Distribution of parafoveal and peripheral cones in emmetropic and myopic subjects

Abstract

Purpose : We measured in vivo cone photoreceptors up to 24° of eccentricity along the horizontal meridian of human healthy retina. We also investigated the impact on cone densities of axial eye length occurring during evolutive myopia.

Methods : Using a flood illumination device (rtx1^TM, imagine Eyes) coupled with an adaptive optics system, sixty right healthy retinas were imaged along the horizontal (i.e. nasal and temporal) meridian over a 48° field (i.e. from 3° to 24° each 3°). Then, cones were manually detected within 80 x 80 pixel regions of interest. Cone density and packing geometry (i.e. number of neighbors) were calculated (AOdetect^TM, imagine Eyes). Subjects were divided into two groups : a group of 36 emmetropic (i.e. refractive error from -0.25 D to +0.50 D) subjects and a group of 24 myopic subjects (i.e. -0.50 D to -8 D). Myopic subjects had a history of evolutive myopia.

Results : Cone density decreased with eccentricity in both semi-meridians. The loss of cone photoreceptors occurred mainly in the first nine degree. The difference of cone density between the nasal and temporal semi-meridian increased with eccentricity from 0.1% at 3° to 25% at 24°. The emmetropic group exhibited higher cone density than the myopic group when expressed cone density in metric unit (i.e. cone/mm²). The difference ranged from 7% to 18% (i.e. 13% in average) and was statistically significant excepted near the optic nerve cup (i.e., 12° and 21°) where no difference was observed. If expressed in visual unit (i.e. cones/deg²), there was no significant difference between the two groups whatever the eccentricity.
The number of cells with exactly six neighbors (i.e. hexagonal packing arrangement) decreased from 50% to 35% from 3° to 24°.

Conclusions : The difference of cone density (i.e. metric unit) between emmetropes and myopes supports the hypothesis that the retina is stretched with the eyeball elongation. The retina seems to have a global expansion (i.e. quite comparable difference within the central 48-degree of the horizontal meridian) except near the optic nerve.

This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.

 

Cone density as a function of eccentricity (°). Orange and green lines correspond respectively to the emmetropic and myopic group. Left column - cone density expressed in metric unit. Right column - cone density expressed in viual unit. Whiskers correspond to standard errors.