Editors Selection (IGR: 12-1) | International Glaucoma Review #809

Editors Selection IGR 12-1

Outflow: Outflow facility and body position

Comment by Albert Alm on:

25210 Aqueous humor outflow facility by tonography does not change with body position, Selvadurai D; Hodge D; Sit AJ, Investigative Ophthalmology and Visual Science, 2010; 51: 1453-1457

What happens to aqueous humor dynamics when we lie down? Previous studies have shown that intraocular pressure (IOP) and episcleral venous pressure (Pev) increase while there is no effect on aqueous flow. However, the effect of body position on outflow is not known. Selvadurai et al. (155) have tried to fill this gap. There are two reasons to try to find out if outflow is changed in the supine position or not. First, it has not been proven that all of the increase in IOP could be explained by the increase in Pev. Second, and more intriguing, there are reports that the increase in IOP is larger in eyes with glaucoma than in normal eyes, suggesting that something happens to outflow in eyes with glaucoma when body position is changed. In this carefully executed study the investigators focused on normal eyes. They found no significant effect on outflow facility with a change in body position. However, as pointed out by the authors, the lack of significance only means that the study failed to provide evidence for a change in outflow facility.

There is no significant effect on outflow facility with a change in body position

The change in IOP was only 2 mmHg. This could certainly be explained by a combination of an increase in Pev, in the range reported by previous investigators, and a small increase in outflow facility, smaller than the detection threshold for the present study. Considering the lack of precision of several of the techniques involved in studies on aqueous humor dynamics it will be very difficult, if not impossible, to sort out the cause/causes behind such a small change in IOP. Assuming a larger change in eyes with glaucoma it could, of course, be worthwhile to repeat the study in such eyes. As it is this particular aspect of aqueous humor dynamics in glaucoma eyes remains a challenge.

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