Reliability of peripheral corneal pachymetry with the Oculus Pentacam
Article Outline
In their recent paper, Khoramnia et al.1 overlooked mentioning their stated reference point when measuring peripheral corneal thickness. Did they use the default setting of pupil center as the reference point or the more reliable corneal vertex? We emphasize this point because in our study,2 peripheral corneal thickness measurements showed poor repeatability (mean coefficient of repeatability [COR] ±95% limits of agreement3) ±26.28 μm [range 22.37 to 30.04 μm]) using the default pupil center, whereas a marked improvement in reliability (mean COR ±16.00 μm [range 13.71 to 19.85 μm] was evident when corneal vertex was used as the reference point. We found that repeatability of peripheral corneal thickness measurements were comparable to repeatability of central thickness measurements using the corneal vertex as the reference but peripheral repeatability worsened twofold using the pupil center as the reference point. The poor reliability findings of Khoramnia et al. suggest that they used the default pupil center in acquiring their peripheral measurements.
The problem is inherent because the pupil center is an unreliable measure.2 Because of the dynamic nature of pupil size and shape and the image acquisition time of 2 seconds, we propose that the pupil center is constantly moving so the results are variable. If the reference point keeps changing, it follows that the corneal sampling for peripheral thickness also changes between measurements. The end result is poor reliability.
The default standard of pupil center as the reference point when measuring corneal thickness, in the tradition of ultrasonic probe pachymetry,4 is an anachronism when using an automated scanner such as the Oculus Pentacam. We recommend that the default be altered to use corneal vertex as the more stable reference point.
References
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- . Anterior segment biometry with the Oculus Pentacam: a comprehensive assessment of repeatability of automated measurements. J Cataract Refract Surg. 2008;34:95–102
- . Measuring agreement in method comparison studies. Stat Methods Med Res. 1999;8:135–160
- Evaluation and comparison of sources of variability in the measurement of corneal thickness with ultrasonic and optical pachymeters. Ophthalmic Surg. 1983;14:750–754
PII: S0886-3350(07)01803-2
doi:10.1016/j.jcrs.2007.08.042
© 2008 ASCRS and ESCRS. Published by Elsevier Inc. All rights reserved.
