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Editors Selection IGR 12-2

Clinical examination methods: Outflow facility

Comment by Makoto Aihara on:

67303 Measurement of Outflow Facility Using iPerfusion, Sherwood JM; Reina-Torres E; Bertrand JA et al., PLoS ONE, 2016; 11: e0150694

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Aqueous humor dynamic is still based on the classic formula known as the Goldmann equation. So far, there are many discussions as to this formula. However, this most fundamental mechanism of IOP has not been clarified. Animal models are needed to develop this question, but big challenges are present in small eyes, especially in mouse eyes. One of the factors can be measured in aqueous dynamics, outflow facility, is still under debate in its pressure dependency.

Sherwood et al. developed the new device to measure mouse outflow facility incorporating an actuated pressure reservoir, thermal flow sensor, differential pressure measurement and a computer interface called iPerfusion^TM in enucleated eyes. As a result, using several tens of B6 mice, outflow facility was best fit in the lognormal distribution and was individually different with six-fold variability.

In this study, outflow facility was well calculated based on the calibrated flow system and statistical analysis. However, there are many limitations as well discussed in the article. As mentioned, one of the issues is that this perfusion system was done in the enucleated eyes. In this incubated eyes, the episcleral venous pressure and the intraorbital pressure have been ignored. Denervation and no blood circulation are also big issues. Subchoroidal negative pressure will be definitely affected. Thus, in in-vivo eyes, the result will change. Hopefully, in future, these systems will contribute to clarify the aqueous dynamics in animal models. However, fundamental issues abound. Especially, still we do not histologically know the real outflow pathway otherwise than the conventional outflow. Even in the conventional outflow, the flow may be variable in the parts of trabecular meshwork, Schlemm's canal, and collector channel. Even more, the unconventional outflow and the pressure in its unknown destinations should be investigated.

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