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C. G. Hoffmann, E. Spoerl, R. Koch, L. E. Pillunat, A. G. Boehm; Astigmatism Induced by the Cannulation of the Anterior Chamber Alters IOP Readings by Pascal Dynamic Contour Tonometer During Intracameral Measurements. Invest. Ophthalmol. Vis. Sci. 2008;49(13):685.
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© ARVO (1962-2015); The Authors (2016-present)
To examine the effect of astigmatism induced by cannulating the anterior chamber on Pascal Dynamic Contour Tonometer (DCT) readings during intracameral measurements.
In a prospective clinical trial 99 eyes of 99 patients scheduled for cataract surgery were examined. Before phacoemulsification the anterior chamber was cannulated at the temporal corneal limbus. In a closed system the IOP was directly set to 20 and 35 mmHg by manometric water column in a random sequence. Before and after the cannulation astigmatism (Nidek Autokeratomter KM500) and corneal topography (Schwind Corneal Wavefront Analyzer) was measured. After measuring 45 patients (group1: M:F = 1:2; mean age 68±10y; CCT 566±38 µm; corneal curvature 42.97±1.22D, axial length 23.34±1.52mm) a preliminary evaluation of the corneal topography showed a significant astigmatism with a steepening of the cornea towards the temporal site of cannulation. Therefore at the following 54 patients (group 2: M:F = 5:6; mean age 71±9y; CCT 573±35 µm; corneal curvature 42.88±1.45D, axial length 23.67±1.66mm) the tonometer tip was decentered minimally nasal to minimize the effect of the mechanically induced astigmatism.
In group 1 the induced astigmatism was -3.68D at 20mmHg and 2.73D at 35mmHg, in group 2 4.6D and 3.28D, respectively. In both groups differences between intracameral IOP and IOP measured by DCT were statistically significant correlated with the induced astigmatism (Group 1: R² =0.312, slope of the regression line -0.57mmHg/D; group 2: R² = 0,098 and the slope -0.2mmHg/D, respectively). The difference between intracameral IOP and IOP measured by DCT was smaller in group 2 with -0.23±1.54mmHg, compared to -1.75±2.84mmHg in group 1 (P<0.01).
Mechanically induced astigmatism by cannulation of the anterior chamber effects measurements of DCT. Higher astigmatism leads to falsely low DCT readings. By adjusting the position of the tonometer tip according to the corneal topography measurement errors can be reduced in intracameral measurements. For the evaluation of DCT measurements during intracameral measurements the mechanically induced astigmatism needs to be considered to avoid data misinterpretation.
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