A team from the University of British Columbia and the University of Toronto says it has identified a previously unknown POLO pathway eye drainage system, revealing a route through which fluid and waste leave the back of the eye and enter the body’s lymphatic network. The findings, published in Translational Vision Science & Technology, were demonstrated in mice and have not yet been validated in humans.
The pathway has been named the posterior ocular lymphatic outflow, or POLO pathway. The researchers say it appears to carry fluid and waste from the choroid (a thin, blood vessel-rich layer beneath the retina) into the surrounding orbital tissues and, from there, into nearby lymph nodes. Whether the same anatomy and function exist in the human eye remains an open question the team says must be addressed before any clinical implications can be drawn.
What the POLO pathway eye drainage study actually found
To trace fluid movement, the researchers combined magnetic resonance imaging, near-infrared fluorescence imaging, and microscopic examination in mice. They introduced fluorescent tracer molecules into a narrow space at the rear of the eye, then observed where those tracers moved. The results showed tiny lymphatic vessels in the choroid, vessels that, the team says, had not previously been established in that region.
Within minutes of injection, the tracers reached nearby lymph nodes. The team interprets this as evidence of an active drainage route rather than passive diffusion. ‘We were surprised to see such a direct route for fluid to leave the eye and connect with the lymphatic system,’ said Dr Yeni Yücel, a professor and director of ophthalmic pathology at the University of Toronto and a pathologist-scientist at St Michael’s Hospital. ‘It suggests the back of the eye has an active clearance pathway, which could play an important role in removing fluid, proteins and inflammatory material that build up in disease.’
The word ‘could’ is carrying some weight there. The study describes a previously unrecognised anatomical feature and demonstrates its apparent function in mice. What it does not do (and the researchers acknowledge this) is establish what happens when the pathway is disrupted, or whether it can safely be modified.
A century of assumptions, and a prior finding from the same team
For more than a century, the prevailing view held that the eye lacked the lymphatic drainage systems found in nearly every other organ. That view began to shift in 2009, when Dr Neeru Gupta and Dr Yücel identified what they described as a lymphatic-related drainage route at the front of the eye, naming it the uveolymphatic pathway. The current discovery extends that line of inquiry to the back of the eye, a region the team describes as particularly underexplored despite being the site of some of the most serious vision-loss diseases.
‘The retina is one of the most metabolically active parts of the body, constantly generating byproducts that need to be cleared,’ said Dr Gupta, professor and head of UBC’s department of ophthalmology and visual sciences. ‘This discovery helps explain how the eye flushes this waste and promises to transform how we think about and treat a range of eye conditions.’
The conditions most often cited in connection with fluid and waste accumulation at the back of the eye include glaucoma and age-related macular degeneration. The report states that age-related macular degeneration alone affects approximately 2.5 million Canadians, which puts the potential reach of any future treatment in context, though no treatment is yet in prospect.
Claims worth separating from the evidence
‘This gives us a whole new framework for understanding these diseases and a potential target for therapeutics,’ Dr Gupta said. ‘The question now is: How can we enhance or exploit this cleanup system to treat or even prevent disease.’ That framing is reasonable as a statement of research intent. As a guide to near-term clinical development, it is premature, by the researchers’ own account.
The team lists several possibilities that further research might eventually pursue: therapies designed to improve fluid clearance, improved drug delivery to deep ocular structures, and better understanding of how pressure and inflammation interact in glaucoma and macular degeneration. Each is described as a potential future development, not an established or planned treatment.
‘Because lymphatic vessels in the choroid were thought not to exist, the team used multiple techniques to demonstrate both their presence and function,’ Dr Yücel said. That methodological caution (using several independent imaging approaches to confirm the same result) is worth noting. A single imaging modality showing an unexpected structure would be far easier to dismiss.
The research was supported by the Canadian Institutes of Health Research, the Glaucoma Research Society of Canada, the Canadian Space Agency, the Canada Foundation for Innovation Leaders Opportunity Fund, and several named endowment funds. The paper’s authors include Babishaa Sauntharrajan, Dr Gupta, Xun Zhou, Ekim Gümeler, Greg Stanisz, Wilfred W. Lam, Margaret Koletar, and Dr Yücel. Next steps, as the team frames them, involve determining how the POLO pathway functions in humans and whether changes in its activity are associated with eye disease, a research programme rather than a therapy.
