POLO pathway: a hidden drain at the back of the eye

A team from the University of British Columbia and the University of Toronto has found a drainage route at the back of the eye that had not been described before, ScienceDaily reports. The researchers call it the posterior ocular lymphatic outflow, or POLO pathway. Fluid and waste leave the rear of the eye through it and join the lymphatic network, the vessels and tissues that keep fluid levels steady, carry off waste and help coordinate immune responses.

The work was done in mice, and the results appear in Translational Vision Science & Technology, ScienceDaily reports. The finding does not change any treatment available today. What it does is give researchers a named structure to study in a part of the body where that structure was thought to be absent.

The retina sits at the back of the eye and turns incoming light into signals the brain can read. That work takes a lot of energy and leaves waste that has to be cleared. When fluid and waste build up, they are associated with glaucoma, age-related macular degeneration and other retinal disorders. Age-related macular degeneration alone affects about 2.5 million Canadians, ScienceDaily reports.

"The retina is one of the most metabolically active parts of the body, constantly generating byproducts that need to be cleared," said Dr. Neeru Gupta, a professor who heads the department of ophthalmology and visual sciences at UBC. Gupta said the discovery helps explain how the eye clears that waste.

A century of thinking the eye was sealed

For more than a century, the standard view held that the eye had no lymphatic drainage of the kind found in almost every other organ, ScienceDaily reports. That started to shift in 2009, when Gupta and Dr. Yeni Yücel described a drainage route at the front of the eye and named it the uveolymphatic pathway.

Yücel, a professor at the University of Toronto who directs ophthalmic pathology there and is a pathologist-scientist at St. Michael's Hospital, kept looking for similar routes elsewhere in the eye. The back of the eye was the biggest gap, because that is where several of the most serious causes of permanent vision loss begin.

"The retina is responsible for vision and it's also where many of the most serious vision-loss diseases occur," Yücel said.

How the route was traced

The team combined magnetic resonance imaging, near-infrared fluorescence imaging and microscopy, all in mice, ScienceDaily reports. It put fluorescent tracers into a narrow space behind the eye and followed where the molecules traveled.

The tracers showed small lymphatic vessels in the choroid, the thin layer under the retina that is packed with blood vessels. Such vessels had not been established there before. Fluid left the rear of the eye and moved into the tissue around the eyeball, and tracers reached nearby lymph nodes within minutes. That links the eye's new drain to the body's wider lymphatic system.

What is still unknown

The experiments were in mice. Scientists still need to establish how the pathway works in people, whether changes in it contribute to eye disease and whether its activity can be modified safely, according to ScienceDaily. This is one study, not a consensus.

If those questions are answered, a few possibilities open up. Therapies could aim at improving fluid clearance, which might reduce the buildup tied to inflammation and tissue damage. Knowing the route could also change how drugs are delivered to structures deep inside the eye. It may also help explain how pressure, inflammation and fluid movement interact in glaucoma and age-related macular degeneration. All of that is future work, not a treatment available now.

Gupta said the finding gives researchers a new map, a new mechanism and a new set of questions to explore.

This is a foundational discovery that shows the eye is not as closed a system as we previously thought.

Gupta described the result that way. The findings were published in Translational Vision Science & Technology, ScienceDaily reports. Funding came from Canadian research organizations, including the Canadian Institutes of Health Research, the Glaucoma Research Society of Canada and the Canadian Space Agency.

What this means for a reader today is limited but concrete. A part of the eye long treated as cut off from the body's waste-clearing network now has a name, and a named structure is something future treatments can aim at. For anyone with glaucoma or macular degeneration in the family, that is the part worth remembering.

• eye
• glaucoma
• macular degeneration
• POLO pathway
• University of British Columbia
• University of Toronto