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M. T. Feng, R. H. Silverman, R. Ursea; Corneal Pachymetry by Anterior Segment OCT Versus Very High Frequency Digital Ultrasound. Invest. Ophthalmol. Vis. Sci. 2010;51(13):5680.
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© ARVO (1962-2015); The Authors (2016-present)
To compare in vivo pachymetry of normal human corneas as measured by anterior segment optical coherence tomography (ASOCT) and very high frequency digital ultrasound (VHFUS).
Cross-sectional study. We randomized 22 healthy subjects with normal corneas to receive bilateral ASOCT (Visante, Zeiss Meditec, Dublin, CA) followed by VHFUS (Artemis 2, ArcScan, Golden, CO), or vice versa. Parameters included high resolution cornea mode for ASOCT and corneal transducer position for VHFUS. We centered each eye on its vertex and acquired three scans in the horizontal meridian, which we then analyzed using the provided software. We measured corneal thickness centrally (CCT) and at 0.5, 1.0, 1.75, and 2.5 mm in both temporal and nasal quadrants. Two-tailed paired t-tests provided statistical analysis of mean ASOCT and VHFUS data from each eye.
We imaged 42 eyes of 22 subjects, consisting of 9 (41%) men and 13 (59%) women with a mean age (± SD) of 43.7 ± 17.7 (range 18-85). Among right eyes (n = 22), mean CCT was 541 ± 35 µm by ASOCT and 540 ± 35 µm by VHFUS (P = .26). Nasally, no difference existed between ASOCT and VHFUS pachymetry at 0.5 (P = .81), 1.0 (P = .57), and 1.75 mm (P = .069); however, at 2.5 mm, variation between ASOCT (582 ± 37 µm) and VHFUS (577 ± 38 µm) approached significance (P = .052). Temporally, ASOCT measurements were less than VHFUS at 0.5 (-4.2 µm; P = .0008), 1.0 (-11 µm; P < .0001), 1.75 (-17 µm; P < .0001), and 2.5 mm (-25 µm; P < .0001). Left eye (n = 20) data were similar. Overall (n = 42), mean CCT was 540 ± 35 µm by ASOCT and 540 ± 36 µm by VHFUS (P = .98) with agreement nasally within 2 µm (.20 < P < .78). Temporally, ASOCT differed by -7 at 0.5 mm to -28 µm at 2.5 mm (each P < .0001). Pachymetry at 2.5 mm was more asymmetric for ASOCT (P < .0001) than VHFUS (P = .99).
Both ASOCT and VHFUS allow high resolution imaging of the cornea and anterior segment. Each modality uses distinct mechanics to arrive at similar measures of CCT. To our knowledge, paracentral congruency has not been examined in prior studies. We found significant disagreement temporally from 0.5 to 2.5 mm. Given that VHFUS pachymetry was more symmetric over the central 5 mm, one might conclude that VHFUS is more reliable and, in turn, that ASOCT underestimates temporal corneal thickness and an adjustment factor is needed. Alternatively, ASOCT measurements trended higher than VHFUS approaching the nasal 2.5 mm, suggesting that ASOCT data were displaced nasally and standard alignment protocols require reevaluation. Further study is needed to follow the behavior of these trends into the corneal periphery.
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