GDx-VCC vs GDx-ECC in glaucoma diagnosis

Purpose To compare results provided by scanning laser polarimetry variable corneal compensation (VCC) vs enhanced corneal compensation (ECC) and evaluate correlation to visual field results in glaucoma patients. Methods Study included 339 eyes of 182 patients screened by the glaucoma unit of the University Eye Clinic of Pavia (Italy). Patients were submitted to complete ophthalmic examination, standard automated perimetry (SAP), scanning laser polarimetry with GDx-VCC and GDx-ECC. Quality image (Q), typical scan score (TSS), nerve fibers index (NFI), nerve fibers layer average thickness in a band around the optic nerve head (TSNIT average) and in the upper (TSNIT sup) and lower sector (TSNIT inf) were evaluated for each exam. Q, TSS and morphometric parameters provided by VCC and ECC were compared using Wilcoxon signed-rank test and Lin correlation coefficient. Correlation between GDx and perimetric global indexes and was evaluated with Pearson correlation index “r”. Results 204 images out of 339 (60%) were of good quality (Q>7) with VCC and 325 out of 339 (96%) with ECC. 140 images out of 339 (41%) were atypical (TSS<80) with VCC but only 20 out of 339 with ECC (6%). ECC vs VCC constantly displays lower TSNIT thickness and higher NFI. All comparisons between ECC and VCC parameters showed statistically significant differences confirmed by a moderate to poor concordance between the two instruments. Structure/function correlation was better for ECC parameters and first of all for NFI. Conclusion ECC provides better quality images than VCC. High quality exam is the first condition to reproduce a more reliable RNFL structure. ECC points out lower RNFL thickness and higher NFI and reveal a better correlation to perimetric indexes MD and PSD as compared to VCC. GDx-ECC could improve early glaucoma diagnosis.