Estimation of normal and glaucomatous optic nerve morphology from perfusion

To estimate the optic nerve head morphology from its haemoglobin (Hb) distribution.

The optic disc of 189 normal eyes and 292 with confirmed and suspected glaucoma were analysed with Spectralis-OCT and the Laguna ONhE application using the Topcon NW400 fundus camera. Topographic Hb values were correlated with OCT tissue thicknesses from Bruch's membrane. The neuroretinal rim volume (RV), the cup volume (CV) and their relation to the globin distribution function (GDF) index of Laguna ONhE were analysed. The results were applied to 1 163 241 optic nerve images obtained in a glaucoma screening setting. Differences in segmentation and the presence of vessels without local nutritional function are the main limitations of the comparison.

The correlation between local Hb density and the tissue thicknesses from Bruch's membrane was R=0.953 (p<0.0001). RV could be estimated from pixel-to-pixel Hb values with an R=0.650 (p<0.0001) and from six Laguna ONhE indices with an R=0.786 (p<0.0001). CV could be estimated with an R=0.762 (p<0.0001). RV had a mean value of 0.396 mm³ (SD=0.187) for positive GDF values and usually below 0.210 mm³ for negative GDF. The distribution of RV values in the screening series was congruent with that found in the prospective series, with a higher value in large nerves.

The volume and shape of the optic nerve tissue are closely related to its perfusion and can be deduced from it. The relationship between the RV and GDF is curvilinear and suggests that perfusion measurement may bring the diagnosis forward in earlier stages.