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G. Wollstein, R. A. Bilonick, L. Kagemann, H. Ishikawa, M. L. Gabriele, R. J. Noecker, J. S. Schuman; Retinal Nerve Fiber Layer (RNFL) and Visual Function Loss: The Tipping Point. Invest. Ophthalmol. Vis. Sci. 2009;50(13):3511.
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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.
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.
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.
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.
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