Residual and Dynamic Range of Retinal Nerve Fiber Layer Thickness in Glaucoma: Comparison of Three OCT Platforms

Abstract

Purpose: To estimate visual field (VF) sensitivity at which retinal nerve fiber layer (RNFL) thinning reaches the measurement floor and at which RNFL stops thinning (change points), the dynamic range of RNFL thickness, and the number of steps from normal to RNFL floor among three optical coherence tomography (OCT) devices.

Methods: Glaucomatous patients (n = 58) and healthy subjects (n = 55-60) prospectively underwent VF testing and RNFL thickness measurement with Cirrus, Spectralis, and RTVue. Change points and corresponding RNFL thicknesses were estimated with simple linear regression (SLR) and Bayesian change point (BCP) analyses. The dynamic range and number of steps to RNFL floor were determined.

Results: The average VF change points and corresponding residual thickness at the time RNFL stopped thinning were -22.2 dB and 57.0 μm (Cirrus), -25.3 dB and 49.2 μm (Spectralis), and -24.6 dB and 64.7 μm (RTVue). The RNFL dynamic ranges derived from SLR values were wider on Spectralis (52.6 μm) than on Cirrus (35.4 μm) and RTVue (35.5 μm); the corresponding number of steps to reach the RNFL floor were 9.0 on Cirrus, 10.6 on Spectralis, and 8.3 on RTVue.

Conclusions: The relative VF sensitivity at which average RNFL thickness reaches the measurement floor, the residual layer thickness, and RNFL dynamic measurement range differ among the three devices. However, the number of steps from normal to the RNFL thickness floor is comparable.

Figure 1

Bayesian change point plots (top row) and simple linear regression plots (bottom row) of the structure–function relationship of average RNFL thickness measured with Cirrus (A), Spectralis (B), and RTVue (C) with SAP average relative VF sensitivity. On simple linear regression plots, the meeting point between the two regression lines provides the coordinates of the amount of VF loss and corresponding residual layer thickness.

Figure 2

Staircase representation of the number of steps from normal average RNFL thickness to average RNFL floor on Cirrus, Spectralis, and RTVue. The upper and lower dashed lines indicate the values of normal RNFL thickness and floor, respectively; the space between them is the dynamic range. The height of the steps on each plot represents the TRTV. The patterned area represents the residual layer thickness. The straight line connecting the stairs in the Cirrus plot indicates that RNFL actually thins continuously rather than in steps.