Abstract

SO.16.05

Andrea Wilhelm, Thomas Meigen, Franz Grehn
Universitäts-Augenklinik Würzburg

Objective
Various instruments are in use for quantification and follow-up of nerve fiber layer thickness in clinical routine of glaucoma or ocular hypertensive patients. In the present investigation, the agreement between measurements of laser polarimetry and optical coherence tomography were investigated.
Methods
In 31 patients the retinal nerve fiber layer around the optic disc was measured with the Spectral Domain OCT (Cirrus – OCT, Zeiss) and the GDx (GDx ecc = enhanced corneal compensation, Zeiss) at the same time. The curves were scanned and processed using image software (GIMPPortable) as the software of the instruments does not provide numerical values of nerve fiber layer thickness at individual locations. These values were transferred into numerical nerve fiber layer thickness values (in µm) that formed the basis for comparison. The relative coefficient of variation RCV =  was used to quantify the deviation between the two measurements.
Results
(1) The GDx ecc values of nerve fiber layer thickness were systematically smaller than the SD-OCT values (RCV=31,3% ± 17,6%, n=2170). (2) The regression analysis yielded smaller GDx values (by 1,8 on average) than OCT values. (3) The Bland-Altmann analysis showed that the deviation increased with increasing nerve fiber layer thickness, even if the GDx values were scaled according to the systematic difference. In this case the resulting RCV values were 16,9% ± 12,6%. (4) By using an exponential regression analysis the accordance could only be slightly improved (RCV=15,0% ± 10.3%).
Conclusions
(1) A direct comparison of GDx- and OCT-measurements of nerve fiber layer thickness resulted in significant differences. (2) Differences between the two measurement methods are smaller in thinner nerve fiber layers, as compared to thicker nerve fibers layers. (3) Even when compensating for the systematic difference, a mean deviation of 2*RCV= 30% was found. (4) When taking OCT as a reference measurement, the discrimination with the GDx instrument between healthy and glaucomatous nerve fiber layers is hampered by underestimation of thick nerve fiber layers in the early phase of the disease.

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