An Assessment of Intraocular Pressure Change in Healthy Subjects during Air Flight

Dear Editor:

I read with interest the article about the effect of air flight on intraocular pressure (IOP).¹ I would like to suggest a mechanism that could explain this finding based on a physiologic concept of outflow facility.

If the eye is compressed with honan balloon or excessive outside pressure, what will happen? Extra pressure leads to more drainage for a fixed secretion of aqueous and return of IOP to its initial amount. To do so, the anterior chamber (A/C) volume decreases. In this instance, we assume that outflow facility, which is measured by micL/min/mmHg, remains constant. It means more pressure leads to more drainage of aqueous via conventional outflow through the trabecular meshwork to restore the initial IOP.²

If A/C volume decreases because of more drainage, the A/C angle and its configuration will change, so the assumption of constant outflow facility is not correct.² In this situation, because of decrease in outflow facility, IOP does not return to its initial amount but gains a somewhat higher amount. The reverse is true for decrease in external pressure with air flight. Decrease in external pressure leads to more A/C volume and depth and increase in outflow facility. It means decrease in IOP. Decrease in IOP following cataract surgery could be explained by the same mechanism of increase in outflow facility by increasing in A/C volume. Physical activity and sympathetic stimulation also could deepen and increase A/C volume. Increase in outflow facility by this mechanism might augment the IOP drop in high altitude to a degree that IOP over-reading, because of hypoxic corneal edema, could not be seen.

Why some researchers do not take this result? Hypoxia leads to corneal thickening and over-reading in IOP. This will mask the true IOP-lowering effect of high altitude as stated by Bayer et al. (2008). I would like to suggest the same mechanism for retinal vascular dilation. Retinal hypoxia may result in retinal vascular dilation, but not the increase in altitude by itself. This picture is also seen in anemia and other hypoxemic states.³

It is suggested comparing IOP in people who are living at sea level and mountains and also their A/C parameters as further research for this theory.

Sincerely, Dr. Morteza Mehdizadeh

Declaration of interest: The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.