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Andrew Tatham, Robert Weinreb, Linda Zangwill, Jeffrey Liebmann, Christopher Girkin, Felipe Medeiros; The relationship between cup/disc ratio and estimated number of retinal ganglion cells. Invest. Ophthalmol. Vis. Sci. 2013;54(15):5915.
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To investigate the relationship between cup/disc ratio (CDR) and estimates of retinal ganglion cell (RGC) number derived from spectral domain optical coherence tomography (SD-OCT) and standard automated perimetry (SAP).
A cross-sectional study of 336 eyes of 209 subjects from the African Descent and Glaucoma Evaluation Study (ADAGES) and the Diagnostic Innovations in Glaucoma Study (DIGS). The study included 156 healthy eyes, 53 glaucoma suspects and 127 eyes with glaucoma tested with SAP, Cirrus SD-OCT and stereoscopic optic disc photography within 6 months. CDR was determined from stereoscopic photographs by two or more experienced masked graders. Severity of glaucoma was defined using the Hodapp-Anderson-Parrish classification. The number of RGCs in each eye was estimated using a previously published model that combines estimates of RGC number from SAP sensitivity thresholds and SD-OCT retinal nerve fiber layer thickness measurements.
The mean estimated RGC count was 1,062,105 in healthy eyes, 828,062 in glaucoma suspects, 778,888 in early glaucoma, 483,176 in moderate glaucoma, and 228,229 in advanced glaucoma. Healthy eyes had a mean vertical CDR of 0.45 ± 0.15 compared to 0.80 ± 0.16 in glaucomatous eyes. The relationship between estimated RGC counts and vertical CDR was best represented using a 3rd degree polynomial regression model, including age and optic disc area, which accounted for 83.6% of the variation in estimated RGC counts (Figure 1). The non-linear relationship between the RGC estimates and CDRs indicated that eyes with a large CDR would require loss of large numbers of RGCs for a small increase in CDR.
The relationship between estimated RGC counts and CDR suggests that assessment of change in CDR is an insensitive method for evaluation of progressive neural losses in glaucoma. Even relatively small changes in CDR may be associated with large losses of RGCs, especially in eyes with large CDRs.
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