Intraocular and episcleral venous pressure increase during inverted posture.

We measured intraocular and episcleral venous pressure in 11 subjects in both the supine and the head-down vertical position. Gonioscopy was performed in eight subjects. The intraocular pressure changes were correlated with the alterations in episcleral venous pressure using linear regression analysis. We found that for each 0.83 +/- 0.21 mm Hg increase in episcleral venous pressure there was a rise of 1 mm Hg in intraocular pressure (r = .80, P = .003). Upon inversion, blood appeared in Schlemm's canal in half of the eyes studied with gonioscopy, suggesting that it refluxed into the canal from increased episcleral venous pressure. The mechanism of a sustained intraocular pressure rise during gravity inversion appears to be closely related to increased venous pressure in the orbit.