Since its introduction in 1991, OCT technology has experienced a dramatic evolution and has quickly become a popular resource in ophthalmic imaging and diagnostics. Despite its popularity and role in glaucoma research, its role in diagnostics is still developing. The correlation between glaucoma progression and RNFL thinning is well-documented.
8,11 Evidence that the thinning of RNFL precedes the loss of VF function indicated that OCT could help detect the onset of glaucomatous changes more sensitively than by VF testing.
12 An earlier diagnosis of glaucomatous changes gives clinicians more time to establish an effective therapy and may help maintain patients' VF function.
Previous studies have kept record of the increasing reproducibility of RNFL thickness measurements performed with first-,
25,26 second-
27,28 and third-
29 generation TD-OCT technology, as well as for the different SD-OCT devices
30 –35 that are currently available. The purpose of this study was to investigate the contribution of two specific software applications to achieve more reliable and more highly reproducible RNFL thickness measurements. The improvements of SD-OCT have been well documented in the literature.
30 –35 Although other studies have already been published on the reproducibility of RNFL thickness measurements with SD-OCT, the uniqueness of the present study lies in the differentiation between the two measurement modalities (methods A and B). The higher resolution and faster scanning speed of SD-OCT have made the measurements more reproducible. The present study demonstrates that reproducibility can be further improved by the use of specific software for retest recognition and compensation for involuntary eye movement (eye tracker).
The Spectralis software algorithm automatically detects the RNFL. In some cases, the software has problems detecting the correct boundary of the RNFL. In these cases, it is possible to manually correct the boundary in the Spectralis software. To avoid bias by a glaucoma specialist who uses manual correction of the RNFL boundary, we did not use manual correction in the present study. If it was obvious that the automatic detection had failed, those study eyes were excluded.
Our results show good reliability for measurements obtained with the eye tracker and retest function (method A). The Bland-Altman plot with 95% limits of agreement (
Fig. 2) shows that RNFL thicknesses for G were comparable between the two methods. The only significant difference in RNFL thickness was found in control patients where method A resulted in higher values for the NI and N sectors and for G and in lower values in the TS sector. As the differences are as low as 0.2 to 3.0 μm, we consider them to be clinically irrelevant. Measurements with the new eye tracker and retest function (method A) are reliable and comparable to the thickness values measured without the eye tracker and retest function (method B).
The results of this study show excellent reproducibility and significant improvement of reproducibility of RNFL measurements when using the eye tracker and retest software. Using the eye tracker and retest software (method A) of Spectralis SD-OCT enhanced reproducibility significantly (COV 2.7%–1.3% for G in glaucomatous eyes; P = 0.000). In glaucoma patients, the improvement in reproducibility was significantly higher than in control eyes. To our knowledge the present study is the first to report on the reproducibility of RNFL measurements using the Spectralis SD-OCT device.
Budenz et al.
29 studied reproducibility of RFNL measurements with TD-OCT Stratus OCT3 (Carl Zeiss Meditec, Dublin CA) in 88 normal and 59 glaucomatous eyes and found COVs ranging from 3.7% (G) to 11.9% (N) in glaucomatous eyes and from 1.7% (G) to 8.25% (N) in normal eyes, respectively. Similar to the present study, N measurements were the least, whereas T measurements were the most reproducible. Also in that study, glaucomatous eyes showed less reproducibility than normal eyes, which was also reported by Blumenthal et al.
27 Although reproducibility found in the present study is still higher in normal eyes than in glaucomatous eyes, our results show that the eye tracker and retest software (method A) had a higher impact on reproducibility in glaucomatous eyes than in normal eyes. This finding indicates that such software may help to reduce the gap in reproducibility previously found between measurements of glaucomatous and normal eyes. Mwanza et al.
35 studied intra- and intervisit reproducibility of RNFL thickness and optic nerve head parameters measured with the SD-OCT Cirrus HD-OCT (Carl Zeiss Meditec, Inc.) in 55 glaucomatous eyes. Similar to the present study, the mean RNFL thickness showed the best intravisit reproducibility, with a COV of 1.9%. Menke et al.
34 reported on the reproducibility of the 3D Fourier domain OCT (3D OCT1000; Topcon, Tokyo, Japan) in 38 normal subjects by having two operators perform three RNFL thickness measurements. The mean COV was 4.1%. Highest reproducibility was found for the inner ring area (ETDRS-scheme) with a COV of 1.9%, which compares with results of the present study for measurement method B in normal eyes (1.6%). Lee et al.
32 used the test–retest function of Spectral OCT/SLO (Ophthalmic Technologies Inc., Toronto, ONT, Canada) to investigate the reproducibility of RNFL thickness measurements in 98 normal and 79 glaucomatous eyes performing three measurements within one session. RNFL measurements showed good reproducibility for that device. As in the present study, best reproducibility was found for the global mean RNFL thickness, with a COV of 1.9% in normal and 2.0% in glaucomatous eyes. As all scans were performed with one method (retest) only, no conclusion regarding the specific effect of the retest function to enhance reproducibility could be drawn.
Reproducibilities found in different studies are not directly comparable, as different eyes and different study protocols were used. The results of the present study showed excellent reproducibility for measurements using the eye tracker and retest protocol (method A) of Spectralis SD-OCT. With lowest COVs of 1.0% in normal eyes and 1.3% in glaucomatous eyes, the results show one of the best reproducibilities ever reported for RNFL thickness measurements for any OCT device available today. The HS mode used in the present study used only half the transverse resolution the device is capable of. If one were to use the full resolution of the HR mode, even higher reproducibility may be achieved. Whereas other currently available OCT devices provide a scanning resolution comparable to the device used in the present study and also include test–retest software, the Spectralis SD-OCT is the first device to integrate real-time eye tracking. The significant improvement of reproducibility attained by using this software in the present study indicates that improvement in reproducibility cannot entirely be accounted for by higher resolutions and faster scanning speeds of the latest SD-OCT devices, but also has to be understood as a result of more sophisticated software applications. These findings may have implications for the design and development of the next generation of OCT devices.