Relationship between ganglion cell layer thickness and estimated retinal ganglion cell counts in the glaucomatous macula

Abstract

Purpose: To investigate the relationship between macular ganglion cell-inner plexiform layer (mGCIPL) thickness and estimated macular retinal ganglion cell (RGC) counts in glaucoma.

Design: Observational cohort study.

Participants: Cross-sectional study of 77 healthy, 154 glaucoma suspect, and 159 glaucomatous eyes from the Diagnostic Innovations in Glaucoma Study.

Methods: All eyes underwent 24-2 standard automated perimetry (SAP) and optic nerve and macular imaging using high-definition optical coherence tomography (OCT). The total number of RGCs was estimated using a previously described model that uses SAP and OCT circumpapillary retinal nerve fiber layer (cpRNFL) measurements. The number of macular RGCs was estimated from the temporal cpRNFL and SAP test points within the central 10°.

Main outcome measures: The correlation between mGCIPL thickness and estimates of macular RGC counts.

Results: The average estimated macular RGC count in glaucomatous eyes was 306 010 ± 109 449 cells, which was significantly lower than the estimate of 520 678 ± 106 843 cells in healthy eyes (P < 0.001). Glaucomatous eyes had 41% fewer estimated macular RGCs than healthy eyes and suspects had 21% fewer estimated macular RGCs. There was strong correlation between estimated macular RGC counts and mGCIPL thickness (R(2) = 0.67; P < 0.001). Macular RGC counts performed better than average mGCIPL thickness in discriminating healthy and glaucomatous eyes with receiver operating characteristic curve areas of 0.873 and 0.775, respectively (P = 0.015).

Conclusions: The strong association between estimated macular RGC counts and mGCIPL thickness and the better diagnostic performance of the macular RGC counts compared with mGCIPL thickness provides further evidence that estimates of RGC number from cpRNFL thickness and SAP sensitivity can be used to assess neural losses in glaucoma.

Figure 1

Illustration of the 24-2 visual field map for the right eye showing test locations used for retinal ganglion cell estimates for total (black and grey points) and macular (16 black points corresponding to the central 10 degrees) estimates (A). Illustration of the 6 clock hours of the optical coherence tomography circumpapillary retinal nerve fiber layer thickness scan used for the structural component of the macular retinal ganglion cell number estimate (the black sectors in the figure) (B).

Figure 2

Boxplot illustrating the distribution of estimated number of macular retinal ganglion cells and total number of retinal ganglion cells in healthy, glaucoma suspects and glaucomatous eyes.

Figure 3

Scatterplots and ordinary least squares (OLS) regression lines showing the relationship between estimated number of macular RGCs and the macular ganglion cellinner plexiform layer (mGCIPL) thickness (A) and estimated total number of RGCs and mGCIPL thickness(B).

Figure 4

Receiver operating characteristic curves showing the ability of estimated number of macular retinal ganglion cells and the macular ganglion cell-inner plexiform layer (mGCIPL) thickness to differentiate glaucomatous and healthy eyes.

Figure 5

Example of a healthy left eye of a 66-year-old subject showing macular ganglion cell-inner plexiform layer (mGCIPL) thickness, deviation and thickness sectors maps (A), circumpapillary retinal nerve fiber layer (cpRNFL) thickness, deviation maps and RNFL thickness sector map (B), and standard automated perimetry (SAP) results (C). The average mGCIPL thickness was 76 µm and the estimated numbers of total and macular retinal ganglion cells (RGCs) were 941,711 and 448,087 cells, respectively.

Figure 6

Example of a glaucomatous left eye of 64-year-old subject showing macular ganglion cell-inner plexiform layer (mGCIPL) thickness, deviation and thickness sectors maps (A), circumpapillary retinal nerve fiber layer (cpRNFL) thickness, deviation maps and thickness sectors maps (B), and standard automated perimetry (SAP) results (C). The mean mGCIPL thickness was 67 µm and the estimated numbers of total and macular retinal ganglion cells were 558,922 and 282,981 cells, respectively.