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Kavitha Ratnam, Joseph Carroll, Travis Porco, Jacque L. Duncan, Austin Roorda; Visual Acuity and Foveal Sensitivity are not Reliable Measures of Cone Density at the Fovea. Invest. Ophthalmol. Vis. Sci. 2012;53(14):4646.
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To study the relationship between cone spacing and clinical measures of visual function near the fovea.
High-resolution images of the photoreceptor mosaic were obtained with Adaptive Optics Scanning Laser Ophthalmoscopy (AOSLO) from 21 patients with inherited retinal degenerations, including 6 patients with retinitis pigmentosa, 7 with Usher syndrome, 4 with neuropathy, ataxia and retinitis pigmentosa (NARP) syndrome, 2 choroideremia carriers, and 2 X-linked retinitis pigmentosa carriers. Cone spacing measures were made close to or at the foveal center (with a maximum eccentricity of 0.63 deg and an average eccentricity of 0.14 deg). Since cone-spacing measurements were not all made at the exact foveal center, they were converted to z-scores (# of standard deviations from expected mean spacing for the specific eccentricity tested, based on data collected from 27 normal eyes). The cone spacing at the tested locations was also converted into the fraction of cones (FOC) compared to normal expected values for each location. We then compared the z-scores and FOC with best-corrected visual acuity (ETDRS) scores, and foveal sensitivity (Humphrey Visual Field Analyzer), assuming that the z-scores reflected departure from normal cone spacing throughout the foveal region.
Visual function was correlated with cone loss. The Spearman rank correlation was -0.79 for cone spacing z-score vs ETDRS score (p<0.0001), -0.77 for cone spacing vs visual acuity (p<0.0001), and -0.60 for cone spacing z-score vs foveal sensitivity (p=0.006). However, the same clinical measures plotted vs FOC revealed that clinical vision tests remained at normal levels (better than 80 letters (20/25) and better than 35 db sensitivity) despite cone counts that were 50% below normal.
Although conventional tests of visual function show a strong relationship with cone loss, the relationship is not necessarily linear. As such, cone losses of up to 50% might have to take place in the fovea before detectable changes in visual acuity or sensitivity are detected.
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