On a crisp September day, Zoe Aldrich walked onto a rugby pitch on a college campus in upstate New York. With her teammates surrounding her, she got ready for the kickoff and the pitch became a blur of colored jerseys. A teammate passed Aldrich the ball and she started running, but an opposing player tackled her to the ground. Players collided above her, competing for the ball. As Aldrich tried to crawl out from under them, one of her teammates accidentally kicked her in the head. "I never lost consciousness," she says, "but I didn't feel well."

Trainers diagnosed her with a concussion, and for the next year and a half, Aldrich suffered from a feeling of fogginess, like her brain wasn't working correctly. People told her she would feel better in two weeks, then four, then six. Eventually, she says, "I had to give up on this notion that I had to wait a certain number of weeks and then things would go back to normal."

Each year in the United States, there are around 3.8 million concussions, and sports- and recreation-related activities are responsible for a significant number of them. Most patients experience symptoms similar to Aldrich's — headaches, nausea, fatigue, dizziness, and memory loss. There is no single test to diagnose a concussion; instead, doctors examine balance, coordination, ability to pay attention, and memory. If the symptoms are severe, they'll also conduct brain scans to check for swelling or bleeding.

For around 80 percent of patients, symptoms go away within two weeks. But others, like 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.

Italian physician Paolo Mascagni created a detailed atlas of the lymphatic system — including this drawing from his 1787 Vasorum Lymphaticorum Corporis Humani Historia et Ichnographic. He depicted the vessels in the meninges around the brain, but other scientists dismissed the idea, arguing that the lymphatic system was separate from the central nervous system. It took more than two centuries for scientists to show that Mascagni was right. | (PAOLO MASCAGNI / WELLCOME IMAGES / Courtesy Knowable Magazine)

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 to 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.

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."

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