Purpose: The mechanism of IOP change during a body position change is poorly understood. In this study, we investigated changes in episcleral venous pressure (EVP) between two body positions, sitting and inclined, and compared this with changes in IOP.
Methods: This study was a prospective, comparative case series of 43 eyes of 24 healthy volunteers. IOP was measured using a pneumatonometer in the seated position. EVP was then measured in a selected episcleral vein by using an automated, slit-lamp-mounted venomanometer. Thirty minutes later, the subject was placed in an inclined position with the neck extended and the head resting on the chin rest of the slit lamp. After 5 minutes, IOP and EVP in the same vein were remeasured. EVP in the inclined position was compared with EVP in the seated position, and the change in IOP was compared with the change in EVP. Statistical significance was determined using generalized estimating equation models.
Results: Mean IOP increased from 11.4 ± 3.0 mm Hg (mean ± SD) in the sitting position to 13.1 ± 3.4 mm Hg in the inclined position (P < 0.001). Mean EVP increased from 6.4 ± 1.4 mm Hg in the sitting position to 7.8 ± 1.7 mm Hg in the inclined position (P < 0.001). The postural rise in IOP was not different from the rise in EVP (P = 0.18).
Conclusions: In the inclined position, IOP and EVP are higher than they are in the sitting position. The posture-induced rise in IOP can be attributed to an increase in EVP.