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Editors Selection IGR 10-4

IOP, VF, Imaging and Electrophysiology: Origin of atypical retardation patterns

Comment by Qienyuan Zhou on:

22320 Analysis of the origin of atypical scanning laser polarimetry patterns by polarization-sensitive optical coherence tomography, Götzinger E; Pircher M; Baumann B et al., Investigative Ophthalmology and Visual Science, 2008; 49: 5366-5372

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GDx^TM VCC is a confocal scanning laser polarimeter (SLP) for measuring the retinal nerve fiber layer (RNFL) retardance in which the anterior segment birefringence is cancelled with a variable corneal. compensator Atypical retardation pattern (ARP), also referred to as atypical birefringence pattern (ABP), characterized by irregularly elevated retardation patterns that do not correspond to expected RNFL anatomy, has been reported in some GDx VCC images. Götzinger et al. (1463) attempt to identify the origin of ARP by using polarization-sensitive optical coherence tomography (PS-OCT) to image the birefringent layers of the retina. PS-OCT has several potential advantages. It has much higher axial resolution than SLP and, potentially, could achieve more accurate retardation measurements of the RNFL. The simultaneous RNFL thickness and retardation measurements with PS-OCT could allow assessment of RNFL birefringence with a single device. Potential clinical benefits of PS-OCT may be assessed when good repeatability and reproducibility are achieved with the device.

The scleral contribution was identified as the origin of the artifacts for most cases of GDxVCC atypical birefringence

Study eyes were selected to enrich the prevalence of ARP based on their GDx retardation image appearance; approximately equal number of eyes with (22) and without (19) ARP were included, consisting of 11 glaucomatous eyes, 10 healthy eyes, and 20 glaucoma-suspect eyes. PS-OCT images that included and excluded the signal from the sclera were compared side-by-side with GDx VCC images of the same eyes. In most ARP cases (15 out of 22), the scleral contribution was identified as the origin of the artifacts. Five GDx ARP cases appeared to arise from sources other than sclera. Two cases with normal VCC images, but exhibiting ARP in PS-OCT images, had abnormally large RPE depolarization in OCT images. Thus, Götzinger, et al. have demonstrated a scleral contribution to most examples of ARP in GDx VCC. Another method of corneal compensation, ECC, was introduced into the GDx in 2004 and has been reported to significantly reduce ARP (Refs 9 &10 to the article). It would be interesting, therefore, to see a follow-up PS-OCT study that includes GDx ECC images and an appropriate analysis of the origins of their retardation components.

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