Impact of atypical retardation patterns on detection of glaucoma progression using the GDx with variable corneal compensation

Abstract

Purpose: To evaluate the impact of atypical retardation patterns (ARP) on detection of progressive retinal nerve fiber layer (RNFL) loss using scanning laser polarimetry with variable corneal compensation (VCC).

Design: Observational cohort study.

Methods: The study included 377 eyes of 221 patients with a median follow-up of 4.0 years. Images were obtained annually with the GDx VCC (Carl Zeiss Meditec Inc, Dublin, California, USA), along with optic disc stereophotographs and standard automated perimetry (SAP) visual fields. Progression was determined by the Guided Progression Analysis software for SAP and by masked assessment of stereophotographs by expert graders. The typical scan score (TSS) was used to quantify the presence of ARPs on GDx VCC images. Random coefficients models were used to evaluate the relationship between ARP and RNFL thickness measurements over time.

Results: Thirty-eight eyes (10%) showed progression over time on visual fields, stereophotographs, or both. Changes in TSS scores from baseline were significantly associated with changes in RNFL thickness measurements in both progressing and nonprogressing eyes. Each 1-unit increase in TSS score was associated with a 0.19-microm decrease in RNFL thickness measurement (P < .001) over time.

Conclusions: ARPs had a significant effect on detection of progressive RNFL loss with the GDx VCC. Eyes with large amounts of atypical patterns, great fluctuations on these patterns over time, or both may show changes in measurements that can appear falsely as glaucomatous progression or can mask true changes in the RNFL.

FIGURE 1

Scatterplot showing the relationship between typical scan scores (TSS) at baseline and temporal–superior–nasal–inferior–temporal (TSNIT) average values at baseline. Lower TSS values were associated with higher TSNIT average measurements. A locally weighted scatterplot smoother (LOWESS) shows that the relationship between TSS and TSNIT average values was approximately linear.

FIGURE 2

Scatterplot showing the relationship between absolute change in TSS over time and baseline TSS scores. Lower TSS scores at baseline had greater absolute changes in TSS scores over time. A LOWESS shows that the relationship was approximately linear.

FIGURE 3

GDx variable corneal compensation (VCC) scans (Top) of a glaucomatous eye that progressed by visual fields (VF; Middle) and optic disc stereophotographs (Bottom). The optic disc photographs show progressive inferior neuroretinal rim thinning with associated disc hemorrhages. The VFs show corresponding worsening on the superior hemifield. However, GDx VCC scans show an apparent increase in retinal nerve fiber layer (RNFL) thickness measurements over time as indicated by the parameter TSNIT average, in disagreement with optic disc and VF findings. This could be explained by the worsening of atypical retardation patterns over time, as indicated by decreasing TSSs.

FIGURE 4

GDx VCC scans (Top) of an eye that remained stable on VFs (Middle) and optic disc stereophotographs (Bottom). The GDx VCC scans show an improvement in the amount of atypical patterns with an apparent decrease in RNFL thickness measurements over time.