The impact of retardance pattern variability on nerve fiber layer measurements over time using GDx with variable and enhanced corneal compensation

Abstract

Purpose: To examine the impact of retardance pattern variability on retinal nerve fiber layer (RNFL) measurements over time using scanning laser polarimetry with variable (GDxVCC) and enhanced corneal compensation (GDxECC; both by Carl Zeiss Meditec, Inc., Dublin, CA).

Methods: Glaucoma suspect and glaucomatous eyes with 4 years of follow-up participating in the Advanced Imaging in Glaucoma Study were prospectively enrolled. All eyes underwent standard automated perimetry (SAP), GDxVCC, and GDxECC imaging every 6 months. SAP progression was determined with point-wise linear regression analysis of SAP sensitivity values. Typical scan score (TSS) values were extracted as a measure of retardance image quality; an atypical retardation pattern (ARP) was defined as TSS < 80. TSS fluctuation over time was measured using three parameters: change in TSS from baseline, absolute difference (maximum minus minimum TSS value), and TSS variance. Linear mixed-effects models that accommodated the association between the two eyes were constructed to evaluate the relationship between change in TSS and RNFL thickness over time.

Results: Eighty-six eyes (51 suspected glaucoma, 35 glaucomatous) of 45 patients were enrolled. Twenty (23.3%) eyes demonstrated SAP progression. There was significantly greater fluctuation in TSS over time with GDxVCC compared with GDxECC as measured by absolute difference (18.40 ± 15.35 units vs. 2.50 ± 4.69 units; P < 0.001), TSS variance (59.63 ± 87.27 units vs. 3.82 ± 9.63 units, P < 0.001), and change in TSS from baseline (-0.83 ± 11.2 vs. 0.25 ± 2.9, P = 0.01). The change in TSS over time significantly (P = 0.006) influenced the TSNIT average RNFL thickness when measured by GDxVCC but not by GDxECC.

Conclusions: Longitudinal images obtained with GDxECC have significantly less variability in TSS and retardance patterns and have reduced bias produced by ARP on RNFL progression assessment.

Figure 1.

Absolute difference in TSSs obtained with GDxVCC and GDxECC, distributed among the study population.

Figure 2.

Scatterplots demonstrate the relationship between the change in typical scan score (TSS) from baseline and the change in TSNIT average RNFL from baseline in GDxVCC (A) and GDxECC (B) scans in all eyes. A significant inverse relationship was observed between the change in TSS and the change in TSNIT average RNFL thickness from baseline in GDxVCC (P < 0.001) but not in GDxECC (P = 0.07) measurements.

Figure 3.

A glaucomatous eye falsely identified as having apparent RNFL loss over time by GDxVCC, despite nonprogressing visual fields (top left) and optic disc stereophotographs (top right). On the detection software map (Progressor Medisoft, London, UK), each bar represents one test with the bar length showing to the depth of the defect and the color showing the P value of the regression slope. None of the locations demonstrated progression in this patient. The baseline GDxVCC (middle) image (TSS = 14) demonstrated ARP, and during 4 years of follow-up, there was a 32-unit increase in TSS and a 5.19-μm reduction in the TSNIT average RNFL thickness. The baseline GDxECC image (bottom) demonstrated a normal retardance pattern (TSS 91), and the absolute change in TSS was 7 units with less fluctuation in RNFL thickness.

Figure 4.

A glaucomatous eye with superior visual field progression (top left) as judged with progression detection software (Progressor; Medisoft, London, UK) and a corresponding inferiorly located optic disc hemorrhage (top right, arrow) at 4 years of follow-up. The progressing point is highlighted in red on the gray-scale image and is seen as a series of progressively lengthening bars on the software-generated map, which are color coded in red when the regression slope becomes significant (P < 0.01). The baseline GDxVCC (middle) image showed a normal retardance pattern (TSS = 91), and during 4 years of follow-up, there was a 24-unit reduction in TSS and an apparent 3.43-μm increase in the TSNIT average RNFL thickness. GDxECC (bottom) showed no change in the absolute TSS over time, and a 9.9-μm reduction in TSNIT average was observed with atrophy in the inferior region (black arrow) corresponding to the location of the optic disc hemorrhage and visual field change.