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Harriet Lloyd, Mara Berganovsky, Ronald Silverman, Raksha Urs; Effect of Intraocular Pressure on Speed-of-Sound and Thickness in Ex Vivo Cornea in Intact Globes. Invest. Ophthalmol. Vis. Sci. 2013;54(15):1644. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
Ultrasound is regarded as the ‘gold standard’ for the determination of corneal thickness, but standard methods for measuring this parameter have required determination on excised corneas, which is non-physiologic. We developed a means for measurement of speed-of-sound in intact globes. Our objective was to determine the effect of intraocular pressure (IOP) on corneal speed of sound and thickness.
We acquired high-resolution ultrasound data on four ex vivo pig corneas. The eyeball was mounted in a custom apparatus, which included a sharpened, thin, flat metal surface that was inserted across the anterior chamber. An 18 gauge needle attached to a saline drip bag was inserted through the optic nerve. IOP was raised and lowered by raising or lowering the saline bag and monitored with a digital pressure gauge attached to the IV line. The eye and apparatus was submerged in 20% dextran solution. Data of the cornea and of the metal surface on either side of the globe were obtained using a single-element focused transducer with a center frequency of 35 MHz. Pulse/echo ultrasound data was acquired at a 400 MHz sample rate. We measured the shift in the metal flat echo compared to its expected, interpolated position, and from this and the speed-of-sound in aqueous and the dextran solution solved for the speed-of-sound and thickness of the cornea.
The speed-of-sound averaged over all cases showed relatively little dependence on intraocular pressure. At 0 mm the speed-of-sound averaged Hg 1556 m/s, at 40 mm Hg it was 1560 m/s and upon return to 0 mmHg it was 1548 m/s. These differences were not statistically significant. The thickness of the cornea at 0mmHg measured 1.01 mm, at 40 mmHg it was 0.91 mm, and it recovered to 0.96mm as the pressure was gradually reduced to 0mmHg.
IOP had little effect on the speed-of-sound in the cornea, indicating that ultrasound pachymeters would not be required to be recalibrated to compensate for IOP. However, as pressure increased, the cornea stretched and became thinner, recovering gradually and partially as IOP was decreased.
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