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Adam M. Dubis, Sean O. Hansen, Robert F. Cooper, Benjamin R. Hansen, Joseph Carroll; Assessing the Relationship Between Cone Density and Foveal Morphology. Invest. Ophthalmol. Vis. Sci. 2012;53(14):5672.
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Defining features of the human fovea include an excavation of inner retinal layers (forming the pit), an avascular zone, increased cone density, and an absence of rod photoreceptors. Previous data has demonstrated a correlation between the size of the foveal avascular zone and the volume of the foveal pit. As foveal cone density has been reported to be highly variable, here we sought to examine how cone density is related to these gross anatomical specializations of the fovea.
Thirteen subjects had their right eye dilated with 1% Tropicamide and 2.5% Phenylephrine. They were imaged with Cirrus HD-OCT and foveal pit metrics for depth, diameter, slope and volume were extracted using previously published methods. Each subjects’ foveal avascular zone (FAZ) was imaged using either a custom built adaptive optics scanning light ophthalmoscope (AOSLO) with focus set at the inner retina or Retinal Function Imager (RFI). From these images, the FAZ diameter and area were calculated. Foveal and parafoveal photoreceptor mosaics were imaged using the AOSLO with focus set at the photoreceptor layer. Cone photoreceptor density was assessed over a ~3 deg field of view using semi-automated cone identification software in Matlab. After photoreceptor identification, density was calculated across the image using an 800 µm2 search window.
In our subjects, axial length ranged from 22.75 to 27.45 mm, FAZ area ranged from 0.072 to 0.802 mm2, foveal volume ranged from 0.022 to 0.133 mm3, and peak foveal cone density ranged from 169,000 to 221,000 cones/mm2. In this population, FAZ and foveal volume were positively correlated (R2 = 0.472, p =0.0094). Axial length was not correlated with FAZ size or foveal pit morphology. Peak cone density was negatively correlated with axial length (R2 = 0.572, p = 0.003); positively correlated with foveal pit volume (R2 =0.381, p=0.025) and FAZ area (R2 =0.449, p=0.012). In other words, subjects with larger FAZ and foveal pits had higher cone density.
While previous reports have demonstrated anatomical links between the FAZ and foveal pit morphology, our data provide an important additional piece of data (cone density). Developing an understanding of how all of the anatomical specializations of the fovea are related is an important first step in refining existing models of foveal development.
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