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Helen A Swarbrick, Pauline Kang, Rajini Peguda; Comparing total corneal thickness and epithelial thickness measurements between Sonogage ultrasound pachometry and high resolution OCT. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):5656.
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The Sonogage or Corneo-Gage Plus 1 pachometer (Sonogage Inc., USA) uses A-scan ultrasound to measure total corneal thickness. It is also claimed that this instrument is capable of measuring corneal epithelial thickness. Results obtained using this instrument have been used to question the reduction in epithelial thickness reported during orthokeratology (OK) contact lens wear. We sought to validate this claim by comparing total and epithelial thickness measurements obtained with the Sonogage to those obtained with high resolution optical coherence tomography (OCT).
9 non-contact lens wearers and 11 subjects who wore Paragon CRT OK lenses (Paragon Vision Sciences Inc, USA) overnight for a minimum of one month were recruited.<br /> Three OCT measurements and 5 ultrasound measurements were taken in one eye of each subject. Paired t-tests and Wilcoxon tests were used to compare total and epithelial corneal thicknesses measured with the Sonogage ultrasound pachometer and the Tomey Casia OCT (Tomey Corp., Nagoya, Japan). Pearson and Spearman correlations were used to examine the relationship between measurements obtained with the two instruments.
There was a significant difference in total corneal thickness measurements between the two instruments. Although a significant correlation was found (r=0.894, p<0.001), the Sonogage consistently measured greater total corneal thickness compared with the OCT (+60.7 ± 9.9 μm; p<0.001). Interestingly, epithelial thickness measured using the Sonogage showed little variation, ranging between 46.4 and 50.0 μm, whereas epithelial thickness ranged from 28.0 to 53.7 μm using the OCT. There was no significant correlation between epithelial thickness measurements obtained with the Sonogage and the OCT (r=-0.194, p=0.414).
The Sonogage tends to give thicker measurements for total corneal thickness than high resolution OCT. The Sonogage seems not to be capable of epithelial thickness measurement in vivo, returning an almost identical measurement over a range of epithelial thicknesses measured by OCT.
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