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Arash Kazemi, Jay W McLaren, Sayoko Eileen Moroi, Carol B Toris, Shuai-Chun Lin, Arthur J Sit; Comparison of Aqueous Outflow Facility Measurement by Pneumatonography and Digital Schiøtz Tonography. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):4851.
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© ARVO (1962-2015); The Authors (2016-present)
Electronic Schiøtz tonography was once the standard method for measuring outflow facility but it is no longer commercially available. This method has been replaced by a commercial pneumatonometer with a tonography option. In this study we compared outflow facility measured by pneumatongraphy to outflow facility measured by digital Schiøtz tonography.
Fifty eyes from 25 healthy adult subjects (age 41-67 years, mean 50 years) were examined as an ancillary study to an existing clinical trial. Intraocular pressure (IOP) was measured in the supine position by pneumatonometry (Model 30 Classic, Mentor). Outflow facility was then measured by pneumatonography (10-gm weight, 2 minutes) and 45 minutes later by custom digital Schiøtz tonography (5.5-gm weight, 4 minutes) in the supine position. The pneumatonographic printed tracings were digitized, a second-order polynomial was fitted to pressure vs time, and outflow facility was determined from the IOP on the fitted curve at 0 and 2 minutes. A second-order polynomial was fitted to pressures measured by Schiøtz tonography and outflow facility was determined from pressure on the curve at 0 and 4 minutes by using standard tables and nomograms. Ocular rigidity was calculated for each eye from the supine IOP and IOP at the beginning of the pneumatonogram and used to determine outflow facility for both methods. Significance of the difference between instruments was determined by using generalized estimating equation models to account for possible correlation between fellow eyes of the same subject, and relationships were illustrated by Pearson correlation.
Mean outflow facility was 0.23± 0.07 µL/min/mm Hg (± SD) by Schiøtz tonography and 0.21± 0.07 µL/min/mm Hg by pneumatonography (p=0.003). Outflow facilities by the two methods were correlated (r=0.68, p<0.001). Mean ocular rigidity was 0.028.
Outflow facilities measured by Schiøtz tonography and pneumatonography are correlated, but Schiøtz tonography reports slightly higher values. This study cannot determine which method is more accurate. Difference in shape and size of the two tonographer tips may be responsible for differences in outflow facility between these instruments.
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