Chu and Racette are to be commended for tackling a complex topic in the field of glaucoma diagnostics by investigating the presumed lag between structural and functional progression. One hundred twenty eyes of 120 subjects with definite or suspected glaucoma and 11 testing sessions over a period of five to ten years were enrolled from two prospective cohorts. The structural and functional outcomes of interest were the global rim area (RA) derived from HRT2 and mean threshold sensitivity (MS) from 24-2 visual fields. To make the two structural and functional measures more consistent, the RA and MS were transformed and expressed as percentage of mean normal values based on a separate database of normal eyes. The correlation of RA and MS was calculated at varying time intervals so as to determine which one modality was more likely to demonstrate change earlier in individual eyes.
The findings confirmed that either structural or functional damage could potentially precede the other measure and that structural damage did not necessarily precede functional damage. The results enhance our understanding of the temporal patterns of structural vs. functional damage in glaucoma. The authors properly acknowledge the shortcomings of their approach including the fact that the dynamic range of structural measurements may not encompass the entire available numerical range due to presence of a measurement floor.
The results enhance our understanding of the temporal patterns of structural vs. functional damage in glaucoma
Other caveats need to be considered interpreting the results of this study. Any normalization scheme would be imperfect due to the high variability of measurements in normal individuals and a smaller normative group such as the one used in this study, may introduce bias into the percentage estimations. It is also not clear if the normalization was done in the linear or dB scale for MS. As the data provided on lag are based on the number of visits rather than time between visits, it makes it harder to draw conclusions on the exact timing of lags. However, the authors do mention that the visits were on average ten months apart. The investigators are to be lauded for exploring the effect of measurement noise on the results. While the conclusions are based on multiple cross-sectional correlation analyses, it would be of great interest to investigate the correlation between longitudinal changes in structure vs function in this or other cohorts. It would also be important to see the results on the relative lags as a function of baseline disease severity. The main take-home message for clinicians is that both structural and functional tests are needed for a timely detection of change in glaucoma as neither modality is sensitive enough alone to detect all progressors.
The main take-home message for clinicians is that both structural and functional tests are needed for a timely detection of change in glaucoma as neither modality is sensitive enough alone to detect all progressors