Comparison of Aqueous Outflow Facility Measurement by Pneumatonography and Digital Schiotz Tonography

PURPOSE. It is not known if outflow facilities measured by pneumatonography and Schiotz tonography are interchangeable. In this study we compared outflow facility measured by pneumatonography to outflow facility measured by digital Schiotz tonography. METHODS. Fifty-six eyes from 28 healthy participants, ages 41 to 68 years, were included. Intraocular pressure (IOP) was measured in the sitting and supine positions with a pneumatonometer. With the subject in the supine position, IOP was recorded for 2 minutes by using a pneumatonometer with a 10-g weight and for 4 minutes by using a custom digital Schiotz tonometer. Outflow facility was determined from the changes in pressure and intraocular volume and a standard assumed ocular rigidity coefficient for each instrument, respectively, and by using an ocular rigidity coefficient calculated by measuring pressure without and with a weight added to the pneumatonometer tip. RESULTS. The outflow facility was 0.29 +/- 0.09 mu L/min/mm Hg by Schiotz tonography and 0.24 +/- 0.08 mu L/min/mm Hg by pneumatonography (P < 0.001) when using the standard assumed constant ocular rigidity coefficient. Mean calculated ocular rigidity coefficient was 0.028 +/- 0.01 mu L-1, and outflow facility determined by using this coefficient was 0.23 +/- 0.08 mu L/min/mm Hg by Schiotz tonography and 0.21 +/- 0.07 mu L/min/mm Hg by pneumatonography (P = 0.003). Outflow facilities measured by the two devices were correlated when the ocular rigidity was assumed (r = 0.60, P < 0.001) or calculated (r = 0.70, P < 0.001). CONCLUSIONS. Outflow facilities measured by pneumatonography were correlated with those measured by Schiotz tonography, but Schiotz tonography reported approximately 10% to 20% higher facilities when using the standard method. When ocular rigidity was determined for each eye, differences were smaller. Measurements from these devices cannot be compared directly.