Author Response: Dependence of Outflow Facility on Intraocular Pressure?

We would like to thank Peter Koch Jensen ¹ for the kind remarks and his interest in our paper. In our article ² we provided original data on ocular rigidity, outflow facility C, ocular pulse amplitude, and pulsatile ocular blood flow, based on measurements in living human eyes using an invasive experimental setting. Due to the inherent difficulties in the measurement procedure and the fact that all measurements were performed in patients just before cataract surgery, ² it would be difficult to test reproducibility. However, reproducible results were found with the same technique in animal experiments (data not shown). 

Contrary to what originally was thought, that C is constant and outflow resistance R is not affected by the intraocular pressure IOP, Brubaker ³ as well as other investigators ^4,5 suggested that R changes in relation to the IOP. Brubaker ³ introduced his technique to overcome ocular stretching and anterior chamber deepening, and found that R increases directly and linearly with IOP. His results pertain to measurements in the enucleated normal human eye, whereas Armaly ⁴ performed his experiments in rabbit and cat eyes. 

As Jensen ¹ has already pointed out, there are discrepancies between our results, and the results from Brubaker ³ and Armaly. ⁴ Since our data represented original measurements in the living human eye, these differences compared to previous investigations could be explained by differences in methods, and fundamental differences between the living and cadaver eye. Indeed, analysis of the outflow curves, which were recorded continuously for 4 minutes in our study, do not support a linear dependence of R on IOP. Importantly, the difference between our values and data observed by Brubaker ³ increases as the IOP increases and this may explain the negative estimates calculated assuming a linear approach. 

As a result, we did not base the analysis of our data on the calculation of an equivalent Q or a1 coefficients, as that would imply a linear relationship between resistance to outflow and IOP. However, in our original paper, in the second paragraph in the Results section, we did report that there was no difference in the quotients of C at an IOP of 35 mm Hg and C at 25 mm Hg between patients and controls (P = 0.497). ²  

In addition, another difference in methodology between our study and the one by Brubaker ³ is that in our measurement technique we allowed for changes in anterior chamber dimensions and the volume of the eye, and also measured this relation between pressure and volume. 

Overall, it could be argued that the procedure followed in our investigation is invasive and bears inherent difficulties as to reproducibility testing, as well as limitations to the questions that it could answer, but it is clear that it provides data in living human eyes in clinically meaningful levels of IOP.