The Relationship between Visual Field Index and Estimated Number of Retinal Ganglion Cells in Glaucoma

Amir H Marvasti,Christopher A Girkin,Felipe A Medeiros,Robert N Weinreb,Andrew J Tatham,Linda M Zangwill,Jeffrey M Liebmann,Richard Libby

doi:10.1371/journal.pone.0076590

Abstract

PurposeTo evaluate the relationship between visual field index (VFI) and the estimated number of retinal ganglion cells (RGCs) in glaucoma.MethodsA multicenter study of 1,245 healthy, glaucomatous and suspected glaucomatous eyes of 1,245 subjects recruited from the Diagnostic Innovations in Glaucoma Study (DIGS) and African Descent and Glaucoma Evaluation Study (ADAGES). All eyes underwent standard automated perimetry (SAP) and time-domain optical coherence tomography (TD-OCT). Estimates of RGC count and percentage of RGCs remaining, compared to age-matched healthy eyes, were calculated from TD-OCT using a previously described formula. Smoothing spline curves were fitted to examine the relationship between VFI and the percent remaining RGCs. The first derivative (i.e., slopes) of these curves was used to explore the relationship between changes in these measures.ResultsThe relationships between the VFI and both estimated RGC counts and the percent remaining RGCs were nonlinear. A unit number of VFI loss corresponded to substantially greater loss of estimated RGCs and estimated percentage of RGCs remaining in early compared to late disease.ConclusionsThe relationship between VFI and estimated RGC counts is nonlinear and the index substantially underestimates the amount of neural loss early in the disease. Disease severity should be taken into account when interpreting rates of VFI change over time.

Highlights

Glaucoma is a progressive optic neuropathy, characterized by loss of retinal ganglion cells (RGCs), and associated morphological changes to the optic nerve and retinal nerve fiber layer (RNFL). [1] Structural damage is usually accompanied by decrease in visual function, which can lead to blindness
Change in visual field index (VFI) is used to estimate the rate of progression using so-called ‘‘trend-based’’ analysis. [4,5,6] A trend-based analysis using a linear regression analysis of VFI values over time is calculated as part of the Guided Progression Analysis (GPA), from which the patient’s rate of progression, in percent VFI loss per year, can be determined [8]
[9] Estimates derived using these formulas have shown good correlation to histologic RGC counts [9] and have demonstrated good ability for staging disease. [10,11,12] The aim of the present study was to evaluate the relationship between VFI and estimated numbers of RGCs obtained from optical coherence tomography (OCT) data in the same subjects

Summary

Introduction

Glaucoma is a progressive optic neuropathy, characterized by loss of retinal ganglion cells (RGCs), and associated morphological changes to the optic nerve and retinal nerve fiber layer (RNFL). [1] Structural damage is usually accompanied by decrease in visual function, which can lead to blindness. A major challenge in the management of glaucoma is how to best determine severity of disease and estimate the rate of progression. [7] the VFI of an eye with a completely normal visual field is 100% and the VFI of a perimetrically blind eye is 0%. [4,5,6] A trend-based analysis using a linear regression analysis of VFI values over time is calculated as part of the Guided Progression Analysis (GPA), from which the patient’s rate of progression, in percent VFI loss per year, can be determined [8]. It is claimed that by using this weighting procedure, the VFI would more closely reflect underlying loss of RGCs [7]

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