Retinal Nerve Fiber Layer (RNFL) and Visual Function Loss: The Tipping Point

Abstract

Purpose: : It is believed that RNFL loss precedes visual function loss in glaucoma. The purpose of the present study is to determine the RNFL thickness measured using spectral domain optical coherence tomography (SD-OCT) at which visual field damage accelerates.

Methods: : Visual fields (VF) exams and SD-OCT optic disc 200x200 cube scans (Humphrey field analyzer and Cirrus HD-OCT respectively; Carl Zeiss Meditec, Dublin, CA) were obtained in both eyes in 10 healthy and 29 glaucoma subjects. Comparison of average and quadrant RNFL thicknesses with VF mean deviation (MD) showed a plateau of MD at high RNFL thicknesses and a sharp decrease at thin RNFL thicknesses. A "broken stick" analysis model was fitted to the data while ignoring clustering to provide an initial estimate of the tipping point. The change point is a knot between the two straight-line segments. The method of maximum likelihood was used to estimate the mixed effect model parameters including the knot location.

Results: : The inflection point for VF loss was 72.34µm, 23% below the healthy population mean of 92.60µm, which was an RNFL thickness equal to a loss of approximately 50% of the OCT dynamic range (Figure). Above the tipping point, MD is lost at a rate of 0.076 (95% confidence interval -0.013-0.165) dB/µm RNFL thickness lost. Below the tipping point, MD is lost at a rate of 0.568 (0.453-0.683) dB/µm. Inferior, superior, nasal, and temporal RNFL tipping point thicknesses were 88.36, 67.75, 53.96, and 57.10µm respectively.

Conclusions: : Substantial structural loss occurs prior to the appearance of functional VF loss. At mean RNFL thicknesses above 72µm, RNFL assessment is more sensitive to disease progression.

Keywords: imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound) • visual fields • nerve fiber layer