Imaging instruments are a powerful tool to assist clinicians in the diagnosis and management of glaucoma. However, image quality is an important variance component that can affect the outcomes of any imaging device, including Stratus OCT (Carl Zeiss Meditec, Dublin, California, USA). Based on our findings and the findings from other groups, signal strength can influence measurements’ accuracy and reliability. Therefore, scan quality should always be assessed before interpreting Stratus OCT results for clinical management decisions.
There are several possible explanations as to why the authors’ results differed from ours. They did not find a consistent association between signal strength and retinal nerve fiber layer (RNFL) thickness measurements in their study; an association was found only in the temporal region. First, our study was designed specifically to look at the effect of signal strength and scan misalignment on RNFL thickness measurements’ variability. We prospectively evaluated differences in signal strength between baseline scans and follow-up scans. A prospective study is more suited to look at the effect of signal strength than a cross-sectional study, such as the one that the authors seem to have performed. In addition, analyses between continuous variables usually have more power to detect associations than analyses that convert continuous variables into categories, such as moderate, good, and excellent signal strength.
Regardless, we agree that a signal strength cutoff for acceptable quality has yet to be defined for Stratus OCT. This should be taken into consideration particularly when assessing glaucomatous structural change, as we have shown that changes in signal strength may have a significant effect on the average RNFL thickness measurements. Conversely, in 2 recent reports, we found that the newer spectral-domain optical coherence tomography method may incorporate more robust algorithms and that RNFL thickness measurements may be less dependent on signal strength. In addition, these new devices incorporate numerous software enhancements to ensure that only good-quality scans are available for analysis. Future studies will confirm whether spectral-domain optical coherence tomography, while providing features designed to ensure better image quality, also will offer a significant advantage over previous technology for monitoring and assessing scan quality.