Measurement of intraocular pressure using the Tono-Pen in comparison with Goldmann applanation tonometry - a clinical study in 100 eyes

Background: In clinical practice ophthalmologists often need a tonometer which is independent of a slit lamp. Such a hand-held device is the Tono-Pen. We compared the precision of two equal Tono-Pens with Goldmann applanation tonometry. Material and methods: Measurement of intraocular pressure (IOP) was done in 100 eyes of 51 patients (mean age 63 15 years) suffering from ocular hypertension or glaucoma. According to a random table either the right or left eye was measured using Goldmann tonometer first and the Torio-Pen second. For the other eye the measurement was reversed. One of the two equal Tono-Pens (So-Ian/USA) was used according to a second random table. Three measurements were obtained with each instrument on both eyes within 15 minutes subsequently. Patients were placed in an upright position for all measurements. Results: Even for well-trained ophthalmologists a learning curve of approximately 10 measurements was observed using the Tono-Pen. The Tono-Pen measured an average IOP of 16.9 mm Hg in all 100 eyes. The Goldmann tonometer measured an average IOP of 17.7 mm Hg. The difference was not statistically significant. The standard deviation for all measurements was better for the Tono-Pen (4.7 mm Hg vs 5.8 mm Hg for Goldmann tonometer). No reduction of the IOP after Tono-Pen measurement was observed (in contrast to the Goldmann tonometer). The reproducibility of the Tono-Pen on the same eye was inferior to the Goldmann tonometer by a factor of 2. There was an almost significant difference in reproducibility between two equal Tono-Pens. Conclusions: Measurement of IOP with the Tono-Pen is comparable to Goldmann applanation tonometry if an average of 3 measurements is used. The difference between two equal Tono-Pens indicates the need for improvement of the quality check during production.