The eye has a hidden cleanup system scientists never knew existed

Key points

• Scientists at the University of British Columbia and the University of Toronto identified a previously unknown drainage route at the back of the eye called the posterior ocular lymphatic outflow, or POLO pathway.
• The pathway, demonstrated in mice, carries fluid and waste from the back of the eye into the lymphatic system, challenging the long-held belief that the eye lacked lymphatic drainage.
• Researchers say the discovery could lead to new treatments for glaucoma, age-related macular degeneration and other causes of blindness, though human studies are still needed.

Scientists have identified a previously unknown cleanup system at the back of the eye that lets fluid and waste drain into the body's lymphatic system. The route, called the posterior ocular lymphatic outflow, or POLO pathway, was demonstrated in mice by a team from the University of British Columbia and the University of Toronto.

The finding challenges more than a century of assumptions that the eye lacked the lymphatic drainage found in nearly every other organ. It could eventually lead to new ways to treat glaucoma, age-related macular degeneration and other causes of permanent vision loss, which are linked to the buildup of fluid, metabolic waste and inflammatory material in the back of the eye.

How the pathway was found

The researchers used MRI, near-infrared fluorescence imaging and microscopic examination in mice. They placed fluorescent tracer molecules into a narrow space at the rear of the eye and tracked where the fluid traveled.

The tracers revealed tiny lymphatic vessels in the choroid, a thin, blood vessel-rich layer beneath the retina. Fluid moved from the back of the eye into the surrounding orbital tissues and reached nearby lymph nodes within minutes.

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

— Dr. Neeru Gupta

What researchers said

Dr. Neeru Gupta, professor and head of UBC's department of ophthalmology and visual sciences, said the retina constantly generates byproducts that need to be cleared. "This discovery helps explain how the eye flushes this waste," she said.

Dr. Yeni Yucel, a professor and director of ophthalmic pathology at the University of Toronto, said the team was surprised to see such a direct route for fluid to leave the eye. The researchers say further studies are needed to establish how the pathway works in humans.