A family of anterior segment angiographic methods
There are currently only a handful of anterior segment angiographic methods besides aqueous angiography. Dr Huang explained that scleral angiography and iris angiography are similar to retinal IFA in that tracers are placed into the peripheral vein to observe blood flow, in this case for the anterior segment.
More specifically, iris angiography is useful for visualising neovascular membranes in the anterior segment, which are often associated with neovascular diseases such as diabetes. Scleral angiography has been reported to be useful in evaluating scleral ischaemia in scleritis12. Alternatively, Dr Huang’s group has used scleral angiography to visualise abnormal blood vessels in anterior segment ocular tumours13.
For anterior segment aqueous humour outflow, canalography is another angiographic method14. In canalograms, an external surgical approach is taken to access Schlemm’s canal similar to canaloplasty glaucoma surgery. Tracers are then directly delivered into the canal via a cannula or a canaloplasty fiberoptic probe.
“Unlike aqueous angiography, canalograms bypass the trabecular meshwork,” Dr Huang noted. “Therefore, the final angiographic image shows the surgeon what aqueous humour outflow looks like without the trabecular meshwork filter.”
A mathematical model including canalograms and aqueous angiography would be: canalogram – (aqueous angiography) = trabecular meshwork. In this way, studying outflow with both techniques will be important to reveal insights into the function of the trabecular meshwork in both normal and diseased eyes.
Separately, a new and popular angiographic method is OCT angiography (OCTA)15. OCTA is advantageous as it is noninvasive and does not require tracer introduction. However, as Dr Huang stated, OCTA requires particulate motion to generate a signal.
The blood corpuscle motion in the blood vessels is what is being visualised in retinal OCTA, and in the anterior chamber there is no equivalent structure that can be detected for aqueous humour outflow. Therefore, Dr Huang believes that OCTA is currently not yet relevant for visualising aqueous humour outflow.