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R.C. Becker, M.H. Graef; Comparison of Landolt–C and Snellen–E–Acuity in Strabismus Amblyopia . Invest. Ophthalmol. Vis. Sci. 2006;47(13):2454.
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Assesment of visual acuity depends on the optotypes used for measurement. The ability to recognize different optotypes differs even if their critical detail appears under the same visual angle. Since optotypes are evaluated on individuals with good visual acuity and without eye disorders, differences in the lower visual acuity range cannot be excluded. In this study, visual acuity measured with the Snellen E was compared to the Landolt C acuity.
100 patients (age 8–90 years, median 60.5 years) with different eye disorders, among them 39 with amblyopia due to strabismus, and 13 healthy volunteers (age 18–33 years, median 24 years) were tested. Visual acuity assessment was performed using charts with the Snellen E and the Landolt C (Precision Vision) which mimic the ETDRS charts. 3 out of 5 optotypes per line had to be correctly identified, while wrong answers were monitored. In the group of patients, the eyes with the lower visual acuity, and for the healthy subjects, the right eyes were evaluated.
Differences between Landolt C (LR) and Snellen E (SE) acuity were small. The mean decimal values for LR and SE were 0.25±0.4 and 0.29±0.4 in the entire group and 0.14±0.8 and 0.16±0.8 for the eyes with strabismus amblyopia. The mean difference between LR and SE was 0.55 lines in the entire group and 0.55 lines for the eyes with strabismus amblyopia, with higher values for the Snellen E in both groups. In the acuity range below 0.1, the mean difference between LR and SE was 0.58 lines for the entire group and 0.58 lines for the eyes with strabismic amblyopia with higher values for the Snellen E in both groups.
Using the charts described, there was only a slight overestimation of visual acuity by the Snellen E compared to the Landolt C, even in strabismus amblyopia. Small differences in the lower visual acuity range have to be considered.
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