Precise Measurements of Pressure in Schlemm’s Canal

A laptop is connected to an endoscope via a camera to produce an image of Schlemm’s canal (the reddish-orange band).

 

Aqueous humor drains from the anterior chamber (front) of the eye, primarily through the trabecular meshwork (TM), and into Schlemm’s canal (SC), and then into collector channels, aqueous veins, the episcleral veins and finally into the general venous system. Episcleral venous pressure (EVP) is an important contributor to intraocular pressure (IOP) but is notoriously difficult to measure accurately and precisely in humans and experimental animals. Results obtained using non-invasive techniques have been varied and imprecise in both humans and experimental animals and results using more invasive techniques in experimental animals have also been inexact. The discrepancies in EVP values have made it very difficult to determine the effects of drugs, such as those used to treat glaucoma, on EVP. As part of our current research, we are devising a method for obtaining accurate, reproducible EVP measurements, using non-invasive and/or minimally invasive techniques in an experimental animal model.

When the IOP is lowered to approach the level of EVP, blood will reflux or fill the SC and collector channels. The SC can be visualized using a specialized lens (goniolens) under a microscope, which is attached to a video camera. Alternatively, the SC can also be observed non-invasively using a small endoscope attached to a digital camera that is connected to a computer (see photo). Using either of these systems, the SC is readily visible as a bright reddish-orange band when it is filled with blood. The ability to observe the deflation and inflation of the SC with changes in IOP in real time provides the possibility of measuring the pressure in SC (SCP). The IOP and appearance of the SC can be recorded and time-stamped at specific time points so that the IOP and images are synchronized. These images can later be analyzed using specialized image analysis software. This method can be used to obtain a very precise description of the effect of IOP on the pressure in the SC, which reflects EVP and may be clinically more relevant than EVP. Obtaining accurate SCP and EVP values is critical for examining their roles in ocular hypertension and glaucoma, and for determining whether the IOP-lowering action of therapeutics are related to effects on these parameters.