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R. S. Harwerth, J. L. Wheat, N. V. Rangaswamy; Age-Related Losses of Retinal Ganglion Cells and Axons. Invest. Ophthalmol. Vis. Sci. 2007;48(13):492.
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Age-related losseses in retinal nerve fiber layer thickness (RNFLT) areare assumed to be the result of an age dependent reduction of retinal ganglion cells (RGCs), but the rates are much different; published age-related losses in RGCs are about 0.65%/year, compared to 0.25%/year for thinning of RNFLT. To determine whether the neural bases for these aging effects are equivalent, the aging effects on estimates of RGCs from standard automated perimetry (SAP) and estimates of RGC axons by optical coherence tomography (OCT) were compared.
Preliminary data were obtained from visual fields (normal GHT on SAP 24-2 fields) and RNFLT (standard OCT scan) of 32 patients (ages 20 - 71 years, median = 46). The SAP measures of visual sensitivity and OCT measures of RNFLT were used to estimate neuron counts by each procedure.
The age-related thinning of RNFLT was 0.3%/year when a constant axon density was used to derive axon counts from RNFLT, compared to 0.6%/year for the age-related loss of RGCs from SAP. Concordance between losses of axons and soma required an age-dependent reduction of 0.46%/year in the density of axons in the RNFLT.
The results suggest that the proportion of RNFLT that is comprised of RGC axons is not constant across age, but rather a greater proportion of the total thickness is non-neural tissue for older patients and, thus, the dynamic range of RNFLT thinning from glaucoma is smaller. The difference in RNFLT changes from aging and glaucoma may reflect a difference in the site (soma vs. axon) of initial injury leading to cell death.
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