Abstract
Purpose::
Axial elongation of the myopic eye might lead to stretching of the retina, thereby reducing the sampling density of retinal neurons. We are testing the prediction that neural sampling density is reduced in myopic eyes by comparing cone photoreceptors in emmetropic eye and highly myopic eyes.
Methods::
A high resolution adaptive optics scanning laser ophthalmoscope was used to image the cones of human eyes. One emmetrope and three high myopes were tested. Due to poor images quality in two of the myopic eyes, only one subject’s data was selected for data analysis. Scans are made along the vertical meridian by using programmable displacements of a set of scanning mirrors while the subject fixated a single fixation point. Subject’s eye size was measured using A-scan ultrasonography. Cone packing densities are computed using a custom Matlab program (MathWorks, Natick, MA) (Li and Roorda, 2006. In press). Cone densities at the corresponding retinal areas were compared between subjects after correcting the retinal magnification factor induced by different axial lengths. To date we have complete measurement sets in an emmetropic eye and a myopic eye (-7.50D). Montages of the entire cone mosaic were created from 0.4mm to 2.3mm (1.4 degrees to 8 degrees) retinal eccentricity and 1.0mm to 2.4mm (3 degrees to 7 degrees) retinal eccentricity in the emmetropic eye and the myopic eye respectively.
Results::
Cone density declined from 21,700cells/mm2 to 14,000cells/mm2 along the retinal eccentricity of 0.5 mm to 2.0mm in the emmetropic eye. Results obtained at corresponding retinal areas showed that cone density was lower in myopic eye than that of emmetropic eye. From 1mm to 2mm retinal eccentricity, cone density decreased from 18,500cells/mm2 to 14,000cells/mm2 with an average of 17,500cells/mm2 in the emmetropic eye and decreased from 9,100cells/mm2 to 8,200cells/mm2 with an average of 8,700cells/mm2 in the myopic eye. The reduction rates of cone density in emmetropic eye and myopic eye were 22.5% and 10.2% respectively.
Conclusions::
As predicted by retinal stretching, our result indicates that cone density is lower in a highly myopic eye than in an emmetropic eye, which implies that cones are more widely spaced in a highly myopic eye. The cone density data for the emmetropic eye are consistent with a previous anatomical study on human photoreceptor topography (Curcio et al., 1990). From the current experiments testing even higher myopes will be difficult in the current system, but the ability to test subjects between +2.00D and -7.50 D will allows us to test models of the how cone packing varies with the degree of myopia.
Keywords: myopia • retina