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R. P. Tornow, F. Lauterwald, D. Bendschneider, F. K. Horn, C. Y. Mardin, R. Lämmer; Birefringence of the Peripapapillary Retinal Nerve Fibre Layer. Invest. Ophthalmol. Vis. Sci. 2010;51(13):4363.
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© ARVO (1962-2015); The Authors (2016-present)
To measure the birefringence of the peripapillary retinal nerve fiber layer (RNFL) in normal subjects and glaucoma patients using the results of scanning laser polarimetry (SLP) and spectral domain optical coherence tomography (SOCT). Using an research polarization sensitive OCT, it was shown that birefringence is reduced in glaucoma before the RNFL thickness is reduced [Götzinger et al ARVO 2009].
Birefringence B of the RNFL can be calculated from phase retardation P measured by SLP and thickness T measured by SOCT: B = P/T. In 185 subjects (77 normal subjects, 49 ocular hypertensive, 35 preperimetric, and 24 perimetric glaucoma patients) retardation maps (GDxVCC) and RNFL thickness at circular B-Scans (diameter 3.4 mm, 768 A-Scans) around the optic nerve head (ONH) (Spectralis HRA&OCT, Heidelberg Engineering) were measured. For all subjects the GDxVCC typical scan score was ≥ 90, and the quality ≥ 8. Raw data of both instruments were exported for further analysis. Using SLP-IR-images, phase retardation maps were aligned to SOCT-IR-images (ImageJ, Turboreg). In the aligned images, phase retardation values were extracted from the retardation maps at exactly the position of the circular OCT B-scans. Relative birefringence was calculated for 768 positions around the ONH, averaged to 32 sectors (11.25° each) and to peripapillary total mean. Positions with retinal vessels were excluded from the calculation. As the calibration factor for the phase retardation maps was unknown, only relative birefringence could be calculated.
In normal subjects, relative birefringence showed a variation around the ONH with relative maxima superior and inferior and relative minima temporal and nasal to the ONH, as was shown previously [Huang et al 2004]. In our study the birefringence increases in glaucoma patients. Assuming a mean relative birefringence of 1 in normal subjects, the mean relative birefringence (± SD) for the patient groups is (1.04 ± 0.13) in ocular hypertensive, (1.07 ± 0.10) in preperimetric, and (1.40 ± 0.14) in preperimetric glaucoma patients.
Using a combination of commercially available SLP and SOCT, in our study the earlierer described reduced birefringence in glaucoma could not be confirmed. This might be due to artifacts of the GDxVCC phase retardation results caused by phase retardation introduced by the sclera and/or not completely compensated corneal birefringence.
Clinical Trial: :
www.clinicaltrials.gov NCT 00494923
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