Relationship between corneal thickness and measured intraocular pressure in a general ophthalmology clinic1
Section snippets
Materials and methods
Data were collected prospectively by a review of 235 normal control eyes with no corneal/ocular surface disorder and no evidence of glaucomatous disease, 335 eyes diagnosed as having POAG, 52 eyes with NTG, 12 eyes with PXE, 42 eyes with CACG, and 232 GS eyes. The eyes had previously been classified into the subgroups above on prior appointments. All glaucoma/GS eyes had previously undergone full ophthalmic examination, including applanation tonometry, gonioscopy, funduscopy, and repeated
Results
The CCT (mean and 95% confidence intervals) for both eyes of each patient individually and together is given in Table 1. A boxplot showing the spread of corneal thickness (all eyes) is given in Figure 1. The distribution of corneal thickness by diagnosis is given in Figure 2, Figure 3, Figure 4, Figure 5, Figure 6, Figure 7, which are drawn to the same scale to aid comparison between the different graphs. The histograms show that the CCT of eyes with a diagnosis of normal, POAG, and GS follow a
Discussion
The IOP measurement by applanation tonometry is based on the Imbert-Fick principle, which asserts that the pressure inside a liquid-filled sphere can be determined by measuring the force required to flatten the surface of the sphere.2 The original formulation of this principle assumed that the cornea is infinitely thin, perfectly elastic, and perfectly flexible and ignored the effects of surface tension (i.e., it assumed that the only force acting on it is the force of the applanating surface).
Acknowledgements
The authors thank B. Faragher, Department of Medical Statistics, University of Manchester, England, for his statistical advice and R. K. Mehta, A. M. Morrison, and S. Wallis, Eye Department, Royal Bolton Hospital, Bolton, England, for referring patients for this study.
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The authors have no proprietary interest in the development or marketing of any pachymeter.