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P.W. DeBry, R. Krishna, T.L. Willoughby; Comparison of Intraocular Pressure Measurement by Dynamic Contour Tonometry and Goldmann Applanation Tonometry . Invest. Ophthalmol. Vis. Sci. 2003;44(13):2185.
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Purpose: The Dynamic Contour Tonometer (DCT) is a new tonometric device. Rather than operating on the principle of applanation, this device has a pressure sensor built into a small concave tip. By coupling the tip to the corneal surface direct intraocular pressure can be measured. This study is designed to compare IOP measurements obtained with DCT to Goldmann applanation (GAT) measurements, and to look for patient factors related to pressure differences between the techniques. Methods: Patients with normal corneas and no recent intraocular surgery were enrolled. Demographic information including age, race, glaucoma diagnosis, lens status, and medication use was recorded. Corneal curvature was measured with a manual keratometer. Patients were randomly divided into GAT first or DCT first groups; all measurements were done on the right eye. For GAT readings the tonometer dial is randomly set and a pressure taken. This was repeated for 3 readings, which were averaged to represent the GAT pressure. For the DCT readings the tip is apposed to the cornea until a pressure tracing appears on the LCD screen. The tip is left in place for 5-10 seconds as the ocular pulse pressure tracing is collected. The tip is then retracted and reapposed in a similar manner for a total of three readings. The calculated intraocular pressure and ocular pulse amplitude is displayed on the LCD screen for each tracing. The average of three DCT measurements was used to represent the DCT pressure. Corneal thickness was measured with an ultrasound pachymeter. Results: Data was collected on 75 individuals during initial enrollment. DCT mean IOP (19.4 mmHg) was statistically different (p<0.001) from GAT IOP (17.5 mmHg). However, the DCT and GAT values correlated very highly (r=0.951; p<0.0001). Coefficient of variation (100*SD/mean) was computed for each set of three readings. Comparing the proportion of data sets with a COV <5%, DCT showed better reproducibility within an individual set of readings as compared to GAT (75% vs 51% respectively). A signed delta (DCT-Goldmann) and absolute value delta were calculated and the relationship with other variables was determined in order to find factors that contributed to the DCT-Goldmann differences. Corneal thickness was found to be correlated to IOP differences, with thicker corneas having smaller differences between the two techniques. Conclusions: DCT measurements correlated well with Goldmann applanation. A direct comparison cannot be made, as no true gold standard is available in standard clinical practice. Corneal thickness seems to be partially responsible for pressure differences between the two devices.
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