Measurement Floors and Dynamic Ranges of OCT and OCT Angiography in Glaucoma

Abstract

Purpose: To determine if OCT angiography (OCTA)-derived vessel density measurements can extend the available dynamic range for detecting glaucoma compared with spectral-domain (SD) OCT-derived thickness measurements.

Design: Observational, cross-sectional study.

Participants: A total of 509 eyes from 38 healthy participants, 63 glaucoma suspects, and 193 glaucoma patients enrolled in the Diagnostic Innovations in Glaucoma Study.

Methods: Relative vessel density and tissue thickness measurement floors of perifoveal vessel density (pfVD), circumpapillary capillary density (cpCD), circumpapillary retinal nerve fiber (cpRNFL) thickness, ganglion cell complex (GCC) thickness, and visual field (VF) mean deviation (MD) were investigated and compared with a previously reported linear change point model (CPM) and locally weighted scatterplot smoothing curves.

Main outcome measures: Estimated vessel density and tissue thickness measurement floors and corresponding dynamic ranges.

Results: Visual field MD ranged from -30.1 to 2.8 decibels (dB). No measurement floor was found for pfVD, which continued to decrease constantly until very advanced disease. A true floor (i.e., slope of approximately 0 after observed CPM change point) was detected for cpRNFL thickness only. The post-CPM estimated floors were 49.5±2.6 μm for cpRNFL thickness, 70.7±1.0 μm for GCC thickness, and 31.2±1.1% for cpCD. Perifoveal vessel density reached the post-CPM estimated floor later in the disease (VF MD, -25.8±3.8 dB) than cpCD (VF MD, -19.3±2.4 dB), cpRNFL thickness (VF MD, -17.5±3.3 dB), and GCC thickness (VF MD, -13.9±1.8 dB; P < 0.001). The number of available measurement steps from normal values to the CPM estimated floor was greatest for cpRNFL thickness (8.9), followed by GCC thickness (7.4), cpCD (4.5), and pfVD (3.8).

Conclusions: In late-stage glaucoma, particularly when VF MD is worse than -14 dB, OCTA-measured pfVD is a promising tool for monitoring progression because it does not have a detectable measurement floor. However, the number of steps within the dynamic range of a parameter also needs to be considered. Although thickness parameters reached the floor earlier than OCTA-measured pfVD, there are more such steps with thickness than OCTA parameters.

Figure 1.

Plot of change point analysis for vessel density-function relationship between circumpapillary vessel density (cpVD), perifoveal vessel density (pfVD) and visual field mean deviation (MD) (A, B) and thickness-function relationship between the circumpapillary retinal nerve fiber layer (cpRNFL) thickness, ganglion cell complex (GCC) thickness, and visual field MD (C, D). The plain thick line represents the course of the predicted vessel density or thickness reduction as a function of visual field loss, the dotted lines are the upper and lower 95% confidence intervals and the solid blue line indicates the locally weighted scatterplot smoothing curve fitting the data. R2 represents the coefficient of determination. AIC: Akaike’s information criterion.