GLAUCOMA RESEARCH

The lymphatic system appears to play a hitherto unappreciated role in aqueous outflow, a finding that could lead to new therapeutic approaches for glaucoma, Professor Neeru Gupta, Chief of Glaucoma at the University of Toronto, Canada, told a session of the World Ophthalmology Congress in Tokyo.

The lymphatic system pumps three litres of lymph throughout the human body every day. This is important because it drains extracellular solutes, removes fluids from our extracellular space, and plays an important role in immune surveillance, she explained.

The lymphatic system accomplishes its functions through capillaries in the tissues. These capillaries have one-way valves that propel fluid through the smaller lymphatic capillaries, then merge to form larger lymphatic vessels, pass through the lymph nodes and eventually drain back into the subclavian vein at the base of the neck.

“We don’t often talk about lymphatics in the eye. It was believed for a long time that they didn’t exist. But if you think about it, the aqueous is transparent and ocular tissues are highly metabolically active. For example, the accommodative mechanism in the ciliary body is very dynamic. This means that surely there must be metabolites that are being released, there must be some system for removing waste products,” said Dr Gupta.

Prof Gupta described a series of experiments in collaboration with Prof Yeni Yucel, Director of Eye Pathology at the University of Toronto, that appear to confirm the existence of lymphatics in the eye. Coining this the “uveolymphatic outflow pathway”, with the potential to manipulate the process for therapeutic purposes, they pursued a series of investigations.

First she and colleagues reviewed pathological section studies demonstrating the presence of lymphatic vessels in human ciliary body. These were subsequently confirmed with electron microscopy studies (Yucel YH et al, Exp Eye Research, 89: 810-819, 2009).

But does aqueous flow through lymphatics? To address this question, her group designed studies with living sheep. These are a preferred animal model for studying the lymph system because of their large size. Radioactive tracer studies indicated that radioactive albumin injected in the eye was cleared from the eye and ended up in the cervical lymph nodes.

Can lymphatic drainage from the eye be measured? Additional studies in sheep showed that this was indeed the case (Kim M et al, Exp Eye Research. Nov;93(5):586-91, 2011).

“So we know the lymphatic vessels are in the eye. We know that they have a role in aqueous flow. We have developed a method for measuring this, and this is the beginning of much more work that is needed to understand this system.

“We believe the lymphatic system in the eye offers an exciting potential to improve our understanding of aqueous outflow, the role of the uveolymphatic pathway in glaucoma, and to develop new therapeutic targets. We look forward to seeing many new studies in this area,” Dr Gupta told the conference.

Neeru Gupta: GuptaN@smh.ca