Assessing an instrument's repeatability is an important first step in the evaluation of a new technology for use in clinical practice. Current results suggest that SD-OCT RNFL thickness measurements are highly repeatable and that reproducibility is excellent in both healthy participants and patients. In addition, strong correlations and a good agreement were found between RNFL thickness measurements performed using the SD-OCT and the TD-OCT, suggesting that the two instruments may provide similar estimates of RNFL thickness. This information is important because TD-OCT has already demonstrated the ability to detect glaucomatous structural damage.12–19
These results show that this technology is promising for measuring RNFL thickness in healthy and glaucomatous eyes.
Several earlier studies have shown that TD-OCT measurements are reproducible, with RNFL thickness in glaucomatous eyes being slightly more variable than in normal eyes.8
However, results from the current study showed that SD-OCT measurements were similarly repeatable in both healthy and patient eyes. This is despite the fact that differences in disc area between healthy and patient eyes favoured a lower repeatability in patient eyes (due to measurements obtained closer to the disc margin in patient eyes that had larger optic discs on average).22
It is possible that SD-OCT provides more robust algorithms than TD-OCT for RNFL delineation, producing less variability in diseased eyes. Alternately, results may be due to differences in the study populations and the criteria applied for classifying healthy and glaucoma. In this study, most of the glaucomatous eyes had early to moderate disease based on the Glaucoma Staging System (GSS) classification for visual-field abnormalities;23
of the 78 patient's eyes, 24 (30.8%) were classified as GSS Stage 0, 35 (44.9%) were classified as Stage 1, 17 (21.8%) were classified as Stage 2, and two (2.5%) were classified as Stage 3. It is possible that repeatability may be worse in eyes with more advanced glaucoma.
One of the advantages of the Cirrus SD-OCT is that it does not require manual centring of the scan on the optic disc as long as the peripapillary region is included in the Optic Disk Cube 200×200 scan. The scan registration process performed by the Cirrus algorithms is fully automated, reducing the likelihood of operator error. Previous studies with TD-OCT have shown that misalignment of the circle scan by the operator can significantly affect RNFL measurements, particularly in quadrants and clock hours.24
However, in the absence of an eye-tracking system, Cirrus SD-OCT scan acquisition might still be affected by eye movements. In this study, Cirrus Optic Disk Cube 200×200 maps were checked for the presence of eye-movement artefacts such as vessel misalignment and/or image distortion. Nevertheless, as previously found using the TD-OCT, the nasal quadrant RNFL thickness was the most variable sector. As previously suggested for TD-OCT, it is possible that the angle of incidence of the illuminating beam is such that the RNFL is dimmer nasally, thus limiting the ability of the detection algorithm to consistently identify the RNFL at the same location over time.21
Our results showed that SD-OCT and TD-OCT thickness measurements are highly correlated. As expected, the correlation was particularly strong for average RNFL thickness compared with quadrant thickness. These results were confirmed in multivariate analysis using GEE adjusting for the correlation between two eyes of the same subject (data not shown). Although a good agreement was found for all parameters, Bland–Altman plots showed that TD-OCT RNFL measurements were consistently thicker than SD-OCT measurements, particularly in eyes with thick RNFL measurements. The reason for this finding is unclear. Because signal strength has been shown to affect RNFL thickness measurements using TD-OCT,26–31
it is possible that differences in signal strength between the two instruments may be one of the causes. However, in this study, only scans with adequate signal strength were included, and no differences in signal strength were observed between scans taken using the two devices (average signal strengths for SD-OCT and TD-OCT were 8.4 (1.9) and 8.7 (1.1), respectively; t test, p = 0.73). Nevertheless, in healthy participants, the average RNFL thickness measured with SD-OCT was not associated with signal strength (R2
= 0.02, p = 0.11), while TD-OCT RNFL thickness showed a positive, although weak association with signal strength (ie, the greater the signal strength, the greater the thickness, R2
= 0.11, p<0.05). Alternatively, the discrepancy in the measurements could be related to an intrinsic difference between the instruments and software edge-detection algorithms for measuring the RNFL, and the precise location of the RNFL measured. It is likely that SD-OCT, by providing scans at a higher resolution than TD-OCT, may also provide measurements that reflect a more accurate delineation of the RNFL margins. Future studies might clarify which instrument offers greater accuracy in estimating the “true” in vivo RNFL thickness.
This study evaluated the reproducibility of RNFL thickness measurements using the commercially available Cirrus SD-OCT in healthy eyes and glaucoma patient eyes. A previous report on SD-OCT measurement variability was obtained using a prototype device on a limited sample of healthy eyes.32
In conclusion, this study showed that SD-OCT provides excellent reproducibility suggesting that this technology is promising for detecting small structural changes over time in most eyes. As RNFL measurements with TD-OCT are thicker than with SD-OCT, measurements with these instruments should not be considered interchangeable. Further studies with longer follow-up are needed in order to assess intervisit variability and to better evaluate variance components that could affect measurements repeatability.