Re-evaluation of using cup-to-disc ratio to identify structural and functional damage in glaucoma patients and suspects

Abstract

Purpose : Controversy exists regarding the continued use of vertical cup-to-disc ratio (CDR) as a means of identifying early glaucomatous changes. We performed a retrospective cross-sectional study to investigate the association between CDR and clinically relevant structural and functional outcomes related to glaucoma.

Methods : Among all Mass. Eye and Ear Glaucoma Service patients between 2011 and 2021, each eye’s most recent measurement of clinician-estimated CDR from electronic medical records (EMR) and associated Cirrus HD-optical coherence tomography (OCT) volume scan derived CDR, average circumpapillary retinal nerve layer thickness (RNFLT), and Humphrey 24-2 visual field mean deviation (MD) and visual field index (VFI) were selected if reliable according to device manufacturer’s criteria. To check for floor or ceiling effects, sigmoidal regression models (Fig.1) of CDR were fitted to MD, VFI and RNFLT. Pairwise Bayesian model comparisons were calculated using logarithms of the Bayes Factor (logBF), with logBF>5 indicating very strong evidence for one model versus the other.

Results : 10,420 patients and a total of 17,509 eyes fulfilled the selection criteria. There was strong evidence for sigmoidal regression compared to linear regression (logBF>650) for both clinically estimated and OCT measured CDR as regressors and all evaluated outcomes. Model fits (Fig.2) demonstrated ceilings for functional outcomes below the values denoting normal visual fields (MD=0 and VFI=100, verified by comparisons with models fixed to these levels; all logBF>250). Plateaus for clinician estimated CDR towards 0 at MD=-2.34dB and VFI=94.8 indicate that CDR cannot discriminate early stage functional damage. Clinician estimated CDR had a stronger association with functional outcomes (all logBF>347) when compared to OCT measured CDR, however, OCT measured CDR had a stronger association with RNFLT (logBF=243).

Conclusions : CDR, one of the most frequently recorded parameters for glaucoma diagnostics as per EMR and billing requirements, is subject to considerable ceiling effects for structural and functional glaucoma-related outcomes and therefore poor at discriminating early glaucomatous damage (e.g. MD>2.3dB or VFI>94.8). Therefore, CDR values should be interpreted with caution, particularly in screening settings for early disease detection.

This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.

 

Figure 1: Function

 

Figure 2: CDR vs MD, VFI and RFNLT