Correspondence of Tono-Pen intraocular pressure measurements performed at the central cornea and mid-peripheral cornea.

ABSTRACT

BACKGROUND: As the awareness of the influence of central corneal thickness (CCT) on Goldmann tonometry has increased, many publications have questioned the accuracy of Goldmann intraocular pressure (IOP) measurement. The Tono-Pen, because it indents a much smaller surface area when compared to a Goldmann probe, may be less affected by corneal thickness variations when compared with Goldmann tonometry. METHODS: Forty human subjects with no history of refractive surgery participated in this study. The IOP of the right eye of each subject was measured with the Goldmann tonometer, the Tono-Pen at the central cornea, and the Tono-Pen at the mid-peripheral cornea. An ultrasonic DGH Pachette pachymeter was used to measure the central and mid-peripheral corneal thickness at the location of the IOP readings. RESULTS: Tono-Pen measurements at the central and mid-peripheral cornea highly correlated (r = 0.933), and did not significantly differ (p = 0.646). The IOP readings with the Goldmann tonometer (r= 0.406), the Tono-Pen at the central cornea (r = 0.453), and the Tono-Pen at the mid-peripheral cornea (r = 0.321) showed a positive correlation to corneal thickness. The Goldmann and Tono-Pen tonometers differed significantly in the measurement of IOP at the central cornea (p = 0.007), but were positively correlated (r = 0.674). CONCLUSIONS: The Tono-Pen IOP measurement at the central cornea highly approximated Tono-Pen IOP measurement at the mid-peripheral cornea. Furthermore, although not highly correlated, both the Goldmann and Tono-Pen tonometers showed a significantly positive correlation between IOP and corneal thickness measurements.