Abstract
Purpose: :
Intraocular pressure (IOP) varies with body position, and these variations are assumed to be in part caused by changes in episcleral venous pressure (EVP). However, the effect of body position on EVP is poorly understood. In this study, we investigated changes in EVP between two body positions, sitting and prone.
Methods: :
IOP was measured by using a pneumatonometer in 25 eyes of 13 healthy volunteers (mean age, 43 years; range 24 to 73 years) in a seated position. EVP was then measured 4 times in a selected episcleral vein by using an automated, slit-lamp mounted, venomanometer, based on the pressure chamber method. Venous pressure was assumed to be equal to the pressure in the chamber when the vein first began to collapse as the chamber was inflated at a constant rate. After 30 minutes, the subject was placed in a prone position. After five minutes, IOP was remeasured and EVP in the same vein was remeasured twice with the neck extended and the head resting on the chin rest of the slit lamp. EVP in the prone position was compared to EVP in the seated position by using a paired t-test. The change in IOP between the seated and prone positions was also compared to the change in EVP by using a paired t-test.
Results: :
Mean IOP in the sitting and prone positions were 13.0 ± 2.6 mmHg (mean ± SD) and 15.2 ± 2.6 mmHg respectively and were significantly different from each other (p<0.001). Mean EVP in the sitting and prone positions were 6.4 ± 1.4 mmHg and 7.7 ± 1.7 mmHg and were significantly different from each other (p<0.001). The rise in IOP in the prone position from IOP in the seated position (2.1 ± 1.0 mmHg) was greater than the rise in EVP in the prone position from EVP in the seated position (1.3 ± 1.5 mmHg, p=0.015).
Conclusions: :
In the prone position IOP and EVP are higher than they are in the sitting position. The rise in EVP when changing from the upright to prone position could partly explain the rise in IOP. However, the Goldmann equation predicts that the change in IOP should be equal to the change in EVP if aqueous humor flow, outflow facility and uveoscleral flow remain constant. This suggests that other factors, in addition to the increase in EVP, may contribute to the rise in IOP when changing from upright to recumbent body position. Understanding the origin of the positional changes in IOP would advance our understanding of physiologic factors that determine IOP and the origin of pressure elevation in glaucoma.
Keywords: aqueous • intraocular pressure • outflow: trabecular meshwork