Relating retinal nerve fiber layer thickness and functional estimates of ganglion cell sampling density in healthy eyes and in early glaucoma.

PURPOSE To investigate the relationship between retinal nerve fiber layer (RNFL) thickness and peripheral grating resolution acuity (PGRA) as well as differential light sensitivity (DLS) in healthy subjects and patients with early glaucoma. The agreement between estimates of retinal ganglion cell (GC) density from each functional test is explored. METHODS PGRA was measured in 24 patients with early glaucoma (mean deviation [MD] > -8 dB) and 26 healthy subjects using achromatic Gabor stimuli in 4 diagonal visual field locations at 10° eccentricity. DLS for a Goldmann size III equivalent was obtained from individual spatial summation functions and expressed in Humphrey Field Analyzer-equivalent decibel values. RNFL thickness was measured around the optic nerve head using Zeiss Stratus optical coherence tomography and related to functional measures using a retinotopic map. Functional GC density was estimated using structure/function models for both tests. Passing-Bablok regression was used to investigate the structure/function relationships. RESULTS A positive and statistically significant association was found between PGRA and RNFL thickness, and separately between DLS and RNFL thickness, for combined glaucoma and healthy data (both P < 0.05). The slope of the structure/function association in healthy subjects was not significantly different to that in glaucoma patients using either functional measure (both P > 0.05). Agreement between estimates of GC density from psychophysical data was moderate. CONCLUSIONS The relationship between PGRA and RNFL thickness is at least as great in magnitude as that between DLS and RNFL thickness; a significant structure/function association is also observed in healthy subjects alone.