Determining the cone:ganglion cell ratio in glaucoma using psychophysical measures of retinal ganglion cell density and in vivo imaging of the retinal cone mosaic by a modified Heidelberg Retina Angiograph (HRA2)

Abstract

 

Purpose

 

To determine the parafoveal cone:ganglion cell (GC) density ratio in glaucoma patients compared to age-matched normal controls using a modified Heidelberg Retina Angiograph (HRA2) to image cones and localized measurements of peripheral grating resolution acuity (PGRA) to estimate GC density.

 

Methods

 

A group of glaucoma patients with early to moderate visual field loss and healthy age-matched controls underwent in vivo cone imaging at 4 retinal locations at 6° eccentricity using a modified HRA2 (scan angle of 3°). Retinal cones were identified and counted using custom software. PGRA measurement was performed at the same retinal locations and converted to GC density. PGRA values were converted to minimum angle of resolution (MAR) and GC density was calculated using the equation MAR = 0.93/√D. A conversion factor [1] was used to calculate the number of ganglion cells per square millimetre of the retina.

 

Results

 

The mean cone count within the 3° window was 4545 (6678 cells/mm², range 6235-7287 cells/mm²) for glaucoma patients and 4808 (7064 cells/mm², range 6601-8007 cells/mm²) for healthy controls. Mean ganglion cell density was 3110 cells/mm² (range 420-5634 cells/mm²) for glaucoma patients compared to 6139 cells/mm² (range 4670-8775 cells/mm²) for healthy controls. This gives a mean cone:GC ratio of 2.15:1 for glaucoma patients and 1.15:1 for healthy controls. PGRA, but not cone density, was reduced in the scan locations of the maximum visual field loss in glaucoma patients (mean cone:GC ratio 4.59:1).

 

Conclusions

 

The number of cones does not differ in glaucoma patients compared to age-matched normal controls, despite large differences in ganglion cell density. Since large inter-individual differences in GC density exist even in normal subjects [2], cone:GC ratio may prove to be a useful indicator of GC loss from baseline in glaucoma.<br /> <br /> References:<br /> 1. Drasdo N, Fowler CW. Non-linear projection of the retinal image in a wide-angle schematic eye. Br J Ophthalmol 1974;58:709-714.<br /> 2. Curcio CA, Allen KA. Topography of ganglion cells in human retina. J Comp Neurol 1990;300:5-25.  

 

Figure shows the location of the cone images with a retinal eccentricity of approximately 6 degrees.

 

 

Figure is an example of an image of the retinal cone mosaic of a glaucoma patient using the modified HRA2.