The effect of corneal edema on dynamic contour and goldmann tonometry.

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The effect of corneal edema on dynamic contour and goldmann tonometry. Optom Vis Sci. 2008 Jun;85(6):451-6 Authors: Hamilton KE, Pye DC, Kao L, Pham N, Tran AQ PURPOSE.: To determine the effect of contact lens-induced corneal edema on measurements of intraocular pressure (IOP) using the Pascal dynamic contour tonometer (DCT), compared with the Goldmann tonometer. METHODS.: Thirty young healthy subjects (23.0 +/- 3.0 years) were recruited from the student population at the University of New South Wales. Thick hydroxyethyl methacrylate contact lenses were worn monocularly for 2 hours under closed-eye conditions to induce corneal edema via hypoxia. IOP (Goldmann and Pascal DCT), ocular pulse amplitude (OPA), and central corneal thickness (CCT) were measured in both eyes before and after lens wear. Paired t-tests, Pearson correlation, and Bland-Altman plots were used to identify changes in, and relationships between, these parameters resulting from corneal edema. RESULTS.: Lens wear resulted in statistically significant changes in CCT (+48.3 +/- 14.4 mum, p < 0.001), Goldmann IOP (+1.5 +/- 2.8 mm Hg, p = 0.007), and Pascal DCT IOP (-0.7 +/- 1.1 mm Hg, p = 0.001) but not OPA (0.0 +/- 0.3 mm Hg, p = 0.721, two-tailed paired t-test). The Pascal DCT provided IOP readings that were 1.3 +/- 2.0 mm Hg higher than the Goldmann IOP readings when hydration was normal, but the Goldmann tonometer provided readings that were 0.8 +/- 2.5 mm Hg higher than the Pascal DCT readings when the cornea was edematous. The variation between the two instruments was weakly correlated to the change in CCT (r = -0.261, p = 0.044). CONCLUSIONS.: Contact lens-induced corneal edema caused a small underestimation error in IOP measurements by the Pascal DCT, and an overestimation error in Goldmann tonometry measurements. The OPA measurement provided by the Pascal DCT is insensitive to corneal edema-induced changes in corneal properties. PMID: 18521023 [PubMed - in process]

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[Gary R. Bell]

Good Methodology

Some surprises were found here. Conventional wisdom is that corneal edema softens the cornea, which should produce lower IOP readings. However, these researchers found a 1.5 mm increase in IOP with Goldmann readings, but a small decrease with the DCT. Is conventional wisdom wrong? Was there a methodology problem with this study?

I think there could be a methodology flaw in this study. They said they used young subjects in this study, which means most of them had active accommodation in play. It has been known for forty years, but largely ignored, that active accommodation can lower IOP, and can do so 3-4 mm Hg. This is not to say that accommodation affected one set of readings more than another. But, it does say that if you don't relax accommodation on all readings, you have an uncontrolled factor that is contaminating your results. So, I would say if accommodation wasn't relaxed on all the readings, the accuracy of the results may have been seriously compromised. If that makes me a disagreeable S.O.B., then so be it.