Purpose: The objective of the study was to investigate the influence of signal strength (SS) on retinal nerve fiber layer (RNFL) measurement and interpretation of the RNFL thickness deviation map obtained with a spectral-domain optical coherence tomography.
Design: Cross-sectional study.
Methods: Thirty-seven eyes from 37 normal subjects were included. Six cube scans, each with 200 × 200 RNFL measurement pixels covering an area of 6 × 6 mm at the optic disc region with SS from 5 to 10, were collected with the Cirrus HD-optical coherence tomography (Carl Zeiss Meditec Inc, Dublin, Calif) by varying the level of polarization. The differences in RNFL measurements and the number of abnormal (outside 95% of the centile ranges) superpixels (1 superpixel = 4 × 4 pixels) at different levels of SS were compared with repeated-measures analysis of variance.
Results: The RNFL thickness generally increased with the SS. There were significant differences in RNFL thickness between images with SS of 10 and those with SS of 5 to 7 at the superonasal (1-3 o'clock) and inferotemporal (7-9 o'clock) sectors. One o'clock was the most frequent location that showed abnormal RNFL measurement. The average number of abnormal superpixels ranged between 99 (SS = 5) and 34 (SS = 10). There was no significant difference in the number of abnormal superpixels between images with SS of 10 and those with SS of 8 or 9.
Conclusions: Although SS changes induced by polarization adjustment may not be the same as those induced by media opacity or corneal irregularity, it is evident that the level of SS has a significant impact on RNFL measurement.