Purpose: Age-related losses in retinal nerve fiber layer (RNFL) thickness have been assumed to be the result of an age-dependent reduction of retinal ganglion cells (RGCs), but the published rates differ: age-related losses of RGCs of approximately 0.6%/year compared to 0.2%/year for thinning of the RNFL. An analysis of normative data for standard automated perimetry (SAP) sensitivities and optical coherence tomography (OCT) measures of RNFL thickness showed that the apparent disagreement in age-dependent losses of RGCs and axons in the RNFL can be reconciled by an age-dependent decrease in the proportion of the RNFL thickness that is composed of axons. The purpose of the present study was to determine whether the mechanisms of age-related losses that were suggested by the normative data can be confirmed with data from healthy, normal eyes.
Methods: Data were obtained from visual fields (normal results in a Glaucoma Hemifield Test [GHT] on standard automated perimetry [SAP] 24-2 fields) and RNFL thickness measurements (standard OCT scan) of 55 patients (age range, 18-80 years; mean, 44.5 +/- 17.3). The SAP measures of visual sensitivity and OCT measures of RNFL thickness for one eye of each patient were used to estimate neuron counts by each procedure.
Results: The age-related thinning of RNFL was 0.27%/year when a constant axon density was used to derive axon counts from RNFL thickness, compared with 0.50%/year for the age-related loss of RGCs from SAP. In agreement with the model developed with normative clinical data, concordance between losses of axons and soma was achieved by an age-dependent reduction of 0.46%/year in the density of axons in the RNFL.
Conclusions: The results suggest that the proportion of RNFL that is composed of RGC axons is not constant with age; rather, the proportion of the total thickness from non-neuronal tissue increases with age. If a similar compensation occurs in the RNFL thickness with axon loss from glaucoma, then a stage-dependent correction to translate OCT measurements to neuronal components is needed, in addition to the age-dependent correction.
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