Structure-function relationship depends on glaucoma severity

Background/aims: To determine the structure-function relationship throughout the different stages of glaucoma. Methods: Subjects: 228 controls and 1007 suspected, early, moderate, or advanced glaucomas. In controls, Pearson coefficient of variation (PCV) of standard automated perimetry mean sensitivity (SAP-MS) and global mean retinal nerve fibre layer thickness (RNFL, Heidelberg Retina Tomograph) were calculated. Piecewise linear regression was used to evaluate if the data were related better to two separate regression lines in all population. Results: PCV for SAP-MS and RNFL was 6.19% and 29.27%, respectively. For SAP-MS.22.42 dB (piecewise linear regression-obtained breakpoint, p<0.05), the logarithmic and linear relationships between SAP-MS and mean RNFL thickness were not different (r = 0.182, r = 0.185 respectively; p = 0.950). For SAP-MS<22.42 dB, the logarithmic and linear relationships between SAP-MS and mean RNFL thickness were not different (r = 0.353, r = 0.344 respectively; p = 0.884). Conclusion: The logarithmic and linear relationships between SAP-MS and mean RNFL thickness did not differ when individuals were stratified using the piecewise linear regression-obtained cut-off point. The curvilinear relationship between the morphological and perimetric results may be due to the wide variability in normal morphology and limitations in the dynamic range of the morphological tests in cases with moderate and severe defects.