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Ann E Elsner, Joel A Papay, Kirby D. Johnston, Brett King, Samantha L. Polster, Toco Yuen Ping Chui, Stephen A Burns; Aging, age-related macular degeneration, and the neural economy hypothesis. Invest. Ophthalmol. Vis. Sci. 2016;57(12):No Pagination Specified. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
To investigate mechanisms that may help cones to survive without retinal pigment epithelium (RPE), as in atrophy and outer retinal tubulation in age-related macular degeneration (AMD) (King, ARVO, 2014). We hypothesize that several morphological and physiological changes to cones and their support cells allow more economical existence, termed the neural economy hypothesis. Cone density and photoreceptor length are hypothesized to decrease in aging or AMD, allowing cone longevity.
To measure the outer, inner, and total photoreceptor lengths at the fovea and +/- 4.9 deg horizontally in equal steps, we used SD-OCT (Spectralis) and new custom software (Matlab). Mean age was 27.2 yr for 8 younger subjects and 56.2 yr for 8 older subjects with healthy retinas. The older group subjects were known to have thicker outer nuclear+Henle layer, but lower cone densities on average, from an adaptive optics scanning laser ophthalmoscope (AOSLO). Four additional subjects with AMD and atrophy were measured using AOSLO for cone density, and SD-OCT and GDx for photoreceptor length, foveal position, and RPE status.
The ratio of photoreceptor length for older:younger subject groups was significantly less than 1 if averaged across all 15 locations for the inner, outer, and total (inner+outer) length measurements ( p = .0015, .037, and .0015, respectively). Changes across the retina were gradual. For foveal measures of outer segment length, which included only cones, 5 of 8 older subjects were less than the confidence limits for younger subjects. The aging effects were small: foveal cone outer segment length was 63.5 vs. 60.5 microns for younger vs. older groups. There was considerable variability in the density of cones atrophy in the patients with AMD, but all had at least on location with > 10,000 cones/mm. Cone length, when the retinal layers were intact enough to permit measurements, was also variable.
Photoreceptor length and cone density decrease with aging, consistent with the neural economy hypothesis, but the variability in AMD is much greater. The decrease in retinal layer thickness with increasing aging is not found for all layers, as these subjects were previously found to have thicker ONL+Henle layers. Structural or functional changes may help the survival of cones without RPE in AMD.
This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.
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