Influence of corneal thickness on comparative intraocular pressure measurements with goldmann and non-contact tonometers in keratoconus

Background: The influence of corneal thickness and curvature on the difference between intraocular pressure (IOP) measurements obtained with non-contact (NCT) and those with the Goldmann applanation tonometer (GAT) was studied in patients with keratoconus. Materials and methods: We examined 113 patients with keratoconus. IOP was measured by the Canon TX 10 non-contact tonometer and the Goldmann tonometer, corneal curvature and thickness were obtained by Humphrey Automatic Refractometer Keratometer and Orbscan Version 3.0 Bausch & Lomb Surgical. Results: The IOP measured by NCT was significantly lower than that measured by GAT. The mean pachymetry of the thinnest point was 423.15 +/- 98.43 mum for the right eyes and 426.7 +/- 93.88 mum for the left eyes. The difference between NCT-GAT and corneal thickness showed a significant negative correlation (r = -0.427, p < 0.0001; t = -3.677, p < 0.0001). Values of NCT measurements were significantly increasing with corneal thickness (F = 6.505, p < 0.0001 for right eyes and F = 4.37, p = 0.004 for left eyes), whilst GAT measurements did not show a significant influence of the corneal thickness. The keratometry had no effect on the difference between NCT-GAT measurements (t = 1.090, p = 0.278). Conclusions: The thin cornea has more influence on the measurement with NCT than GAT. The relative precision of NCT compared with GAT seems to be influenced by the corneal thickness.