Effect of body position on intraocular pressure and aqueous flow

The relationship between intraocular pressure and the rate of aqueous humor formation was studied in human subjects. Intraocular pressure was altered by changing the subject's gravity-dependent body position. Aqueous humor flow was measured fluorophotometrically. An average change of intraocular pressure of 2.4 +/- 1.2 mmHg and 11.2 +/- 2.7 mmHg was recorded in subjects whose body position was alternated +/- 15 degrees from horizontal and +/- 50 degrees from horizontal. No change of the rate of fluorescein clearance was observed for the milder change of position. A small change of fluorescein clearance was noted in response to the steeper change of position. Changing intraocular pressure seems not to induce compensatory changes in aqueous flow. Therefore we conclude that aqueous formation is relatively pressure-insensitive, a conclusion one would draw from clinical instances of angle-closure glaucoma or as in other systems of secretion, such as the toad bladder or the cerebrospinal fluid, where alterations in pressure do not necessarily influence flow. If there is a regulator of aqueous flow it is not responsive to the changes in intraocular pressure brought about in this study. If aqueous flow regulates intraocular pressure or compensates for changes in intraocular pressure, the signal is not small changes in pressure.