Improving MIGS Outcomes with Aqueous Outflow Imaging

Multiple approaches described, all still evolving. Cheryl Guttman Krader reports from the 39th ESCRS Congress in Amsterdam

Minimally invasive glaucoma surgery (MIGS) procedures that aim to increase conventional aqueous outflow have benefits because of their ease and safety but can fall short in their IOP lowering effect.

While various imaging approaches have been successfully used to improve aqueous humour outflow understanding, none of the techniques tried so far has all the characteristics that describe a clinically practical and effective approach that can improve surgical methods and outcomes, according to Alex A Huang MD, PhD.

“An ideal imaging method would be one used pre- or perioperatively so that it gives knowledge just prior to the actual surgery. It should be non-invasive and physiologic, meaning it is done without creating wounds or needing any visualisation agents. And it should be based on positive imaging [so] the user is looking to identify a signal rather than for something lacking or disappearing,” Dr Huang said.

“In addition, the method should be informative to guide the surgery performance and predict success and easy in terms of technical feasibility, the number of steps required, and time intensiveness. Together, these criteria make the ideal aqueous humour outflow imaging methods for improving patient care.”

TRIED BUT CARRYING LIMITATIONS Describing his group’s work to evaluate OCT imaging and automated segmentation of Schlemm’s canal and collector channels, Dr Huang said drawbacks include very time-intensive imaging acquisition and processing. Furthermore, the clinical meaning of the information it provides is unclear.

“If we see a dilated pathway, does that mean there is more flow or a trapped pocket of fluid that can’t get out?” he asked.

Fluorescein channelographic/canalographic examination is an approach used during viscocanalostomy or canaloplasty to visualise outflow pathways. This approach, however, is performed after trabecular MIGS and suffers from being invasive and nonphysiologic, Dr Huang said.

Imaging of the episcleral venous fluid wave created by a surge of BSS has been used to evaluate trabecular outflow after trabectome surgery and suggested as a possible predictor of surgical outcomes. However, this is a nonphysiologic measure and relies on a negative imaging technique to look for loss of signal after the surgery.

Haemoglobin video imaging of aqueous outflow into the vascular system using a monochromatic camera is preoperative, non-invasive, and physiologic. However, it is not yet a real-time assessment owing to custom analyses and loses the big picture because it is magnified on a small field with examination of individual veins.

Addressing the latter limitation, Dr Huang and colleagues worked on developing aqueous angiography to evaluate aqueous humour outflow and designed a two-tracer method using indocyanine green followed by fluorescein to show outflow improvement after trabecular MIDS. On the plus side, this technique can be done before and after the surgery, has positive imaging, and is “sort of” physiologic, Dr Huang said. But their method has limitations as well as invasive and particularly complex in terms of steps and personnel needed.

“What would be nice is if we could image something that is endogenous and in a positive way,” he said.

He proposed vitamin C imaging as one possibility, but this presents a technical challenge.

“The concentration of vitamin C is 100-fold higher in the aqueous than in the serum. However, vitamin C is a very bland molecule, so it is hard to image,” he explained.

“An ideal imaging method would be one used pre- or perioperatively so that it gives knowledge just prior to the actual surgery.”

Alex A Huang MD, PhD recently joined the Shiley Eye Institute at the University of California, San Diego, USA, as Alfred Vogt Chair in Ophthalmology. ahuang@doheny. org or alhuanga@yahoo.com