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Jun Liu, Junhua Tang, Xueliang Pan, Paul A. Weber; Initial Results on the Correlation between Corneal Acoustic Impedance and Tonometry Errors in Enucleated Human Eyes. Invest. Ophthalmol. Vis. Sci. 2012;53(14):5114.
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Previous studies have shown that corneal stiffness may significantly affect tonometric readings. Corneal acoustic impedance, measurable using non-invasive ultrasound, was found to correlate well with corneal tensile modulus [He and Liu, IOVS, 2011, 52(2)]. The purpose of this study was to experimentally examine the correlation between corneal acoustic impedance and tonometric measurements of IOP in cadaveric human eyes.
Eight enucleated human globes were obtained within 48 hours postmortem. Central corneal thickness was measured by using an ultrasound pachymeter (DGH-550BPL, DGH Technology). Corneal acoustic impedance was measured using a bench-top immersion ultrasound system described elsewhere. IOP was manometrically controlled by using a saline column adjusted to 10, 15, 20, 30 or 40 mmHg and confirmed by a pressure sensor that was connected to the anterior chamber. Three IOP readings were taken using a Goldmann Applanation Tonometer (GAT, AT900, Haag Streit, Switzerland) and a Tonopen (Tono-pen XL, Reichert, Inc.).
The mean CCT was 750±44 µm prior to other measurements suggesting moderate postmortem corneal swelling. At a true IOP of 10, 15, 20, 30, 40 mmHg, the GAT readings were 6.9±2.3, 13.0±1.8, 18.9±2.1, 30.4±1.6, and 40.4±1.6 mmHg, respectively. The corresponding Tonopen readings were 11.3±1.8, 16.7±1.5, 21.7±1.5, 30.2±2.3, and 39.5±1.6 mmHg. The mean corneal acoustic impedance was 1.73±0.11 MPa•s/m. Significant correlation between corneal acoustic impedance and GAT was found when the true IOP was 20 or 30 mmHg (R=0.72 or 0.78, P=0.046 and 0.021). Significant correlation between corneal acoustic impedance and Tonopen readings were found when the true IOP was 30 or 40 mmHg (R=0.74 or 0.87, P=0.038 or 0.005). No significant correlation was found between CCT and tonometric readings.
This study provides preliminary experimental evidences for a correlation between corneal acoustic impedance and tonometric errors in the human eye. The results suggest that the noninvasive ultrasonic acoustic impedance measurement of the cornea may help calibrate tonometry measurements for individual eyes.
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