Intraocular Pressure Measurement Using Rebound Tonometer for Deviated Angles and Positions in Human Eyes

Purpose: To investigate the influence of positional deviations on rebound tonometer measurement of intraocular pressure in humans. Materials and methods: Intraocular pressure was measured using the Icare rebound tonometer on the right eyes of 53 subjects in various conditions as follows: first, at a distance of 4, 6, or 8 mm from the center of the cornea with the probe perpendicular to the corneal plane; then, at 2 mm from the limbus in the nasal and temporal regions with the probe perpendicular to the corneal plane or along the visual axis; and lastly, with the angled probe touching the central cornea at angles of 10 degrees or 20 degrees. Results: Bland-Altman plots between the Goldmann applanation tonometer and rebound tonometer at various conditions revealed 95% limits of agreement ranging from +/- 4.5 to +/- 5.6 mm Hg. Intraocular pressures measured using the rebound tonometer were significantly lower than those measured using the Goldmann applanation tonometer when the rebound tonometer probe was placed 2 mm from the limbus in the temporal or nasal regions with the probe along the visual axis or when the probe was angled to touch the central cornea at an angle of 10 degrees or 20 degrees. In other positions, the intraocular pressures measured using the rebound tonometer were not significantly different. The rebound tonometer, noncontact tonometer, and Tonopen XL showed good agreement with the Goldmann applanation tonometer for intraocular pressure readings under optimal conditions. Conclusions: The intraocular pressures determined using the rebound tonometer were approximately equal to those obtained using the Goldmann applanation tonometer when the rebound tonometer measurements were made with the probe perpendicular to the corneal plane, irrespective of the location, that is, at the central cornea or 2 mm from the limbus.