Tono-Pen tonometry in normal and in post-keratoplasty eyes

Oculab Tono-Pen tonometry was compared with Goldmann applanation tonometry in 82 eyes of 82 patients with normal corneas and in 54 eyes of 54 patients who had undergone penetrating keratoplasty and whose corneas did not preclude the use of Goldmann tonometer. We found that the intraocular pressure (IOP) in 48% of the eyes with normal corneas and in 57% after keratoplasty has different measurements with Goldmann and Tono-Pen pressures of 3 mm Hg or more. Despite the correlation between the Goldmann tonometer and the Tono-Pen in the group of eyes with normal corneas (r = 0.83) as well as in the group of eyes after keratoplasty (r = 0.79) the Tono-Pen tended to significantly overestimate the Goldmann tonometer reading (p < 0.0001). The mean difference between the two instruments was highest across the lower IOP range (< 9 mm Hg) in the group of eyes after keratoplasty. Because the mean absolute values of the paired differences between Goldmann and Tono-Pen measurements varied significantly across all IOP intervals it was not possible to establish a correction factor which could be used when comparing the two measurements. Based on this study the Tono-Pen consistently overestimated the actual IOP in an unpredictable manner. Where possible Goldmann measurements of the IOP are still to be preferred.