April 2013
Volume 54, Issue 4
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Letters to the Editor  |   April 2013
Influence of the Eye-Tracking–Based Follow-Up Function in Retinal Nerve Fiber Layer Thickness Using Fourier-Domain Optical Coherence Tomography
Author Notes
  • Department of Neurology, VU University Medical Centre, Amsterdam, The Netherlands. 
  • E-mail: L.balk@vumc.nl  
Investigative Ophthalmology & Visual Science April 2013, Vol.54, 3045. doi:10.1167/iovs.13-12073
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      Lisanne J. Balk, Axel Petzold; Influence of the Eye-Tracking–Based Follow-Up Function in Retinal Nerve Fiber Layer Thickness Using Fourier-Domain Optical Coherence Tomography. Invest. Ophthalmol. Vis. Sci. 2013;54(4):3045. doi: 10.1167/iovs.13-12073.

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      © ARVO (1962-2015); The Authors (2016-present)

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We read with interest the paper by Costa et al 1 on the influence of using the eye-tracking function on the accuracy of retinal optical coherence tomography (OCT) measurements. It was suggested that the eye-tracking function (EBF) of the Heidelberg Spectralis OCT device was essentially irrelevant for the accuracy of quantitative retinal OCT in healthy subjects. This observation is counterintuitive and in contrast with our own data, obtained in a comparable setting. As summarized in the Table, EBF improved the reliability of the OCT measurements in high-speed (HS) mode, both in terms of inter- and intraobserver intraclass correlation coefficients (ICCs) (6 HS-mode scans per eye, 3 with EBF and 3 without, 4 healthy subjects, 3 different observers). Importantly, all these measurements were performed with respect to a single, registered baseline scan. This situation reflects the real-life or clinical study setting. 
In contrast, the protocol by Costa et al 1 suggests that the two observers used for their analysis individually placed the reference (baseline) scans which were used in order to acquire the follow-up scans using the eye-tracking function. Consequently, their Table 2 reported interobserver ICCs for the baseline scans, which in fact is a comparison of how both observers chose to place the baseline ring scan. 1 Because using a different reference scan already introduced an observer bias at baseline (shown in their Table 2), an effect on interobserver ICCs can be expected. 
It also follows that the statistics on the follow-up scans without using the EBF function represent in fact a comparison of two arbitrarily placed follow-up scans. 
In other words, it is not clear what the difference is between the interobserver ICC for the baseline scan and the interobserver ICC for the follow-up scans without using the information from the baseline scan (e.g., without EBF). 1 It seems that the only difference between the two is that the authors used data from the first scan and from the second scan (independent from the first), respectively. In that case, are the baseline and follow-up (without EBF) interobserver ICCs not, in essence, the same and represent merely a measure for the agreement between the two observers in placing the ringscan correctly on the optic nerve head? Admittedly, the differences in accuracy appear to be small but will still be relevant to using retinal OCT as a biomarker for neurodegeneration in multiple sclerosis where changes of only 1 to 2 μm per year are expected. 2  
Table. 
 
Interobserver and Intraobserver ICC, With and Without EBF Function
Table. 
 
Interobserver and Intraobserver ICC, With and Without EBF Function
Retinal Sector Interobserver ICC Intraobserver ICC
No EBF With EBF No EBF With EBF
Global mean 0.94 0.96 0.98 1.00
Temporal, superior 0.96 0.94 0.99 1.00
Temporal 0.79 0.85 0.94 1.00
Temporal, inferior 0.94 0.94 0.96 1.00
Nasal, superior 0.90 0.83 0.95 1.00
Nasal 0.45 0.53 0.71 1.00
Nasal inferior 0.66 0.73 0.89 1.00
Papillomacular bundle (PMB) 0.67 0.69 0.89 1.00
References
Costa AMC Costa RA Melo LAS Jr Calucci D Orefice JL Cronemberger S. Influence of the eye-tracking–based follow-up function in retinal nerve fiber layer thickness using Fourier-domain optical coherence tomography. Invest Ophthalmol Vis Sci . 2013; 54: 1958–1963. [CrossRef] [PubMed]
Talman LS Bisker ER Sackel DJ Longitudinal study of vision and retinal nerve fiber layer thickness in multiple sclerosis. Ann Neurol . 2010; 67: 749–760. [PubMed]
Table. 
 
Interobserver and Intraobserver ICC, With and Without EBF Function
Table. 
 
Interobserver and Intraobserver ICC, With and Without EBF Function
Retinal Sector Interobserver ICC Intraobserver ICC
No EBF With EBF No EBF With EBF
Global mean 0.94 0.96 0.98 1.00
Temporal, superior 0.96 0.94 0.99 1.00
Temporal 0.79 0.85 0.94 1.00
Temporal, inferior 0.94 0.94 0.96 1.00
Nasal, superior 0.90 0.83 0.95 1.00
Nasal 0.45 0.53 0.71 1.00
Nasal inferior 0.66 0.73 0.89 1.00
Papillomacular bundle (PMB) 0.67 0.69 0.89 1.00
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