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A.H. Bloom, H.D. Perry, E.D. Donnenfeld; Applanation Tonometry and Pneumatonometry in Post-keratoplasty Eyes with High Astigmatism . Invest. Ophthalmol. Vis. Sci. 2003;44(13):4713.
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Purpose: To assess the effect of astigmatic axis on applanation tonometry measurements, and to evaluate the reliability of pneumatonometry, in post-keratoplasty eyes with high regular astigmatism. Methods: Twenty-one evaluations were made of post-keratoplasty eyes at various stages of suture removal. The axis and magnitude of corneal astigmatism was determined using a corneal topographer (Humphrey Corneal Topographer Model 991). Only eyes with high regular astigmatism by topography were included in the study. The intraocular pressure was then measured using a pneumatonometer (Mentor Model 30 Classic Pneumatonometer). Goldmann applanation tonometry was performed in the flattest axis, and then in the steepest axis. Results: The average corneal astigmatism was 12.5 D (range, 6 D – 19.8 D), and the average difference between the steep axis and flat axis applanation tonometry readings was 6.4 mmHg (range, 1 mmHg – 14 mm Hg). Since the true intraocular pressure was assumed to be the average of the readings in the steep axis and the flat axis, only measuring one of the two would cause an average error of 3.2 mmHg (range, 0.5 mmHg – 7 mmHg). This corresponds to an average error of 1 mmHg for every 3.9 D of astigmatism. Pneumatonometry readings correlated the best with the steep axis applanation tonometry (Pearson correlation coefficient = 0.86), followed by the average applanation tonometry (Pearon correlation coefficient = 0.81), and correlated worst with the flat axis applanation tonometry (Pearson correlation coefficient = 0.62). Conclusions: Astigmatic axis in post-keratoplasty eyes with high astigmatism may have a clinically significant effect on applanation tonometry readings. Similar to previous reports for eyes without corneal transplants, the average maximum error that could be made was about 1 mmHg for every 4 D of astigmatism. However, there was great variability in this relationship, and in some cases the maximum error was as large as 1 mmHg for every 1.7 D of astigmatism, demonstrating the need to take astigmatism into account when performing applanation tonometry post-keratoplasty. Pneumatonometry correlated better with the steep axis applanation tonometry than the flat axis applanation tonometry. Pneumatonometry may therefore be more likely to overestimate than underestimate the true intraocular pressure in post-keratoplasty eyes with high astigmatism.
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