The effect of moderate myopia on rod and cone photoreceptor densities in human eyes using AO-SLO imaging

Abstract

Purpose : It is known from histology and adaptive optics (AO)-scanning laser ophthalmoscopy (SLO) imaging that cone photoreceptor density tends to be lower in myopic eyes compared to emmetropes. The aim of this study was to use AO-SLO imaging to further examine the role of myopia on photoreceptor packing by examining variations in both rod and cone densities at retinal locations ranging from the fovea to mid-periphery (30°) in healthy human subjects.

Methods : The right eyes of 6 heathy human subjects age 22–27 with spherical refractive error ranging from +0.25 D to -4.25 D were imaged using AO-SLO at retinal locations ranging from 30° temporal (TR) to 30° nasal (NR) of the fovea. Subjects were grouped as either emmetropic (n=3, spherical equivalent (SE) = +0.25 D to -0.75 D) or myopic (n=3, SE = -2.375 D to -4.25 D). The AO system used a Shack-Hartmann wavefront sensor and a 97 actuator deformable mirror to measure and correct for aberrations, with 680 nm light used for both imaging and AO correction. Field of view was 0.7° x 1.0°. Frames were registered to remove eye motion and averaged to increase signal to noise ratio. At each location, cone and rod densities were measured.

Results :
Rod density was lower for myopes at all locations except 3° TR and 3° NR (where density was higher by 3.8% and 2.2% respectively). The greatest differences were observed at 15° TR, 5° NR, and 10° NR where rod densities were 27.5%, 27.1%, and 27.2% lower for myopes (85,500 vs 118,000 rods/mm²; 57,700 vs 79,100 rods/mm²; and 84,600 vs 116,100 rods/mm²). Differences were statistically significant at 15° TR, 10° NR, 25° NR, and 30 ° NR (p<0.1). Averaged over all locations, rod density was 16.4% lower for myopes (standard deviation 10.4%). For cones, density was lower for myopes at all locations except 3°NR and 20°NR (where cone densities were higher by 1.9% and 0.2%, respectively). The greatest difference was observed at 15° TR, where cone density was 29.8% lower for myopes (5,300 vs 7,560 cones/mm²). Differences were statistically significant at 10° TR, 15° TR, 5° NR, and 25°NR (p<0.1). Averaged over all locations, cone density was 10.8% lower for myopes (standard deviation 8.7%).

Conclusions : The ability to accurately measure and monitor changes in photoreceptor density may lead to better understanding of the progression of myopia in adults and children, and also has implications for the study of retinal diseases.

This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.