March 2014
Volume 55, Issue 3
Free
Letters to the Editor  |   March 2014
Dependence of Outflow Facility on Intraocular Pressure?
Author Notes
  • Department of Ophthalmology, Roskilde Hospital, Roskilde, Denmark. 
Investigative Ophthalmology & Visual Science March 2014, Vol.55, 1857. doi:10.1167/iovs.13-13860
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      Peter Koch Jensen; Dependence of Outflow Facility on Intraocular Pressure?. Invest. Ophthalmol. Vis. Sci. 2014;55(3):1857. doi: 10.1167/iovs.13-13860.

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      © ARVO (1962-2015); The Authors (2016-present)

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I read with great interest the recent paper of Dastiridou et al. 1 In their paper, the dependence of outflow facility (C) on IOP (P) was reported for healthy and glaucomatous, anaesthetized, cycloplegic human eyes. 
The mean values of C show large variances and are, presumably, skewed as a result of calculating C from the ratio of small differences. No indications of reproducibility are given. The paired values of C allow calculation of its sensitivity to changes in IOP: 
Brubaker's formulation2 of the dependence of outflow resistance (Rp) on P states:  where Pe is the pressure in the episcleral veins and Q is a parameter with an experimental value in human cadaver eyes of 0.012 mm Hg−1. Solution of the equations by measurement1 of C at two pressure levels gives:     
Estimating values of Pe in the range of 5 to 14 mm Hg yields negative calculated values of Q = [−0.1; −3.3] and [−0.1; −1.7] mm Hg−1 and R 0 = [−5.1; −0.31] and [−3.1; −0.19] mm Hg min/μL for glaucomatous and healthy eyes, respectively. 
Mathematically, to obtain a value of Q = 0.012 mm Hg−1, P e must be assumed to be 100 mm Hg, which is physiologically impossible. 
In Armaly's formulation3:  where the constants a1 = 0.03 min/μL and a2 = 1.4 mm Hg min/μl.4 Measurements of C at two pressure levels gives:   with calculated values a1 = 0.53 and 0.33 min/μL and a2 = −7.5 and −4.8 mm Hg min/μL for glaucomatous and healthy eyes, respectively. Findings of negative values for resistance are not meaningful and indicate that the either the uveal outflow and aqueous secretion are not constant or the assumption of linearity is not fulfilled.  
In the article in question, 1 the calculated values of the sensitivity of outflow resistance to IOP (Q and a 1) differ markedly from previously published results. 2,3 Furthermore, there is no indication of variation between individuals. Therefore, it would be of interest to show an x-y scatter plot of the values of C at the two pressure levels plotted against each other for each eye in both groups. The results, 1 however, indicate that the outflow pathway is extremely sensitive to high IOPs under cycloplegia, presumably as result of partial collapse of Schlemm's canal. 5  
References
Dastiridou AI Tsironi EE Tsilimbaris MK Ocular rigidity, outflow facility, ocular pulse amplitude, and pulsatile ocular blood flow in open-angle glaucoma: a manometric study. Invest Ophthalmol Vis Sci . 2013; 54: 4571–4577. [CrossRef] [PubMed]
Brubaker RF. The effect of intraocular pressure on conventional outflow resistance in the enucleated human eye. Invest Ophthalmol . 1975; 14: 286–292. [PubMed]
Armaly MF. The effect of intraocular pressure on outflow facility. Arch Ophthalmol . 1960; 64: 125–132. [CrossRef] [PubMed]
Collins R van der Werff TJ. Mathematical models of the dynamics of the human eye. In: Levin S ed. Lecture Notes in Biomathematics . Berlin: Springer-Verlag; 1980: 1–90.
Tandon PN Autar R. Flow of aqueous humor in the canal of Schlemm. Math Biosci . 1989; 93: 53–78. [CrossRef] [PubMed]
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