Accuracy of GDx variable corneal compensation polarization measurements in normal human eyes: effect of accommodation, cycloplegia, focus, pupil size, and eye selection on reproducibility

The effect of pupil size on the measurement of corneal birefringence properties: preliminary study

We used a partial Mueller matrix polarimeter to measure the corneal anisotropic properties at three pupil sizes (dilated, natural, and constricted). The geometrical parameters of first order isochromes were described by quadrilaterals. These parameters are statistically significantly different between the three pupil sizes. The pupillary size changes do not influence the azimuth angle distribution α. The retardation R and birefringence distributions show asymmetry in the nasal-temporal cross-section. There are differences between pupil sizes for both nasal and temporal parts of the cornea for these distributions. Iridial light scattering and diffraction might be the reason for these differences.

Screening for glaucoma using GDx-VCC in a population with ≥1 risk factors

OBJECTIVE

To estimate the diagnostic accuracy of the GDx-VCC for glaucoma screening in a population with ≥1 risk factors for glaucoma.

DESIGN

Cross-sectional evaluation of a diagnostic test for screening.

PARTICIPANTS

Two hundred forty-seven community-based volunteer participants with risk factors for glaucoma.

METHODS

The peripapillary retinal nerve fibre layers (RNFL) of participants' eyes were scanned using the GDx-VCC. Based on an ophthalmologic examination and frequency doubling perimetry, eyes were classified into 4 categories: normal, possible glaucoma, probable glaucoma, and definitive glaucoma. The sensitivities, specificities, positive and negative predictive values, and positive and negative likelihood ratios of the RNFL parameters were calculated.

RESULTS

The right eyes were retained for analyses. After excluding 5 eyes because of missing data and 23 eyes because of poor scan quality, the data of 219 right eyes were analyzed. Four eyes had definitive glaucoma. The best performing parameter was the nerve fibre indicator using a cutoff of 35 with a sensitivity of 75% (95% CI 19.4-99.4) at a specificity of 95% (95% CI 91.3-97.3), a positive predictive value of 25 (95% CI 4.3-48.1), a negative predictive value of 99 (95% CI 97.5-100.0), a positive likelihood ratio of 16 (95% CI 6.69-32.5), and a negative likelihood ratio of 0.20 (95% CI 0.05-1.44).

CONCLUSIONS

The GDx-VCC has inadequate sensitivity for screening of definitive glaucoma.

Birefringence of the central cornea in children assessed with scanning laser polarimetry

Corneal birefringence is a well-known confounding factor with all polarization-sensitive technology used for retinal scanning and other intraocular assessment. It has been studied extensively in adults, but little is known regarding age-related differences. Specifically, no information is available concerning corneal birefringence in children. For applications that are geared towards children, such as retinal birefringence scanning for strabismus screening purposes, it is important to know the expected range of both corneal retardance and azimuth in pediatric populations. This study investigated central corneal birefringence in children (ages three and above), by means of scanning laser polarimetry (GDx-VCC™, Carl Zeiss Meditec, Inc.). Children's measures of corneal retardance and azimuth were compared with those obtained in adults. As with previous studies in adults, corneal birefringence was found to vary widely in children, with corneal retardance ranging from 10 to 77 nm, and azimuth (slow axis) ranging from -11° to 71° (measured nasally downward). No significant differences in central corneal birefringence were found between children and adults, nor were significant age-related differences found in general. In conclusion, establishing knowledge of the polarization properties of the central cornea in children allows better understanding, exploitation, or bypassing of these effects in new polarization-sensitive pediatric ophthalmic applications.

The influence of posterior capsular opacification removal on anterior segment birefringence parameters as measured by scanning laser polarimetry

PURPOSE

To determine the effect of posterior capsular opacification (PCO) removal on anterior segment birefringence (ASB) and its influence on peripapillary retinal nerve fibre layer (RNFL) retardation measurements obtained by means of scanning laser polarimetry.

METHODS

In this prospective interventional study, scanning laser polarimetry was performed using GDx variable corneal compensation on 26 eyes of 26 patients who developed clinically significant PCO (after uncomplicated cataract surgery and with no other ocular pathology) both before and between 1 and 4 weeks after Nd:YAG capsulotomy. Best-corrected visual acuity (BCVA), intraocular pressure, corneal polarization axis (CPA), corneal polarization magnitude (CPM) were compared using the Student t-test and Wilcoxon signed ranks test. Spearman correlations between changes (differences between values after and before capsulotomy) in the CPA, CPM, BCVA and RNFL data were also performed.

RESULTS

PCO removal is associated with a shift in CPA (from 10.86 to 15.03 degrees, P = 0.004) and CPM (from 28.54 to 37.92 nm, P = 0.004). Significant correlations were found between changes in the parameters of ASB and BCVA. Furthermore, RNFL measurements (nerve fibre indicator, temporal-superior-nasal-inferior-temporal average and superior average) were also well related to the CPA and CPM shifts.

CONCLUSIONS

PCO induces an inaccurate compensation of ASB which affects RNFL assessment. Thus, it is necessary to recompensate ASB after posterior capsulotomy.