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Letter

Assessment of corneal thickness measurement using swept-source Fourier-domain anterior segment optical coherence tomography and Scheimpflug camera

Huang, Jinhai MD1; Feng, Yifan MD1; Wang, Qinmei MD1; Pesudovs, Konrad PhD2

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Journal of Cataract & Refractive Surgery: July 2012 - Volume 38 - Issue 7 - p 1305-1306
doi: 10.1016/j.jcrs.2012.05.004
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We have some comments to share with Szalai et al.1 to broaden the discussion about swept-source Fourier-domain anterior segment optical coherence tomography and Scheimpflug camera. In the conclusion of their article, Szalai et al. state that “Bland-Altman plots showed good agreement between the measurements of the 2 devices.” However, the 95% limits of agreement (LoA) between swept-source Fourier-domain optical coherence tomography (OCT) (Casia SS-1000, Tomey Corp.) and Scheimpflug imaging (Pentacam HR, Oculus, Inc.) were very large. For anterior axial keratometry measurements, the 95% LoA ranged from −1.27 to +1.45 diopters (D) and −1.05 to +1.23 D for normal subjects in the steep meridian and flat meridian, respectively. In simple terms, this means that keratometry measured by the 2 devices can vary up to 2.72 D; for any application, this would be a clinically important difference. The LoAs were wider again in the keratoconus group (ranging from −6.09 to +5.81 D and −6.08 to 6.29 D in the steep meridian and flat meridian, respectively). These results are similar to those found by Nakagawa et al.,2 who report the 95% LoA between swept-source Fourier-domain OCT and Scheimpflug imaging ranged from −1.00 to +1.90 D in control subjects and from −8.4 to 8.9 D in eyes with keratoconus. It should be concluded that the agreement of anterior keratometry readings obtained by both devices is poor. A trend similar to that of the corneal thickness assessments was found. These aforementioned parameters are in poor agreement and cannot be used interchangeably, especially in keratoconic eyes.

Surprisingly, the authors found that with the Oculus Pentacam HR, the repeatability of corneal thickness at pupil center (12.34 μm) and the thinnest point (11.45 μm) was better than at the corneal vertex (20.32 μm). However, repeatability studies by Shankar et al.3 and McAlinden et al.4 report that repeatability of corneal thickness measurements at pupil center are equivalent or worse than that at the vertex or the thinnest point. Given the dynamic nature of pupil size and shape and the image acquisition time of 2 seconds, any measure that uses the pupil as a reference instead of a fixed point must carry increased variability in its measurement. Therefore, it seems unlikely that the repeatability of corneal thickness at pupil center was better than at the corneal vertex.

REFERENCES

1. Szalai E, Berta A, Hassan Z, Modis L Jr. Reliability and repeatability of swept-source Fourier-domain optical coherence tomography and Scheimpflug imaging in keratoconus. J Cataract Refract Surg. 2012;38:485-494.
2. Nakagawa T, Maeda N, Higashiura R, Hori Y, Inoue T, Nishida K. Corneal topographic analysis in patients with keratoconus using 3-dimensional anterior segment optical coherence tomography. J Cataract Refract Surg. 2011;37:1871-1878.
3. Shankar H, Taranath D, Santhirathelagan CT, Pesudovs K. Anterior segment biometry with the Pentacam: comprehensive assessment of repeatability of automated measurements. J Cataract Refract Surg. 2008;34:103-113.
4. McAlinden C, Khadka J, Pesudovs K. A Comprehensive evaluation of the precision (repeatability and reproducibility) of the Oculus Pentacam HR. Invest Ophthalmol Vis Sci. 2011;52:7731-7737.
© 2012 by Lippincott Williams & Wilkins, Inc.