Sectoral First Peak Fractal Analysis of Optical Coherence Tomography Angiography in Glaucomatous Eyes.

Abstract

Purpose : To assess whether microvascular dropout as measured by fractal self-similarity breakdown of optical coherence tomography angiography (OCTA) occurs in a sectoral fashion in eyes with primary open angle glaucoma (POAG) compared to control patients.

Methods : A retrospective study using OCTA images obtained on 40 eyes with POAG, and 14 control eyes. OCTA images with peripapillary scans of 4.5mm x 4.5mm diameters were obtained using RTVue XR Avanti (Optovue Inc., Fremont, CA, USA), and standardized and binarized using ImageJ (National Institutes of Health, Bethesda, Maryland, USA). Fractal dimension by means of box-counting algorithm using box sizes with increasing exponential factor of two with grid algorithm on Fractalyse (ThéMA, Besançon Cedex, France) was plotted against linear box dimension and first local peak (FLP) representing smallest box size resolution before breakdown of self-similarity was recorded. The ratio of superior to inferior (SI Ratio) of FLP was the main outcome measure.

Results : There is significant difference between control and POAG eyes (p = 0.01), with POAG having larger superior to inferior (SI Ratio) of FLP (1.40 +/- 0.93) compared to control (1.00 +/- 0.0). The increased SI FLP ratio of the POAG eyes reflects a preferential loss of self-similarity in the superior quadrant (i.e. higher FLP values) rather than increased self-similarity in the inferior quadrant (FLP inferior = 8.0 +/- 0 and FLP superior = 11.2 +/- 7.4 in POAG, FLP inferior = 8.0 +/- 0 and FLP superior = 8.0 +/- 0 in controls).

Conclusions : The measurement of box size of the first local maximum of fractal dimension as a function of increasing box size represents the smallest box size resolution prior to loss of uniformity of the vascular pattern’s fractal dimension. While this loss of uniformity may be related to resolution in normal eyes, the preferential loss of microvascular complexity inferior to the optic disc in POAG eyes suggests asymmetric small vessel dropout in the POAG. This accords well with the asymmetric loss demonstrated in a prior OCTA study. Our study was not designed to determine whether microvascular loss occurred as a consequence or as a cause of axonal loss. We propose FLP SI Ratio as a useful measure of POAG-associated microvascular dropout.

This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.