Moghimi et al. explored the measurement floor and dynamic range of measurements for circumpapillary RNFL and macular GCC thickness and compared those to perifoveal and circumpapillary capillary vessel densities (cpCD and pfVD, respectively) derived from OCTA in a group of normal eyes and eyes with suspected or established glaucoma. They hypothesized that OCTA parameters may provide additional information in later stages of the disease when thickness measurements have reached their floor.
They reported that the change point, i.e., the point where any given OCT parameter reaches its measurement floor, occurred either later (cpCD) or could not be detected (pfVD) with OCTA parameters as compared to thickness measures. However, when the number of potential steps of change within the dynamic range of thickness and OCTA parameters were compared, there was actually a smaller number of steps with OCTA measures compared to OCT parameters. The investigators concluded that OCTA measures may be useful for detection of change in advanced glaucoma.
Can OCTA parameters predict functional deterioration with high enough accuracy to be clinically useful for confirmation or forecasting of functional glaucoma progression?
The findings are promising but should be considered preliminary. One issue is that the number of eyes with advanced glaucoma (i.e., beyond -15 dB of MD) was small; under such circumstances the 95% CI for the change point can be quite wide or a change point may not be detectable and therefore, it is difficult to prove a later change point for any outcomes of interest. A direct comparison of the performance of macular measures to RNFL measures may have been affected by the composition of the study sample and the proportion of glaucoma eyes with early central damage; only a small number of test locations (12 or less) within the 24-2 VF actually correspond to the central macula. A smaller number of steps within the dynamic range for the OCTA parameters could potentially limit their utility. On the other hand, one might argue that with newer iterations of OCTA hardware and software, there would be less variability and the rates of change of OCTA measures may become more accurate or the number of step changes detectable within the dynamic range may increase.
Further confirmation of these findings is needed. If confirmed, additional questions will need to be addressed before the findings can be clinically applied. The main question is whether a change in OCTA parameters can predict functional deterioration with high enough accuracy to be clinically useful for confirmation or forecasting of functional glaucoma progression? We will be looking forward to additional studies on this cohort.