For 80 to 90 percent of patients, symptoms go away within two weeks. But others, such as Aldrich, experience symptoms for months or even years. A history of multiple concussions may increase the risk of more serious problems later in life, including Alzheimer’s disease and chronic traumatic encephalopathy, or CTE, a neurodegenerative disorder with dementia-like symptoms.
Although it’s clear that concussions damage the brain, exactly how they do so is still largely a mystery—especially when it comes to long-term problems. An intriguing new clue focuses on tiny tubes sandwiched between the meninges, a set of membranes that surrounds and protects the brain and spinal cord. These tubes, called meningeal lymphatic vessels, help clear cellular and molecular waste from the brain. A mouse study published in September in the journal Nature Communications reported that after minor blows to the head, the brain swells and pins these vessels up against the skull. Like putting a kink in a hose, this diminishes their ability to drain properly.
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This damaged drainage system, the researchers speculate, may be what leads to more severe and longer-lasting symptoms.
“We know that most of the time, a concussion is a limited process; most people recover and don’t have long-term effects,” says Ann McKee, a neuropathologist who directs Boston University’s CTE Center and was not involved in the study. But in autopsies of people who had suffered from CTE during their life, McKee has found scarring in the meninges. “The idea that meningeal lymphatic channels may contribute to inflammation and persistent symptoms, I think, is a very interesting idea—it makes a lot of sense to me.”
The existence of lymphatic vessels surrounding the brain was confirmed only recently—in 2015—but scientists have long known that they exist elsewhere in the body, mingled among the veins and arteries that supply oxygen and nutrients to tissues and organs. This network of tiny, thin-walled tubes acts as the body’s molecular trash-collection system. The vessels collect the clear fluid called lymph that leaks from the bloodstream; it’s filled with immune cells, as well as proteins and molecular debris. These are transported to lymph nodes: small structures throughout the body that filter out the harmful waste.
In the 18th century, Italian physician Paolo Mascagni created a detailed atlas of the lymphatic system and depicted these vessels in the meninges around the brain. But other scientists dismissed the idea, arguing that the lymphatic system was completely separate from the central nervous system. That remained the consensus for more than 200 years.
Then, in 1996, scientists looking at brain tissue with an electron microscope discovered structures in the meninges that looked like lymphatic vessels. Other researchers still weren’t convinced, writing off the tubes as capillaries, which are also thin-walled vessels. Finally, in 2015, neuroimmunologist Jonathan Kipnis (now at Washington University in St. Louis) and his colleagues at the University of Virginia examined the brains of mice and uncovered a web of channels that resembled the lymphatic vessels found in the rest of the body.