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A. Kotecha, E.T. White, J.M. Shewry, D.F. Garway–Heath; The Relative Effects of Corneal Thickness and Age on Goldmann Applanation Tonometry and Dynamic Contour Tonometry . Invest. Ophthalmol. Vis. Sci. 2005;46(13):4838.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Goldmann applanation tonometry (GAT) is influenced by corneal biomechanical properties. Pascal dynamic contour tonometry (DCT) uses a direct transcorneal method to measure IOP and is said to be unaffected by corneal biomechanics. The aims of this study are to assess agreement between GAT and DCT, to evaluate the effect of central corneal thickness (CCT) and age on GAT and DCT, and to compare the repeatability and reproducibility (intra– and inter–observer variation) of GAT with DCT. Methods: GAT and DCT measurements were made on 130 eyes of 130 patients. Agreement between IOP measurements was determined using Bland–Altman plots– mean difference and 95% limits of agreement were calculated. The effect of CCT and age on GAT– and DCT–measured IOP, and on GAT/DCT IOP differences, was assessed by linear regression analysis.The power to detect a correlation of r=0.3 between CCT and IOP was 90% at p<0.05. Precision of GAT and DCT measurements was assessed in 100 eyes of 100 patients in whom IOP was measured with GAT and DCT in a random order by an experienced clinician and two technicians. The repeatability of GAT and DCT was determined from calculations of the within–subject standard deviation. Bland Altman plots were used to assess the reproducibility of each technique. Results: The mean difference (95% limits of agreement) between GAT and DCT was –0.7 (–6.3 to 4.9) mmHg. There was a significant relationship between GAT–measured IOP and CCT, with a 3.2–mmHg increase in IOP per 100–µm CCT change (slope 0.032, r2 =0.06, p=0.004). There was no significant relationship between DCT–measured IOP and CCT (slope 0.013, r2 =0.02, p=0.156). For every 100–µm increase in CCT, there was a 1.7–mmHg increase in GAT/DCT IOP differences (slope 0.017, r2 =0.05, p=0.01). GAT/DCT IOP differences increased significantly with age (slope 0.05, r2 =0.05, p=0.01). The intra–observer variability, or repeatability, of GAT and DCT was 1.7 and 3.2 mmHg, respectively. The inter–observer variability, or reproducibility, was [mean difference between observers (95% limits of agreement)] 0.4 (–3.5 to 4.2) mmHg for GAT and 0.2 (–4.9 to 5.3) mmHg for DCT. Conclusions: GAT is affected significantly more than DCT by both CCT and subject age. Subject age accounted for as much inter–subject variation in GAT/DCT differences as did CCT (5% each); age and CCT both accounted for a difference of 3.0 mmHg between the youngest and oldest eyes and between the thinnest and thickest corneas. These effects were independent of each other. The DCT has greater measurement variability than the GAT.
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