We read with great interest the article by Di Staso and Gaudio about measurement of aqueous humor outflow facility using the Falck Medical Applanation Tonometer (Falck Medical, Inc., Mystic, CT).1 Our group has extensive experience over decades measuring aqueous humor dynamics in humans, including outflow facility measurements using constant-weight tonography and fluorophotometry.2,3 While these measurements are noninvasive, they can be technically challenging and time-consuming. We were therefore excited at the prospect of a much quicker method for measurement of outflow facility in humans.
Unfortunately, their study does not compare the outflow facility measurements to other methods, which is typical for most new data and results generated from new technologies. Furthermore, to the best of our knowledge, there are no published comparisons of this device with an accepted method of measuring outflow facility. Our examination of their data in the manuscript suggests that outflow facility is not being measured based on a novel, proprietary technique, which the authors describe as “the device identifies aqueous humor by capturing the optical signature of an intrinsic compound and measures its egress through the trabecular meshwork.” Instead, comparing the intraocular pressure (IOP) measurements from the Falck device with the associated outflow facility values shows a near-perfect correlation using a power relationship (Fig. 1). This is unheard of in biological systems.
Fig. 1.
Relationship between outflow facility measurements and IOP from the Falck Medical Applanation Tonometer (Falck Medical, Inc., Mystic, CT) device showing a near-perfect correlation with a power relationship
A relationship of this form between IOP and outflow facility can only be achieved using the Goldmann equation if episcleral venous pressure is assumed to be zero, and aqueous humor production rate and uveoscleral outflow rate are assumed to be constant across all eyes under all conditions. Both assumptions are of course completely invalid and are contrary to decades of research in aqueous humor dynamics.4,5 Simplifying this relationship, it appears that outflow facility may be calculated as C = 3.5/IOP in the Falck device (R = −0.944). While disappointing, this calculation is a much more realistic explanation of the data than the notion that this device can image and record the rate of egress of a novel “intrinsic compound” not previously identified, using techniques not previously described. If such a breakthrough existed, then it should not be hidden in a device that does not fully disclose the technology as it would fundamentally alter the measurement of fluid dynamics in medicine and beyond.
Orcid
Arthu J Sit https://orcid.org/0000-0002-3904-2625
Carol B Toris https://orcid.org/0000-0003-3794-6142
David M Reed https://orcid.org/0000-0002-4772-6138
Sayoko E Moroi https://orcid.org/0000-0002-0418-0657
Footnotes
Source of support: Nil
Conflict of interest: None
References
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