjects with amblyopia (r=0.62) and with cataract (r=0.64) between subjective and objective VA. Although correlation was high, for amblyopic eyes the objective acuity was overestimated by more than a factor of 2 (or 0.3 logMAR) in 65% and for eyes with cataract in 19%.
Both measurements of visual acuity with FrAVEP and FrACT were highly reproducible. In amblyopia, however, acuity is often markedly overestimated and never underestimated by FrAVEP. We ascribe this result to the periodic stimuli (checkerboards), which resembles the high subjective acuities obtained with gratings. FrAVEP needs to be interpreted with caution in amblyopia.
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