An area of great excitement in lymphatic research - and I think great confusion - is the ‘new discovery’ of ‘lymph vessels’ in the brain. The idea of functioning lymph vessels inside the brain is fascinating, as is every new discovery in lymphology, but the research doesn't exactly show this. The evidence for lymphatic endothelium in the dural spaces is exciting, but is it vastly different to what we have been teaching in Vodder Courses for the last 20 years? and will it change the way we perform clinically? No and no.
But it does offer an opportunity to review what we know about fluid and waste removal and the passage of immune cells through the cerebral spaces. The paper that started all the hype was published in Nature in 2015 and stated that the authors had found
“...... functional lymphatic vessels lining the dural sinuses. These structures express all of the molecular hallmarks of lymphatic endothelial cells, are able to carry both fluid and immune cells from the cerebrospinal fluid, and are connected to the deep cervical lymph nodes. The unique location of these vessels may have impeded their discovery to date, thereby contributing to the long-held concept of the absence of lymphatic vasculature in the central nervous system.”
Louveau A, Smirnov I, Keyes TJ, Eccles JD, Rouhani SJ, Peske JD, et al. Structural and functional features of central nervous system lymphatic vessels. Nature. 2015;523:337.
Delving a little deeper into the paper we find that this work has been done in murine models (mice), not humans, so care should be taken not to immediately assume that humans will be the same. What was found are vessels in the dural spaces which appear to have the characteristics of initial lymph vessels and that these vessels are connected to lymph pathways in the deep cervical system. The endothelial cells form in tubules with a lumen but there is no smooth muscles and no valves,
Image of a mouse brain showing evidence of lymphatic endothelial cells lining the dural sinuses
This adds an extra layer of detail to what we already know about removal of the lymph obligatory load (LOL) from the cerebral spinal fluid (CFS). We already know that the LOL, which includes circulating immune cells and waste products such as the amyloids responsible for Alzheimer’s disease, are removed through the subarachnoid sheath via ‘arachnoid granulations’. Dyes and tracers used to image this pathway show pathways through the deep cervical lymphatics, the perineural sheaths of olfactory, optic nerves and spinal nerves and in suboccipital lymph nodes. The new findings suggest that there is a formed, lymphatic endothelium along the dural sinuses of mice, which resemble initial lymph vessels. If these vessels are also present in human they may add a connecting piece in the puzzle of how the LOL is transported from the subarachnoid sheath to the external collecting lymphatics.
Does this influence the way we perform MLD for drainage of the cerebral spaces as we do after stroke etc? Not at all! Clearing proximal pathways via the cervical lymph nodes and drainage of the soft palate to clear the olfactory pathway are still the best technique to reduce cerebral oedema. New lymphatic discoveries are always welcome – and there is so much still to discover! For MLD therapists, this is one of the less startling discoveries of late, partly because it doesn’t really add anything to our clinical practice.
Diagram of cerebral drainage pathways from the 1997 Textbook of Dr. Vodder’s Manual Lymph Drainage by former Vodder School Medical Director, Dr Ingrid Kurz. Volume 2: Therapy 4th Edition, Haug Publishers