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Yiyi Wang, Nicolas Bensaid, Pavan Tiruveedhula, Jianqiang Ma, Austin Roorda, Sowmya Ravikumar; The Relationship between Cone Density and Axial Length: CAL Study. Invest. Ophthalmol. Vis. Sci. 2018;59(9):731.
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© ARVO (1962-2015); The Authors (2016-present)
In vivo imaging studies of human myopic retina show that increased axial length of the eye is associated with decrease in linear cone photoreceptor density (cones/mm2). These findings were typically made at parafoveal and peripheral retinal locations. However, histological evidence from a marmoset model of myopia suggests that longer eyes have a higher linear cone density at the fovea. The purpose of our study was to measure cone density at the human fovea as a function of axial length and discuss the implications of our findings.
We used a new generation Adaptive Optics Scanning Laser Ophthalmoscope (AOSLO) to obtain images at 680 nm wavelength, of 28 eyes. An improved optical design, higher stroke mirror and the use of a larger pupil diameter enabled imaging of the smallest cones at the center of the fovea. Nine high signal-to-noise images, each ~0.9 degree2, were stitched together to create a 1.8 degree2 montage of the central retina. Automated cone marking software, with manual refinement, was used to identify the location of every cone in the montage. Cone density and other metrics were computed using custom MATLAB software. To make accurate conversions from angular to linear dimensions, we calculated the Retinal Magnification Factor of each eye computed from biometry data (axial length, anterior chamber depth and corneal curvature) measured with the IOL Master (Carl Zeiss Meditec).
The range of axial lengths of the subject group was 22.26 to 27.06 mm. Spherical Equivalent Refraction was closely correlated with axial length. Angular peak cone density (cones/deg2) increased significantly as a function of axial length (R2=0.334, P<0.01). Longer eyes had higher angular cone density than shorter eyes at all eccentricities from the foveal center to 40 minutes of arc. Linear peak cone density (cones/mm2) decreased significantly as a function of axial length (R2=0.393, P<0.05), from the fovea out to 100 microns eccentricity.
The cone mosaic in longer eyes is expanded at the fovea, but not in proportion to eye length. In other words, despite retinal stretching, myopes generally have a higher angular sampling density in and around the fovea compared to emmetropes. Therefore, the potential for better acuity vision exists in human myopes. Reports of reduced best-corrected central visual acuity in myopes compared to emmetropes cannot be explained by retinal stretching during myopic progression.
This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.
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