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J.P. Levine, K. Wals, P.S. Rosenbaum, J.S. Schultz; Measuring Central Corneal Thickness by Ultrasound Pachymetry After Applanation Tonometry: Are the Results Valid? . Invest. Ophthalmol. Vis. Sci. 2005;46(13):2738.
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Purpose: The influence of central corneal thickness (CCT) on measured intraocular pressure (IOP) has gained increasing clinical significance in recent years. Ultrasound pachymetry is often performed after applanation tonometry to confirm ocular hypertension in patients who present with borderline or elevated IOP. As corneal tissue is compressible, applanation of the cornea may theoretically result in temporary thinning of the cornea and may lead to a falsely low CCT if pachymetry were immediately performed afterward. The purpose of this study is to determine if pachymetry performed immediately after applanation is valid. Methods: Central corneal thickness was measured with an ultrasonic pachymeter (DGH 500 Pachette; DGH Technology, Inc., Exton, PA) in 36 eyes of 19 subjects with no known corneal pathology. There were 26 eyes in the applanation group and 10 control eyes not applanated. Ten measurements were taken for each eye; five prior to applanation and five immediately after applanation in the experimental group. Control eyes had a five–minute break in between the first five and second five CCT measurements. Five consecutive CCT measurements were averaged to determine an individual mean CCT (IMCCT). IMCCTs were averaged to calculate a group mean CCT (GMCCT). Group means were compared using a paired t–test to assess statistical significance. A linear model according to Argus (1995) was used in this study to calculate IOP adjustment given an IMCCT. The IOP adjustment before applanation was compared to the IOP adjustment after applanation for each eye. As per our literature review, a clinically significant outcome was defined as a difference in IOP adjustment of 1.5 mm Hg or greater. Results: The GMCCT for patients in the applanation group was 529.59 ± 48 µm before applanation compared to 526.98 ± 46 µm after applanation. The decrease in CCT of 2.61 µm was statistically significant (p < 0.05). The baseline GMCCT for patients in the control group was 534.62 ± 46 µm compared to 534.6 ± 46 µm after a five minute break (p = 0.98). Using the linear model for IOP correction, none of the 36 eyes in our study had a clinically significant difference in IOP adjustment between their first and second set of pachymetry measurements. Conclusions: Central corneal thickness measurements taken immediately after applanation are valid. Although applanation may cause a statistically significant decrease in corneal thickness the difference is of no clinical significance.
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